New Zealand Journal of Crop and Horticultural Science abstracts
Inhibition of hexokinase and expression of asparagine synthetase and
[[beta]]-galactosidase genes during sugar feeding and starvation of
asparagus (Asparagus officinalis) callus cultures
DONALD E. IRVING*
GLEN J. SHINGLETON
PAUL L. HURST+
JOHN F. SEELYE
BEN K. SINCLAIR
New Zealand Institute for Crop & Food
Research Limited
Food Industry Science Centre
Private Bag 11 600
Palmerston North, New Zealand
email: hurstp@crop.cri.nz
*Present address: School of Land and Food, University of
Queensland, Gatton College, Qld 4345, Australia.
Abstract Hexokinase has an important role in hexose
metabolism and in signalling sugar status in plants. The aim of this study was
to inhibit hexose phosphorylation using a hexokinase inhibitor (glucosamine),
and to determine the effects on expression of asparagine synthetase (AS) and
[[beta]]-galactosidase during glucose feeding and starvation of asparagus
(Asparagus officinalis L.) callus cultures. After 48 h without
glucose and a further 24-h incubation with a range of hexoses and analogues,
expression of both AS and [[beta]]-galactosidase was repressed by D-glucose,
D-galactose, and 2-deoxyglucose, but the genes were not repressed when glucose
was absent, or when 3-O-methylglucose or L-glucose were supplied. Glucose- and
fructose-phosphorylating activity was determined in extracts from callus
cultures which had been exposed to glucose, 2-deoxyglucose, 2-deoxyglucose with
glucosamine or mannitol (as an osmotic control), or glucose-free media. After
48 h on glucose-free media and 48-h incubation with 2-deoxyglucose and
glucosamine, glucose- and fructose-phosphorylating activities were reduced by
68 and 83%, respectively. When glucose was present in the cultures, there was
no expression of AS transcripts, but when glucose was absent, AS was highly
expressed. AS expression was reduced when 2-deoxyglucose was present for
48 h, even when glucosamine or mannitol was also present in the culture
media. [[beta]]-galactosidase was highly expressed when glucose was absent, but
expression was very low in all of the treatments which contained 2-deoxyglucose
(including the glucosamine and mannitol treatments). The results suggest AS and
[[beta]]-galactosidase are sugar regulated, but inconsistencies, particularly
reduced AS expression in the presence of glucosamine, are discussed in relation
to the possibilities that multiple forms of hexokinases exist which might be
differentially affected by glucosamine, and that uptake and distribution of
2-deoxyglucose and glucosamine are limited.
Keywords Asparagus officinalis; asparagine synthetase;
[[beta]]-galactosidase; glucosamine; glucose; hexokinase; sugar sensing
+Corresponding author.
H99033
Received 20
September 1999; accepted 20 January 2000
PDF file of entire paper: medium quality (1752K); (scanned from paper original: notes about this process)
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