The transcription factors GmBBX17 and GmSTFs antagonistically regulate shade avoidance in soybean by oppositely modulating GmSRG1 transcription

High-density planting represents a promising strategy to enhance crop productivity. However, this agronomic practice inevitably triggers shade avoidance responses (SAR), manifested as stem and petiole elongation, suppressed branching, and altered petiole angles. Elucidating the molecular underpinnings of shade avoidance remains imperative for rationalizing planting density strategies in crop production. Here, we report that soybean SHADE-REDUCED GENE 1 (GmSRG1) inhibits the stem and petiole growth both in white light (WL) and low ratio of Red:Far-red light (Low R:FR) conditions in soybean. GmSRG1 likely represents a catalytically inactive homolog of xyloglucan endotransglucosylase/hydrolase (XTH) that lacks the conserved active site. Despite its enzymatic deficiency, it is associated with enhanced xyloglucan-degrading activity and prevents xyloglucan accumulation in soybean. GmBBX17 binds to the GmSRG1 promoter to repress its transcription, whereas GmSTFs (GmSTF1 and GmSTF2) transcriptionally activate GmSRG1. GmBBX17 physically interacts with GmSTFs to antagonistically regulate GmSRG1 expression. Unlike GmSRG1, its close homolog GmXTH23 is associated with increased xyloglucan accumulation and reduced xyloglucan-degrading activity, while GmSRG1 physically interacts with GmXTH23 and counteracts this GmXTH23-associated effect. Our work reveals an integrated regulatory network comprising the GmBBX17-GmSTFs transcriptional module and the GmSRG1-GmXTH23 regulatory module, which collectively govern shade avoidance architecture in soybean.