Litchi LcAP1-1 and LcAP1-2 Exhibit Different Roles in Flowering Time

Litchi (<i>Litchi chinensis</i> Sonn.) is a kind of evergreen fruit tree with good flavor and taste which has high economic value. Sufficiently low temperature in winter is essential for the successful flower formation of litchi. Therefore, in the context of global warming, litchi often experiences unstable flower formation, ultimately resulting in a decrease in litchi production. Our previous research has highlighted the pivotal role of the <i>LcFT1</i> gene in regulating the flower formation of litchi and identified two <i>AP1</i> homologous genes associated with <i>LcFT1</i> (named <i>LcAP1-1</i> and <i>LcAP1-2</i>) based on RNA-Seq and weight gene co-expression network analysis (WGCNA). In this study, the functions of the two <i>AP1</i> homologous genes in regulating flowering time were investigated. Result showed that <i>LcAP1-1</i> and <i>LcAP1-2</i> were expressed in all litchi tissues. <i>LcAP1-1</i> was more highly expressed in mature leaves compared to other tissues, while the <i>LcAP1-2</i> has the highest expression level in flower buds. Both of them exhibited upregulation in the terminal bud of litchi under low temperature. The expression of <i>LcAP1-1</i> and <i>LcAP1-2</i> was highly correlated with the initiation of flower buds and the development of flower organs. They increased gradually during the floral initiation but decreased gradually during flower bud development. The transgenic tobacco of <i>LcAP1-2</i> flowered about 55 days earlier than wild-type, while tobacco overexpressing the <i>LcAP1-1</i> gene had no significant changes in flowering time compared to the wild-type. These results indicate that the two genes have divergent regulatory functions, and that the <i>LcAP1-2</i> gene may be involved in the regulation of flower transformation and flower organ development in litchi. Our research will further reveal the molecular regulatory mechanisms of flower formation in litchi and will also provide theoretical guidance for the molecular breeding of litchi.