Ethylene and the responses of plants to phosphate deficiency

The consideration as to how plants uptake and transport phosphorus (P) is of significant agronomic and economic importance, in part driven by finite reserves of rock phosphate. Our understanding of these mechanisms has been greatly advanced, particularly with respect to the responses of plants to P deficiency and the genetic dissection of the signalling involved. Further, the realization that there are two tiers of transcriptional responses, the local, in which inorganic P (Pi) acts as an external signal independent of the endogenous P level, and the systemic involving root–shoot signalling, has now added a dimension of both clarity and complexity. Notwithstanding, it is now clear that the hormone ethylene plays a key role in mediating both levels of responses. This review, therefore, covers the role of ethylene in terms of mediating responses to P deficiency. The evidence that Pi supply regulates ethylene biosynthesis and sensitivity, and that this, in turn, regulates changes in root system architecture and in Pi-deprivation responses is examined here. While ethylene is the focus, the key interactions with auxin are also assessed, but interactions with the other hormone groups, which have recently been reviewed, are not covered. The emerging view that ethylene is a multi-faceted hormone in terms of mediating responses to P deficiency invites the dissection of the transcriptional cues that mediate changes in ethylene biosynthesis and/or sensitivity. Knowledge of the nature of such cues will subsequently reveal more of the underpinning interactions that govern P responses and provide avenues for the production of germplasm with an improved phosphate use efficiency.