PLANT PHYSIOLOGY , Vol 101, Issue 3 965-968, Copyright © 1993 by American Society of Plant Biologists
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DEVELOPMENT AND GROWTH REGULATION |
Brassinosteroid Stimulation of Hypocotyl Elongation and Wall Relaxation in Pakchoi (Brassica chinensis cv Lei-Choi)
T. W. Wang, D. J. Cosgrove and R. N. Arteca
Department of Horticulture (T.-W.W., R.N.A.) and Department of Biology (D.J.C.), The Pennsylvania State University, University Park, Pennsylvania 16802
Hypocotyl elongation of pakchoi (Brassica chinensis cv Lei-Choi) was
stimulated by applying 300 ng of brassinosteroid
(2[alpha],3[alpha],22[beta],23[beta]-tetrahydroxy-24[beta]-methyl-B-homo-7-
oxa-5[alpha]-cho-le stan-6-one, BR) in 1 [mu]L of 50% ethanol to the apex
of hypocotyls. BR had its greatest effect on elongation of the apical 3-mm
region below the cotyledonary node (75% stimulation) between 6 and 18 h
after treatment. Stress/strain (Instron) analysis of this 3-mm region
revealed that plastic and elastic components of extension were not
significantly different between BR-treated and control seedlings. In
pressure-block experiments, the initial rate of relaxation was 2-fold
faster in BR-treated plants as compared with controls, whereas after 125
min the total amount of relaxation and the relaxation rate were the same
for the two treatments. Osmotic pressure of cell sap expressed from this
3-mm region showed a large decrease (28%) in BR-treated seedlings compared
to the controls. We conclude that BR stimulates growth in pakchoi by
accelerating the biochemical processes that cause wall relaxation, without
inducing a large change in wall mechanical properties.
