Introduction

In this chapter, I discuss the current state of knowledge about the biochemistry and molecular biology of phosphate starvation inducible (psi) enzymes and proteins in higher plants. Special attention will be paid to the excreted phosphate starvation inducible (epsi, pronounced ee Ψ) acid phosphatase (APase) of higher plants. A significant amount of data now exists to support the theory that the epsi-APase, an epsi-RNase, several intracellular RNases and other proteins of unknown function form a family of co-induced proteins that act, at least in part, as a phosphate starvation rescue mechanism for higher plants. In addition, we have conducted experiments to show that some of these proteins can apparently affect phosphate use-efficiency metabolism under non-starvation conditions. While crucial experiments remain to be done, our data further suggest that the epsi-APase genes as well as genes for other psi proteins can be regulated at the mRNA level (possibly by transcriptional activation). We have proposed, as a working model, that these psi genes are part of the higher plant pho stimulon and may be co-regulated by the same trans-acting element(s) to form a pho regulon (Goldstein et al., 1989a).

Phosphorus is an essential nutrient for all cells. For organisms that absorb their mineral nutrients directly from the external medium, ionic inorganic phosphate (Pi, usually H2PO4− or HPO42−) is the preferentially absorbed form of phosphorus. A macronutrient based on its contribution to biomass, Pi is one of the least available mineral nutrients in many environments.