Plants detoxify toxic metals through a GSH-dependent pathway. GSH homeostasis is maintained by the g-glutamyl cycle, which involves GSH synthesis and degradation and the recycling of component amino acids. The enzyme g-glutamyl cyclotransferase (GGCT) is involved in Glu recycling, but the gene(s) encoding GGCT has not been identified in plants. Here, we report that an Arabidopsis thaliana protein with a cation transport regulator-like domain, hereafter referred to as GGCT2;1, functions as g-glutamyl cyclotransferase. Heterologous expression of GGCT2;1 in Saccharomyces cerevisiae produced phenotypes that were consistent with decreased GSH content attributable to either GSH degradation or the diversion of g-glutamyl peptides to produce 5-oxoproline (5-OP). 5-OP levels were further increased by the addition of arsenite and GSH to the medium, indicating that GGCT2;1 participates in the cellular response to arsenic (As) via GSH degradation. Recombinant GGCT2;1 converted both GSH and g-glutamyl Ala to 5-OP in vitro. GGCT2;1 transcripts were upregulated in As-treated Arabidopsis, and ggct2;1 knockout mutants were more tolerant to As and cadmium than the wild type. Overexpression of GGCT2;1 in Arabidopsis resulted in the accumulation of 5-OP. Under As toxicity, the overexpression lines showed minimal changes in de novo Glu synthesis, while the ggct2;1 mutant increased nitrogen assimilation by severalfold, resulting in a very low As/N ratio in tissue. Thus, our results suggest that GGCT2;1 ensures sufficient GSH turnover during abiotic stress by recycling Glu.

In animals, PIEZOs are plasma membrane–localized cation channels involved in diverse mechanosensory processes. We investigated PIEZO function in tip-growing cells in the moss Physcomitrium patens and the flowering plant Arabidopsis thaliana. PpPIEZO1 and PpPIEZO2 redundantly contribute to the normal growth, size, and cytoplasmic calcium oscillations of caulonemal cells. Both PpPIEZO1 and PpPIEZO2 localized to vacuolar membranes. Loss-of-function, gain-of-function, and overexpression mutants revealed that moss PIEZO homologs promote increased complexity of vacuolar membranes through tubulation, internalization, and/or fission. Arabidopsis PIEZO1 also localized to the tonoplast and is required for vacuole tubulation in the tips of pollen tubes. We propose that in plant cells the tonoplast has more freedom of movement than the plasma membrane, making it a more effective location for mechanosensory proteins.

A key aspect of plant growth is the synthesis and deposition of cell walls. In specific tissues and cell types including xylem and fibre, a thick secondary wall comprised of cellulose, hemicellulose and lignin is deposited. Secondary cell walls provide a physical barrier that protects plants from pathogens, promotes tolerance to abiotic stresses and fortifies cells to withstand the forces associated with water transport and the physical weight of plant structures. Grasses have numerous cell wall features that are distinct from eudicots and other plants. Study of the model species Brachypodium distachyon as well as other grasses has revealed numerous features of the grass cell wall. These include the characterisation of xylosyl and arabinosyltransferases, a mixedlinkage glucan synthase and hydroxycinnamate acyltransferases. Perhaps the most fertile area for discovery has been the formation of lignins, including the identification of novel substrates and enzyme activities towards the synthesis of monolignols. Other enzymes function as polymerising agents or transferases that modify lignins and facilitate interactions with polysaccharides. The regulatory aspects of cell wall biosynthesis are largely overlapping with those of eudicots, but salient differences among species have been resolved that begin to identify the determinants that define grass cell walls.

Summary Plants are continuously exposed to diurnal fluctuations in light and temperature, and spontaneous changes in their physical or biotic environment. The circadian clock coordinates regulation of gene expression with a 24 h period, enabling the anticipation of these events. We used RNA sequencing to characterize the Brachypodium distachyon transcriptome under light and temperature cycles, as well as under constant conditions. Approximately 3% of the transcriptome was regulated by the circadian clock, a smaller proportion than reported in most other species. For most transcripts that were rhythmic under all conditions, including many known clock genes, the period of gene expression lengthened from 24 to 27 h in the absence of external cues. To functionally characterize the cyclic transcriptome in B. distachyon, we used Gene Ontology enrichment analysis, and found several terms significantly associated with peak expression at particular times of the day. Furthermore, we identified sequence motifs enriched in the promoters of similarly phased genes, some potentially associated with transcription factors. When considering the overlap in rhythmic gene expression and specific pathway behavior, thermocycles was the prevailing cue that controlled diurnal gene regulation. Taken together, our characterization of the rhythmic B. distachyon transcriptome represents a foundational resource with implications in other grass species.

Plant growth is mediated by the integration of internal and external cues, perceived by cells and transduced into a developmental program that gives rise to cell division, elongation, and wall thickening. Extra-, inter-, and intra-physical cellular forces contribute to this regulation. Across the plant kingdom thigmomorphogenesis is widely observed to alter plant morphology by reducing stem height and increasing stem diameter. The transcriptome is highly responsive to touch, including components of calcium signalling pathways and cell wall modification. One aspect of this cascade involves gibberellins and bZIP family transcription factors. Here, we present data connecting thigmomorphogenesis with secondary cell wall synthesis by gibberellin (GA) inactivation and bZIP translocation into the nucleus. Brachypodium distachyon SECONDARY WALL INTERACTING bZIP (SWIZ) protein translocated into the nucleus in response to mechanical stimulation. This translocation was mitigated by supplement with exogenous bioactive GA, and induced by chemical inhibition of GA synthesis. Misregulation of SWIZ expression resulted in plants hypersensitive to mechanical stimulation, with reduced height and increased secondary wall thickness. Classical touch responsive genes were upregulated in B. distachyon following touch, and we observe significant induction of the glycoside hydrolase 17 family, which may be indicative of a grass specific facet of thigmomorphogenesis. SWIZ binding to an E-box variant in exons and introns was associated with immediate activation followed by repression of gene expression. Thus, SWIZ is a transcriptional regulatory component of thigmomorphogenesis, which includes the thickening of secondary cell walls.