Interaction of Brassinosteroids with Light Quality and Plant Hormones in Regulating Shoot Growth of Young Sunflower and <Emphasis Type="Italic">Arabidopsis</Emphasis> Seedlings |
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Authors: | Leonid V Kurepin Se-Hwan Joo Seong-Ki Kim Richard P Pharis Thomas G Back |
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Institution: | (1) Department of Biological Sciences, University of Calgary, Calgary, AB, T2N 1N4, Canada;(2) Laboratory of Plant Physiology, Department of Life Sciences, Chung-Ang University, 221 Huksuk-dong, Dongjak-ku, Seoul, 156-756, Republic of Korea;(3) Department of Chemistry, University of Calgary, Calgary, AB, T2N 1N4, Canada;(4) Department of Biology, Biological & Geological Sciences Building, University of Western Ontario, 1151 Richmond Street, London, ON, N6A 5B7, Canada |
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Abstract: | Sunflower hypocotyls elongate as light quality changes from the normal red to far-red (R/FR) ratio of sunlight to a lower
R/FR ratio. This low R/FR ratio-induced elongation significantly increases endogenous concentrations of indole-3-acetic acid
(IAA) and also of three gibberellins (GAs): GA20, GA1, and GA8. Of these, it is likely GA1 that drives low R/FR-induced growth. Brassinosteroids are also involved in shoot growth. Here we tested three R/FR ratios:
high, normal, and low. Significant hypocotyl elongation occurred with this stepwise reduction in R/FR ratio, but endogenous
castasterone concentrations in the hypocotyls remained unchanged. Brassinolide was also applied to the seedlings and significantly
increased hypocotyl growth, though one that was uniform across all three R/FR ratios. Applied brassinolide increased hypocotyl
elongation while significantly reducing (usually) levels of IAA, GA20, and GA8, but not that of GA1, which remained constant. Given the above, we conclude that endogenous castasterone does not mediate the hypocotyl growth
that is induced by enriching FR light, relative to R light. Similarly, we conclude that the hypocotyl growth that is induced
by applied brassinolide does not result from an interaction of brassinolide with changes in light quality. The ability of
applied brassinolide to influence IAA, GA20, and GA8 content, yet have no significant effect on GA1, is hard to explain. One speculative hypothesis, though, could involve the brassinolide-induced reductions that occurred
for endogenous IAA, given IAA’s known ability to differentially influence the expression levels of GA20ox, GA3ox, and GA2ox, key genes in GA biosynthesis. |
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