OsNLA1, a RING-type ubiquitin ligase, maintains phosphate homeostasis in Oryza sativa via degradation of phosphate transporters

Inorganic phosphate (Pi) transporters (PTs) play vital roles in Pi uptake and translocation in plants. Under Pi sufficient conditions, PTs are degraded to prevent excess Pi accumulation. The mechanisms targeting PTs for degradation are not fully elucidated. In this study, we found that the Oryza sativa (rice) ortholog of Arabidopsis thaliana nitrogen limitation adaptation (NLA), OsNLA1 protein, a RING-type E3 ubiquitin-ligase, was predominantly localized in the plasma membrane, and could interact with rice phosphate transporters OsPT2 and OsPT8. Mutation of the 265th cysteine residue in OsNLA1 that was required for ubiquitination prevented breakdown of OsPT2/PT8, suggesting OsNLA1 targeted OsPT2/PT8 for degradation. Mutation in OsNLA1 (osnla1) led to a significant increase of Pi concentration in leaves in a nitrate-independent manner. Overexpression of OsNLA1 or repression of OsPT2/PT8 restored the high leaf Pi concentration in osnla1 mutants to a level similar to that of wild-type plants. In contrast to what has been observed in Arabidopsis, the transcript abundance of OsNLA1 did not decrease under Pi limited conditions or in OsmiR827 (microRNA827)- or OsPHR2 (PHOSPHATE STARVATION RESPONSE 2)-overexpressing transgenic lines. Moreover, there was no interaction of OsNLA1 and OsPHO2, an E2 ubiquitin-conjugase, suggesting that OsPHO2 was not the partner of OsNLA1 involved in ubiquitin-mediated PT degradation. Our results show that OsNLA1 is involved in maintaining phosphate homeostasis in rice by mediating the degradation of OsPT2 and OsPT8, and OsNLA1 differs from the ortholog in Arabidopsis in several aspects.