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The plant cell wall plays a critical role in signaling responses to environmental and developmental cues, acting as both the sensing interface and regulator of plant cell integrity. Wall-associated kinases (WAKs) are plant receptor-like kinases located at the wall-plasma membrane-cytoplasmic interface and implicated in cell wall integrity sensing. WAKs in Arabidopsis thaliana have been shown to bind pectins in different forms under various conditions, such as oligogalacturonides (OG)s in stress response, and native pectin during cell expansion. The mechanism(s) WAKs use for sensing in grasses, which contain relatively low amounts of pectin, remains unclear. WAK genes from the model monocot plant, Brachypodium distachyon were identified. Expression profiling during early seedling development and in response to sodium salicylate and salt treatment was undertaken to identify WAKs involved in cell expansion and response to external stimuli. The BdWAK2 gene displayed increased expression during cell expansion and stress response, in addition to playing a potential role in the hypersensitive response. In vitro binding assays with various forms of commercial polysaccharides (pectins, xylans, and mixed-linkage glucans) and wall-extracted fractions (pectic/hemicellulosic/cellulosic) from both Arabidopsis and Brachypodium leaf tissues provided new insights into the binding properties of BdWAK2 and other candidate BdWAKs in grasses. The BdWAKs displayed a specificity for the acidic pectins with similar binding characteristics to the AtWAKs.

Plant responses to drought are regulated by complex genetic and epigenetic networks leading to rapid reprogramming of plant growth. miRNAs have been widely indicated as key players in the regulation of growth and development. The role of miRNAs in drought response was investigated in young leaves of Brachypodium distachyon, a drought-tolerant monocot model species. Adopting an in vivo drought assay, shown to cause a dramatic reduction in leaf size, mostly due to reduced cell expansion, small RNA libraries were produced from proliferating and expanding leaf cells. Next-generation sequencing data were analyzed using an in-house bioinformatics pipeline allowing the identification of 66 annotated miRNA genes and 122 new high confidence predictions greatly expanding the number of known Brachypodium miRNAs. In addition, we identified four TAS3 loci and a large number of siRNA-producing loci that show characteristics suggesting that they may represent young miRNA genes. Most miRNAs showed a high expression level, consistent with their involvement in early leaf development and cell identity. Proliferating and expanding leaf cells respond differently to drought treatment and differential expression analyses suggest novel evidence for an miRNA regulatory network controlling cell division in both normal and stressed conditions and demonstrate that drought triggers a genetic reprogramming of leaf growth in which miRNAs are deeply involved.

The drought-tolerant grass Brachypodium distachyon is an emerging model species for temperate grasses and cereal crops. To explore the usefulness of this species for drought studies, a reproducible in vivo drought assay was developed. Spontaneous soil drying led to a 45% reduction in leaf size, and this was mostly due to a decrease in cell expansion, whereas cell division remained largely unaffected by drought. To investigate the molecular basis of the observed leaf growth reduction, the third Brachypodium leaf was dissected in three zones, namely proliferation, expansion, and mature zones, and subjected to transcriptome analysis, based on a whole-genome tiling array. This approach allowed us to highlight that transcriptome profiles of different developmental leaf zones respond differently to drought. Several genes and functional processes involved in drought tolerance were identified. The transcriptome data suggest an increased energy availability in the proliferation zones, along with an up-regulation of sterol synthesis that may influence membrane fluidity. This information may be used to improve the tolerance of temperate cereals to drought, which is undoubtedly one of the major environmental challenges faced by agriculture today and in the near future.