Abstract

SUMMARY Seed size is a key agronomic trait that critically influences the commercial quality of fruit. In litchi ( Litchi chinensis Sonn.), small-seeded cultivars are highly desirable due to their higher pulp-to-seed ratio. However, the molecular mechanisms controlling small-seed development remain poorly understood. Here, we identify LcTINY, an ERF/DREB transcription factor, that plays an essential role in seed development in litchi. LcTINY expression is both cultivar-specific and developmentally regulated, showing a strong correlation with seed morphological traits. Functional analyses show that silencing LcTINY reduces seed weight and size, whereas its overexpression promotes seed expansion, confirming its positive role in seed growth. Mechanistically, we demonstrate that LcTINY directly binds to the promoter of LcbZIP53 , a bZIP transcription factor, and activates its expression, thereby coordinating seed growth though modulating hormone signaling and cell proliferation. We further show that the LcTINY-LcbZIP53 regulatory module is evolutionarily conserved across the Sapindaceae family. Our results reveal a novel transcriptional mechanism underlying litchi seed development and establish a theoretical foundation for breeding small-seeded litchi cultivars with enhanced fruit quality.