Abstract: Increasing plant density has been recognized as an effective strategy for boosting maize yields over the past few decades. However, dense planting significantly reduces the internal light intensity and the red to far-red (R:FR) light ratio in the canopy, which subsequently triggers shade avoidance responses (SAR) that limit further yield enhancements, particularly under high-density conditions. In this study, we identified double B-box containing protein DBB2, a member of the ZmBBX family that is rapidly induced by shade, as a crucial regulator of plant height and SAR. Disruption of DBB2 resulted in shorter internodes, reduced plant height, decreased cell elongation, and diminished sensitivity to shade in maize, effects that can be largely alleviated by external treatment with gibberellins (GA). Furthermore, we discovered that DBB2 physically interacted with the transcription factor HY5, inhibiting its transcriptional activation of ZmGA2ox4, a gene encoding a GA2 oxidase that can deactivate GA. This interaction positively influences maize plant height through the GA pathway. Additionally, we found that the induction of ZmDBB2 by shade is mediated by the transcription factor PIF4. Interestingly, DBB2 then interacted with PIF4 to enhance the transcriptional activation of cell elongation-related genes, such as ZmEXPA1, thereby establishing a positive feedback loop promoting cell elongation under canopy shade conditions. Our findings highlight the critical role of BBX proteins in modulating plant height and SAR, presenting them as key genetic targets for developing maize varieties suited to high-density planting conditions. This study also provides new insights into the molecular mechanisms underlying SAR and offers potential strategies for the genetic improvement of maize plant architecture and grain yield.

Key words: BBX, density tolerance, maize, plant height, shade avoidance response