As most soil phosphates exist as insoluble inorganic phosphate and organic phosphates, higher plants have developed several strategies for adaptation to low phosphorus (P). These include the secretion of acid phosphatase and organic acids, induction of the inorganic phosphate (Pi) transporter and the substitution of some enzyme activities as alternative pathways to increase P utilization efficiency. It has been proposed that plants also have a ' pho regulon' system, as observed in yeast and Escherichia coli ; however, the detail of the regulation system for gene expression on P status is still unclear in plants. To investigate the alteration of gene expression of rice roots grown under P-deficient conditions, a transcriptomic analysis was conducted using a cDNA microarray on rice. Based on the changes of gene expression under a -P treatment, the up-regulation of some genes due to P deficiency was confirmed. Some new important metabolic changes are suggested, namely: (1) acceleration of carbon supply for organic acid synthesis through glycolysis; (2) alteration of lipid metabolism; (3) rearrangement of compounds for cell wall; and (4) changes of gene expression related to the response for metallic elements such as Al, Fe and Zn.

Summary Plants have developed several methods of adapting to conditions of low phosphorus (P). However, details of the regulation of the gene expression system that responds to P status of plants is unknown. Here, a phosphorus limitation inducible novel gene was isolated and characterized to provide further information of plant adaptation to low P. Rice plants (Oryza sativa) were grown hydroponically with or without P. A novel gene was isolated by cDNA microarray analysis and designated as OsPI1 (Oryza sativaPhosphate‐limitation Inducible Gene 1). mRNA accumulation was examined by Northern blot and quantitative real time PCR. The OsPI1 gene was rapidly induced by phosphate starvation in both shoots and roots. When phosphate was supplied to phosphate‐deficient plants, the OsPI1 transcripts rapidly disappeared. OsPI1 cDNA consisted of 375 bp and contained several small open reading frames (ORFs). The OsPI1 gene shows the same characteristics as the TPSI1/Mt4 family (the phosphate starvation inducible novel gene family). It is suggested that OsPI1 acts as riboregulator, that is, it binds with other molecules under phosphate starvation and regulates their function.

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