Primary multistep phosphorelay activation comprises both cytokinin and abiotic stress responses: Insights from comparative analysis of Brassica type-A response regulators.

Multistep phosphorelay (MSP) signaling integrates hormonal and environmental signals to control both plant development and adaptive responses. The type-A RESPONSE REGULATORs (RRAs), the downstream members of the MSP cascade and cytokinin primary response genes are supposed to mediate primarily the negative feedback regulation of (cytokinin-induced) MSP signaling. However, the transcriptional data suggest the involvement of RRAs in stress-related responses as well. By employing evolutionary conservation with the well-characterized Arabidopsis thaliana RRAs, we identified 5 and 38 novel putative RRAs in Brassica oleracea and Brassica napus, respectively. Our phylogenetic analysis suggests the existence of gene-specific selective pressure, maintaining the homologs of ARR3, ARR6, and ARR16 as singletons during the evolution of Brassicaceae. We categorized RRAs based on the kinetics of their cytokinin-mediated upregulation and observed both similarities and specificities in this type of response across Brassicaceae species. Using bioinformatic analysis and experimental data demonstrating the cytokinin and abiotic stress responsiveness of A. thaliana-derived TCSv2 reporter, we unveil the mechanistic conservation of cytokinin- and stress-mediated upregulation of RRAs in Brassica rapa and Brassica napus. Notably, we identify partial cytokinin dependency of cold stress-induced RRA transcription, thus corroborating the role of cytokinin signaling in the crop adaptive responses.