Input Template for Content Writers. (e-text and Learn More) 1. Details of Module and its Structure

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Input Template for Content Writers (e-Text and Learn More)

1. Details of Module and its Structure

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Library Science Management of Library and Information Network Network

Module Detail

Subject Name <BOTANY>

Paper Name <Plant Physiology II>

Module Name/Title <Plant cell expansion and cell elongation>

Module Id <4d>

Pre-requisites Basic knowledge about expansion of plant cell and cell elongation

Objectives To make the students aware of the components of plant cell expansion and cell elongation patterns

Keywords Anisotrophy, auxin, acid growth theory, expansins, cell wall creep, isotrophy, sugar signalling, cell elongation, cellulose, cell wall loosening, organ formation, Pectin, Pectin

methyltransferase,Endoglucanases, Endotransglycosylases

2. Development Team

Structure of Module / Syllabus of a module (Define Topic / Sub-topic of module )

<Sub-topic Name1>, <Sub-topic Name2>

Role Name Affiliation

Subject Coordinator <Dr. Sujata Bhargava> Savitribai Phule Pune University, Pune Paper Coordinator <Dr. Sujata Bhargava>

Content Writer/Author (CW)

<Dr. Dhiraj Naik > JB Science College Wardha Content Reviewer (CR) <Dr. Sujata Bhargava>

Language Editor (LE) <Dr. Sujata Bhargava>

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TABLE OF CONTENTS (for textual content) 1. Introduction- Plant cell wall

2. Plant cell wall content- Cellulose, Pectin, Pectin methyltransferase, Endoglucanases, Endotransglycosylase, Expansins

2. Type-I and Type-II plant cell wall

4. Acid growth theory for plant cell elongation

5. Cell wall loosening is key response to organ formation

6. Mechanism of cell wall loosening is related to anisotrophic plant cell growth 7. Role of auxin and sugar in plant cell expansion

Introduction

Plant growth and development is an important aspect of life on earth. Understanding the time and space dependent regulation of plant growth and its constituent organs is an important aspect in biological world. Plant growth forms basis for various crop growth and yield, ecosystem primary producers and the means for which plant to adapt tovarious environmental conditions and experimental treatments. The development of leaf, root, stem, flower, fruits and seeds in plant species is an intriguing complex resulting from outcomes of regulatory pathways. As seed germinates, it produces two important

structures which play critical role in further plant development. The shoot apical meristem and root apical meristem undergoes continuous cell division. These continuously dividing cells enlarge

By coordinated expression of individual cells as well as the surrounding cells of the individual cell. For example the large storage cells in potatoes or metaxylem elements or germinated pollen tube or elongated root hair, the final cell size can be 1000000 or 10000000 times greater thant the original cell size. This is an interesting component of plant cell at the stage of elongation where most of volumetric increased in protoplast is

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accounted for by the vacuolar expansion. Such expansion could be up 95% of total cell volume. In elongation phase of cell, the existing cell wall architecture must change during cell expansion to incorporate new cellulosic and other important polysaccharide material, increasing the surface area of the cell by as much as 10000 times. Such cell enlargement make radical changes in cell physiology particularly increased water uptake by the

protoplast.

The rate of cell expansion and cell elongation is rate limiting for growth. This conditions derives two important processes: first establishing the plane of cell division in cell s, thus determining the positions in which new cell walls are created and second, targeting new cell wall material to the particular area of cell surface. The primary cell wall is essential during cell division and is capable to support cell expansion and cell elongation. A

separate secondary cell wall is added to cell’s function. Typically mature plant cell have then both a primary and a distinct secondary cell wall.

Plant Cell Wall:

The plant cell wall is consists of cellulose, non-cellulosic wall polysaccharides such as hemicelluloses and pectin and a small amount of proteins. Cellulose forms the main framework of cell wall and accounts for 20 % to 30 % of the dry mass of elongated plant cell walls. The architecture of plant cell wall is not only rigid and strong but also provided structural support for the plant organs and also able to control cell expansion in isotrophic and anisotrophic way. Cellulose is synthesize at the plasma membrane by PM-resident cellulose synthase (CSA) complexes, Non-cellulosic polysaccharides are assembled within Glogi, later secreted into apoplast by fusion Golgi derived vesicles within the plasma

membrane (Figure 1).

