ISSN 10214437, Russian Journal of Plant Physiology, 2013, Vol. 60, No. 1, pp. 84–90. © Pleiades Publishing, Ltd., 2013.
Nitrate is the major source for nitrogen supply .
The regulation of nitrate uptake has attracted much
attention because it is necessary to increase the effi
ciency of nitrogen use, to improve the content of pro
tein, and to minimize fertilizer waste and pollution.
This requires the understanding regularities that deter
mine the rates of transport and utilization of nitrogen.
In recent years, the effect of sugars on nitrate
uptake by roots has been regarded primarily as a signal,
although sugars also supply respiratory substrates and
carbon skeletons for reductive assimilation of nitrate,
which could have indirect effects on nitrate uptake.
The old idea that sucroseinduced stimulation is due
to increasing respiratory energy supply is contradicted
by theoretical considerations  and experimental
evidence . One type of evidence for signaling comes
from identifying pathways from sugars to nitrate trans
port, in which sugars act as a signal rather than a sub
strate, and several possible steps have been character
ized, although sucrose sensors have not been recog
nized . Other evidence would come from the time
course of the effect of sugar on nitrate uptake, because
a change in the concentration of a signal sugar is likely
to exert an immediate or very rapid response. A lag of
even a few minutes would suggest that an effect was not
a signal, and for this reason accurate measurements of
the time course of nitrate uptake are needed.
In addition to sucrose, three other sugars, namely
glucose, fructose, and mannose, have been shown to
This text is published in original.
stimulate nitrate uptake when supplied directly to
roots [5, 6], raising the question of specificity. Some
investigations indicated the possibility of glucose sig
naling, particularly those concerning hexokinase ,
some other have shown the specificity of sucrose ,
but none have shown their nonspecificity. A second
method of testing specificity is to examine the relative
rates of the effects exerted by various candidate sugars.
If fed to root cells in the bathing solution, a signaling
sugar would have a prominent and rapid effect on
nitrate uptake, as suggested above. Any nonsignaling
sugar would have a lower effect after it would be con
verted via metabolism into a “signal” sugar, and so
occurring only after a lag. We have therefore compared
the time courses of the effects of a wide range of sugars
on the nitrate uptake rate, looking to see whether
sucrose, or any other sugar, has an effect distinctly
larger and faster than those of other sugars.
In order to obtain the time courses with a high
enough resolution to answer the questions posed
above, we have used a new technique that enables very
detailed measurement of changes in the net nitrate
uptake by the root system of intact, growing wheat
plants. We present the first systematic set of detailed
time courses of the response of net nitrate uptake to
the fundamental experimental perturbations—the
supply of a selected range of sugars and mannitol to the
MATERIALS AND METHODS
EM18) plants were grown in a glass beakers of 2.5 L in
Identifying Sucrose as a Signal for Nitrate Uptake by Wheat Roots
J. Z. Li
, G. Y. He
, and W. J. Cram
School of Life Science, Taizhou University, Taizhou, Zhejiang 318000, China;
fax: 8657688324314; email: firstname.lastname@example.org
ChinaUK HUSTRRes Genetic Engineering and Genomics Joint Laboratory, College of Life Science and Technology,
Huazhong University of Science & Technology (HUST), Luoyu Road 1037, Wuhan, Hubei 430074, China
Received December 13, 2011
—Some sugars supplied directly to roots can stimulate nitrate uptake by wheat (
roots. To identify a signaling molecule, we compared the response of net nitrate influx to sugar supply. A
method with a high time resolution (minutes) enabled to make a comparison. A signaling sugar should cause
a faster and greater response than other compounds. Among nine sugars and mannitol tested, sucrose alone
caused an immediate active stimulation of net nitrate influx. Glucose, fructose, and raffinose caused weak
responses with a lag. Other carbohydrates had no effect. Sucrose behaves as a specific signal for nitrate uptake,
which has long been supposed but not supported experimentally.
Keywords: Triticum aestivum
, net nitrate influx, sucrose, signal molecule