Sensing and Signaling of Phosphate Starvation: From Local to Long Distance.

Phosphorus (P) is an essential nutrient, but low concentrations of phosphate (Pi), the predominant form in which it is acquired, in the soil often limits plant growth and reproduction. To adapt to low Pi availability, plants have developed intricate regulatory mechanisms that integrate the environmental stimuli with internal cues in order to exploit the use of P. These mechanisms include sensing external and internal Pi concentrations along with co-ordination between local and long-distance signaling pathways. The downstream actions governed by these signaling pathways include local responses for remodeling the root system architecture and systemic responses for modulating the activities of Pi uptake, remobilization and recycling. As an initially acquired molecule, Pi is considered to be a primary signal that directly regulates Pi starvation responses and sets in motion the generation of subsequent signals, such as hormones, sugars, P-containing metabolites, peptides and mobile RNAs. In this review, we summarize recent progress in understanding the regulatory pathways mediated by these signaling molecules that underlie both local and systemic responses to Pi deprivation, and discuss the potential cross-talk among these signaling pathways.

Phosphite-Mediated Suppression of Anthocyanin Accumulation Regulated by Mitochondrial ATP Synthesis and Sugars in Arabidopsis.

Despite the essential role of phosphate (Pi) in plant growth and development, how plants sense and signal the change of Pi supply to adjust its uptake and utilization is not yet well understood. Pi itself has been proposed to be a signaling molecule that regulates Pi starvation responses (PSRs) because phosphite (Phi), a non-metabolized Pi analog, suppresses several PSRs. In this study, we identified a phosphite-insensitive1 (phi1) mutant which retained anthocyanin, a visible PSR, in Phi-containing but Pi-deficient medium. phi1 mutants were impaired in the gene encoding an FAd subunit of mitochondrial F1Fo-ATP synthase and showed a reduced mitochondrial ATP level in roots, growth hypersensitivity to oligomycin and an increased mitochondrial membrane potential, suggesting that this gene has a crucial role in mitochondrial ATP synthesis. phi1 mutants accumulated a high level of sugars in shoots, which may account for the increased accumulation of anthocyanin and starch in Phi-containing conditions. Gene expression analysis showed that a subset of genes involved in carbohydrate metabolism in phi1 was misregulated in response to Phi. The majority of genes were repressed by Pi starvation and, unlike wild-type plants, their repression in phi1 was not affected by the addition of Phi. Our findings show that defective mitochondrial ATP synthesis results in sugar accumulation, leading to alteration of Phi-mediated suppression of PSRs. This study reinforces the role of sugars, and also reveals a cross-talk among ATP, sugars and Pi/Phi molecules in mediating PSRs.

Long-distance call from phosphate: systemic regulation of phosphate starvation responses.

Phosphate (Pi) is an essential nutrient for plants but is normally fixed in soil, which limits plant growth and reproduction. In response to low availability of Pi, shoots and roots react differently but cooperatively to improve Pi acquisition from the rhizosphere and adjust Pi distribution and metabolism within plants. Shoot and root responses are coordinated by the trafficking of various kinds of systemic signals through the vasculature. Mutual communication between different tissues is necessary to integrate the environmental stimuli with the internal cues at the whole-plant level. Different approaches have been used to monitor or manipulate components in the vascular stream to reveal several candidates of systemic signals from roots or shoots, including photosynthates, phytohormones, microRNAs, and Pi. In addition, the downstream signalling pathways mediated by these signals have been discovered. The crosstalk among different signalling pathways has been revealed, showing the complexity of the Pi signalling network. In this review, we summarize the approaches used for studying systemic signalling and discuss recent progress and challenges in investigating the systemic signalling pathway that integrates Pi starvation responses to maintain Pi at physiological concentrations. Knowledge gained from this study may help improve the phosphorus use efficiency of crops.