Abstract

The structural plasticity of carbohydrates is greater than that of amino acids but our understanding of the implications of such plasticity and how it relates to a potential biological function is still limited. We have examined in depth the fundamental role that carbohydrates play in the growth of root hairs of Arabidopsis thaliana. Root hairs are single cells specialized in the absorption of water and nutrients. Growing root hairs require intensive cell-wall changes to accommodate cell expansion at the apical end by a process known as tip growth. The cell wall of plants is a very rigid structure comprised largely of polysaccharides and hydroxyproline-rich glycoproteins (HRGPs) that include extensins (EXTs) and arabinogalactan-proteins (AGPs). However, during plant growth these cells increase 200 times their original size by addition of more sugars and proteins. It has been assumed that chemical changes of wall constituents and wall networks orchestrated by enzymes and cell-wall modifying genes are implicated in the process. To investigate the issue, our group focused on root hairs as our main cell model. Proline hydroxylation, an early posttranslational modification of cell wall HRG Ps that is catalyzed by prolyl 4-hydroxylases (P4Hs), defines the subsequent O-glycosylation sites in EXTs (which are mainly arabinosylated) and AGPs (which are mainly arabinogalactosylated). We explored the biological function of P4Hs, arabinosyltransferases, and EXTs in root hair cell growth. Biochemical inhibition or genetic disruption resulted in the blockage of polarized growth in root hairs and reduced arabinosylation of EXTs. On the other hand, over-expression of P4Hs doubles the length and increases the density of root hairs. Our results demonstrate that correct O-glycosylation on EXTs is essential for cell-wall self-assembly and, hence, root hair elongation. The changes that O-glycosylated cell-wall proteins undergo during growth represent a starting point to unravel the entire biochemical pathway involved in plant growth and development. Velásquez et al 2012. Recent advances on the post-translational modifications of EXTs and their roles in plant cell walls. Frontiers in Plant Science 3(93):1-6. Velásquez & J.M. Estevez. Root hair sweet growth. Plant Signaling & Behavior 2011. 6 (10): 1600-1602. Velasquez et al. 2011. Essential role of O-glycosylated plant cell wall extensins for polarized root hair growth. Science 332, 1401-1403.