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Why will happen if plant phloem does not contain sieve plates?

Why will happen if plant phloem does not contain sieve plates?


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What are the primary functions of the sieve plates that make them so crucial? I've done a bit of reading online and found "Sieve plates are perforated end walls separating the component cells (sieve elements) that make up the phloem sieve tubes in vascular plants. The perforations permit the flow of water and dissolved organic solutes along the tube and are lined with callose." However, wouldn't the flow of water or organic diluted be faster without the extra sieve plates obstructing the flow? Or is there some other functions of the sieve tubes?


First of all, phloem sieve cells (opposing to xylem vessels) are live cells. In the phloem, a transport occurs not like in water pipes (with free movement of liquids) but through cells transport structures, particularly through plasmodesmata. As live cells, sieve elements need all standard structures, including cell wall, to sustain functionality.

Moreover, the phloem transport is (at least partially) an active process. I.e. to transport something you have to spend some energy. Today the most supported hypothesis of phloem transport is Pressure Flow Hypothesis . According to Wikipedia:

While movement of water and minerals through the xylem is driven by negative pressures (tension) most of the time, movement through the phloem is driven by positive hydrostatic pressure. This process is termed translocation, and is accomplished by a process called phloem loading and unloading. Cells in a sugar source "load" a sieve-tube element by actively transporting solute molecules into it. This causes water to move into the sieve-tube element by osmosis, creating pressure that pushes the sap down the tube. In sugar sinks, cells actively transport solutes out of the sieve-tube elements, producing the exactly opposite effect. The gradient of sugar from source to sink causes pressure flow through the sieve tube toward the sink.

So, a plant needs live cells to support such mechanism.

Here we come to sieve plates. In plants, live cells interacts by means of plasmodesmata. In simple words, they are extensions of endoplasmatic reticulum which connect the cytoplasm of two cells and act as transport channels. And sive plates are simply places of highly hypertrophied plasmodesmata region.

So in conclusion:

  1. Phloem transport is a partially active transport which occurs with an involvement of cells transport structures like plasmodesmata.
  2. The region of hyper-developed plasmodesmata on sieve elements wall appears as sieve plate.
  3. That is why an absence of such structure will not provide any gains in conductivity.

UPD. In response to Jim Young comment. Damage protection is not a major role of sieve plates. Again, sieve plates are areas where plasmodesmata penetrate cell wall. Plasmodesmata act as the main vector of intercellular transport. So, the transport is the main purpose (or a reason) of sieve plates. The damage protection is an additional property and not an aim of these structures. As to tyloses, yes it is a common mechanism of vertical transport prevention in xylem. But xylem vessels consist of dead cells. They are literally pipes. Therefore it is possible to cork up xylem vessels by simply inserting packs of fibre. BTW, that is why cavitation in vessels can cause embolization. In contrast, phloem cells are live cells (with cytoplasm and EPR). They conduct water and sugars not by free flaw but with an involvement of intercellular mechanisms. That is why mechanical embolization is not a case for phloem cells.


Developmental Signaling in Plants

E. Saplaoura , F. Kragler , in The Enzymes , 2016

Abstract

Phloem serves as a highway for mobile signals in plants. Apart from sugars and hormones, proteins and RNAs are transported via the phloem and contribute to the intercellular communication coordinating growth and development. Different classes of RNAs have been found mobile and in the phloem exudate such as viral RNAs, small interfering RNAs (siRNAs), microRNAs, transfer RNAs, and messenger RNAs (mRNAs). Their transport is considered to be mediated via ribonucleoprotein complexes formed between phloem RNA-binding proteins and mobile RNA molecules. Recent advances in the analysis of the mobile transcriptome indicate that thousands of transcripts move along the plant axis. Although potential RNA mobility motifs were identified, research is still in progress on the factors triggering siRNA and mRNA mobility. In this review, we discuss the approaches used to identify putative mobile mRNAs, the transport mechanism, and the significance of mRNA trafficking.


Abstract

Bark is an important functional structure in woody plants. However, fossil tree axes are commonly decorticated. The development of bark during the evolutionary history of fossil plants thus remains poorly understood. Here, we describe exceptionally well-preserved extraxylary tissues of the Lopingian (Late Permian) conifer Ningxiaites specialis Feng, including vascular cambium and bark, from the Sunjiagou Formation (Changhsingian) of northern China. The vascular cambium bears one or two layers of cambial cells. The bark comprises secondary phloem and periderm. The secondary phloem consists of rays, axial parenchyma and sieve cells. The rays of the secondary phloem are uniseriate and continuous from the rays of the xylem. They are more frequently present in the inner zone of the secondary phloem. Axial parenchyma cells are vertically aligned and appear more regularly distributed in the outer zone of the secondary phloem. Elliptical or subcircular sieve areas are placed on the radial walls of the sieve cells. The periderm located outside the secondary phloem is composed of imbricate flattened cork cells. The cork cells show suberised cell walls and are generally filled with dark contents. Remains of the secondary phloem present between layers of periderm indicate the formation of rhytidome-type bark. This is the first detailed report of the bark anatomy of a conifer from the upper Palaeozoic of Cathaysia, and shed light on the early diversity of bark structure during the evolutionary history of conifers.


