Molecular regulation of phosphate starvation-induced processes in plants

Phosphate has a profound influence on plant growth, development and carbon metabolism. Plants respond to changes in soil phosphate status by differential expression of genes. The goal of this research was to investigate the involvement of phytohormones, phosphite (a Pi analog) and carbon in regulation of phosphate starvation-inducible (PSI) genes. The first objective was to look for differences in the expression patterns of the two predominant Arabidopsis high-affinity Pi transporters AtPT1 and AtPT2 ( Arabidopsis thaliana phosphate transporter). Using the reporter gene (β-glucuronidase) the spatial regulation of the Pi transporters was examined. Although both the Pi transporters were induced in roots in the case of AtPT2, flowers and siliques also showed expression of reporter gene. Thus AtPT2 may mobilize Pi into reproductive organs as well. In addition responses to Pi deficiency mimic the effect of phytohormones. Thus the effect of auxin, ethylene and cytokinin on the expression of PSI genes was investigated. External application of auxins and cytokinins suppressed the expression of AtPT1. In contrast inhibition of auxin transport or auxin response did not affect its expression. None of these phytohormones affected the expression of the other PSI genes analyzed. Taken together these results indicate that phytohormones may be involved in regulation of some component(s) of Pi starvation response pathway. Recent studies in yeast and higher plants suggest that phosphite (Phi) selectively inhibits Pi starvation responses and may serve as a useful molecular tool to dissect Pi sensing. Thus the second objective was to study the effect of Phi on the coordinated plant phosphate starvation responses. Phi suppressed Pi deficiency symptoms and the expression of PSI genes. Hence Phi enabled the isolation of Arabidopsis T-DNA tagged mutants that were altered in their response to Pi deficiency. There is growing evidence for the role of carbon in nutrient acquisition. Pi deficiency has a profound influence on plant carbon partitioning suggesting a link between Pi and carbon metabolism. Certain metabolizable sugars induced the PSI genes. Exposure of plants to light leading to increased sugar synthesis also induced the luciferase reporter gene driven by the AtPT2 promoter.