PLANT PHYSIOLOGY , Vol 104, Issue 2 569-580, Copyright © 1994 by American Society of Plant Biologists
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MOLECULAR BIOLOGY AND GENE REGULATION |
Dark and Circadian Regulation of mRNA Accumulation in the Short-Day Plant Pharbitis nil
S. D. O'Neill, X. S. Zhang and C. C. Zheng
Division of Biological Sciences, Section of Plant Biology, University of California, Davis, Davis, California 95616
The developmental transition of the meristem from vegetative to
reproductive growth is controlled by the cyclic alternation of light and
darkness in photoperiodic plants. Photoperiod is perceived in the leaves or
cotyledons, where a flower-inducing signal is produced and transmitted to
the apex. To begin to understand the molecular basis of the photoperiodic
induction of flowering, we investigated changes in gene expression at the
level of mRNA abundance that occur in association with dark induction of
flowering in the short-day species Pharbitis nil. Several cDNAs were
isolated that corresponded to mRNAs whose abundance is altered after the
transition to darkness. The pattern of increase in mRNA levels
corresponding to one cDNA clone, PN1, showed a dark-induced maximum at 8 h
of darkness, whereas a second clone, PN9, showed a dark-induced
accumulation of mRNA with peak levels at 12 to 16 h of darkness. When
plants were held in continuous darkness, both PN1 and PN9 exhibited
rhythmic patterns of mRNA accumulation with an approximate circadian
periodicity, suggesting that their expression is under the control of an
endogenous clock. The observed pattern of expression of PN1 and PN9 in
cotyledon tissue was unusual in that darkness rather than light promoted
mRNA accumulation, which is a temporal pattern of expression distinct from
that of several other Pharbitis genes, including Cab, PsaG, and actin,
whose mRNAs were most prevalent or equally prevalent in the light. Brief
illumination of an inductive dark period by a red light night break
strongly inhibited the accumulation of both PN1 and PN9 mRNA. The
expression of both PN1 and PN9 was spatially regulated in that mRNA
transcripts were detected in the cotyledons and stems, but not the roots,
of photoperiodically competent seedlings. Both PN1 and PN9 appeared to be
present as single-copy genes in the Pharbitis genome. Sequence analysis has
not determined the identity of these genes. Overall, the accumulation of
mRNAs corresponding to both PN1 and PN9 closely paralleled the process of
photoperiodic floral induction in P. nil, but a clear involvement with this
process cannot be established from our findings because of the difficulty
of separating photoperiodic events from other light-regulated processes,
especially those involved in photosynthesis, such as Cab gene expression.
These results identify the products of circadian-regulated genes in
photoreceptive tissue of P. nil and support the concept that
circadian-regulated gene expression interacting with darkness may be
involved in the regulation of photoperiodically controlled physiological
processes, including flower induction.
