Loss of galactan synthesis in pea (Pisum sativum) causes defects in organ expansion and is associated with increased extensin content

Pectic galactan is a rhamnogalacturonan-I side chain, composed of β-1,4-linked D-galactose residues. Its multifunctional role in cell wall assembly and properties has primarily been proposed in various developmental contexts. In this study, we investigated the role of galactan in pea ( Pisum sativum syn. Lathyrus oleraceus ). First, we used glycan profiling to examine stems with different auxin-induced elongation rates for changes in cell wall composition. The galactan epitopes, recognised with the LM5 monoclonal antibody, were more abundant in the convex, more elongated part of the pea stem. This finding was supported by monosaccharide composition analysis and epitope detection chromatography (EDC). To evaluate the impact of disrupting galactan synthesis on pea development, we used Virus-Induced Gene Silencing (VIGS) to downregulate the pea ortholog of Arabidopsis β-1,4-galactosyltransferases of the GT92 family ( PsGALS3 ). Plants with silenced PsGALS3 exhibited severe defects in elongation and vegetative organ growth. Microarray polymer profiling (MAPP) revealed that, alongside the reduction in galactan epitopes, the cell wall proteoglycan extensin content increased in these plants, as confirmed by in situ immunohistological analysis. Our results provide new evidence linking galactan to cell and organ expansion and the relation between pectin and structural cell wall glycoproteins, specifically extensin.