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Figure 1: Structure and biogenesis of cellulose. Adapted from: Cosgrove; Nature 407(21):321-326(2000).

This structure later form association with newly synthesized cellulose microfibrils. The cellulose microfibrils is a thin ribbon life structure about 5μm in width and many

micrometers in length. Cellulose microfibrils consists an ordered structure of many parallel chains of unbrached glucose polymer. Hemicelluloses are typically branched

polysaccharides. They have strong tendency to bind cellulose.

Pectins:

Pectins are generally acidic polysaccharides with a strong tendency to form ionic gels.

Pectins form a dense aqueous wall matrix and connect cell wall polymers around and between cells. Pectins based on structure are typically classified into three classes.

1)Homogalacturonans (HGs), 2) rhamnogalacturonan I (RGI) and 3) rhamnogalacturonan II (RGII) (Figure 2). Pectins are preferentially synthesized based on α- (14)- linked D- galacturonic acid backbones. This backbone can be diversely substituted.

Homogalacturonans carries linear chains of α- (14)-linked D-galacturonic acid which can be methyl- or acetyl esterified (Figure 1). Rhamnogalacturonan I consists of α- (14)- linked D-galacturonic acid-α-rhamnose- (12)-linked repeats with galactose and

arabinose side chains (Figure 2). Rhamnogalacturonan II is complex in nature and carries diverse polymers including a series of sugars and sidechains, with α- (14)- linked D-

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galacturonic acids serving as the central structure. Pectins are synthesized in the golgi bodies. After synthesis, pectin polymers are transported to the developing cell wall.

Pectin Methyltransferase:

Pectinolytic enzymes or pectinases are a heterogeneous diverse group of enzymes that hydrolyze pectic and pectic-similar substances present in plants. Pectin methyltransferase (3.1.1.11), hydrolytic enzymes that uses methyl ester group which is produced by plants, pathogenic fungi, and bacteria. Pectin methyltransferase catalyzes reactions leading to deeterification through transferring C6 carboxyl groups in pectin-PME complexes to water molecules altering degree and pattern of methyl esterification and transacylation through transferring C6 carboxyl group to carboxyl groups of another pectin molecule. That results in formation of high molecular weigh pectins with new non-methoxy ester linkages. Plant pectin methyltransferases belong to large multigene families. For examples, in

Arabidopsis, 66 open reading frames have been annotated as full length PMEs. PMEs have been shown potential cell wall loosening activity at the site of leaf initiation on shoot apical meristems are softer as result of deesterification of pectin mainly

homogalacturonans (HGs). HGs are synthesized by Golgi apparatus and released in developing cell wall. After delivery of HG to cell wall, methyl esters are removed by action of pectin methyltransferase (PME) encoded in plants by large gene family. Under in vitro conditions, de-esterified pectin form stiffer gels than do methyl-eserified pectin and

therefore pectin de-esterification in vivo is associated with cell wall stiffening as plant cells ceases cell elongation process.

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Figure 1: Major cell wall polymers: A) Cellulose backbone, B) Hemicellulose xyloglucan, C) Hemicellulose xylan, D) Pectin Hemigalacturonan, E) Pectin Rhamnogalacturonan-I, F) Pectin Rhamnogalacturonan-II. Adapted from: Somssich, Khan and Persson; Frontiers in Plant Science 7:1242 (2016)

Expansin:

Expansins are cell wall proteins (a large super-family which codes for 225-330 amino acid residues) that consist of four sub-families based on genome and protein characterization;

α-expansin, β-expansin, expansin-like A and expansin-like B. Expansins like X (hereafter referred to as “EXLX) as another group of expansions which are phylogenetically related to expansin genes. These are bacterial expansions found both insided and outside the plant kingdom. These proteins induced wall creep and wall relaxation, yet expansions neither hydrolyzed the cell wall nor exhibited other enzyme activities.

