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.