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The Mechanism of Rhythmic Ethylene Production in
Sorghum.
The Role of Phytochrome B and Simulated
Shading1
Scott A. Finlayson,
In-Jung Lee,
John E. Mullet, and
Page
W. Morgan*
Department of Soil and Crop Sciences (S.A.F., I.-J.L.,
P.W.M.), and Department of Biochemistry and Biophysics (J.E.M.), Texas
A&M University, College Station, Texas 77843-2474
Mutant sorghum (Sorghum
bicolor [L.] Moench) deficient in functional phytochrome B
exhibits reduced photoperiodic sensitivity and constitutively expresses
a shade-avoidance phenotype. Under relatively bright, high red:far-red
light, ethylene production by seedlings of wild-type and phytochrome
B-mutant cultivars progresses through cycles in a circadian
rhythm; however, the phytochrome B mutant produces ethylene peaks with
approximately 10 times the amplitude of the wild type. Time-course
northern blots show that the mutant's abundance of the
1-aminocyclopropane-1-carboxylic acid (ACC) oxidase mRNA SbACO2 is
cyclic and is commensurate with ethylene production, and that ACC
oxidase activity follows the same pattern. Both SbACO2 abundance and
ACC oxidase activity in the wild-type plant are very low under this
regimen. ACC levels in the two cultivars did not demonstrate
fluctuations coincident with the ethylene produced. Simulated shading
caused the wild-type plant to mimic the phenotype of the mutant and to
produce high amplitude rhythms of ethylene evolution. The circadian
feature of the ethylene cycle is conditionally present in the mutant
and absent in the wild-type plant under simulated shading. SbACO2 abundance in both cultivars demonstrates a high-amplitude diurnal cycle
under these conditions; however, ACC oxidase activity, although elevated, does not exhibit a clear rhythm correlated with ethylene production. ACC levels in both cultivars show fluctuations
corresponding to the ethylene rhythm previously observed. It appears
that at least two separate mechanisms may be involved in generating
high-amplitude ethylene rhythms in sorghum, one in response to the loss
of phytochrome B function and another in response to shading.
1
This work was supported by the U.S. Department
of Agriculture National Research Initiative Competitive Grants Program
(grant no. 97-35304-4820 to P.W.M.), by the Texas Higher Education
Board (ATP grant no. 999902-87 to P.W.M.), by a Predoctoral Overseas Korean Government Scholarship to I.-J.L., and by the Texas Agriculture Experiment Station.
*
Corresponding author; e-mail p-morgan{at}tamu.edu; fax
1-409-845-0456.
Plant Physiol. (1999) 119: 1083-1090
Copyright Clearance Center: 0032-0889/99/119//08
© 1999 American Society of Plant Physiologists
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