Using the yeast (Saccharomyces cerevisiae) two-hybrid system and a potato (Solanum tuberosum) KNOX protein, designated POTH1, as bait, we have identified seven distinct interacting proteins from a stolon library of potato. All seven cDNAs are members of the BEL1-like family of transcription factors. Among these proteins, there are at least four regions of high sequence conservation including the homeodomain, the proline-tyrosine-proline three-amino acid loop extension, the SKY box, and a 120-amino acid region upstream from the homeodomain. Through deletion analysis, we identified a protein-binding domain present in the carboxy end of the KNOX domain of POTH1. The protein-binding domain in the BEL1 protein is located in the amino-terminal one-half of the 120-residue conserved region of the BELs. RNA-blot analysis showed differential patterns of RNA accumulation for the BELs in various potato organs. The level of StBEL5 mRNA increased in response to a short-day photoperiod in both leaves and stolons. Similar to sense mutants of POTH1, transgenic lines that overexpressed StBEL5 exhibited enhanced tuber formation even under noninductive conditions. Unlike POTH1 sense lines, however, these BEL lines did not exhibit the extreme leaf and stem morphology characteristic of KNOX overexpressers and displayed a more rapid rate of growth than control plants. Both StBEL5 and POTH1 sense lines exhibited an increase in cytokinin levels in shoot tips. StBEL5 lines also exhibited a decrease in the levels of GA 20-oxidase1 mRNA in stolon tips from long-day plants. Our results demonstrate an interaction between KNOX and BEL1-like transcription factors of potato that may potentially regulate processes of development.

BELL1-like transcription factors interact with their protein partners from the KNOTTED1 family to bind to target genes and regulate numerous developmental and metabolic processes. In potato (Solanum tuberosum), the BELL1 transcription factor StBEL5 and its protein partner POTH1 regulate tuber formation by affecting hormone levels. Overexpression of StBEL5 in transgenic lines produces plants that consistently exhibit enhanced tuber formation, and the mRNA of this gene moves through phloem cells in a long-distance signaling pathway regulated by photoperiod. Whereas photoperiod mediates the movement of StBEL5 RNA, activation of transcription of the StBEL5 gene in leaves is regulated by white light, regardless of photoperiod or light intensity. Illumination with either red or blue light induces the StBEL5 promoter, whereas far-red light had no effect. As expected, the StBEL5 promoter harbors numerous conventional light-responsive cis-acting elements like GT1, GATA, and AT1 motifs. Deletion constructs were analyzed to determine what sequences are involved in light activation. Transcriptional activity was also mediated by wounding on stems, insect predation on leaves, and photoperiod in stolons. These results demonstrate that StBEL5 gene activity in the leaf is correlated with wavelengths optimal for photosynthesis. The number of factors that affect the StBEL5 promoter supports the premise that the BELL1-like genes play a role in a wide range of functions.