LtSGR1 Inhibits LtPSY Expression, Lowers Strigolactones and Abscisic Acid Levels, Thereby Promoting Shoot Branching

The STAY-GREEN (SGR) genes play pivotal roles in chlorophyll degradation across diverse plant species; however, their functions in the fast-growing tree Liriodendron tulipifera remain largely unknown. In this study, we isolated and characterized the LtSGR1 gene from L. tulipifera. Bioinformatics analysis indicated that LtSGR1 encodes a chlorophyll a-Mg dechelatase and is a member of the staygreen superfamily. Subcellular localization analysis demonstrated that LtSGR1 localizes specifically to the chloroplast. Overexpression of LtSGR1 in Arabidopsis thaliana accelerated chlorophyll degradation and led to a significant reduction in carotenoid accumulation. Yeast one-hybrid and dual-luciferase reporter assays demonstrated that LtSGR1 directly binds to the promoter region of LtPSY and negatively regulates its transcription. Transgenic lines displayed an increase in the number of rosette leaves and enhanced shoot branching. Additionally, RT-qPCR analysis indicated that the expression levels of genes associated with the biosynthesis of strigolactones (SL) and abscisic acid (ABA) were significantly downregulated. These findings suggested that LtSGR1 modulates carotenoid biosynthesis by directly repressing LtPSY transcription, thereby influencing SL and ABA levels, which ultimately regulate shoot branching. Our findings provide novel insights into the molecular mechanisms mediated by LtSGR1, laying a solid foundation for future investigations into the regulation of shoot branching in L. tulipifera.