ABSTRACT
During phosphate starvation, Snf1-related kinase 1 (SnRK1) activity significantly decreases compared with plants growing under normal nutritional conditions. An analysis of the expression of the genes encoding for the catalytic subunits of SnRK1 showed that these subunits were not affected by phosphate starvation. Transgenic Arabidopsis (Arabidopsis thaliana) plants overexpressing the AKIN10 and AKIN11 catalytic subunits fused with green fluorescent protein (GFP) were produced, and their localizations were mainly chloroplastic with low but detectable signals in the cytoplasm. These data were corroborated with an immunocytochemistry analysis using leaf and root sections with an anti-AKIN10/AKIN11 antibody. The SnRK1 activity in transgenic plants overexpressing AKIN11-GFP was reduced by 35% to 40% in phosphate starvation, in contrast with the results observed in plants overexpressing AKIN10-GFP, which increased the activity by 100%. No differences in activity were observed in plants growing in phosphate-sufficient conditions. Biochemical analysis of the proteins indicated that AKIN11 is specifically degraded under these limited conditions and that the increase in AKIN10-GFP activity was not due to the phosphorylation of threonine-175. These results are consistent with an important role of AKIN10 in signaling during phosphate starvation. Moreover, akin10 mutant plants were deficient in starch mobilization at night during inorganic phosphate starvation, and under this condition several genes were up-regulated and down-regulated, indicating their important roles in the control of general transcription. This finding reveals novel roles for the different catalytic subunits during phosphate starvation.
Subject(s)
Arabidopsis Proteins/metabolism , Arabidopsis/enzymology , Phosphates/deficiency , Protein Serine-Threonine Kinases/metabolism , Arabidopsis/drug effects , Arabidopsis/genetics , Arabidopsis/growth & development , Arabidopsis Proteins/genetics , Blotting, Western , Catalytic Domain , Chloroplasts/drug effects , Chloroplasts/enzymology , Gene Expression Regulation, Enzymologic/drug effects , Gene Expression Regulation, Plant/drug effects , Gene Silencing/drug effects , Isoenzymes/genetics , Isoenzymes/metabolism , Mutation/genetics , Phosphates/pharmacology , Plant Leaves/cytology , Plant Leaves/drug effects , Plant Leaves/enzymology , Plants, Genetically Modified , Protein Serine-Threonine Kinases/genetics , Starch/metabolism , Transcription, Genetic/drug effectsABSTRACT
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.