Creation of two hyperactive variants of phytochrome B1 for attenuating shade avoidance syndrome in maize

Increasing the planting density of maize is an effective measure to improve its yield. However, plants under high planting density tend to trigger shade avoidance syndrome (SAS), reducing lodging resistance and ultimately yield drop. Phytochrome B (phyB) plays a dominant role in mediating shade avoidance response. This study constructed two hyperactive mutated alleles of maize PHYB1: ZmPHYB1Y98F (mimicking Y104F of AtPHYB) and ZmPHYB1Y359F (mimicking Y361F of AtPHYB). Ectopic expression of ZmPHYB1Y98F and ZmPHYB1Y359F under the control of the ZmPHYB1 promoter in the Arabidopsis phyB-9 background rendered enhanced activity on complementing the phyB-9 related phenotypes compared with ZmPHYB1WT. Moreover, similar to the behavior of ZmPHYB1WT, ZmPHYB1Y98F and ZmPHYB1Y359F proteins are localized to the nucleus after red light exposure, and could interact with PIF proteins of maize. In addition, expression of ZmPHYB1Y98F and ZmPHYB1Y359F variants under the control of the native ZmPHYB1 promoter attenuated SAS of maize seedlings subjected to simulated shade treatment. It effectively reduced mature maize's plant height and ear height in field conditions. The results combined demonstrate the utility of ZmPHYB1Y98F and ZmPHYB1Y359F for attenuating SAS and breeding high density-tolerant varieties of maize.