PD-MEDIATED TRANSPORT OF MACROMOLECULES

Direct Evidence Provided by Viral Movement Proteins

A considerable body of genetic evidence has now accumulated to support the concept that plants use a combination of NCAPs and PD to communicate between cells. Experimental support for the concept that PD have the capacity to mediate the cell-to-cell trafficking of macromolecules was provided by studies into the mechanisms by which plant viruses move within host tissues ( Deom et al., 1992 Lucas and Gilbertson, 1994 Carrington et al., 1996). Genetic studies identified viral-encoded proteins, termed movement proteins (MPs), which were shown to be essential for the cell-to-cell spread of infection. The link between these viral MPs and PD was established when it was discovered that expression of the Tobacco mosaic virus (TMV)–MP, within transgenic tobacco plants, resulted in an alteration in the functional properties of mesophyll PD. Under normal conditions, such PD restrict the size of molecules that can diffuse cell to cell to ∼800 D ( Robards and Lucas, 1990). However, in the presence of the TMV-MP, this size exclusion limit (SEL) was increased to a value in the range of 15 kD ( Wolf et al., 1989).

PD Potentiate Selective Cell-to-Cell Transport of Viral MPs/MP–Nucleic Acid Complexes and Endogenous Transcription Factors.

(A) and (B) Bright-field and fluorescent images, respectively, illustrating extensive cell-to-cell movement of a FITC-labeled viral MP after its injection into a Phaeolus vulgaris (bean) mesophyll cell. Arrows indicate injected cells. IAS, intercellular air space. (Adapted from Noueiry et al., 1994.)

(C) A mutation in this MP blocked its ability to move out of the injected cell. Arrows indicate injected cells. (Adapted from Noueiry et al., 1994.)

(D) Expression of TMV-MP-GFP in a tobacco epidermal cell, after biolistic bombardment, leads to cell-to-cell movement of this fluorescent probe. (Adapted from Crawford and Zambryski, 2001.)

(E) Control GFP bombardment experiment in which free GFP (27 kD) was produced in a tobacco epidermal cell (source leaf). Limited GFP diffusion into adjacent cells likely reflects low-frequency trafficking of endogenous NCAPs. (Adapted from Kotlizky et al., 2001.)

(F) Presence of viral MP (vMP) or MP–nucleic acid complexes (vNA-MP) (microinjected or produced in the infected cell) causes the dilation of PD microchannels, thereby permitting cell-to-cell movement of MP, MP–nucleic acid, and F-dextran/GFP probes (yellow circles). CW, cell wall N, nucleus.

(G) Cell-to-cell trafficking of a tetramethylrhodamine isothiocyanate (TRITC)–labeled NCAP (left) permitted the simultaneous spread of an 11-kD FITC-labeled dextran (center) the yellow signal resulting from merged images (right) highlights the coupled nature of the TRITC-NCAP and FITC-dextran movement. Arrows indicate injected cell. (Adapted from Kragler et al., 1998b.)

(H) KN1 displays NCAP properties microinjection of KN1-FITC (left) or KN1 plus 20-kD FITC-labeled dextran (center) resulted in the spread of fluorescence signal into the surrounding mesophyll cells, but movement was blocked in the case of the M6 KN1 mutant (right). Arrows indicate injected cell. (Adapted from Lucas et al., 1995.)

(I) to (K) Biolistic bombardment experiments confirm the capacity of NCAPs to traffic through PD. (Adapted from Kim et al., 2002.)

(I) Confinement to the target cell of the fluorescent signal associated with expression of GFP-YFP (52 kD) in epidermal cells of Arabidopsis.

(J) Parallel experiment to (I) demonstrating limited cell-to-cell movement of GFP-KN1 (∼69 kD).

(K) Parallel experiment to (J) illustrating cell-to-cell movement of GFP-KN1 in onion root epidermal cells.

(L) Endogenously expressed or microinjected NCAPs interact with PD to induce microchannel dilation, thereby permitting their entry into the next cell as well as the co-diffusion of F-dextran/GFP probes (yellow circles). Cell-autonomous proteins (CAPs) lack this capacity to interact with PD. CW, cell wall N, nucleus.

(M) and (N) Schematic illustrations of the patterns of NCAP cell-to-cell movement after delivery by microinjection or plasmid bombardment, respectively. In microinjection experiments, an NCAP generally spreads through some five cells within 1 min by 10 min it will have moved out in a radial direction through some 10 cells. In bombardment experiments, NCAP-GFP expression takes 24 to 48 hr before a fluorescent signal can be detected, and then radial movement is often restricted to one or two cells.

PD Potentiate Selective Cell-to-Cell Transport of Viral MPs/MP–Nucleic Acid Complexes and Endogenous Transcription Factors.

(A) and (B) Bright-field and fluorescent images, respectively, illustrating extensive cell-to-cell movement of a FITC-labeled viral MP after its injection into a Phaeolus vulgaris (bean) mesophyll cell. Arrows indicate injected cells. IAS, intercellular air space. (Adapted from Noueiry et al., 1994.)

(C) A mutation in this MP blocked its ability to move out of the injected cell. Arrows indicate injected cells. (Adapted from Noueiry et al., 1994.)

(D) Expression of TMV-MP-GFP in a tobacco epidermal cell, after biolistic bombardment, leads to cell-to-cell movement of this fluorescent probe. (Adapted from Crawford and Zambryski, 2001.)

(E) Control GFP bombardment experiment in which free GFP (27 kD) was produced in a tobacco epidermal cell (source leaf). Limited GFP diffusion into adjacent cells likely reflects low-frequency trafficking of endogenous NCAPs. (Adapted from Kotlizky et al., 2001.)

(F) Presence of viral MP (vMP) or MP–nucleic acid complexes (vNA-MP) (microinjected or produced in the infected cell) causes the dilation of PD microchannels, thereby permitting cell-to-cell movement of MP, MP–nucleic acid, and F-dextran/GFP probes (yellow circles). CW, cell wall N, nucleus.

