Regulation of potato tuberization by daylength and gibberellins

Tuberization of S. tuberosum ssp andigena is induced by short days (SD) and prevented by long days (LD) or by interruption of the long nights with a 3 0 min night break (SD+NB). Antisense inhibition of the StGA20ox1 gene, enco ding the GA 20-oxidase biosynthetic enzyme results in early tuberization in SD, but does not overcome the need of SD conditions for tuber induction. U sing RT-PCR differential display of leaves from plants grown under inductiv e (SD) and non-inductive (SD+NB) conditions we have isolated clone PHOR1 en coding a novel arm-repeat containing protein with homology to Drosophila ar madillo. PHOR1 mRNA oscillates with a diurnal rhythm and is up-regulated in plants induced for tuberization. Transgenic lines inhibited in PHOR1 expre ssion are semi-dwarf and show a reduced response to GAs, which is consisten t with a function of PHOR1 in GA signaling. These results indicate that cha nges in GA sensitivity are involved in mediating tuber induction.