Rice circadian clock regulator Nhd1 controls sucrose transporter OsSUT1 expression and impacts carbon-nitrogen balance.

Interdependent metabolic and transport processes of carbon (C) and nitrogen (N) regulate plant growth and development, while the regulatory pathways remain poorly defined. We previously reported that a rice circadian clock N-mediated heading date-1 (Nhd1) regulates growth-duration-dependent N use efficiency. Here, we report that knockout of Nhd1 in rice reduced photosynthesis rate and sucrose ratio of sheaths to blades but increased total C to N ratio and free amino acids. Leaf RNA-seq analysis indicated that mutation of Nhd1 dramatically altered expression of the genes linking to starch and sucrose metabolism, circadian rhythm, and amino acid metabolic pathways. We identified that Nhd1 can directly activate the transcriptional expression of sucrose transporter-1 (OsSUT1). Knockout of Nhd1 suppressed OsSUT1 expression and both nhd1 and ossut1 mutants showed similar shorter height, lower shoot biomass and sucrose concentration in comparison to wild type, while overexpression of OsSUT1 can restore the defective sucrose transport and partially ameliorate the reduced growth of nhd1 mutants. Nhd1-binding site of OsSUT1 promoter is conserved in all known rice genomes. The positively related variation of Nhd1 and OsSUT1 expression among randomly selected Indica and Japonica varieties suggests a common regulatory module of Nhd1-OsSUT1 mediated C and N balance in rice.