Phytochrome A resets the circadian clock and delays tuber formation under long days in potato

Transgenic potatoes (Solanum tuberosum) with either increased (sense transf ormants) or reduced (antisense transformants) phytochrome A (phyA) levels w ere used, in combination with specific light treatments, to investigate the involvement of phyA in the perception of signals that entrain the circadia n clock. Far-red or far-red plus red light treatments given during the nigh t reset the circadian rhythm of leaf movements in wild-type plants and phyA over-expressors, but had little effect in phyA under-expressors. Far-red l ight was also able to reset the rhythm of leaf movement in wild-type Arabid opsis thaliana but was not effective in mutants without phyA. Blue light wa s necessary to reset the rhythm in phyA-deficient potato plants. Resetting of the rhythm by far-red plus red light was only slightly affected in trans genic plants with reduced levels of phytochrome B. The production of tubers was delayed by day extensions with far-red plus red light, but this effect was reduced in transgenic lines deficient in phyA. We conclude that phyA i s involved in resetting the circadian clock controlling leaf movements and in photoperiod sensing in light-grown potato plants.