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Breeding of High-yield and Early-maturation Rice Could Be Achieved
The application of nitrogen (N) fertilizers is one of the most effective forces driving crop yield improvements. It is estimated that more than 120 million tons of nitrogen is used worldwide as fertilizer annually. However, application of N at high rates brings detrimental effects to crops such as delayed flowering and thus prolonged maturation times, greatly increasing risks of yield loss. In breeding practice, high-yield and early-maturation is also a long-standing paradox, and it is extremely difficult to make these two most important traits achieved simultaneously. As for one single gene, it seems to be even more impossible.
Recent work published on the Plant Cell from Prof. CHU Chengcai’s laboratory in Institute of Genetics and Developmental Biology, Chinese Academy of Sciences reported a rice nitrate transporter (NRT1.1A) that may provide a solution to the nitrogen use/flowering problem.
CHU and colleagues showed that overexpression of this nitrate transporter in different rice varieties increased rice yields up to 60% by improving N use efficiency (NUE), and also matured up to 18 days earlier than wild-type controls. In addition, they observed even larger yield increases and shortened maturation times when we overexpressed this transporter in the model eudicot Arabidopsis.
It is widely recognized that increasing NUE is a key strategy to increase the sustainability of agricultural production, and large-scale programs for improving NUE have been underway for several decades. Important functional and structural studies in Arabidopsis have demonstrated the role of AtNRT1.1, a member of the nitrate transporter 1/peptide transporter family, as an essential component in nitrate signaling.
CHU’s laboratory previously demonstrated that OsNRT1.1B is a functional homologue of AtNRT1.1 that is involved in nitrate utilization, and showed that a single polymorphism in this gene contributes the long-noted divergence in NUE between the indica and japonica subspecies of Asian rice (Nature Genetics, 2015). In addition to OsNRT1.1B, rice has two additional NRT1.1 homologues, OsNRT1.1A and OsNRT1.1C. Whereas the expression of OsNRT1.1B is induced by nitrate, they found here that the expression of OsNRT1.1A is actually suppressed by nitrate but induced by ammonium. This functional divergence is particularly important for rice production, as ammonium is the preferred form of N for rice and other plants that grow under anaerobic conditions.
Application of N at high rates brings detrimental effects to crops such as delayed flowering and thus prolonged maturation times, which significantly increases the risk of yield losses, especially in high-latitude regions where late-season temperatures can severely restrict grain filling.
“High yield and early maturation are two of the most desirable, if seemingly contradictory, traits sought in plant breeding efforts”, says Dr. HU Bin, one of the co-first authors. “We demonstrate that overexpression of OsNRT1.1A results in the improvement of three agronomically-critical traits: NUE, yield, and maturation time. We believe that the groundbreaking discovery reported here will have a rapid and widespread impact on agricultural production.”  
The multi-year, multi-site large-scale field tests showed that transgenic rice overexpressing OsNRT1.1A had a ~50% increase in NUE and grain yield, and maturation times shortened up to 18 days. Importantly, they found that expression of OsNRT1.1A in the eudicot Arabidopsis had an even more dramatic effect in yield improvement and early maturation than in the monocot rice (as high as ~90% increase in seed weight per plant), indicating a potentially-enormous influence of this discovery for the development of early-maturation high-yield varieties in a great many crop species.
Dr. CHU Chengcai
Institute of Genetics and Developmental Biology, Chinese Academy of Sciences