PHYTOCHROME-B MEDIATES THE PHOTOPERIODIC CONTROL OF TUBER FORMATION IN POTATO

To determine whether phytochrome B is involved in the response of pota to plants to photoperiod, a potato PHYB cDNA fragment was inserted in the antisense orientation behind the 35S CaMV promoter in Bin19 and th is construct was transformed into Solanum tuberosum ssp, andigena plan ts which normally require short days for tuberization. Two independent transformants were obtained that had much lower levels of PHYB mRNA a nd protein, and which exhibited phenotypes characteristic of phyB muta nts, for example, elongated stems and decreased chlorophyll content. T he level of phyA, and of several phytochrome A-controlled responses, w as unaffected in these plants. The photoperiodic control of tuberizati on in these antisense PHYB plants was abolished, the plants tuberizing in short day, long day, or short day plus night break conditions. Thi s result shows that phytochrome B is required for the photoperiodic co ntrol of tuberization in potato (Solanum tuberosum ssp. andigena) and that it regulates this developmental process by preventing tuber forma tion in non-inductive photoperiods rather than by promoting tuberizati on in inductive photoperiods.