Involvement of Ureides in Nitrogen Fixation
Inhibition in Soybean |
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Authors: | Rachid Serraj Vincent Vadez R Ford Denison and Thomas R Sinclair |
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Institution: | United States Department of Agriculture-Agricultural Research Service, Agronomy Physiology Laboratory, Institute of Food and Agricultural Sciences, University of Florida, P.O. Box 110965, Gainesville, Florida 32611–0965 (R.S., V.V., T.R.S.);Department of Agronomy and Range Science, University of California, Davis, California 95616 (R.F.D.) |
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Abstract: | The
sensitivity of N2 fixation to drought stress in soybean
(Glycine max Merr.) has been shown to be associated with
high ureide accumulation in the shoots, which has led to the hypothesis
that N2 fixation during drought is decreased by a feedback
mechanism. The ureide feedback hypothesis was tested directly by
measuring the effect of 10 mm ureide applied by stem
infusion or in the nutrient solution of hydroponically grown plants on
acetylene reduction activity (ARA). An almost complete inhibition of
ARA was observed within 4 to 7 d after treatment, accompanied by
an increase in ureide concentration in the shoot but not in the
nodules. The inhibition of ARA resulting from ureide treatments was
dependent on the concentration of applied ureide. Urea also inhibited
ARA but asparagine resulted in the greatest inhibition of nodule
activity. Because ureides did not accumulate in the nodule upon ureide
treatment, it was concluded that they were not directly inhibitory to
the nodules but that their influence mediated through a derivative
compound, with asparagine being a potential candidate. Ureide treatment
resulted in a continual decrease in nodule permeability to
O2 simultaneous with the inhibition of nitrogenase activity
during a 5-d treatment period, although it was not clear whether the
latter phenomenon was a consequence or a cause of the decrease in the
nodule permeability to O2.The physiological basis of N2 fixation
inhibition by water deficits in legume nodules is not clearly
understood. A potential physiological basis for this water-deficit
sensitivity may be that drought stress decreases the
Po (Weisz et al., 1985), as has been shown with
other stresses such as temperature, salinity, or nitrate (Hunt and
Layzell, 1993; Serraj et al., 1994; Denison and Harter, 1995). The role
of O2 limitation in the response of nitrogenase
activity to drought stress has been discussed extensively (Diaz del
Castillo and Layzell, 1995; Purcell and Sinclair, 1995; Serraj and
Sinclair, 1996b; Serraj et al., 1999). However, the mechanisms by which
drought affects Po have
not been elucidated. It is not clear whether drought stress has a
direct effect on Po, or whether the decrease in
Po is a consequence of a decrease in nodule
activity.An alternative explanation for the decrease in nitrogenase activity
under drought could be a feedback mechanism involving the accumulation
of N compounds. Pate et al. (1969) proposed that lower rates of water
movement out of the nodule during drought stress may restrict export of
products of N2 fixation, and the accumulation of
these products would inhibit nitrogenase activity. Others have
suggested that N2 fixation in legumes might be
regulated by a feedback mechanism involving N metabolism and the pool
of reduced N in the plant (Silsbury et al., 1986; Parsons et al., 1993;
Hartwig et al., 1994). Oti-Boateng and Silsbury (1993) reported an
inhibition of nitrogenase activity in fava bean after plant uptake of
Asn or Gln.Soybean (Glycine max Merr.) usually exports more
than 80% of the N compounds out of the nodules in the form of the
ureides Aln and Alac. They are transported in the xylem to the shoots,
where they are catabolized (Winkler et al., 1987). High accumulation of
petiole ureides has been measured during the inhibition of
N2 fixation by drought in both controlled (de
Silva et al., 1996; Serraj and Sinclair, 1996a) and field (Purcell et
al., 1998) environments. Furthermore, in a comparison of grain legume
species, Sinclair and Serraj (1995) reported that those species
exporting ureides from the nodules had N2
fixation that was drought sensitive. Those species that exported little
or no ureide had N2 fixation that was relatively
drought tolerant.An important possibility is that the accumulation of ureides in soybean
nodules under soil-water deficits might trigger a feedback mechanism
that results in decreased N2 fixation activity
(Sinclair and Serraj, 1995; Serraj et al., 1999). This paper reports a
series of experiments to investigate the hypothesis of a ureide
feedback inhibition of N2 fixation in soybean.
First, ureide levels were measured in plant tissue (nodules, roots, and
shoots) upon the imposition of water deficits to confirm that ureide
levels increased in the nodules themselves, and not just in the shoot.
Second, the influence of ureides on nodule activity was examined by
introducing ureides, along with other compounds, into soybean plants.
These experiments were designed to examine the time course of the
response and to determine the concentration response. Third, data were
collected to determine if Po and the response of
N2 fixation to pO2 were
also sensitive to introduced ureides. |
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