The Effects of High Steady State Auxin Levels on Root Cell Elongation in Brachypodium

Abstract

The long-standing Acid Growth Theory of plant cell elongation posits that auxin promotes cell elongation by stimulating cell wall acidification and thus expansin action. To date, the paucity of pertinent genetic materials has precluded thorough analysis of the importance of this concept in roots. The recent isolation of mutants of the model grass species Brachypodium distachyon with dramatically enhanced root cell elongation due to increased cellular auxin levels has allowed us to address this question. We found that the primary transcriptomic effect associated with elevated steady state auxin concentration in elongating root cells is upregulation of cell wall remodeling factors, notably expansins, while plant hormone signaling pathways maintain remarkable homeostasis. These changes are specifically accompanied by reduced cell wall arabinogalactan complexity but not by increased proton excretion. On the contrary, we observed a tendency for decreased rather than increased proton extrusion from root elongation zones with higher cellular auxin levels. Moreover, similar to Brachypodium, root cell elongation is, in general, robustly buffered against external pH fluctuation in Arabidopsis thaliana. However, forced acidification through artificial proton pump activation inhibits root cell elongation. Thus, the interplay between auxin, proton pump activation, and expansin action may be more flexible in roots than in shoots.

Neighbor Detection Induces Organ-Specific Transcriptomes, Revealing Patterns Underlying Hypocotyl-Specific Growth

“…Expression of several AGPs also increases at the time of rapid hypocotyl elongation in the dark and in the root elongation zone ( Figure 7C). Of note, several AGP genes are upregulated in roots of a Brachypodium mutant with enhanced root cell elongation (Pacheco-Villalobos et al, 2016). Moreover, during deetiolation, the expression of numerous AGPs increases in the expanding cotyledon, while it decreases in hypocotyls where growth is rapidly inhibited ( Figure 7C) (Sun et al, 2016).…”

A Synthetic Glycan Microarray Enables Epitope Mapping of Plant Cell Wall Glycan-Directed Antibodies

“…Thus, we conclude that the AGstructures in metaphloem cells contain a low degree of arabinosylation, whereas all other root cells display highly arabinosylated AGPs. The complexity of the AG-structure in AGPs has previously been associated with growth phenotypes in Brachypodium mutants and an Arabidopsis mutant (Knoch et al 2013;Pacheco-Villalobos et al 2016). Both studies used glycosidic linkage analyses on whole roots or root segments to show differences in type II AG related galactose linkages (Knoch et al 2013;Pacheco-Villalobos et al 2016 which cell types display the differences in AGP complexity, more detailed analyses are now feasible based on the newly derived epitope information for type II AG-specific antibodies and the possibility for integrating specific glycosyl hydrolases into cell wall analyses.…”

“…To obtain high spatial resolution, a large number of monoclonal antibodies (mAbs) that bind distinct classes of cell wall polysaccharides have been developed (Pattathil et al 2010;Classen et al 2004;McCartney, Marcus, and Knox 2005;Meikle et al 1994;Ralet et al 2010;Willats, Marcus, and Knox 1998). These mAbs are widely used to localize polysaccharides in cells and tissues of various plant species (Guillon et al 2004;Classen et al 2004;da Costa et al 2017), and to characterize mutant plants with alterations in cell wall composition (Pacheco-Villalobos et al 2016;Gendre et al 2013). Yet, the limited information on the precise molecular structures bound by the mAbs has hindered a comprehensive interpretation of immunological cell wall analyses.…”

Auxin steers root cell expansion via apoplastic pH regulation in Arabidopsis thaliana

“…Moreover, exogenous auxin application has been described to trigger apoplast alkalization in roots, which is the opposite effect as in shoots (18)(19)(20). Notably, a recent study provides substantial transcriptomic insight into auxin-triggered cell wall modification and cell expansion in Brachipodium distachion roots (21). However, the authors also observed that medium acidification does not correlate with root cell elongation (21).…”