(G) Cell-to-cell trafficking of a tetramethylrhodamine isothiocyanate (TRITC)–labeled NCAP (left) permitted the simultaneous spread of an 11-kD FITC-labeled dextran (center) the yellow signal resulting from merged images (right) highlights the coupled nature of the TRITC-NCAP and FITC-dextran movement. Arrows indicate injected cell. (Adapted from Kragler et al., 1998b.)

(H) KN1 displays NCAP properties microinjection of KN1-FITC (left) or KN1 plus 20-kD FITC-labeled dextran (center) resulted in the spread of fluorescence signal into the surrounding mesophyll cells, but movement was blocked in the case of the M6 KN1 mutant (right). Arrows indicate injected cell. (Adapted from Lucas et al., 1995.)

(I) to (K) Biolistic bombardment experiments confirm the capacity of NCAPs to traffic through PD. (Adapted from Kim et al., 2002.)

(I) Confinement to the target cell of the fluorescent signal associated with expression of GFP-YFP (52 kD) in epidermal cells of Arabidopsis.

(J) Parallel experiment to (I) demonstrating limited cell-to-cell movement of GFP-KN1 (∼69 kD).

(K) Parallel experiment to (J) illustrating cell-to-cell movement of GFP-KN1 in onion root epidermal cells.

(L) Endogenously expressed or microinjected NCAPs interact with PD to induce microchannel dilation, thereby permitting their entry into the next cell as well as the co-diffusion of F-dextran/GFP probes (yellow circles). Cell-autonomous proteins (CAPs) lack this capacity to interact with PD. CW, cell wall N, nucleus.

(M) and (N) Schematic illustrations of the patterns of NCAP cell-to-cell movement after delivery by microinjection or plasmid bombardment, respectively. In microinjection experiments, an NCAP generally spreads through some five cells within 1 min by 10 min it will have moved out in a radial direction through some 10 cells. In bombardment experiments, NCAP-GFP expression takes 24 to 48 hr before a fluorescent signal can be detected, and then radial movement is often restricted to one or two cells.

Two additional lines of evidence confirmed this conclusion. First, a number of mutant viruses lacking functional coat protein have been shown to retain the capacity to establish a local infection ( Lucas and Gilbertson, 1994 Carrington et al., 1996 Gilbertson and Lucas, 1996). In such situations, because viral particles cannot be formed within the cell, the cell-to-cell spread of infection must be based on the transport of a MP–nucleic acid complex. Second, biolistic experiments confirmed that when produced in vivo, a GFP-tagged MP could move into the surrounding cells via PD ( Table 1, Figure 4D). In contrast, bombardment of GFP::YellowFP, which results in the synthesis of an equivalently sized protein to the MP-GFP, led to the confinement of the fluorescent signal to the targeted epidermal cell ( Kim et al., 2002). Collectively, these studies have established that viral MPs have the capacity to interact with PD to (a) induce an increase in SEL (b) mediate their own transport into the neighboring cell and (c) potentiate the cell-to-cell movement of the viral infectious agent, in the form of a MP–nucleic acid complex ( Figure 4F).

Endogenous Proteins on the Move

Studies performed on a number of plant transcription factors, such as KN1, FLO, LFY, GLO, and DEF, provided strong evidence that these endogenous proteins similarly have the capacity to interact with and move through PD ( Table 1). As observed for viral MPs, introduction of such proteins resulted in (a) an increase in PD SEL (b) the cell-to-cell transport of the probe and (c) simultaneous spread of protein and SEL probes ( Figures 4G and 4H, Table 1). Specificity of the interaction between the protein and the PD transport pathway was again confirmed using engineered KN1 mutant proteins ( Figure 4H, Table 1). In a series of experiments using GFP-tagged KN1 expressed after biolistic delivery or tissue-specific expression within transgenic Arabidopis lines also provided independent confirmation that KN1 has the capacity to move cell to cell ( Kim et al., 2002) ( Figures 4I to 4K).

Control experiments performed with a range of fluorescent probes, including fluorescein isothiocyanate (FITC)–labeled dextrans (10 to 40 kD) and heterologous proteins derived from a variety of organisms, confirmed the requirement for specificity in terms of macromolecular trafficking through PD. A representative sampling of these controls is provided in Table 1. An interesting facet of these results was the observation that, whether the control probe is introduced by microinjection or produced within a bombarded cell, there was often a very low but detectable level of restricted movement into cells that adjoin the target cell. This likely represents the presence of cell-to-cell trafficking of endogenous NCAPs that induce an increase in PD SEL, thereby potentiating the diffusion of the control probe. This interpretation gains support from experiments performed with various forms of GFP. Here, it is interesting that both the frequency and extent to which free GFP is found to move appear to depend on the nature of the tissue used in the study. Generally, GFP is confined to single cells when introduced into mature epidermal cells ( Itaya et al., 1998 Canto and Palukaitis, 1999 Lough et al., 2000 Satoh et al., 2000 Rojas et al., 2001 Tamai and Meshi, 2001a, 2001b). However, on occasion, free GFP appears to be able to move into the adjacent cell layer (see Figure 4E). Quite variable results have been obtained with developing leaf tissue ( Table 1). At times, GFP has been reported to undergo very extensive cell-to-cell movement within such tissues ( Oparka et al., 1999). Irrespective of this variation, expression and accumulation of GFP can serve as an effective reporter for the trafficking of NCAPs ( Figure 4L) either within a tissue ( Figure 4E) or at the whole-plant level ( Imlau et al., 1999 Oparka et al., 1999).

