UC Berkeley

The plant cell wall is a complex and integral component of the cell mediating the cellular response to both external and internal stimuli. As the first physical barrier to the environment external to the cell, the cell wall must be able to withstand pressures or attacks while also sensing more subtle environmental perturbations. As the physical support structure and shaper of the cell, the wall must be able to dynamically adapt to necessary growth or developmental conditions as the cell matures, including adapting to stress conditions. Recent transcriptomic and targeted proteomic and metabolomic studies have implicated cell wall-related biosynthetic and modification genes and enzymes in biotic and abiotic stress responses. Targeted manipulation of expression of cell wall-modifying enzymes confers either stress tolerance or susceptibility to transgenic plants. Research coupling the cell wall-related transcriptomic response to chemical wall analyses in field-grown Sorghum bicolor exposed to drought stress shows that while a large and consistent transcriptomic response occurs, large compositional shifts in the cell wall do not occur, the only exception being in the younger leaves of a drought-tolerant genotype, which had increases in matrix monosaccharides. Further, the cell wall response to drought stresses in these S. bicolor leaves did not negatively impact sugar yield, in one genotype increasing sugar yield from non-structural polysaccharides like starch.

Changes in the cell wall-related transcriptomes and cell wall compositions of field-grown, drought stressed S. bicolor leaves and roots frequently implicated changes in pectin biosynthesis and modification. This is consistent with drought and in fact several biotic and abiotic stress studies in both eudicotyledons and commelinid grasses. As a heterogeneous polysaccharide containing domains with distinct branching, architecture, and chemistry, pectin mediates cell wall hydration status, extensibility of the expanding or maturing wall, and even acts as a signaling molecule. Recent studies have demonstrated that targeted pectin modification through reverse genetics confers either tolerance or susceptibility to a variety of biotic and abiotic stresses. Despite its significance in the wall, pectin biosynthesis is still poorly understood, with less than twenty-five of the more than sixty putative transferases necessary for its unique linkages described. One of the most heterogeneous domains of pectin, rhamnogalacturonan I (RG-I), is defined by its repeating -(1,2)-α-L-Rhap-(1,4)-β-D-GalpA- backbone. This backbone can be substituted on the rhamnosyl residues at the O3 or O4 positions with four potential side chains, consisting of either linear arabinan, linear galactan, type I arabinogalactans, or type II arabinogalactans. However, structures other than these four canonical side chains have been described previously. Though galactans comprise the bulk of RG-I, the full biosynthetic pathway is still unknown. To date, two different RG-I galactosyltransferases have been described, both involved in linear (1,4)-β-D-galactan elongation. An additional RG-I galactosyltransferase, GT47F, has been identified that is crucial to normal development in Nicotiana benthamiana and embryo and vegetative development in Arabidopsis thaliana. The silencing of GT47F results in dwarfed N. benthamiana and A. thaliana, with misshapen cells and mildly chlorotic leaves in N. benthamiana. RG-I composition from silenced N. benthamiana plants demonstrated a smaller RG-I with less galactan per RG-I moiety. Although monoclonal antibody binding to isolated pectin from these lines did not demonstrate a difference in linear (1,4)-β-D-galactan or the unsubstituted RG-I backbone, an increase in branched (1,6)-β-galactans was observed in silenced N. benthamiana lines. T-DNA lines in which the third exon is interrupted, and which likely abolish all activity, are homozygous embryo-lethal in A. thaliana. N. benthamiana microsomes over-expressing a YFP-tagged GT47F demonstrated binding of UDP-galactose in microscale thermophoresis experiments. Taken together, GT47F likely encodes a developmentally-required galactosyltransferase that contributes to RG-I galactosylation. Absence of this galactosylation results in increased branched galactan epitopes and smaller molecular weight of RG-I.