IS THE FAR-RED-ABSORBING FORM OF AVENA PHYTOCHROME-A THAT IS PRESENT AT THE END OF THE DAY ABLE TO SUSTAIN STEM-GROWTH INHIBITION DURING THE NIGHT IN TRANSGENIC TOBACCO AND TOMATO SEEDLINGS

Avena phytochrome A (phyA) overexpressed in tobacco (Nicotiana tabacum L.) and tomato (Lycopersicon sculentum Mill) was functionally charact erised by comparing wild-type (WT) and transgenic seedlings, Different proportions of phytochrome in its far-red-absorbing form (Pfr/P) were provided by end-of-day (EOD) light pulses. Stem-length responses occu rred largely in the range of low Pfr/f (3-61%) for WT seedlings and in the range of high Pfr/P (61-87%) for transgenic seedlings. A similar shift was observed when the photoperiod was interrupted by short light pulses providing different Pfr/P ratios and followed by Ih dark incub ation. In other experiments, Avena phyA was allowed to re-accumulate i n darkness and subsequently phototransformed to Pfr but no extra inhib ition of stem extension growth was observed. In transgenic tomato seed lings the response to EOD far-red light was faster and the response to a far-red light pulse delayed into darkness was larger than in the WT . Avena phyA Pfr remaining at the end of the photoperiod appears intri nsically unable to sustain growth inhibition in subsequent darkness. A z,ena phyA modifies the sensitivity and the kinetics of EOD responses mediated by native phytochrome.