Identification and expression analysis of short internodes related sequence (SRS) gene families in litchi and longan, with functional characterization of LcSRS5 and DlSRS4 in dwarfing regulation.

Dwarfism is a significant and desirable trait in the cultivation of horticultural crops. However, the regulatory mechanisms underlying dwarfism in perennial fruit trees remain poorly understood. In this study, we investigated the role of SRS genes in regulating plant height in Sapindaceae fruit trees, specifically litchi and longan. A total of five litchi SHORT INTERNODES (SHI)-related sequence genes (LcSRS) and four longan DlSRS genes were identified. Expression profile analysis in different tissues demonstrated that LcSRS1-LcSRS5 and DlSRS4 genes exhibited distinct expression patterns, with some genes showing tissue-specific expression. qRT-PCR analysis revealed that LcSRS5 and DlSRS4 genes had higher expression levels in the dwarf litchi ('Ziniangxi') and longan ('Shizi19') cultivars, respectively. Overexpression of the litchi LcSRS5 and the longan DlSRS4 in tobacco resulted in transgenic plants displaying a dwarf phenotype. Subcellular localization studies indicated that both LcSRS5 and DlSRS4 are localized in the nucleus of the cell. Hormone testing analysis indicated that the contents of auxin, cytokinin, and gibberellin varied among different cultivars of litchi and longan. GA3 treatment can promote the elongation of shoots in both litchi and longan. Additionally, silencing the LcSRS5 in litchi buds effectively promotes shoot growth, increases internode length, and significantly suppresses the expression of LcGA2ox1, LcGA2ox2, and LcGA2ox3 genes. Further studies confirmed that the LcSRS5 binds to the promoter of LcGA2ox3 to activate its expression. This study provides a foundation for further analysis of the functions of SRS genes in tropical fruit trees.