Abstract
The cellulose- and pectin-rich plant cell wall defines cell structure, mediates defense against pathogens, and facilitates plant cell adhesion. An adhesion mutant screen of Arabidopsis hypocotyls identified a new allele of QUASIMODO2 (QUA2), a gene required for pectin accumulation and whose mutants have reduced pectin content and adhesion defects. A suppressor of qua2 was also isolated and describes a null allele of SABRE (SAB), which encodes a previously described plasma membrane protein required for longitudinal cellular expansion that organizes the tubulin cytoskeleton. sab mutants have increased pectin content, increased levels of expression of pectin methylesterases and extensins, and reduced cell surface area relative to qua2 and Wild Type, contributing to a restoration of cell adhesion.
Highlights
The extracellular matrix (ECM) or cell wall of vascular plants is an interlaced array of cellulose, hemicellulose, pectin, and a variety of proteins that maintains cell structure, is a first barrier to pathogens, and provides a platform for cell adhesion [1,2,3]
Pectin methylesterase inhibitors (PMEIs), a family of 75 isoforms in Arabidopsis, are tightly regulated and may interact in a pH dependent manner with specific pectin methylesterase (PMEs) to create a complex pattern of inhibition [11,12]
Ruthenium red binds to de-esterified pectin, but while it cannot penetrate the cell wall of wild type Arabidopsis hypocotyls, it can penetrate defective cell walls as it does stain hypocotyls of adhesion mutants qua2-1 and frb1-2 [19,25] (Figure S1). Both mutant and wild type (WT) roots stain with ruthenium red
Summary
The extracellular matrix (ECM) or cell wall of vascular plants is an interlaced array of cellulose, hemicellulose, pectin, and a variety of proteins that maintains cell structure, is a first barrier to pathogens, and provides a platform for cell adhesion [1,2,3]. Pectin is laid down in Golgi-derived vesicles between two dividing cells and after the elaboration of a complex cellulose and protein mixture proximal to the cell membrane, the pectin remains enriched in the middle lamella, an area between the two cells [4,5,6] This pectin layer likely mediates cell adhesion, and its modification through de-esterification by esterases and their inhibitors [7], elaboration of side chains [8], or through cleavage, can alter the adhesive properties in a variety of organs [9]. While mutations in QUA1 [16], QUA2 [19], and FRB1 [20] lead to either a reduction in pectin levels (qua1,2) or a change in their esterification and modification (frb1) and a subsequent loss in cell adhesion, it is likely that pectin crosslinking via extensins can affect the organization and adhesion of the cell wall [21,22]. The results further support the essential role of pectin and its modification in cellular adhesion
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