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Expansins have the capacity to non-enzymatically trigger a pH dependent relaxation of plant cell wall which loosens it thus enabling cell expansion (Figure 30. Growing plant cell walls extend faster due to action of expansions at low pH (pH 4.5), a phenomenon referred as “Acid Growth Theory”. This pH changes is achieved by action of pH dependent H⁺- ATPase in the plasma membrane which pumps proton into cell wall. This process leads to rapid induction of cell elongation by auxin induced acidification of cell wall space.

The ability of α-expansin to induce creep without reducing wall stiffness. They do not cut cell wall linkages. The α-expansins does not mechanically weaken cell wall with no specific cell wall loosening mechanism. Another class of plant expansions included the β-expansin group. The β-expansin is expressed at high levels in pollens of grass family. The β-

expansin in grass pollens selectively loosens cell wall of plants in grasses which has different cell wall composition mainly of arabinoxylan and homogalacturonan (HG). Unlike α-expansins, β-expansins has ability to reduce tensile strength of grass cell wall at least in part by weakening the middle lamella between cells. As compared to wall extension

assays of purified α-expansin and β-expansin, bacterial expansions could induce cell wall creep weakly. Bacterial expansions did not weaken cell wall nor did it exhibit lytic activity with isolated cell wall polysaccharides or with cell wall substrates. Based on domain structure and their ability to induced creep of plant cell walls, biological roles of all studied expansions differ:

1) α-expansins triggers acid-induced extension of plant cell walls without mechanically weakening of plant cell walls.

2) β-expansins only cause cell wall creep and solubilised polysaccharides in the middle lamella between cell walls of grasses but not other plants.

3) Mechanism of Bacterial expansions activity is yet to be established but shows weak wall loosening action with bacterial expansions.

Endoglucanases and Endotransglucanases:

These two classes of enzymes are referred as wall loosening enzyme. Based on genome survey and characterization as well as enzymatic assays, all flowering plants possess diverse wall lytic enzymes classified into numerous sub-families. Out of them, two

enzymes namely GH9 and GH16 have capacity to break β 1, 4-glucans (e.g. xyloglucans or cellulose). In plant cell wall studies, GH16 enzymes are usually called xyloglucan

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endotransglucosylase/hydrolases (XTH) and are under the large multigene family. GH16 break xyloglucan and join the new reducing ends to nonreducing end of another

xyloglucan, a process referred as transglucosylation. Previously it was considered as XTH enzymes are cell wall loosening enzymes, but subsequent experimentation proved no ability to induce wall creeping and to weaken the wall.Since breaking the xyloglucan is insufficient for cell wall loosening, these enzymes were considered as indirect or secondary loosening agent that directly catalyzes cell wall creep.GH16 action showed unusual transglucosylation using cellulose as the lytic substrate and xylogucan as the acceptor substrate. Based on XTH and GH15 studies, these enzymes show no direct involvement in cell wall loosening activity. Their activity is likely involved in xyloglucan modeling during primary wall formation and cessation of cell elongation. Another enzyme GH9 often called as endoglucanases or cellulases undergo hydrolysis of xyloglucans, mannans, and xylans as well as cellulose. Korrigan, one well studies HG9 enzymes includes a membrane associated endoglucanase is part of cellulose synthesis complex and that influences organization of cellulose in developing cell wall. Overexpression of poplar GH9 gene showed leaf growth increase in Arabidopsis suggesting cell wall

loosening activity leading to increased growth in leaf. However direct evidences in context of cell wall loosening activity is lacking.

Figure 3: A model of expansin’s wall loosening action. Adapted from: Cosgrove; Nature 407(21):321-326(2000).

