Rapeseed transcription factor BnaC9bHLH35 promotes root growth and improves nitrogen use efficiency under low nitrate conditions.

BnaC9bHLH35 directly regulated the expression of several nitrate uptake and assimilation-related genes and improved nitrogen use efficiency. Nitrogen is one of the most important basic elements in life, which forms proteins, nucleic acids and numerous indispensable compounds. It is critical to improve the nitrogen use efficiency of crops while reduce the amount of fertilizer use. In this study, we have identified a rapeseed transcription factor BnaC9bHLH35 induced by low nitrate stress. Different from regular transcription factors, BnaC9bHLH35 showed a distinct multiple localization and, it plays an important role in the regulatory network of nitrate usage, functioning to enhance the expression of BnaNRT and BnaNR genes and downregulate the expression of BnaCLCB to stimulate nitrate uptake and further increase cytoplasmic nitrate concentration and promote nitrate reduction when plants suffer from low nitrate conditions. Overexpression of BnaC9bHLH35 in Arabidopsis could promote axial root elongation and lateral root density in a low nitrate environment compared with Col-0 and transgenic GFP lines as controls. Through ChIP-qPCR assay, BnaC9bHLH35 was found to directly bind to the promoter regions of its target genes. To sum up, BnaC9bHLH35 is a critical transcription factor in the regulation of the rapeseed low nitrate response and could be used to improve NUE of existing rapeseed varieties.