Characterization of a type A response regulator in the common bean (Phaseolus vulgaris) in response to phosphate starvation.

Type A response regulators are a family of genes in Arabidopsis thaliana involved primarily in cytokinin signal transduction. A member of this family was isolated from a cDNA library constructed from bean plants (Phaseolus vulgaris) grown under conditions of phosphate starvation. The complete cDNA sequence showed the presence of the DDK domain, which is the hallmark of the response regulator family. Expression of the P. vulgaris response regulator 1 (PvRR1) showed clear regulation based on phosphate availability because transcript levels increased during phosphate starvation and returned to basal levels after resupplementation with phosphorus. Nitrogen and potassium starvation also upregulated PvRR1, indicating that cross talk with other nutrient signaling pathways might occur. Addition of cytokinins to plants growing under phosphate-sufficient conditions stimulated PvRR1 transcript levels both in detached leaves and in roots. However, cytokinins strongly inhibited PvRR1 expression in phosphate-starved plants after 24 h of incubation. At the protein level, subcellular localization of PvRR1 indicated that it is a nuclear protein and that phosphate starvation modified protein levels but not the localization.