Agriculture Reference
In-Depth Information
ably far lower than that formed through the GS/GOGAT pathway [143].
Recently the hypothesis that GDH plays an important role in controlling
glutamate homeostasis has been put forward [142]. This function, which
may have a signaling role at the interface of C and N metabolism, may be
of importance under certain phases of plant growth and development when
there is an important release or accumulation of ammonia [144-146].
Over the last two decades, our knowledge of the various pathways in-
volved in the synthesis of the twenty amino acids that are used to build
up proteins, particularly those derived from glutamate and glutamine, has
been increased through the use of mutant and transgenic plants in which
amino acid biosynthesis has been altered. There are excellent reviews
describing extensively our current knowledge on plant amino acid bio-
synthesis and its regulation [136,143]. Therefore, we will not cover this
complex aspect of N assimilation in this review, even though it is of major
importance for plant growth and productivity. However, there are some
examples of genetic modifi cation in crops in which these pathways have
been altered particularly to increase the content of lysine and methionine,
which are often the most limiting for both humans and animal nutrition
[147-149].
Signifi cant progress has been made during the last few years on the
regulation of inorganic N metabolism and the relationships with C me-
tabolism, both at the cellular and organ levels. In particular, attempts to in-
tegrate large transcriptomic and physiological data sets at the whole plant
level have increased our understanding of the regulation of N assimilation
not only under controlled growth conditions but also under the constantly
changing environmental constraints usually occurring in fi eld situations
[6]. This integration is required, because in addition to regulating a range
of cellular processes including N assimilation itself through the co-ordi-
nation of nitrate or ammonia uptake and use, nitrate and N metabolite
levels in the cell can regulate directly or indirectly a number of closely
related metabolic and developmental processes [150,151]. These process-
es, which may also be regulated through the action of hormones [152],
include the synthesis and accumulation of amino acids and organic acids
and the modifi cation of plant development including the extent and form
of root growth and the timing of fl ower induction. All these processes, act-
ing either individually or synergistically, condition N allocation in newly
