Role of the Cytokinin-Activated Type-B Response Regulators in Hormone Crosstalk

Abstract

Cytokinin is an important phytohormone that employs a multistep phosphorelay to transduce the signal from receptors to the nucleus, culminating in activation of type-B response regulators which function as transcription factors. Recent chromatin immunoprecipitation-sequencing (ChIP-seq) studies have identified targets of type-B ARABIDOPSIS RESPONSE REGULATORs (ARRs) and integrated these into the cytokinin-activated transcriptional network. Primary targets of the type-B ARRs are enriched for genes involved in hormonal regulation, emphasizing the extensive crosstalk that can occur between cytokinin, auxin, abscisic acid, brassinosteroids, gibberellic acid, ethylene, jasmonic acid, and salicylic acid. Examination of hormone-related targets reveals multiple regulatory points including biosynthesis, degradation/inactivation, transport, and signal transduction. Here, we consider this early response to cytokinin in terms of the hormones involved, points of regulatory crosstalk, and physiological significance.

Research Progress on the Roles of Cytokinin in Plant Response to Stress

“…B-type ARRs (ARR1, 2, 10-14, 18-21) include an N-terminal Rec domain and a large C-terminal region, and the C-terminal region contains a Myb-like DNA binding domain and a transcription activation domain [36]. In the nucleus, AHPs transfer phosphate groups to Asp residues in the Rec domain of B-type ARRs to activate B-type ARRs, essential for the initial transcription reaction of cytokinins [37][38][39]. B-type ARRs are positive regulators of cytokinin signal response.…”

The Interaction Network and Signaling Specificity of Two-Component System in Arabidopsis

“…Loss of gene function mutants in Type B1 ARR exhibited reduced sensitivity to cytokinin [13,103], and multiple mutant combinations affected the same cytokinin response as described in ahk and ahp mutants, including reduced shoot development, aborted primary root growth, malformed hypocotyl elongation, retarded chloroplast development, enlarged seed size, and female gametophyte developmental defects [104][105][106][107][108][109][110], indicating a central role of Type B1 members in cytokinin signaling pathway. In addition, cytokinin-activated Type B1 RRs also play essential roles in hormone crosstalk, as their primary targets are enriched for hormone-related genes [111]. The expression of Type B2 and Type B3 subfamily members are not as broadly prevalent as Type B1 members, and their functional research is mainly restricted to over-expressed lines.…”

Characterization of the cytokinin sensor TCSv2 in arabidopsis and tomato

“…Cytokinin signalling is mediated via a two-component multistep phosphorelay cascade. As the final step, type-B response regulators (RRs) activate transcription in response to phosphorelay signalling activity, while type-A response regulators are rapidly induced by CK via Type-B RRs, and, in turn, repress signalling via a negative-feedback loop [6][7][8][9][10].…”