A role for pectin-associated arabinans in maintaining the flexibility of the plant cell wall during water deficit stress

Abstract

One of the main components of pectin, a primary constituent of higher plant cell walls, is rhamnogalacturonan I. This polymer comprised of linked alternating rhamnose and galacturonic acid residues is decorated with side chains composed of arabinose and galactose residues. At present, the function of these side chains is not fully understood. Our research on Southern African resurrection plants, plants that are capable of surviving severe dehydration (desiccation), has revealed that their cell walls are capable of extreme flexibility in response to water loss. One species, Myrothamnus flabellifolia, has evolved a constitutively protected leaf cell wall, composed of an abundance of arabinose polymer side chains, suggested to be arabinans and/or arabinogalactans, associated with the pectin matrix. In this article, we propose a hypothetical model that explains how the arabinan rich pectin found in the leaves of this desiccation-tolerant plant permits almost complete water loss without deleterious consequences, such as irreversible polymer adhesion, from occurring. Recent evidence suggesting a role for pectin-associated arabinose polymers in relation to water dependent processes in other plant species is also discussed.

The flowering plant cell wall is a composite structure consisting of a skeletal framework of cellulose and hemicellulose embedded within a matrix of pectin polysaccharides and cell wall glycoproteins.^1,2 The pectin matrix, in turn, is composed of three primary types of polysaccharides, these being rhamnogalacturonan I (RGI), rhamnogalacturonan II (RGII) and homogalacturonan (HG).¹ RGII is a complex polysaccharide, consisting of many unusual sugar moieties, and is not present in large amounts in the wall.³ HG is effectively a linear homopolymer of galacturonic acid and is believed to facilitate the formation of tight junctions, ‘egg boxes’, by complexing with calcium ions present in the cell wall.¹ RGI is a polymer composed of a backbone of alternating glycosidically linked rhamnose and galacturonic acid residues.¹ Side chains, consisting of either arabinogalactan polymers or linear chains of arabinans and/or galactans, are then attached to the rhamnose residues of the RGI backbone.¹ The manner with which these polymers are attached or become entangled with each other and cellulosic polymers to form the pectin matrix has been a matter of debate. The classical theory is that the RGI and HG polymers alternate with each other as block polymers and that the side chains interact with neighbouring polysaccharide chains. Recently, this standard theory has been questioned and an argument whereby the HG polymers are actually side chains of a RGI backbone polymer has been advanced.⁴ Nevertheless, the complexity of pectin polysaccharides is such that ascribing definitive functions to this matrix of polysaccharides has proven quite difficult. The physical properties of the pectin matrix suggest a number of possible functions. The water binding properties of the galacturonic acid residues indicate that polymers containing these groups have the capacity to hydrate and swell and so possibly help maintain polymer separation in the wall.⁵ The side chains of RGI include arabinan and galactan polymers which have been shown to be highly mobile^6,7,8 with the potential to interact with each other forming a temporally entangled matrix.⁹ It is also believed that arabinan chains, which have been shown to contain ferulate residues attached to terminal arabinose groups, are able to oxidatively cross-link via the formation of diferulate bridges between arabinan chains that originate on separate RGI polysaccharides.¹⁰ The pectin matrix is now believed to contain sub-domains of RGI, HG and RGII which may interact with different polysaccharide components of the cell wall such as cellulose or xyloglucan.^11,12 Hence, it is possible that the pectin matrix may form these associations with other polysaccharides via covalent⁹ and/or non-covalent¹¹ (e.g., H-bonding) interactions and in so doing ensure the integrity of the wall and its polymer organisation. Although a number of general functions, such as hydration and ion binding, have been proposed for the pectin matrix, in particular the RGI polymer and its neutral side chains, there has been difficulty in elucidating specific functions for these polysaccharides. A number of molecular genetic studies have been performed with the aim of establishing specific functions for the RGI side chains. A recent study showed that genetic removal of the arabinan side chains in the cell walls of Nicotiana plumbaginfolia results in the formation of a non-organogenic callus culture with loosely attached cells.