SnRK1 Isoforms AKIN10 and AKIN11 Are Differentially Regulated in Arabidopsis Plants under Phosphate Starvation

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.