A significant difference observed between microinjection and biolistic experiments relates to the extent to which the viral MPs and endogenous NCAPs move. When such proteins are introduced into a target epidermal/mesophyll cell within a source leaf, by microinjection, they readily move out into a number of neighboring cells ( Table 1). In such cases, within a minute the protein can delineate a pathway of cell-to-cell movement involving trafficking through approximately five cells ( Figure 4M). With longer times (5 to 10 min), these injected probes continue to move though additional cells, resulting in trafficking into and through ∼10 cells. In contrast, GFP-tagged protein synthesized, in vivo, after plasmid bombardment into epidermal cells (source leaves), generally exhibits limited movement. Here, the fluorescent signal is typically detected in only one or two cells beyond the target cell, resulting in clusters of approximately eight fluorescently labeled epidermal cells ( Figure 4N). These differences in the degree of movement may reflect (a) the nature of the probe (i.e., GFP-tag may impair the function of the protein) (b) the amount of protein present in the cytoplasm (i.e., rapid delivery versus in vivo protein synthesis) (c) specific activity of the fluorescent tag (i.e., multiple fluorochemical tags per protein versus a single chromophore in a GFP-tag) and (d) the cell types involved in assessing protein movement (i.e., nature and density of PD). Finally, the possibility should not be overlooked that environmental conditions may well influence the capacity and/or extent to which the PD, within a specific tissue, can mediate the trafficking of macromolecules.

KN1 and LFY Act as Non-Cell-Autonomous Transcription Factors.

(A) In the Zea mays, SAM KN1 RNA can be detected only in the L2 layer (at left) whereas, by immunolocalization (at right), KN1 could be observed within the nuclei of cells located in the L1 layer. (Adapted from Lucas et al., 1995.)

(B) In wild-type Arabidopsis plants, LFY transcripts are detected in young floral buds of the inflorescence meristem (im) (at left) but are absent in plants carrying mutant lfy alleles (inset) at left, lfy-30 middle, lfy-12 at right, expression of LFY in a ML1::LFY transgenic lfy-30 line resulted in confinement of transcripts to the L1 layer. Numbers indicate stages of flower development ( Smyth et al., 1990). (Adapted from Sessions et al., 2000.)

(C) Immunodetection of LFY in the plant lines described in (B). In wild-type plants, LFY was present in nuclei of all cells of young floral buds (at left), and as expected, LFY was absent in the lfy-12 mutant (middle), but in the transgenic ML1::LFY line, LFY was detected in all cell layers of the IM and floral buds (at right). Numbers indicate stages of flower development. (Adapted from Sessions et al., 2000.)

KN1 and LFY Act as Non-Cell-Autonomous Transcription Factors.

(A) In the Zea mays, SAM KN1 RNA can be detected only in the L2 layer (at left) whereas, by immunolocalization (at right), KN1 could be observed within the nuclei of cells located in the L1 layer. (Adapted from Lucas et al., 1995.)

(B) In wild-type Arabidopsis plants, LFY transcripts are detected in young floral buds of the inflorescence meristem (im) (at left) but are absent in plants carrying mutant lfy alleles (inset) at left, lfy-30 middle, lfy-12 at right, expression of LFY in a ML1::LFY transgenic lfy-30 line resulted in confinement of transcripts to the L1 layer. Numbers indicate stages of flower development ( Smyth et al., 1990). (Adapted from Sessions et al., 2000.)

(C) Immunodetection of LFY in the plant lines described in (B). In wild-type plants, LFY was present in nuclei of all cells of young floral buds (at left), and as expected, LFY was absent in the lfy-12 mutant (middle), but in the transgenic ML1::LFY line, LFY was detected in all cell layers of the IM and floral buds (at right). Numbers indicate stages of flower development. (Adapted from Sessions et al., 2000.)


Phloem

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Phloem, also called bast, tissues in plants that conduct foods made in the leaves to all other parts of the plant. Phloem is composed of various specialized cells called sieve tubes, companion cells, phloem fibres, and phloem parenchyma cells. Primary phloem is formed by the apical meristems (zones of new cell production) of root and shoot tips it may be either protophloem, the cells of which are matured before elongation (during growth) of the area in which it lies, or metaphloem, the cells of which mature after elongation. Sieve tubes of protophloem are unable to stretch with the elongating tissues and are torn and destroyed as the plant ages. The other cell types in the phloem may be converted to fibres. The later maturing metaphloem is not destroyed and may function during the rest of the plant’s life in plants such as palms but is replaced by secondary phloem in plants that have a cambium.


Secondary Growth in Dicot Stem (With Diagram)

Primary growth produces growth in length and development of lateral appendages. Secondary growth is the formation of secondary tissues from lateral meristems. It increases the diameter of the stem. In woody plants, secondary tissues constitute the bulk of the plant. They take part in providing protection, support and conduction of water and nutrients.

Secondary tissues are formed by two types of lateral meristems, vascular cambium and cork cambium or phellogen. Vascular cambium produces secondary vascular tissues while phellogen forms periderm.

Secondary growth occurs in perennial gymnosperms and dicots such as trees and shrubs. It is also found in the woody stems of some herbs. In such cases, the secondary growth is equivalent to one annual ring, e.g., Sunflower.

A. Formation of Secondary Vascular Tissues:

They are formed by the vascular cambium. Vascular cambium is produced by two types of meristems, fascicular or intra-fascicular and inter-fascicular cambium. Intra-fascicular cambium is a primary meristem which occurs as strips in vascular bundles. Inter-fascicular cambium arises secondarily from the cells of medullary rays which occur at the level of intra-fascicular strips.

These two types of meristematic tissues get connected to form a ring of vascular cambium. Vascular cam­bium is truly single layered but appears to be a few layers (2-5) in thickness due to presence of its immediate derivatives. Cells of vascular cambium divide periclinally both on the outer and inner sides (bipolar divisions) to form secondary permanent tissues.

