PLANT PHYSIOLOGY , Vol 112, Issue 3 965-973, Copyright © 1996 by American Society of Plant Biologists
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DEVELOPMENT AND GROWTH REGULATION |
Phytochrome A Enhances the Promotion of Hypocotyl Growth Caused by Reductions in Levels of Phytochrome B in Its Far-Red-Light-Absorbing Form in Light-Grown Arabidopsis thaliana
J. J. Casal
Departmento de Ecologia, Facultad de Agronomia, Universidad de Buenos Aires, Av. San Martin 4453, 1417-Buenos Aires, Argentina
We sought to determine if phytochrome B (phyB)-mediated responses to the
red light (R)/far-red light (FR) ratio are affected by phytochrome A (phyA)
activity in light-grown seedlings of Arabidopsis thaliana. Pulses of FR
delayed into the dark period were less effective than end-of-day (EOD) FR
in promoting hypocotyl growth over a given period in darkness. White light
minus blue light interposed instead of darkness between the end of the
white-light photoperiod and the FR pulse was sufficient to maintain
responsivity to the decrease in phyB in FR-light-absorbing form in
wild-type (WT) seedlings, but not in the phyA mutant. Compared with EOD R,
hourly R+FR pulses provided throughout the night caused a stronger
promotion of stem growth than a single EOD R+FR pulse in WT Arabidopsis,
cucumber, mustard, sunflower, tobacco, and tomato, but not in phyA
Arabidopsis or in the aurea mutant of tomato. WT seedlings of Arabidopsis
responded to a range of high EOD R/FR ratios, whereas the phyA mutant
required stronger reductions in the EOD R/FR ratio. In sunlight, phyA
seedlings of Arabidopsis showed no response to the "early warning" signals
of neighboring vegetation, and hypocotyl-growth promotion occurred at
higher plant densities than in the WT. Thus, under a series of light
conditions, the sensitivity or responsivity to reductions in the R/FR ratio
were larger in WT than in phyA seedlings. A product of phyA is therefore
proposed to enhance the hypocotyl-growth response to decreases in phyB in
FR-light-absorbing form in light-grown seedlings.