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Two types of Plant Cell Wall:

In angiosperms, two major types of plant cell walls are found. These are ”Type I” and

“Type II” cell walls. “Type I” cell walls are found in dicots and monocots excluding plant s from Commelinaceae family contain equal amounts of crosslinking xyloglucans (XyGs), glucan polymer with xylose containing side-chains and cellulose. Xyloglucan (XyG) are the most abundant hemicelluloses found in dicot primary cell walls. XyGs occur in two distinct locations in the cell wall. One location is binding to exposed region of glucan chains in the cellulose microfibrils and the other location is covering the distance between adjacent micrfibrils or linking to other XyGs in space and locks the microfibrils. The combined structure of cellulose-XyG is embedded in a pectin matrix that controls wall porosity apart from other physiological properties. The pectic polysaccharides form gel matrix and fall under two major classes.The two major classes of pectins are homogalacturonans (HGs) and rhamnogalacturonans I (RG-I). HGs are polymers with galacturonic acid residues. RG- 1 is polymers with backbone of alternating rhamnose and galacturonic acid residues. HGs are secreted as highly methyl esterified polymers. The cell wall based enzyme pectin methylesterase (PME) removes methyl group to initiate binding of carboxylate group of HGs to Ca²⁺ ions. Such crosslinking with Ca2+ ions lead to formation of gels. Some HGs and RGs are crosslinked by ester linkages to pectin that tightly bind in cell wall matrix.

Some neutral polymers such as arabinans or galactans are present at one end of pectin polymers and are highly mobile in nature. Some structural proteins are also present in type I cell wall.

Maize and monocots from family Commelinaceae possess other types of cell wall, a ”Type II” cell wall. The “Type II” cell wall contains cellulose microfibrils of same structure which are present in type I cell wall along with glucuronoarabinoxylans (GAXs). GAXs are the principal polymers that tightly bound to cellulose microfibrils in type II cell walls.

Unbranched GAXs form hydrogen bonds to cellulose and sometimes unbranched GAxs bound to each other. In general, plants from graminoid group are poor in pectin content.

However, the pectin that is present in grasses is structurally similar to those of dicots.

When grass cells undergo elongation phase, cell accumulate mixed linked glucans in addition to GAXs. Grasses have very little structural protein. They generally have

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extensive interconnected networks of phenylpropanoids that are formed after cells stop expanding.

Acid growth theory of plant cell wall elongation

Plant meristematic cells are small and of size ~ 5µm and densely packed with cytoplasm.

When some of these cells are displaced from meristem, they typically undergo prolonged phases of elongation, enlargement and differentiation during which cell volume greatly increases. In elongation phase, plant cell proliferate irreversibly only when load-bearing bonds in cell wall are cleaved. Plant hormone auxin stimulate cell elongation of stem and coleoptiles cells by promoting cell wall loosening via cleavage of these bonds (Figure 4).

Simultaneously, this process may be coupled with intercalation of new cell wall polymers.

Since the primary site of auxin action appears to be the plasma membrane or other intracellular site and wall loosening is extracellular, there must be the signaling molecule that communicates between the protoplast and cell wall. Some “wall loosening factor” must be exported from auxin induced cells (Figure 4)..

Figure 4: Proposed mechanism of Acid growth theory. Adapted from: Rayle and Cleland (1992). Plant Physiology 99: 1271-1274.

It was proposed that wall loosening factor is hydrogen ions. This concept led to the

development of Acid Growth Theory. Growing plant cells typically extends faster at low pH, a phenomena and theory referred as”Acid Growth”.

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The experiment was carried out with etiolated cucumber seedlings. This experiment was carried out in isolated walls (excised hypocotyls portion of approximately size of 1cm) clamped at constant tension in an extensometer (as adapted from Figure 5a). At neutral pH the walls soon stop extending but they rapidly elongates as pH is lowered (Figure 5b).

Four major of qualitative evidences support Acid Growth Theory: 1) Auxin-treated stem and coleoptide sections secrete H⁺ ions or protons in response to auxin, lowering the pH of the apoplast; 2)Treatment of tissues with acidic buffers of pH 5.0 can cause cells to

elongate at rate comparable to or greater than that induced by auxin; 3)neutral buffers infiltrated into the apoplast inhibit auxin induced growth; and 4) the toxin fusicoccin, whose main action is to promote extensive acidification of apoplast causes rapid cell elongation.

All above mentioned qualitative evidences provide strong support for acid growth theory.

Acid induced cell wall elongation is not merely a physicochemical property of wall

polysaccharides and other accessory sugar molecules, but requires active wall proteins.

