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In research reported in The Plant Cell, scientists
Antoine Martin and Bertrand Hirel from the National Institute of
Agronomic Research (INRA) in Versailles, France, together with
colleagues from institutions in the U.K., Spain, and Japan, present
new information on the roles of two forms (isoenzymes) of cytosolic
glutamine synthetase (GS) in maize, which underscores the importance
of this enzyme and nitrogen metabolism in cereal crop productivity.
Improving nitrogen use efficiency of crop plants, i.e. reducing the
amount of costly nitrogen fertilizer inputs that farmers need to apply
to crops while at the same time maintaining and even improving yields,
is an important goal in crop research. As noted by Dr. Hirel, “a more
complete understanding of the roles of GS enzymes in nitrogen
metabolism and grain yield in maize and other crop plants (including
rice, wheat and barley) may lead to improvements in fertilizer usage
and crop yield, thus mitigating the detrimental effects of the overuse
of fertilizers on the environment“.
The roles of these two GS isoenzymes, products of
the Gln1-3 and Gln1-4 genes, were investigated by examining the impact
of knock-out mutations on kernel yield. GS gene expression was
impaired in the mutants, resulting in reduced levels of GS1 protein
and activity. The gln1-4 phenotype displayed reduced kernel size
whereas gln1-3 had reduced kernel number, and both phenotypes were
evident in the gln1-3 gln1-4 double mutant. Shoot biomass production
at maturity was not affected in either the single mutants or double
mutants, suggesting that both gene products play a specific role in
grain production. Levels of asparagine increased in the leaves of the
mutants during grain filling, most likely as a mechanism for
circumventing toxic ammonium buildup resulting from abnormally low GS1
activity. Phloem sap analysis revealed that, unlike glutamine,
asparagine is not efficiently transported to developing maize kernels,
which could account for the reduced kernel production in the mutants.
Constitutive overexpression of Gln1-3 in maize leaves resulted in a
30% increase in kernel number relative to wild type, providing further
evidence that GS1 plays a major role in kernel yield.
Some of the major cereals, such as maize, sorghum,
and sugar cane, exhibit C4 photosynthesis, which enhances the
efficiency of photosynthesis at high temperature (most C4 plants
originated in tropical climates). In standard C3 photosynthesis (present
in rice, wheat, and most temperate crop plants), CO2 entering the leaf
is converted to a 3-carbon compound via the C3 pathway, utilizing
energy derived from the light reactions of photosynthesis. In plants
that have C4 photosynthesis, the C3 pathway enzymes are localized in
specialized “bundle sheath” cells which surround the vascular tissue
in the interior of the leaf. CO2 entering mesophyll cells at the leaf
surface initially is converted to a 4-carbon compound, which is
shuttled into the bundle sheath cells and then decarboxylated to
release CO2. CO2 released into bundle sheath cells then enters the
standard C3 pathway. This CO2-concentrating mechanism allows plants in
a hot and dry climate to take up CO2 at night and store it, and
release it again inside bundle sheath cells during the day, thus
solving the problem of how to maintain a high concentration of CO2
inside the leaf during the daylight hours, when stomata often must be
kept closed to prevent water loss. Using cytoimmunochemistry and in
situ hybridization, Martin et al. found that GS1-3 is present in maize
mesophyll cells whereas GS1-4 is specifically localized in the bundle
sheath cells. Thus the two GS1 isoenzymes play non-redundant roles
with respect to their tissue-specific localization, and the activity
of both is required for optimal grain yield. This work illustrates the
close coordination between nitrogen and carbon metabolism in
photosynthetic tissues, and reveals that nitrogen metabolism plays a
critical role in optimizing grain yields. |