The cells of vascular cambium are of two types, elongated spindle-shaped fusiform initials and shorter isodiametric ray initials (Fig. 6.29). Both appear rectangular in T.S. Ray initials give rise to vascular rays.

Fusiform initials divide to form secondary phloem on the outer side and secondary xylem on the inner side (Fig. 6.28 B). With the formation of secondary xylem on the inner side, the vascular cambium moves gradually to the outside by adding new cells.

The phenomenon is called dilation. New ray cells are also added. They form additional rays every year (Fig. 6.28 D). The vascular cambium undergoes two types of divisions— additive (periclinal divisions for formation of secondary tissues) and multi­plicative (anticlinal divisions for dilation).

Ray initials produce radial system (= horizontal or transverse system) while fusiform initials form axial system (= vertical system) of sec­ondary vascular tissues.

The vascular rays or secondary medullary rays are rows of radially arranged cells which are formed in the secondary vascular tissues. They are a few cells in height.

Depending upon their breadth, the vascular rays are uniseriate (one cell in breadth) or multiseriate (two or more cells in breadth). Vascular rays may be homo-cellular (having one type of cells) or hetero-cellular (with more than one type of cells). The cells of the vascular rays enclose intercellular spaces.

The part of the vascular ray present in the secondary xylem is called wood or xylem ray while the part present in the secondary phloem is known as phloem ray. The vascular rays conduct water and or­ganic food and permit diffusion of gases in the radial direction. Besides, their cells store food.

2. Secondary Phloem (Bast):

It forms a narrow circle on the outer side of vascular cam­bium. Secondary phloem does not grow in thickness because the primary and the older sec­ondary phloem present on the outer side gets crushed with the development of new functional phloem (Fig. 6.28 D). Therefore, rings (annual rings) are not produced in secondary phloem. The crushed or non-functioning phloem may, however, have fibres and sclereids.

Secondary phloem is made up of the same type of cells as are found in the primary phloem (metaphloem)— sieve tubes, companion cells, phloem fibres and phloem paren­chyma.

Phloem pairenchyma is of two types— axial phloem parenchyma made up of longitudinally arranged cells and phloem ray parenchyma formed of radially arranged parenchyma cells that constitute the part of the vascular ray present in the phloem.

Elements of secondary phloem show a more regular arrangement. Sieve tubes are comparatively more numerous but are shorter and broader. Sclerenchyma fibres occur either in patches or bands. Sclereids are found in many cases. In such cases secondary phloem is differentiated into soft bast (secondary phloem without fibres) and hard bast (part of phloem with abundant fibres).

It forms the bulk of the stem and is commonly called wood. The secondary xylem consists of vessels, tracheids (both tracheary elements), wood fibres and wood parenchyma.

Wood parenchyma may contain tannins and crystals besides storing food. It is of two types— axial parenchyma cells arranged longitudinally and radial ray parenchyma cells arranged in radial or horizontal fashion. The latter is part of vascular ray present in secondary xylem.

Secondary xylem does not show distinction into protoxylem and meta-xylem elements. Therefore, vessels and tracheids with annular and spiral thicken­ings are absent. The tracheary elements of secondary xylem are similar to those of meta-xylem of the primary xylem with minor differences. They are comparatively shorter and more thick-walled. Pitted thickenings are more common. Fibres are abundant.

Width of secondary xylem grows with the age of the plant. The primary xylem persists as conical projection on its inner side. Pith may become narrow and ultimately get crushed. The yearly growth of secondary xylem is distinct in the areas which expe­rience two seasons, one favourable spring or rainy season) and the other un-favourable (autumn, winter or dry summer).

In favourable season the temperature is optimum. There is a good sunshine and humidity. At this time the newly formed leaves produce hormones which stimulate cambial activity. The activity decreases and stops towards the approach of un-favourable sea­son. Hence the annual or yearly growth appears in the form of distinct rings which are called annual rings (Fig. 6.30).

Annual rings are formed due to sequence of rapid growth (favourable season, e.g., spring), slow growth (before the onset of un-favourable period, e.g., autumn) and no growth (un-favourable season, e.g., winter). Annual rings are not distinct in tropical areas which do not have long dry periods.

Annual Rings (Growth Rings). It is the wood formed in a single year. It consists of two types of wood, spring wood and autumn wood (Fig. 6.31). The spring or early wood is much wider than the autumn or late wood. It is lighter in colour and of lower density. Spring wood consists of larger and wider xylem elements.

The autumn or late wood is dark coloured and of higher density. It contains compactly arranged smaller and narrower elements which have comparatively thicker walls. In autumn wood, tracheids and fibres are more abundant than those found in the spring wood.

The transition from spring to autumn wood in an annual ring is gradual but the transition from autumn wood to the spring wood of the next year is sudden. Therefore, each year’s growth is quite distinct. The number of annual rings corresponds to the age of that part of the stem. (They can be counted by increment borer).

Besides giving the age of the plant, the annual rings also give some clue about the climatic conditions of the past through which the plant has passed. Dendrochronology is the science of counting and analysing annual growth rings of trees.

Softwood and Hardwood:

Softwood is the technical name of gymnosperm wood be­cause it is devoid of vessels. Several of the softwoods are very easy to work with (e.g., Cedrus, Pinus species). However, all of them are not ‘soft’. The softness depends upon the content of fibres and vascular rays. 90-95% of wood is made of tracheids and fibres. Vascular rays constitute 5-10% of the wood.

Hardwood is the name of dicot wood which possesses abundant vessels. Due to the presence of vessels, the hardwoods are also called porous woods. In Cassia fistula and Dalbergia sisso the vessels are comparatively very broad in the spring wood while they are quite narrow in the autumn wood. Such a secondary xylem or wood is called ring porous.

