Plant Molecular Biology 37: 437–444, 1998.
1998 Kluwer Academic Publishers. Printed in Belgium.
Cytokinin regulates the expression of a soybean
-expansin gene by a
Department of Biology, Indiana University-Purdue University at Indianapolis, 723 West Michigan Street,
Indianapolis, IN 46202-5132, USA (
author for correspondence)
Received 26 August 1997; accepted in revised form 16 December 1997
Key words: cytokinin, expansins, mRNA stability, pollen allergen, protein phosphatase, soybean
The cytokinin-inducible soybean mRNA Cim1 accumulates 20–60-fold upon cytokinin addition to cytokinin-
starved soybean suspension cultures. In this report, we demonstrate that cytokinin-induced stability of the Cim1
mRNAplaysan importantrolein the accumulationofthemessage. We alsopresentevidencethat cytokinin-induced
Cim1 stability is blocked by the addition of the protein phosphatase inhibitor okadaic acid. Thus, we suggest that
protein phosphatase activity is required for the cytokinin-induced stability and subsequent accumulation of Cim1
in soybean cells. The deduced amino acid sequence of the Cim1 protein product is similar to the group I pollen
allergensfromvariousplants, which constitute a subfamilyof expansinproteins.TherelatednessbetweenCim1 and
the expansins supports our hypothesis that the protein product of Cim1 is localized to the cell wall and suggests a
roleforCim1in cytokinin-regulatedcell wall expansion.Thus,post-transcriptionalregulationof Cim1by cytokinin
may represent a molecular link between cytokinin and changes in cell shape and size.
Cytokinin is a growth substance that is involved in
a multitude of physiological and developmental pro-
cesses in plants, such as chloroplast development
, shoot organogenesis , lateral bud release
, delayed tissue senescence  and cell division
. Despite the wide range of physiological effects
ascribed to cytokinin, a detailed understanding of the
molecular basis of cytokinin action is lacking.
mRNAs and proteins and causes a decline in the levels
of others . Cytokinin-inducible mRNAs include
those encoding an early nodulin (SrEnod2) from Ses-
bania rostrata , the light-harvesting chlorophyll
a/b-binding protein [15, 24, 38], the small subunit of
Rubisco [12, 15], a ‘multiple stimulus response’ pro-
tein (msr1) from Nicotiana plumbaginifolia , and
nitrate reductase [9, 10, 21, 23].
The roles of both transcriptional and post-
transcriptional mechanisms in cytokinin-induced
mRNA accumulation have been examined. For
example, cytokinin-induced accumulation of msr1
mRNA  and cytokinin-enhanced light induction
of nitrate reductase mRNA  have been shown
to be the result of increased rates of transcription.
Cytokinin-responsive promoters have provided fur-
ther evidence of cytokinin-regulated transcription of
plant genes [2, 22]. However, it has become evident
that post-transcriptional mechanisms are also involved
in cytokinin regulation of plant gene expression .
These conclusions are based on comparative analyses
of rates of mRNA synthesis (nuclear run-on transcrip-
tion) and mRNA accumulation (northern analysis) or
on direct measurements of mRNA half-lives in the
presence of potent inhibitors of transcription (cordy-
cepin or actinomycin D) [17, 37]. Flores and Tobin
the mRNAs encoding the light-harvesting chlorophyll
a/b-binding protein and the small subunit of Rubisco
occurs primarily by a post-transcriptional mechanism
in Lemna gibba [15, 16]. Recently, mRNA stability
was also found to be involved in cytokinin-induced
accumulation of SrEnod2 mRNA .