Analysis of the synergistic effect between CiFTIP1 and CiFT in flowering regulation of citrus

Overexpression of CiFTIP1 enhances the long-distance transport of the florigen protein CiFT, leading to early flowering in tomato, with a synergistic effect when co-expressed with CiFT, and provides a potential strategy for regulating flowering time in fruit trees. FTIP1 is a key regulator involved in the long-distance transport of the florigen protein FT. To investigate the role of citrus CiFTIP1 in regulating the transport of CiFT, transgenic tomato plants were employed as experimental materials in this study. Phenotypic analysis revealed that overexpression of CiFTIP1 promoted early flowering in tomato and upregulated the expression of key flowering genes SlLFY and SlAP1, and downregulated SlTFL. By crossing CiFT and CiFTIP1 overexpression lines, the double-overexpression hybrids exhibited a marked synergistic early-flowering effect, flowering earlier than single-overexpression plants. Grafting experiments further confirmed that double-overexpression plants used as rootstocks possessed a stronger capability to induce early flowering in wild-type scions. Notably, higher CiFT fluorescence signal and protein abundance were detected in wild-type scions grafted onto double-overexpression rootstocks, confirming that CiFTIP1 acts as a molecular chaperone to effectively enhance the transport efficiency of CiFT protein. This study clarified the molecular mechanism by which CiFTIP1 synergistically regulated flowering via enhancing the mobility of CiFT protein between rootstocks and scions. It also provided innovative strategies for flowering time regulation, hybrid breeding efficiency improvement, and production cycle management of fruit trees.