Plant Molecular Biology 35: 633–640, 1997.
1997 Kluwer Academic Publishers. Printed in Belgium.
Expression of glyoxylate cycle genes in cucumber roots responds to sugar
supply and can be activated by shading or defoliation of the shoot
, Luigi De Bellis
, Amedeo Alpi
and Steven M. Smith
Institute of Cell and Molecular Biology, University of Edinburgh, The King’s Buildings, Mayﬁeld Road,
Edinburgh EH9 3JH, UK (
author for correspondence);
Dipartimento di Biologia delle Piante Agrarie, sez.
Fisiologia Vegetale, Universit
a di Pisa, via Mariscoglio 34, 56124 Pisa, Italy
Received 27 February 1997; accepted in revised form 30 June 1997
Key words: Agrobacterium rhizogenes, carbohydrate regulation, cucumber (Cucumis sativus L.), gene expression,
glyoxylate cycle, hairy roots
(MS) mRNAs increase signiﬁcantly in amount. However, if sucrose is added to the excised roots, the mRNAs do
not accumulate. Hairy roots obtained by transformation with Agrobacterium rhizogenes show the same response.
Transgenic hairy roots containing the Icl and Ms gene promoters fused to the GUS reporter gene, have very low
GUS activity which increases dramatically when roots are incubated in the absence of sugar, indicating regulation
at the transcriptional level. Staining of sugar-deprived roots shows that GUS activity is concentrated mainly in root
tips and lateral root primordia, where demand for carbohydrate is greatest. In order to determine if Icl and Ms
genes are expressed in roots of whole plants under conditions which may occur in nature, cucumber plants were
subjected to reduced light intensity or defoliation. In both cases increases were observed in ICL and MS mRNAs.
These treatments also reduced root sugar content, consistent with the hypothesis that sugar supply could control
expression of Icl and Ms genes in roots of whole plants.
The role of the glyoxylate cycle in the conversion
of storage lipids to sugars during germination and
seedling development has long been established ,
after the discovery of two key enzymes, isocitrate
lyase (ICL; EC 188.8.131.52) and malate synthase (MS; EC
184.108.40.206).More recently it has been discoveredthat gly-
oxylate cycle enzymes are synthesised at other stages
of development, including late embryogenesis [6, 29],
pollen development , and senescence of petals,
leaves and cotyledons [7, 14, 23, 24], where a gluco-
neogenic role for the cycle is feasible, but not proven.
Glyoxylatecycleenzymes are also synthesised in cells
or tissues in response to sugar deprivation[15, 22, 27].
The function of the glyoxylate cycle in such cells is
unknown, but an anapleurotic rather than gluconeo-
genic role has been suggested [15, 21]. The control
of gene expression during seed germination and dur-
ing sugar deprivation is mediated by distinct cis-acting
DNA sequences , indicating regulation of gene
expression in response to different signals, possibly
reﬂecting different functions for the glyoxylate cycle.
The regulation of gene expression by sugar meta-
bolism has so far been demonstrated in cultured cells
, protoplasts [16, 22, 27] and detached organs [14,
22]. It has also been demonstrated in whole seedlings
and plants placed into darkness for prolonged periods
[14, 27, 28]. However, in no case has it been shown
that carbohydrate deprivation can lead to an activation
of glyoxylate cycle gene expression in plants growing
under natural conditions. We reasoned that if such reg-
ulationdoesoccur,itmaydo sointhe root, a sinkorgan
which is subject to frequent and signiﬁcant changes in
carbohydrate supply which can in turn affect metabol-
ism and gene expression [3, 5, 31]. The ﬁrst aim of
the present study was to determine if Icl and Ms gene
expression in roots responds to sugar supply. Previ-
Gr.: 201001129, PIPS Nr. 144520 BIO2KAP
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