Changes in GA 20-oxidase gene expression strongly affect stem length, tuber induction and tuber yield of potato plants

Gene StGA20ox1 encoding potato GA 20-oxidase is expressed to relatively hig h levels in leaves and regulated by daylength. To investigate whether this gene is involved in photoperiodic regulation of tuber formation, we have ob tained transgenic potato plants expressing sense and antisense copies of th e StGA20ox1 cDNA. Over-expression of this cDNA resulted in taller plants th at required a longer duration of a short day photoperiod (SD) to tuberize. Tubers from these plants had a decreased time of dormancy and developed spr outs with elongated internodes. Plants expressing antisense copies of the S tGA20ox1 cDNA had shorter stems, a decreased length of the internodes and t uberized earlier than control plants, showing increased tuber yields. Antis ense inhibition of this gene had no visible effect on the time of dormancy of the tubers, although at the end of dormancy these formed sprouts with sh ortened internodes. Decreased levels of endogenous GA(20) and GA(1) were de tected in the apex and first leaves of the antisense lines. These results d emonstrate the involvement of the GA 20-oxidase activity encoded by StGA20o x1 in the control of stem elongation and in tuber induction but not in tube r dormancy, indicating that the latter may be regulated by another member o f the gene family.