One of major discovery suggested that the wall proteins may be added back to denatured cell walls to restore their ability to elongate (Figure 5c).This new proteins subsequently named as expansions. Their action in referred to as ‘wall loosening; a general term that does not explain the exact mechanism of plant cell wall elongation.

Figure 5: Expansion activity with help of extensometer assays and seedling experiment.

Adapted from: Cosgrove; Nature 407(21):321-326(2000).

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Cell wall loosening is key response to organ formation

During growth and development at local level of any plant organ, plant cells need to extend cell walls. Auxin and cytokinins promote cell wall extensibility. Recent work suggests

phytohormones auxin and cytokinins recognized as key factor that determines developmental pattern and cell fates. The first important discovery came from an experiment using apoplastic protein expansin. Expansin relaxes cellulose microfibril network by binding xyloglucan. Expansin protein expression increased in newly formed leaf primordial. As expansin participate in cell wall loosening activity with an acidic pH, it was observed that auxin accumulation in leaf triggers expansin activity. This indicates cell wall loosening of the cell triggers organ formation. Auxin signaling apart from induction of expansion also promotes demethylesterification of pectin and induction of pectin

methyltransferase (PME) activity. PME loosen cell wall by demethylesterifying pectin. PME induces organ formation. Similarly auxin induced polygalacturonase regulates organ

initiation by cleaving pectin polymers. The dynamic role of expansions, PME,

poygalacturonase activated by auxins plays a critical role in cell wall extensibility during organ formation.

Role of cell wall loosening during microtubule orientation and auxin pattern in the shoot apical meristem:

In newly developing organ in plant body, cells are always constrained by the cell wall in a growing tissue. In such situation, cell wall loosening results in cell swelling and

simultaneous pushing the neighboring cells. Recent studies show that such force from the expanding cell regulates cellular behavior affecting two key factors: cortical microtubule (CMT) orientation and PIN1 protein localization (Figure 6).

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Figure 6: Current model of cell wall loosening and organ formation. Adapted from:

Tameshige et al; Frontiers in Plant Science 6, Article 324 (2015).

The first key factor CMT is a type of cytoskeleton microtubules which are located under the plasma membrane. Generally, CMT orientation correlates with stress direction in various plant tissues. By live imaging of CMT, it is now confirmed that stress direction determines CMT orientation. CMT orientation directs the movement of cellulose synthase complex in such way that cellulose microfibrils orients in parallel to CMT. Interacted cellulose

microfibril arrays then restrict cell expansion in its perpendicular directions (Figure 6A).

During this phase, cell wall loosening in one expanding cells confers the growth

anisotrophy on its surrounding cells in response to the initial stress direction leading to a

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local outgrowth of the growing tissue. This response leads to uneven surface curvature formed by local outgrowth and this force anisotrophic growth from inner tissue to outer tissue, forming a positive feedback that contributes to continuous outgrowth in growing tissues.

The second key factor is auxin efflux carrier PIN1 protein which is preferentially localized to a part of plasma membrane where contacting cell wall is most stressed (Figure 6 B).

Simple model for PIN1 characterisation is as follows: The local amount of PIN1 protein is regulated by exocytosis and endocytosis. Exocytosis is activated near the highly stressed portion of cell walls where tension of plasma membrane is high, leading to PIN1

enrichment (Figure 6B).The resulting PIN1 localization facilitates transport of auxin into the wall loosening cell (Figure 6B, 6C). Auxin activates PME and expansin activities to drive cell wall loosening (Figure 6C, right). Thus PIN1 mediates the positive feedback to enhance cell wall loosening and local auxin accumulation.

Looking at the role of both key factors in nutshell, cell wall loosening activates organ formation by two feedback mechanism. One is the feedback between anisotrophic

behavior of CMT and geometric changes in shoot apical meristem (SAM) (Figure 64A, C left). The other feedback mechanism is interaction between PIN1- dependent auxin transport and auxin-induced cell wall loosening (Figure 6B, 6C right).

