Cell wall mechanics and growth control in plants: the role of pectins revisited

Abstract

How is the extensibility of growing plant cell walls regulated? In the past, most studies have focused on the role of the cellulose/xyloglucan network and the enigmatic wall-loosening agents expansins. Here we review first how in the closest relatives of the land plants, the Charophycean algae, cell wall synthesis is coupled to cell wall extensibility by a chemical Ca(2+)-exchange mechanism between Ca(2+)-pectate complexes. We next discuss evidence for the existence in terrestrial plants of a similar "primitive" Ca(2+)-pectate-based growth control mechanism in parallel to the more recent, land plant-specific, expansin-dependent process.

Keywords: Chara corallina; pectin; pectin methylesterase; wall extensibility.

FIGURE 1

Chemorheological control of wall extensibility. (A) Principle: microfibrils (brown), which in most cases form parallel arrays, are cross-linked by load-bearing (violet) and relaxed (light blue) bonds. The number and strength of the load-bearing bonds determines cell wall strength. Wall extensibility is controlled by chemorheological mechanisms that remove load-bearing bonds. The cell wall relaxes and undergoes turgor-driven mechanical deformation until previously relaxed bonds become load-bearing. (B) Cartoon of cell wall architecture showing microfibrils (brown) and XG chains (green). A small portion of the XG is intertwined or complexed with cellulose, thus sticking the microfibrils together at these points. The endoglucanase Cel12A as well as expansin may act on these relatively inaccessible XG–cellulose interaction domains.

FIGURE 2

Cartoon summarizing Ca ²⁺–pectate-dependent growth control in the alga Chara corallina. (A) In this species, cellulose microfibrils are parallel and oriented transversely to the elongation axis. Ca²⁺ (red ovals)-pectate complexes are load-bearing. One stretched and two relaxed Ca²⁺–pectate complexes are shown. Newly deposited pectate will chelate Ca²⁺ preferentially out of the stretched complexes, thus leading to wall relaxation and turgor-driven mechanical deformation until other complexes become load-bearing. (B) Cartoon of an “eggbox” consisting of two antiparallel poly-galacturonic acid chains complexed by Ca²⁺.