The Multiple Signals That Control Tuber Formation

Abstract

Under optimum environmental conditions, tuberization in potato (Solanum tuberosum) is activated by signals that either function in the leaf or arise in this organ and move down into stolon tips to induce tuber formation. Three major signals have been identified: CYCLING DOF FACTOR1 (StCDF1) for earliness and StBEL5 mRNA and SELF-PRUNING6A (StSP6A) protein as mobile signals originating in the leaf. Overexpression of StSP6A produces plants that tuberize earlier and under noninductive long days. Overexpression of StBEL5 leads to enhanced earliness and overall increased tuber yields. During the onset of tuberization, StBEL5 plus its StKNOX partner initiate signal activity in the leaf through transcriptional induction of both StCDF1 and StSP6A. Amplification of signals occurs on site in stolons, where StBEL5 again in tandem with its StKNOX partner induces the transcription of both StSP6A and StBEL5. In stolons, StSP6A interacts with a FLOWERING LOCUS D-LIKE protein to promote tuberization. In this model, StBEL5 is positioned upstream of a regulatory network that controls tuber formation. Overall, the results suggest that StBEL5 functions to directly activate the tuberization program and to amplify other pivotal signals in the pathway.