In others (e.g., Syzygium cumini) larger sized vessels are distributed throughout spring wood and autumn wood. This type of secondary xylem or wood is known as diffuse porous. Ring porous wood is more advanced than diffuse porous wood as it provides for better translocation when the requirement of the plant is high.

Sapwood and Heartwood:

The wood of the older stems (dalbergia, Acacia) gets differentiated into two zones, the outer light coloured and functional sapwood or alburnum and the inner darker and nonfunctional heartwood or duramen (Fig. 6.33). The tracheids and vessels of the heart wood get plugged by the in growth of the adjacent parenchyma cells into their cavities through the pits. These ingrowths are called tyloses (Fig. 6.32).

Ultimately, the parenchyma cells become lignified and dead. Various types of plant products like oils, resins, gums, aromatic substances, essential oils and tannins are deposited in the cells of the heartwood. These substances are collectively called extractives. They provide colour to the heartwood. They are also antiseptic. The heartwood is, therefore, stronger and more durable than the sapwood.

It is resistant to attack of insects and microbes. Heart wood is commercial source of Cutch (Acacia catechu), Haematoxylin (Haematoxylon campechianum), Brasilin (Caesalpinia sappan) and Santalin (Pterocarpus santalinus). Heart­wood is, however, liable to be attacked by wood rotting fungi. Hollow tree trunks are due to their activity.

B. Formation of Periderm:

In order to provide for increase in girth and prevent harm on the rupturing of the outer ground tissues due to the formation of secondary vascular tissues, dicot stems produce a cork cambium or phellogen in the outer cortical cells. Rarely it may arise from the epidermis (e.g., Teak, Oleander), hypodermis (e.g., Pear) or phloem parenchyma.

Phellogen cells divide on both the outer side as well as the inner side (bipolar) to form secondary tissues. The secondary tissue produced on the inner side of the phellogen is parenchymatous or collenchymatous. It is called secondary cortex or phelloderm. Its cells show radial arrangement.

Phellogen produces cork or phellem on the outer side. It consists of dead and com­pactly arranged rectangular cells that possess suberised cell walls. The cork cells contain tannins. Hence, they appear brown or dark brown in colour. The cork cells of some plants are filled with air e.g., Quercus suber (Cork Oak or Bottle Cork). The phelloderm, phellogen and phellem together constitute the periderm (Fig. 6.34).

Cork prevents the loss of water by evaporation. It also protects the interior against entry of harmful micro-organisms, mechanical injury and extremes of temperature. Cork is light, compressible, nonreactive and sufficiently resistant to fire.

It is used as stopper for bottles, shock absorption and insulation. At places phellogen produces aerating pores instead of cork. These pores are called lenticels. Each lenticel is filled by a mass of somewhat loosely arranged suberised cells called complementary cells.

Lenticels are aerating pores in the bark of plants. They appear on the surface of the bark as raised scars containing oval, rounded or oblong depressions (Fig. 6.34 A). They occur in woody trees but not in climbers. Normally they are formed in areas with underlying rays for facilitating gas exchange. Lenticels may occur scattered or form longi­tudinal rows.

A lenticel is commonly produced beneath a former stomate or stoma of the epidermis. Its margin is raised and is formed by surrounding cork cells. The lenticel is filled up by loosely arranged thin walled rounded and suberised (e.g., Prunus) or un-suberised cells called comple­mentary cells (Fig. 6.34 B).

They enclose intercellular spaces for gaseous exchange. The complementary cells are formed from loosely arranged phellogen cells and division of sub-stomatal parenchyma cells. The suberised nature of complementary cells checks excessive evaporation of water.

In temperate plants the lenticels get closed during the winter by the formation of com­pactly arranged closing cells over the complementary cells.

In common language and economic botany, all the dead cells lying outside phello­gen are collectively called bark. The outer layers of the bark are being constantly peeled off on account of the formation of new secondary vascular tissues in the interior. The peeling of the bark may occur in sheets (sheets or ring bark, e.g., Eucalyptus) or in irregular strips (scaly bark).

The scaly bark is formed when the phellogen arises in strips instead of rings, e.g., Acacia (vem. Kikar). Bark formed in early growing season is early or soft bark. The one formed towards end of growing season is late or hard bark.

Bark is insect repellent, decay proof, fire-proof and acts as a heat screen. Commercially it is employed in tanning (e.g., Acacia), drugs (e.g., Cinchona— quinine) or as spice (e.g., Cannamon, vem. Dalchini). The cork of Quercus suber is employed in the manufacture of bottle stoppers, insulators, floats, sound proofing and linoleum.

Significance of Secondary Growth:

1. Secondary growth adds to the girth of the plant. It provides support to increasing weight of the aerial growth.

2. Secondary growth produces a corky bark around the tree trunk that protects the interior from abrasion, heat, cold and infection.

3. It adds new conducting tissues for replacing old non-functioning ones as well as for meeting increased demand for long distance transport of sap and organic nutrients.

Anomalous Secondary Growth:

It is abnormal type of secondary growth that occurs in some arborescent monocots (e.g., Dracaena, Yucca, Agave) and storage roots (e.g., Beet, Sweet Potato). In arborescent monocot stems, a secondary cambium grows in hypodermal region. The latter forms con­junctive tissue and patches of meristematic cells. The meristematic patches grow into sec­ondary vascular bundles.

Anomalous vascular bundles also occur in cortex (cortical bundles, e.g., Nyctanthes) and pith (e.g., Boerhaavia). In storage roots (e.g., Beet), accessory cambial rings appear on the outside of endodermis. They produce less secondary xylem but more secondary phloem. The secondary phloem contains abundant storage parenchyma.