Mechanism of cell wall loosening is related to anisotrophic plant cell growth:

Plant cells display two mechanism of growth based on directionality. These are uniform isotrophic and uniform anisotrophic. The global mechanism is observed in fruit epidermal cells, where uniform and isotrophic plant cell expansion leads to almost spherical shape and in shoot and root epidermal cells, plant cell expansion is anisotrophic. (Figure 7)

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Figure 7: Digrammatic representation explaining two main plant cell growth: global and directional (Adapted from: Guerriero et al. No Stress! Relax! Mechanisms governing growth and shape in plant cells. International Journal of Molecular Science 2014, 15, 5094-5114.

Bast fibres and cells of vascular tissue show diffuse anisotrophic expansion which leads to development of long fusiform cells (Figure 7).

Cellulose microfibrils are generally considered to be the principal load bearing parts of cell wall and determinants of growth anisotrophy. In elongated cells or tissues, the orientation of cellulose microfibrils is typically perpendicular to the main axis of growth and this

cellulose orientation is causal facto for expansion anisotrophy. Anisotrophic process induces the deposition of cellulose in a particular direction. This directive cellulosic deposition is also mediated by microtubules and their orientation. Microtubule orients themselves in such way that results in growth of cells in perpendicular direction.

Microtubules act as enhancer during anisotrophic growth. Cellulose mediated stiffening is initiates morphogenetic process in anisotrophic growth. However, orientation of cellulose is

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causal and initial agent for anisotrophy is challenged by other cell wall components. Using novel techniques of Atomic force microscopy (AFM) to map mechanical changes in

Arabidopsis hypocotyl epidermal cells, Peaucelle and his collegues observed longitudinal aniticlinal wall undergo softening before onset of anisotrophic cell elongation. These longitudinal anticlinal wall preferentially expand during hypocotyl elongation. Changes in mechanical symmetry in elongated cells of hypocotyls undergo softening due to selective de-esterification of pectins in longitudinal cell walls. The difference in mechanical

properties between longitudinal and transverse walls stimulates expansion of the longitudinal cell walls. Because of cell–cell attachments that lead to tight cell

arrangements, the expansive activity in the hypocotyl tissue occurs primarily longitudinally (Figure 8). Due to the newly generated cylindrical geometry of the cells, the primary stress field is transverse to the direction growth. Microtubules align transversally, in the direction of maximum stress to support the structure (Figure 8). Consistent with this, Peaucelle et al.

(2015) showed that microtubule alignment triggers only after softening of longitudinal walls. Deposition of cellulose microfibrils in transverse orientation causes polysaccharide material of the longitudinal walls to become anisotropically reinforced (Figure 8). An increase in stress anisotropy maintains the transverse orientation of microtubules.

Continued deposition of cellulose microfibrils reinforces the cell wall anisotropy (Figure 8).

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Figure 8: General conceptual model of two-step mechanism achieving anisotrophic plant growth.

Role of auxin and sugar in plant cell expansion:

Plant growth at cellular level is driven by primary process of cell expansion. Plant cell expansion is a net outcome of turgor pressure inside cell and irreversible cell wall extension. Normally plant cell accumulates sugars, ions and other osmotically active solutes. This condition assist plant cell to generate a lower osmotic potential to attract water inside the cell, thereby generating a strong turgor pressure to drive cell expansion.

The process of cell wall expansion induces cell wall loosening leading irreversible cell wall extension followed by deposition of new cell wall material. The direction of cell wall

expansion is also regulated by various factors including cytoskeletons.

Auxin plays an important role in plant organ development and further growth. They induce rapid cell elongation in stem, hypocotyls segments and coleoptiles within few minutes of auxin treatment. Current model of auxin induced cell expansion is based acid growth theory, a widely accepted theory for cell wall loosening and plant cell expansion (as shown in Figure 9). In auxin regulated mechanism, an extracellular auxin is perceived by the auxin receptor, auxin binding receptor (ABP1). ABP1 then interact with less known membrance associated proteins which activates plasma membrane based ATPase. This unique ATPase pumps proton into the extracellular space, which lowers the pH in cell wall matrix, the lowering of pH induces cell wall loosening proteins such as expansions and

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xyloglucan endotransglycosylases/ hydrolases (XTH). This complexes then make cell wall relaxed for expansion. Another important cellular process is activation of hyperpolarisation of membrane potential due to plasma membrane based ATPase (PM-H⁺-ATPase). This process activates voltage dependent potassium channels partially contributing to the osmotically driven water uptake by the cell for expansion process.

