uORF and Phosphate-Regulated Expression of Rice OsNLA1 Controls Phosphate Transport and Reproduction

Rice (Oryza sativa) OsNLA1 has been proposed to play a crucial role in regulating phosphate (Pi) acquisition in roots, similar to that of Arabidopsis AtNLA. However, unlike AtNLA, OsNLA1 is not a target of miR827, a Pi starvation-induced miRNA. It is, therefore, of interest to know whether the expression of OsNLA1 depends on Pi supply and how it is regulated. In this study, we provide evidence that OsNLA1 controls Pi acquisition by directing the degradation of several OsPHT1 Pi transporters (i.e., OsPT1/2/4/7/8/12). We further show that OsNLA1 has an additional function in reproduction and uncover the mechanism of its expression regulation. Analysis of mRNA levels, promoter-GUS (β-glucuronidase) activity and protoplast transient expression showed that the expression of OsNLA1.1, the most abundant transcript variant, is upregulated in response to increasing Pi supply. The OsNLA1 promoter region was found to contain an upstream open reading frame (uORF) that is required for Pi-responsive expression regulation. OsNLA1 promoter activity was observed in roots, ligules, leaves, sheaths, pollen grains, and surrounding the vascular tissues of anthers, suggesting that OsNLA1 is important throughout the development of rice. Disruption of OsNLA1 resulted in increased Pi uptake from roots as well as impaired pollen development and reduced grain production. In summary, our study reveals that Pi-induced OsNLA1 expression regulated by a unique mechanism functions in Pi acquisition, Pi translocation, and reproductive success.