The Multiple Signals That Control Tuber Formation1[OPEN]

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. Potato (Solanum tuberosum) is the third most important subsistence crop grown worldwide. Based on calories generated for human consumption per acre, it is the most efficient food crop on the planet. Recent advances in biotechnology have established potato as an ideal crop for improvement through genetic enhancement, but increased yield has still not been addressed through biotech approaches (Halterman et al., 2016). With a better understanding of the signals that control tuberization, a molecular approach to yield enhancement may soon be within reach. Elegant studies over the past century established the existence of a photoperiodic signal responsible for activating the onset of tuberization (Garner and Allard, 1920; Werner, 1934). Under conditions of cool night temperatures and short days (SD), a graft-transmissible substance is transported from the leaves to stolon tips to activate tuber formation (Gregory, 1956; Chapman, 1958; Kumar and Wareing, 1973). In a benchmark experiment, heterografts using florally induced tobacco (Nicotiana tabacum) as the scion and noninduced potato as the stock demonstrated that some component of the mobile floral signal was capable of inducing tuberization (Chailakhyan et al., 1981). Despite years of innovative studies, however, the exact identity of this tuberinducing stimulus, designated tuberigen, was only recently established. A potato tuber is a specialized stem that arises from the underground organ known as the stolon (Jackson, 1999). It is not formed from a root. A stolon also can