The Role of Brachypodium distachyon Wall-Associated Kinases (WAKs) in Cell Expansion and Stress Responses

Abstract

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.

Transcriptome Profiling of Maize (Zea mays L.) Leaves Reveals Key Cold-Responsive Genes, Transcription Factors, and Metabolic Pathways Regulating Cold Stress Tolerance at the Seedling Stage

“…In this study, CaMs, CMLs, CBLs, CDPKs, 21 leucine-rich repeat receptor-like kinase proteins (LRR-RLKs), 8 lectin receptor-like kinases (LecRLKs), 12 protein kinases (PKs), and 5 WAKs were regulated by cold stress (Table S4). In Camellia japonica, protein phosphorylation by CDPKs and CIPKs improve cold tolerance [54], whereas in plants, WAKs play an important function in abiotic stress tolerance [55]. Interestingly, TMK1 (Zm00001d033777, Zm00001d007313), CBL10 (Zm00001d023353, Zm00001d010459), RKL1 (Zm00001d048054), NIK3 (Zm00001d018635), and PSKR (Zm00001d018635) displayed a high expression pattern in the tolerant lines.…”

Cell wall integrity regulation across plant species

“…Their ability to interact with pectin has been confirmed independently for several species, despite the low pectin abundance in grass cell walls (about 20% in primary cell walls of Arabidopsis but only 5–10% in grasses; Chen et al 2021a , b ; Gigli-Bisceglia et al 2020 ; Wu et al 2020 ) suggesting that WAKs have a highly conserved role in connecting pectic compounds in plant cell walls to intracellular responses. In Brachypodium, high expression levels of several WAK genes (specifically BdWAK2, 10 , 42 , 72 and 108 ) in rapidly growing tissue implicates them in cell expansion (Wu et al 2020 ). Vacuolar invertase-dependent regulation of turgor pressure was hypothesized to be the mechanism responsible for WAK-mediated cell wall expansion in Arabidopsis but remains to be experimentally confirmed (Kohorn 2016 ; Kohorn et al 2012 ).…”

“…Whereas this family consists only of 5 WAKs and 21 WAKLs in Arabidopsis (the distinction between WAKs and WAKLs is less clear in other species), the number has increased to 29 in cotton, 125 in rice, 341 in wheat, more than 100 in maize, 91 in Barley and 115 in Brachypodium distachyon (hereafter Brachypodium)(Dou et al 2021 ; Tripathi et al 2021 ; Wu et al 2020 ; Zhang et al 2021 ). Their ability to interact with pectin has been confirmed independently for several species, despite the low pectin abundance in grass cell walls (about 20% in primary cell walls of Arabidopsis but only 5–10% in grasses; Chen et al 2021a , b ; Gigli-Bisceglia et al 2020 ; Wu et al 2020 ) suggesting that WAKs have a highly conserved role in connecting pectic compounds in plant cell walls to intracellular responses. In Brachypodium, high expression levels of several WAK genes (specifically BdWAK2, 10 , 42 , 72 and 108 ) in rapidly growing tissue implicates them in cell expansion (Wu et al 2020 ).…”

“…Vacuolar invertase-dependent regulation of turgor pressure was hypothesized to be the mechanism responsible for WAK-mediated cell wall expansion in Arabidopsis but remains to be experimentally confirmed (Kohorn 2016 ; Kohorn et al 2012 ). While WAKs have been implicated in abiotic and biotic stress responses by activating phytohormone biosynthesis, such as JA, and hypersensitive responses signaling cascades leading to programmed cell death, as exemplified by the activity of Bd WAK2 (Wu et al 2020 ), the mechanisms responsible remain unknown. Interestingly, manipulation of wheat WAK and WAK-like genes confers pathogen resistance by different mechanisms.…”

Mechanisms of plant cell wall surveillance in response to pathogens, cell wall-derived ligands and the effect of expansins to infection resistance or susceptibility

“…The A. thaliana genome encodes five WAKs, 21 WAK-like genes (WAKL), and genes for truncated proteins or with EGF variations ( Verica et al, 2003 ). WAKL genes have also been identified in several angiosperms including wheat, maize, rice, and tomato, and in the first three cases, they are involved in immune responses ( Li et al, 2009 ; Yang et al, 2014 ; Zuo et al, 2015 ; Wu et al, 2020 ). Interestingly, other components of the WAK signaling pathway, the MAPK6 and the transcription factors EDS1 (ENHANCED DISEASE SUSCEPTIBILITY1) and PAD4, (PHYTOALEXIN DEFICIENT4), all of which are involved in the response to pathogens, seem to converge on this pathway ( Joglekar et al, 2018 ; Dongus and Parker, 2021 ).…”