¹³ Furthermore, it has been shown that ‘in muro’ fragmentation of the RG1 backbone in Solanum tuberosum results in abnormal development of the periderm.¹⁴ This suggests that these side chains may play at least some role in normal cell attachment and cell development. However, the real problem is that no obvious phenotypic differences between wild type and mutant plants (in which neutral side chains have been modified) have been observed.^15,16,17 It may be that the conditions under which phenotypic differences between wild type and mutant plants would arise have not yet been investigated. We believe the water binding and attachment properties of the pectin matrix are particularly important. This is especially so given the role pectin plays in the middle lamella ensuring attachment of cells to each other and in the formation of the apoplast where water mediated transport of solutes occurs.¹ Our research has focused on a group of Southern African plants termed ‘Resurrection plants’ because of their unique ability to survive severe dehydration (desiccation) to an almost air-dry state.¹⁸ We have been interested in how the cell walls of angiosperm resurrection plants such as Craterostigma wilmsii^19,20 and Myrothamnus flabellifolia^21,22 may have become adapted to survive this extreme water deficit stress (desiccation). We have shown that in the case of the Myrothamnus flabellifolia leaf cell wall, which becomes considerably folded when dried, does not undergo dramatic changes in composition or polymer location in response to desiccation.²¹ Rather we propose that this plant has evolved a constitutively protected cell wall which is able to undergo repeated cycles of desiccation and rehydration.^21,22 We have observed that the pectin component of the leaf cell wall in this species was unusually rich in arabinose polymers, most likely arabinan and arabinogalactan in nature, which we advanced was the reason that the cell wall of this species was able to tolerate desiccation.²¹ Here we provide a simple model (Fig. 1) whereby the arabinan side chains of the pectin polysaccharides are responsible for possibly buffering/replacing the lost water during desiccation and in so doing prevent the formation of tight junctions (e.g., egg boxes) or strong H-bonding interactions between the normally separate ‘skeletal’ polysaccharides (e.g., cellulose microfibrils and xyloglucan tethers) embedded in the pectin matrix. Our model is supported by the observation that cell wall arabinans play a crucial role in the response of guard cells to turgor pressure.²³ It was shown that removal of arabinans by enzymatic digestion of leaf strips of Commelina communis resulted in locking of the guard cell walls in either the open or closed position.²³ Additional roles for arabinan polymers in cell walls have recently been implied with respect to the salt tolerance of Mesembryanthemum crystallinum,²⁴ ensuring hydration of the seed endosperm of Gleditsia triacanthos during germination²⁵ and the tolerance of tropical legume seeds to dehydration.²⁶ We believe that the arabinan side chains of RGI play a critical role in the ability of cell walls to remain flexible during plant growth and may have important functions in relation to the water content of the cell. Further studies aimed at determining the relationship between wall water content, RGI side chains and cell wall flexibility may reveal hitherto unsuspected functions for these polysaccharides in the life of the plant.

Figure 1.

A model proposing the role of arabinose rich pectin polymers in stabilising the cell wall against water loss. (A) Pectin consisting of short arabinan chains in the hydrated state, (B) pectin consisting of short arabinan chains in the dehydrated state; (C) pectin consisting of long arabinan chains in the hydrated state; (D) pectin consisting of long arabinan chains in the dehydrated state. The likelihood of irreversible tight junctions (e.g., egg boxes) forming in arabinan poor cell walls during dehydration is demonstrated in (B) while the reversible buffering effect of arabinan rich cell walls is proposed in (D) as would possibly occur in Myrothamnus flabellifolia. For simplicity arabinan chains not participating in the buffering interactions between the RGI backbone chains have been shortened to two arabinose residues in length. Note in (A) and (B) all arabinan chains are two arabinose residues in length.

Abbreviations

RGI

rhamnogalacturonan I

RGII

rhamnogalacturonan II

HG

homogalacturonan

Addendum to: Response of the leaf cell wall to desiccation in the resurrection plant Myrothamnus flabellifolius. Moore JP, Nguema-Ona E, Chevalier L, Lindsey GG, Brandt WF, Lerouge P, Farrant JM, Driouich A. Plant Physiol. 2006;141:651–662.