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.