The GLASSY HAIR3 pectate lyase family protein drives papillae formation on trichome cell walls

Numerous papillae structures form on the cell wall surface of Arabidopsis trichomes. Papillae do not develop in the Arabidopsis glassy hair3 (glh3) mutant. Cloning of the GLH3 gene revealed that a member of the pectate lyase gene family is required for papillae formation. GLH3 pectate lyase expression is trichome-specific in leaves and it precedes the appearance of papillae. The transcriptional Mediator complex containing Med25 and Med16 subunits activates GLH3 gene expression in trichomes. Following transport to the apoplast through a canonical secretion pathway, the GLH3-GFP fusion protein accumulates in papillae. Focused accumulation of the GLH3 pectate lyase in papillae suggests that papillae formation is driven by focal pectin hydrolysis. Trichome overexpression of another pectin metabolism gene, pectin methylesterase QRT1 prevented papillae formation. The presented data illustrate the role of spatially restricted pectin catabolism in altering cell surface topography by forming raised cell wall structures.