Importance of Secondary Growth:

1. It is a means of replacement of old non-functional tissues with new active tissues.

2. The plants showing secondary growth can grow and live longer as compared to other plants.

3. It provides a fire proof, insect proof and insulating cover around the older plant parts.

4. Commercial cork is a product of secondary growth. It is obtained from Quercussuber (Cork Oak).

5. Wood is a very important product of secondary growth. It represents secondary xylem.


On the selectivity, specificity and signalling potential of the long-distance movement of messenger RNA

mRNA transport takes place on a genomic scale between plant parasites and their hosts and in heterografted plants.

Transported mRNAs code for proteins covering a range of biological processes and diverse molecular functions.

Secondary structure motifs in 5′ or 3′ UTR influence mRNA transport.

Transcript abundance and stability can explain the bulk of the mRNA transport data.

The mechanisms underlying a signalling role of mRNA remain to be determined.

Messenger RNA (mRNA) can move through the vascular system in plants. Until recently the transport of mRNA had been demonstrated only for a few well-documented cases, leading to the suggestion that transport was selective and specific. The extent of this long-distance transport has now been shown to be on the genomic scale with thousands of transcripts covering broad regions of gene ontological space. In light of this recent data, I revisit proposed mechanisms of transport of mRNA and critically assess their potential role in signalling.


Is translocation active or passive?

Translocation is the movement of organic compounds from where they are made at their source, to where they are required at their sink. It is an active process which can be used to transport phloem up or down the plant.

Subsequently, question is, what are 3 types of active transport? Active Transport. Active Transport is the term used to describe the processes of moving materials through the cell membrane that requires the use of energy. There are three main types of Active Transport: The Sodium-Potassium pump, Exocytosis, and Endocytosis.

Likewise, people ask, is phloem active or passive transport?

Pressure Flow At the sources (usually the leaves), sugar molecules are moved into the sieve elements (phloem cells) through active transport. Water follows the sugar molecules into the sieve elements through osmosis (since water passively diffuses into regions of higher solute concentration).

What is the basic difference between active and passive transport?

Active transport requires energy for the movement of molecules whereas passive transport does not require energy for the movement of molecules. In the active transport the molecules move against the concentration gradient whereas in the passive transport the molecules move along the concentration gradient.


Abstract

Many plants translocate sugar alcohols in the phloem. However, the mechanism(s) of sugar alcohol loading in the minor veins of leaves are debated. We characterized the loading strategies of two species that transport sorbitol (Plantago major and apple [Malus domestica]), and one that transports mannitol (Asarina scandens). Plasmodesmata are abundant at all interfaces in the minor vein phloem of apple, and in one of two types of phloem in the minor veins of A. scandens. Few plasmodesmata are present in the minor veins of P. major. Apple differs from the other two species in that sugar alcohol and sucrose (Suc) are present in much higher concentrations in leaves. Apple leaf tissue exposed to exogenous [ 14 C]sorbitol, [ 14 C]Suc, or 14 CO2 did not accumulate radiolabel in the minor veins, as determined by macroautoradiography. P. major minor veins accumulated radiolabel from [ 14 C]Suc, [ 14 C]sorbitol, and 14 CO2. A. scandens minor veins accumulated 14 C from [ 14 C]Suc and 14 CO2, but not from [ 14 C]mannitol. We conclude that the movement of sugar alcohol from the mesophyll into the phloem in apple and A. scandens is symplastic and passive, but in P. major it involves an apoplastic step and is energized. We also suggest that apple leaves transport sorbitol in high concentrations to avoid the feedback limitation of photosynthesis that would result from driving passive movement of solute into the phloem with high levels of Suc alone. The loading pathways and the mechanisms by which hydrostatic pressure is maintained in the minor vein phloem of these species are discussed.

Many species transport sugar alcohols in the phloem ( Ziegler, 1975 Zimmermann and Ziegler, 1975 Loescher and Everard, 2000 Noiraud et al., 2001b). Sorbitol (glucitol) is transported in the Plantaginaceae and Rosaceae, mannitol in the Apiaceae, Combretaceae, Oleaceae, and Plantaginaceae, and dulcitol (galactitiol) in the Celastraceae. Several roles have been suggested for sugar alcohols, including osmoprotection, quenching of reactive oxygen species, facilitation of boron transport, storage of reducing power, tolerance to salinity or drought, and involvement in plant pathogen interactions ( Loescher and Everard, 2000 Williamson et al., 2002 Pommerrenig et al., 2007). In some plants, sugar alcohol concentrations in phloem sap may considerably exceed those of Suc.

As with Suc, sugar alcohols are synthesized in the mesophyll and subsequently loaded into the minor vein phloem for delivery to sink tissues. Although it is reasonable to assume that sugar alcohols are loaded by the same species-specific strategies as Suc, these strategies have not been well documented. In particular, there is a debate over the possibility that sugar alcohols load through the symplast ( Moing et al., 1997 Nadwodnik and Lohaus, 2008).

In general, solutes can enter the phloem either from the apoplast or through the symplast. Apoplastic loading is driven by the proton motive force and is capable of creating a steep uphill concentration gradient ( Lalonde et al., 2004). Symplastic loading is necessarily passive since the cytosol is continuous through plasmodesmata ( Schulz, 2005 Turgeon and Ayre, 2005). In willow (Salix spp.) leaves, Suc enters the minor vein phloem passively, through the symplast, and flux is driven by high Suc levels in the mesophyll ( Turgeon and Medville, 1998). In other plants, such as the cucurbits, Suc loading is also symplastic, but the Suc is converted to raffinose and stachyose in the minor vein phloem, and these larger sugar molecules accumulate to high levels by polymer trapping, an active process ( Turgeon and Gowan, 1990).

