LcMADS28 and LcMADS27 coordinately regulate early seed development in Litchi chinensis through direct activation of...

Seed abortion is a critical agronomic trait affecting both fruit quality and yield. Cell wall invertase (CWIN) hydrolyzes sucrose into glucose and fructose, which serve as both nutrients and signaling molecules to regulate seed development, with its activity suppressed by invertase inhibitor (INH). However, the transcriptional regulators controlling INH expression remain unknown. Here, we identify the MADS-box transcription factor LcMADS28 as a direct activator of LcINH1. LcMADS28 is highly expressed in the funicle and embryo of the abortive-seeded cultivar 'Nuomici' at 15 days after anthesis, whereas its expression is much lower in the big-seeded cultivar 'Heiye'. Further analysis reveals that LcMADS28 interacts with LcMADS27 to form a heterodimer that directly binds to the LcINH1 promoter and activates its transcription. Silencing LcMADS28 in 'Nuomici' reduces LcINH1 expression, which increases CWIN activity, promotes embryo and seed development, and lowers fruit abscission. RNA-seq analysis indicates that genes associated with auxin and sucrose metabolism are activated after LcMADS28 silencing. In addition, heterologous overexpression of LcMADS28 in tomato and Arabidopsis increases seed abortion. Together, our results indicate that LcMADS28 and LcMADS27 coordinately activate LcINH1 to regulate early seed development in lychee and provide a mechanistic framework linking MADS-box transcription factors with the invertase/INH module.