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Biotic and abiotic stresses induce reactive oxygen species (ROS) production in plants as a signalling strategy. The receptor-like protein kinases (RLKs) are largely responsible for communication between cells and the extracellular environment, and ROS production is a frequent result of RLK signalling in a multitude of cellular processes (Kimura et al., 2017). However, many of the components for extracellular ROS perception, signal transmission, and specificity of downstream responses remain unknown. Cysteine-rich receptor-like kinases (CRKs) represent a subgroup of RLKs, defined by a conserved pattern of cysteines in their extracellular domain. Based on their expression profile, loss-of-function phenotypes, and the potential for redox regulation in their extracellular domain, CRKs are promising candidates to be involved in ROS signalling, potentially as extracellular ROS sensors (Wrzaczek et al., 2010; Idänheimo et al., 2014; Bourdais et al., 2015).
We have identified several CRKs including CRK2, an evolutionarily ancient member of this protein family, as essential components which can directly phosphorylate and thereby activate plasma membrane-localized NADPH oxidases (respiratory burst oxidase homologs; RBOH) in a calcium-independent manner. CRK2 forms a pre-assembled complex with RBOHD to activate ROs production in response to signal perception. Intriguingly, CRK2 also interacts with a number of different proteins to modulate callose deposition and vesicle traffic in response to biotic and abiotic stimuli.
Most genomes of higher plants encode a large number of CRK genes; however, expansion of different subtypes of CRKs has happened very differently in various plant lineages. This variation is an interesting tool for studying the origin and evolution of large protein families. It however also highlights the difficulties in translating results from model species to crop. A combination of physiological, biochemical and evolutionary/genomic approaches using the CRKs could pave the way for future understanding of large families of receptors or other protein families in plants.