In this regard it is important to note that the term loading is sometimes used to signify the use of energy to transfer solute into the phloem against a thermodynamic gradient, and at other times to describe any route or mechanism of entry into the phloem, including an entirely passive one by diffusion or bulk flow through plasmodesmata. In this article, we use the term loading to indicate both active and passive modes of entry, making a distinction between them when necessary.

Several methods have been used to distinguish between symplastic and apoplastic loading pathways for sugar alcohols. Sugar alcohol transporters, including a mannitol transporter from celery (Apium graveolens) leaves ( Noiraud et al., 2001a), and sorbitol transporters from Plantago ( Ramsperger-Gleixner et al., 2004) and apple (Malus domestica) leaves ( Watari et al., 2004) have been cloned and functionally characterized as proton symporters. Both the Plantago ( Ramsperger-Gleixner et al., 2004) and apple ( Watari et al., 2004) transporters are localized in the minor vein phloem. Furthermore, active uptake of mannitol has been demonstrated in isolated phloem strands of celery ( Daie, 1987) and plasma membrane vesicles prepared from such strands ( Salmon et al., 1995). As the respective authors have pointed out, these data are consistent with an energized, apoplastic loading mechanism. However, it must be emphasized that the presence of an active uptake mechanism for a solute in the phloem does not, in itself, prove that the phloem-loading route is apoplastic. Transporters are involved in the recovery of leaked solute from many cell types, including the phloem. This means that transporters are required in the phloem, even if the loading route from the mesophyll is symplastic. Note that Suc transporters are present in phloem in the petioles and stem, and even in sink tissues ( Sauer, 2007).

A functional strategy often used to link Suc transporter activity to loading is to test the effects of p-chloromercuribenzenesulfonic acid (PCMBS), a membrane-impermeant sulfhydryl-modifying compound, on phloem transport ( Giaquinta, 1976). PCMBS severely inhibits the function of Suc transporters. Unfortunately, sugar alcohol transporters tested to date are not affected by PCMBS ( Flora and Madore, 1993 Noiraud et al., 2001a Gao et al., 2003 Ramsperger-Gleixner et al., 2004 Watari et al., 2004 Juchaux-Cachau et al., 2007), with the exception of two sorbitol transporters ( Ramsperger-Gleixner et al., 2004 McQueen et al., 2005).

In another approach to establishing the loading route, Moing et al. (1997) measured the sorbitol concentration in mesophyll cells of peach (Prunus persica) leaves and compared it to the concentration in phloem sap, obtained from severed aphid stylets. They reasoned that the lack of an uphill gradient would indicate passive loading. They found no significant differences in solute levels, but without information on intracellular compartmentation, which could considerably alter the local concentration in the cytosol of mesophyll cells, they were unable to distinguish between active and passive loading mechanisms.

To overcome this difficulty, Nadwodnik and Lohaus (2008) used the nonaqueous fractionation technique to study solute levels in various cellular compartments in three species, and compared them to phloem sap concentrations. In Plantago spp. and celery they found that Suc and sugar alcohol concentrations were substantially higher in the phloem than in the cytosol of mesophyll cells, with ratios of 4.5 to 40, indicating the presence of an energized, concentrating mechanism and strongly suggesting an apoplastic loading process. This conclusion is supported by the qualitative assessment of plasmodesmatal frequencies provided by Gamalei (1989), indicating low symplastic connectivity between the mesophyll and phloem in Plantago spp. and in the Apiaceae (celery).

Nadwodnik and Lohaus (2008) also studied peach leaves. Again they found that Suc and sorbitol levels were higher in phloem sap than in the mesophyll cytosol, but only by a factor of two, leaving the loading mechanism(s) open to question. Adding to the ambiguity is the fact that Prunus spp., according to Gamalei (1989), have intermediate numbers of plasmodesmata in minor veins. Furthermore, electron micrographs of minor veins in these species have not been published, so it is not known if plasmodesmata are abundant at all interfaces along the loading route.

In this study we compared phloem-loading mechanisms in apple (like Prunus, a member of the Rosaceae), Plantago major, and Asarina scandens. Asarina spp. transport Suc, mannitol, raffinose, and stachyose ( Turgeon et al., 1993 Voitsekhovskaja et al., 2006). We conducted a thorough anatomical analysis of the minor veins to determine if there is a structural basis for a symplastic pathway. We also analyzed the capacity of minor veins to accumulate radiolabel when leaf tissue is exposed to 14 CO2, or to exogenous [ 14 C]Suc, [ 14 C]sorbitol, or [ 14 C]mannitol. Our results are consistent with an energized, apoplastic loading mechanism for sugar alcohol in Plantago and a passive, symplastic mechanism in apple and Asarina.


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Seasonal development of secondary xylem and phloem in Schizolobium parahyba (Vell.) Blake (Leguminosae: Caesalpinioideae)

The cambial activity and periodicity of secondary xylem and phloem formation have been less studied in tropical tree species than in temperate ones. This paper describes the relationship between seasonal cambial activity, xylem and phloem development, and phenology in Schizolobium parahyba, a fast growing semideciduous seasonal forest tree from southeastern Brazil. From 2002 to 2003, wood samples were collected periodically and phenology and climate were recorded monthly in the same period. S. parahyba forms annual growth increments in wood, delimited by narrow initial parenchyma bands. The reduction of the cambial activity to a minimum correlates to the dry season and leaf fall. The higher cambial activity correlates to the wet season and the presence of mature leaves. In phloem, a larger conductive region was observed in the wet season, when the trees were in full foliage. The secondary phloem did not exhibit any incremental zone marker however, we found that the axial parenchyma tends to form irregular bands.

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