Regulation of the expression of OsIPS1 and OsIPS2 in rice via systemic and local Pi signalling and hormones

Two genes with a common region that is characteristic of the TPSI1/Mt4 family were cloned from a Pi-starvation-induced cDNA library of rice roots using suppression subtracted hybridization (SSH). Based on the consensus sequence of these two genes, members of the TPSI1/Mt4 family were found in maize, wheat and barley. BLAST and a cluster analysis in the eight members of the TPSI1/Mt4 family showed two classes of four genes each among monocots. The first gene from rice was designated OsIPS1 based on a comparison of the consensus sequence with AtIPS1, and consequently the second gene, which has been previously reported as OsPI1, was designated OsIPS2. Accumulation of the mRNA of OsIPS1/2 was examined by northern blotting and quantitative reverse transcriptase-polymerase chain reaction in whole-root and split-root experiments under treatment with phosphate (Pi) and the Pi analogue phosphite (Phi). OsIPS1 showed much higher mRNA accumulation in roots than OsIPS2, and an opposite trend was seen in shoots. OsIPS1/2 showed both systemic and local responses to Pi starvation, and less than 10% of the overall induced mRNA level was due to the local Pi concentration in roots. The results indicate that Phi may interfere with earlier events in roots that are associated with a local Pi signalling pathway. An analysis of transgenic plants showed that OsIPS1/2 are independently responsive to Pi signalling and are mainly expressed in lateral roots and in the vascular cylinder in the primary root. Exogenous cytokinin (6-BA) almost completely suppressed systemic Pi starvation signalling and partially suppressed local Pi signalling. Exogenous abscisic acid remarkably reduced Pi starvation signalling. In contrast, exogenous auxin enhanced Pi signalling, especially local Pi signalling in roots. Exogenous ethylene (ethyphon) and the ratio of auxin to cytokinins did not appear to affect the expression of these two genes.