Blocking the Expression of the Nitrate Reductase Gene OsNia3 Confers Early Maturation in Rice.

Early flowering is a major contributing factor to early crop maturation and is a prerequisite for the expansion of crop production to high latitudes with short growing seasons as well as for annual double/triple cropping systems. Nitrogen is the mineral element most limiting to plant growth and, consequently, also to crop yield. Nitrogen also plays a role in the regulation of flowering time. Here, we show that OsNia3 is a member of the rice nitrate reductase (NR) gene family. OsNia3 was found to be expressed in stem nodes of rice, while the loss of its function led to a reduction in plant height mainly due to disrupted nitrate metabolism at the site of OsNia3 expression. The expression levels of genes related to nitrate transport, nitrate assimilation, and ammonium assimilation, such as OsNia1, OsNRT2.3A, and OsNGS1, increased in the leaves of osnia3 mutant plants, concomitant with an increase in the activity of the nitrogen metabolism pathway. The ammonium and amino acid contents in the leaves also increased in the osnia3 mutant, as did the chlorophyll content. Phenotypic analysis showed that the osnia3 mutant exhibited a shorter growth period compared with wild-type plants; nevertheless, yield was not significantly affected in mutant plants. The deletion of the OsNia3 gene led to the upregulation of genes related to the photoperiod regulation network (OsGI, OsHd1, and OsHd3a), which likely promoted early flowering. These results indicated that OsNia3 is involved in the floral inhibition pathway (OsGI-OsHd1-OsHd3a) in rice under long-day conditions. Our findings provide a theoretical and material basis for the coordination and improvement of early maturity and high-yield traits in rice breeding. [ABSTRACT FROM AUTHOR]