Cytokinin Signaling System in Plant Immunity

Abstract

Cytokinins play an important role in the modulation of plant innate immunity. Cytokinins regulate the host defense responses either positively or negatively depending on the concentrations of cytokinins available at the infection site. Cytokinin perception and signaling involve two-component phosphorelay system. Binding of cytokinin to sensor histidine kinase (AHKs) receptors initiates a phosphotransfer cascade that culminates in the phosphorylation of response regulator proteins (ARRs) which are responsible for the regulation of cytokinin signaling system. The two types of ARR transcription activators differentially modulate the defense responses. Type-A ARRs negatively regulate cytokinin signaling, and transgenic lines overexpressing type-A ARRs display decreased defense responses, while the type-B ARR transcription factor triggers enhancement of defense responses. Pathogen-associated molecular patterns modulate the expression of both type-A and type-B ARR genes. The ARR transcription activators regulate the expression of various transcription factors belonging to MYB and AP2/ERF families, which play an important role in plant immune responses. Cytokinin homeostasis in the cytokinin signaling pathway regulates plant immune responses. Cytokinin augments PAMP-triggered immunity by enhancing callose deposition. Cytokinin may trigger the phytoalexin accumulation and confer resistance against pathogens. Cytokinins have also been shown to prime plant cells for the potentiated activation of defense-related genes. Cytokinins systemically induce resistance against pathogen infection, and this resistance is orchestrated by endogenous cytokinin and salicylic acid signaling. Cytokinins enhance resistance against pathogens via a direct induction of defense-related genes, in cooperation with SA, by modulating the NPR1/TGA3-mediated signaling. Cytokinin signaling in concert with SA signaling activates defense responses. Type-A ARRs negatively regulate SA-dependent basal immunity. Cytokinin signaling system may also function independent of SA signaling system. ABA modulates the expression of genes encoding proteins involved in cytokinin biosynthesis and degradation. Synergism and antagonism between auxin and cytokinin signaling have been reported. Auxin positively or negatively regulates the expression of the genes involved in cytokinin homeostasis. Auxin also causes repression of cytokinin responses by activating type-A ARRs, which negatively regulate cytokinin signaling.