IMPAIRED STEM-GROWTH RESPONSES TO BLUE-LIGHT IRRADIANCE IN LIGHT-GROWN TRANSGENIC TOBACCO SEEDLINGS OVEREXPRESSING AVENA PHYTOCHROME-A

The effects of blue light (B) on stem extension-growth were compared i n light-grown seedlings of tobacco overexpressing Avena phytochrome A and its isogenic wild type (WT). Under natural radiation, lowering the levels of B reaching the whole shoot promoted stem extension growth i n WT but not in transgenic seedlings. Under controlled conditions, the seedlings were exposed to white light (WL) or WL minus B, each one pr ovided at two different irradiances. In WT seedlings stem extension gr owth was promoted by lowering B at both irradiance levels. In transgen ic seedlings a reduction of B was promotive only at low irradiance lev els. The seedlings were also grown under WL, WL minus B, WL minus red light (R) and Ear-red light (FR) or WL minus R, FR and B. In the WT, l owering B promoted stem extension growth irrespective of R+FR levels. In the transgenics, B was effective only at very low levels of R+FR (i .e. at low phytochrome cycling rates). Lowering the Pfr levels at the end of the day promoted extension growth in wild type and transgenic s eedlings. Responses to B were not observed in transgenic seedlings hav ing low Pfr levels at the end of the day. The results suggest that the overexpressed phytochrome A acts mainly via irradiance-dependent reac tions. When these reactions are highly expressed, B responses are not observed.