PLANT PHYSIOLOGY , Vol 115, Issue 2 511-518, Copyright © 1997 by American Society of Plant Biologists
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DEVELOPMENT AND GROWTH REGULATION |
Separation of Photolabile-Phytochrome and Photostable-Phytochrome Actions on Growth and Microtubule Orientation in Maize Coleoptiles (A Physiological Approach)
K. Fischer and P. Schopfer
Institut fur Biologie II der Universitat, Schanzlestrasse 1, D-79104 Freiburg, Germany
For separating the physiological actions of photolabile (phy-l) and
photostable phytochromes, we compared the effects of red (R) and far-red
(FR) light on elongation growth and microtubule reorientation in segments
of maize (Zea mays L.) coleoptiles raised either in darkness (phy-l
present) or preirradiated with R (phy-l eliminated). In 4.5-d-old
dark-grown seedlings R first promoted growth and induced a transverse
microtubule orientation. In continuous R the phytochrome action responsible
for these responses was replaced by an opposite phytochrome action that
produced a stable growth inhibition and longitudinal microtubule
orientation. In R-preirradiated segments only the second type of
phytochrome action could be observed. Reversion experiments with FR light
pulses demonstrated that both types of phytochrome action were dependent on
the FR-absorbing form of phytochrome and mirrored the actual phytochrome
state after 1 h. We conclude from these and related results that growth
promotion and transverse microtubule orientation are mediated by phy-l,
whereas growth inhibition and longitudinal microtubule orientation are
mediated by photostable phytochrome. The opposite actions of the two
phytochromes can be separated by preirradiating the seedlings with R.
Photoresponsiveness ascribed to phy-l disappeared after 5 d. phy-l appears
to play a distinct but transitory role in coleoptile development.
