POLYGALACTURONASE INVOLVED IN EXPANSION1 Functions in Cell Elongation and Flower Development in Arabidopsis

Abstract

Pectins are acidic carbohydrates that comprise a significant fraction of the primary walls of eudicotyledonous plant cells. They influence wall porosity and extensibility, thus controlling cell and organ growth during plant development. The regulated degradation of pectins is required for many cell separation events in plants, but the role of pectin degradation in cell expansion is poorly defined. Using an activation tag screen designed to isolate genes involved in wall expansion, we identified a gene encoding a putative polygalacturonase that, when overexpressed, resulted in enhanced hypocotyl elongation in etiolated Arabidopsis thaliana seedlings. We named this gene POLYGALACTURONASE INVOLVED IN EXPANSION1 (PGX1). Plants lacking PGX1 display reduced hypocotyl elongation that is complemented by transgenic PGX1 expression. PGX1 is expressed in expanding tissues throughout development, including seedlings, roots, leaves, and flowers. PGX1-GFP (green fluorescent protein) localizes to the apoplast, and heterologously expressed PGX1 displays in vitro polygalacturonase activity, supporting a function for this protein in apoplastic pectin degradation. Plants either overexpressing or lacking PGX1 display alterations in total polygalacturonase activity, pectin molecular mass, and wall composition and also display higher proportions of flowers with extra petals, suggesting PGX1's involvement in floral organ patterning. These results reveal new roles for polygalacturonases in plant development.

Gradients in Wall Mechanics and Polysaccharides along Growing Inflorescence Stems

“…A recent study that combined extensometry with atomic force microscopy and different forms of cell wall strain showed that both cellulose microfibrils and matrix polymers, predominantly pectins, bear the tensile load of elastically stretched onion walls (Zhang et al, 2017). This confirms the mechanical significance of the matrix at the molecular scale and adds impetus to recent interest in the state of pectins in growing cell walls and the need for better models of cell wall mechanics (Peaucelle et al, 2012;Xiao et al, 2014;Levesque-Tremblay et al, 2015).…”

POLYGALACTURONASE INVOLVED IN EXPANSION3 Functions in Seedling Development, Rosette Growth, and Stomatal Dynamics in Arabidopsis thaliana

“…For example, pectin demethylesterification triggers an increase in wall elasticity during shoot meristem initiation (Peaucelle et al, 2011). In two recent studies, we reported that tissue expansion is promoted when pectin molecular mass is reduced (Xiao et al, , 2014, suggesting a link between pectin size and wall stiffness in growing vegetative tissues.…”

“…HG, the predominant form of pectin in primary cell walls of Arabidopsis thaliana (Zablackis et al, 1995), is synthesized in a highly methylesterified form and can be demethylesterified upon delivery to the cell wall by pectin methylesterases (PMEs), generating negatively charged carboxyl groups on its GalA residues. Pectin demethylesterification can occur in continuous blocks or at random GalA residues, resulting in either wall stiffening via the formation of Ca 2+ -cross-linked HG networks (Vincken et al, 2003) or wall loosening by means of pectin-degrading enzymes (Xiao et al, 2014). Pectin methylesterification status and molecular mass can have profound impacts on wall mechanics, affecting both cellular growth and tissue growth (Braybrook and Jönsson, 2016;Hocq et al, 2017).…”

Monitoring Polysaccharide Dynamics in the Plant Cell Wall

“…While the egg-box model predicts that PME activity increases the stiffness of the pectic matrix, the atomic force microscopy experiments found that demethylesterification of HG promoted wall loosening and was a prerequisite for organ initiation (Peaucelle et al, 2011). Therefore, unesterified HG in growing cells is likely targeted by pectincleaving enzymes such as the recently identified polygalacturonanses involved in cell expansion (PGX1, PGX2, and PGX3), which are required for the development of multiple Arabidopsis organs (Xiao et al, 2014Rui et al, 2017).…”