Phosphate-Starvation-Inducible S-Like RNase Genes in Rice Are Involved in Phosphate Source Recycling by RNA Decay

Abstract

The fine-tuning of inorganic phosphate (Pi) for enhanced use efficiency has long been a challenging subject in agriculture, particularly in regard to rice as a major crop plant. Among ribonucleases (RNases), the RNase T2 family is broadly distributed across kingdoms, but little has been known on its substrate specificity compared to RNase A and RNase T1 families. Class I and class II of the RNase T2 family are defined as the S-like RNase (RNS) family and have showed the connection to Pi recycling in Arabidopsis. In this study, we first carried out a phylogenetic analysis of eight rice and five Arabidopsis <i>RNS</i> genes and identified mono-specific class I and dicot-specific class I RNS genes, suggesting the possibility of functional diversity between class I RNS family members in monocot and dicot species through evolution. We then compared the <i>in silico</i> expression patterns of all <i>RNS</i> genes in rice and Arabidopsis under normal and Pi-deficient conditions and further confirmed the expression patterns of rice <i>RNS</i> genes via qRT-PCR analysis. Subsequently, we found that most of the <i>OsRNS</i> genes were differentially regulated under Pi-deficient treatment. Association of Pi recycling by RNase activity in rice was confirmed by measuring total RNA concentration and ribonuclease activity of shoot and root samples under Pi-sufficient or Pi-deficient treatment during 21 days. The total RNA concentrations were decreased by < 60% in shoots and < 80% in roots under Pi starvation, respectively, while ribonuclease activity increased correspondingly. We further elucidate the signaling pathway of Pi starvation through upregulation of the <i>OsRNS</i> genes. The 2-kb promoter region of all <i>OsRNS</i> genes with inducible expression patterns under Pi deficiency contains a high frequency of P1BS cis-acting regulatory element (CRE) known as the OsPHR2 binding site, suggesting that the OsRNS family is likely to be controlled by OsPHR2. Finally, the dynamic transcriptional regulation of <i>OsRNS</i> genes by overexpression of <i>OsPHR2</i>, <i>ospho2</i> mutant, and overexpression of <i>OsPT1</i> lines involved in Pi signaling pathway suggests the molecular basis of <i>OsRNS</i> family in Pi recycling via RNA decay under Pi starvation.