Alternating Inverse Modulation of Xylem K+/NO3 - Loading by HY5 and PIF Facilitates Diurnal Regulation of Root-to-shoot Water and Nutrient Transport.

Diurnal light-dark cycles regulate nutrient uptake and transport; however, the underlying molecular mechanisms remain largely unknown. Transcription factor MYB59 and ion transporter NPF7.3 participate in root-to-shoot K+/NO3 - translocation in Arabidopsis. In this study, transcriptional analyses and western blotting experiments revealed the diurnal expression of the MYB59-NPF7.3 module. ChIP-qPCR and EMSA showed that transcription factors HY5 and PIF directly bind to the MYB59 promoter. Phenotype analyses and ion content measurement indicated that HY5 and PIF antagonistically control root-to-shoot K+/NO3 - translocation through the MYB59-NPF7.3 module. We found HY5 proteins accumulate in roots and repress MYB59 transcription during daytime, while PIF proteins promote MYB59 transcription in the dark. The expression levels of the NPF7.3 transcript and protein are gradually decreased during daytime, but increased at night. The enhancement of K+/NO3 - loading into the xylem mediated by NPF7.3 could increase root pressure at night, which maintained the root-to-shoot water/nutrient translocation. This study reveals a synergistic mechanism between light signaling and nutrient transport in plants, and defines a diurnal molecular switch of driving forces for root-to-shoot water/nutrient translocation.