Phosphate differentially regulates 14-3-3 family members and GRF9 plays a role in Pi-starvation induced responses

The 14-3-3s are phosphoserine-binding proteins that act as key regulators of many metabolic pathways. Several biotic and abiotic stresses have been shown to modulate the expression of 14-3-3 genes. In Arabidopsis thaliana, 15 genes are known to code for 14-3-3 isoforms belonging to epsilon and non-epsilon groups. Since phosphorus is one of the essential macronutrients for plants, we examined its role in the regulation of the expression of 14-3-3 isoforms belonging to epsilon (GRF9, GRF10, GRF11, GRF13) and non-epsilon (GRF1, GRF3, GRF6, GRF8) groups. The effect of Pi deprivation was differential on the members of non-epsilon group ranging from a significant reduction in the transcripts of GRF3 to non-perceptible changes in the transcripts of other members. Suppressive effect of Pi-deficiency was more pronounced on some of the members of epsilon group with transcripts levels of GRF9 and GRF13 barely detectable. A concurrent increase in the transcript levels of GRF9 with an increase in the Pi concentration suggested a correlation between gene expression and Pi availability. However, neither Pi deficiency at low temperature nor Fe and K deficiency failed to suppress GRF9 expression. In planta role of GRF9 was elucidated by the analysis of the loss-of-function mutant under Pi-replete condition. The analyses revealed exaggerated Pi-starvation responses in the form of starch accumulation in the leaves and modulated root system architecture (RSA). An inverse relationship between the abundance of GRF9 transcripts and accumulation of starch in transgenic lines over-expressing this gene provided further evidence towards the role of GRF9 in modulation of metabolic pathways during Pi-starvation responses.