Auxin inside nuclei of plant cells activates transcription of genes enconding PM-ATPase, expansions, XTHs, K⁺ channels and cell wall remodeling enzymes. Also auxin induces exocytosis of vesicles carrying cell wall related polysaccharides (Figure 9). Auxin also participate in asymmetric cell expansion that is characteristics of anisotrophic cell growth.

Auxin affects this type of cell expansion through ABP1 receptor, Rho-dependent GTPase and their interacting molecule CRIB MOTIF containing proteins (RICs) (Figure 9).

As compared to auxin mediated cell expansion, sugars also has capacity to regulate cell expansion. In most polar cell expansion, like cotton fibres, pollen tube elongation and fungal trip growth, sucrose is hybrolyzed into glucose and fructose. In such situation, vacuolar invertase increases the concentration of sucrose into vacuole. This potentially increased cell turgor leading to cell expansion process which is commonly seen in elongating cotton fibers (Figure 9). Similar observation such as high expression of vacuolar invertase gene and its enzyme activity was seen in grape berry, carrot taproot and Maize ovary.

Sucrose acts as signal molecule which increased turgor pressure by doubling osmotic potential. Vacuolar Invertase interact with wall associated kinases (WAKs) during sucrose hydrolysis. WAKs are bound to pectin in cell walls and their activity is required for cell expansion. In contrast to Invertase based cell expansion, surcrose degrading enzyme commonly referred as Sus also contributes to cell expansion in presence of high sucrose levels. In such level Sus tends to bind actin filaments and form multi-protein complex bound to plasma membrane. This multi-protein complex provide UDPG to callose

synthase and cellulose synthase making cell expansion via cellulose/callose biosynthesis.

These two pathways namely vacuolar investase based cell expansion and Sus mediated cellulose synthesis are recent understandings of how cell expansion process is controlled by sugars and auxin levels. Sucrose hydrolysis in vacuoles also regulated nuclear gene

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expression, involving in auxin biosynthesis, distribution and signaling in developing plant cells.

Figure 9: Sugar and auxin signal in regulating sink cell expansion. Adapted from Wang and Ruan. Frontiers in Plant Science. 2013, volume 4.

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References:

Cosgrove, D.J., 2016. Catalysts of plant cell wall loosening. F1000Res 5.

doi:10.12688/f1000research.7180.1

Cosgrove, D.J., 2000. Loosening of plant cell walls by expansins. Nature 407, 321–326.

doi:10.1038/35030000

Mangano, S., Juarez, S.P.D., Estevez, J.M., 2016. ROS regulation of polar growth in plant cells. Plant Physiol 171, 1593–1605. doi:10.1104/pp.16.00191

Marowa, P., Ding, A., Kong, Y., 2016. Expansins: roles in plant growth and potential applications in crop improvement. Plant Cell Rep 35, 949–965. doi:10.1007/s00299- 016-1948-4

Rayle, D.L., Cleland, R.E., 1992. The Acid Growth Theory of auxin-induced cell elongation is alive and well. Plant Physiol 99, 1271–1274.

Somssich, M., Khan, G.A., Persson, S., 2016. Cell wall heterogeneity in root development of Arabidopsis. Front Plant Sci 7, 1242. doi:10.3389/fpls.2016.01242

Tameshige, T., Hirakawa, Y., Torii, K.U., Uchida, N., 2015. Cell walls as a stage for

intercellular communication regulating shoot meristem development. Front Plant Sci 6, 324. doi:10.3389/fpls.2015.00324

Wang, L., Ruan, Y.-L., 2013. Regulation of cell division and expansion by sugar and auxin signaling. Front Plant Sci 4, 163. doi:10.3389/fpls.2013.0016