The bZIP73 transcription factor controls rice cold tolerance at the reproductive stage.

Cold temperature during the reproductive stage often causes great yield loss of grain crops in subtropical and temperate regions. Previously we showed that the rice transcription factor bZIP73 plays an important role in cold adaptation at the seedling stage. Here we further demonstrate that bZIP73 also confers cold stress tolerance at the reproductive stage. bZIP73 was up-regulated under cold treatment and predominately expressed in panicles at the early binucleate and flowering stages. bZIP73 forms heterodimers with bZIP71, and co-expression of bZIP73 and bZIP71 transgenic lines significantly increased seed-setting rate and grain yield under natural cold stress conditions. bZIP73 :bZIP71 not only repressed ABA level in anthers, but also enhanced soluble sugar transport from anthers to pollens and improved pollen grain fertility, seed-setting rate, and grain yield. Interestingly, bZIP73 :bZIP71 also regulated the expression of qLTG3-1 , and qLTG3-1 overexpression lines greatly improved rice tolerance to cold stress during the reproductive stage. Therefore, our work establishes a framework for rice cold stress tolerance through the bZIP71-bZIP73 -qLTG3-1 -sugar transport pathway. Together with our previous work, our results provide a powerful tool for improving rice cold stress tolerance at both the seedling and the reproductive stages.