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THE PLANT CELL, Vol 4, Issue 4 451-461, Copyright © 1992 by American Society of Plant Biologists
Cytokinins and Auxins Control the Expression of a Gene in Nicotiana plumbaginifolia Cells by Feedback Regulation
J. A. Dominov, L. Stenzler, S. Lee, J. J. Schwarz, S. Leisner and S. H. Howell
Boyce Thompson Institute, Tower Road, Ithaca, New York 14853
Both cytokinin (N6-benzyladenine [BA]) and auxin (2,4-dichlorophenoxyacetic
acid [2,4-D]) stimulate the accumulation of an mRNA, represented by the
cDNA pLS216, in Nicotiana plumbaginifolia suspension culture cells. The
kinetics of RNA accumulation were different for the two hormones; however,
the response to both was transient, and the magnitude of the response was
dose dependent. Runoff transcription experiments demonstrated that the
transient appearance of the RNA could be accounted for by feedback
regulation of transcription and not by the induction of an RNA degradation
system. The feedback mechanism appeared to desensitize the cells to further
exposure of the hormone. In particular, cells became refractory to the
subsequent addition of 2,4-D after the initial RNA accumulation response
subsided. A very different response was observed when the second hormone
was added to cells that had been desensitized to the first hormone. Under
such conditions, BA produced a heightened response in cells desensitized to
2,4-D and vice versa. These findings support a model in which cytokinin
further enhances the auxin response or prevents its feedback inhibition.
The hormone-induced RNA accumulation was blocked by the protein kinase
inhibitor staurosporin. On the other hand, the protein phosphatase
inhibitor okadaic acid stimulated expression, and, in particular, okadaic
acid was able to stimulate RNA accumulation in cells desensitized to auxin.
This suggests that hormone activation involves phosphorylation of critical
proteins on the hormone signaling pathway, whereas feedback inhibition may
involve dephosphorylation of these proteins. The sequence of pLS216 is
similar to genes in other plants that are stimulated by multiple agonists
such as auxins, elicitors, and heavy metals, and to the gene encoding the
stringent starvation protein in Escherichia coli. It is proposed that this
gene family in various plants be called multiple stimulus response (msr)
genes.
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