Cytokinin response factor LcARR11 promotes floral bud physiological differentiation by activating <i>LcIPT3</i> and <i>LcFT1</i> in litchi

Cytokinins play crucial roles in regulating the flower bud differentiation in fruit trees. However, the molecular mechanisms by which cytokinins promote flowering in plants are largely unknown. The litchi (<i>Litchi chinensis</i> Sonn.) is a typical subtropical fruit tree that suffers from severe alternate fruiting due to unstable flowering. Here, we developed a novel alternate-fruiting management, which can ensure 100% flowering of the on-year trees, while the off-year trees hardly flower at all. The abundance of two types of cytokinins (tZR, iPR) and <i>LcFT1</i> expression in the leaves of on-year trees were continuously increased throughout the period of floral bud physiological differentiation. In contrast, these corresponding indicators in off-year trees were maintained at a significantly lower level. Exogenous application of 40 mg/kg 6-BA significantly promoted flowering and increased <i>LcFT1</i> expression level in the leaves of the off-year trees. <i>LcIPT3</i>, encoding a pivotal rate-limiting enzyme in cytokinin biosynthesis, was identified as the key gene determining the differences in cytokinin levels between on-year trees and off-year trees. Interestingly, we discovered that both <i>LcIPT3</i> and <i>LcFT1</i> are directly activated by LcARR11, a type-B cytokinin response factor, as demonstrated through both <i>in vitro</i> and <i>in vivo</i> assays. Furthermore, ectopic expression of <i>LcARR11</i> in <i>Arabidopsis</i> resulted in elevated <i>IPT</i> expression and cytokinin content, alongside increased <i>FT</i> expression and earlier flowering. Collectively, our findings suggest that cytokinin-mediated promotion of flowering in litchi might be orchestrated by LcARR11, which could promote floral bud physiological differentiation by activating both <i>LcIPT3</i> and <i>LcFT1</i>.