Future Prospects
Genetic engineering approaches have contributed significantly to understand the
regulation of amino acid metabolism in plants. Such approaches can be expected
to become major tools in future research on plant amino acid metabolism.
So far, detailed studies on amino acid metabolism, using genetic engineering
approaches, were limited to a narrow range of pathways, particularly the pathway
of AAAM, the aspartate family pathway, and to some extent the pathways of
proline and tryptophan metabolism (Kishor
et al., 1995; Li and Last, 1996; Nanjo
et al., 1999; Tozawa
et al., 2001; Zhang
et al., 2001). Similar approaches for dissecting
metabolic pathways of other amino acids are needed.
Many of the studies discussed here have focused on biosynthetic pathways,
while less effort has been devoted to amino acid catabolic pathways. As in the
emerging progress of lysine catabolism (Galili
et al., 2001), amino acid catabolic
pathways may be important metabolic components in plant development, reproduction,
and responses to stress. Therefore, in future research, more efforts should
be devoted to the dissection of amino acid catabolic pathways.
Amino acid metabolism is strongly regulated by various metabolites, many of
which are non-amino acids, which serve not only as signaling molecules but also
as intermediate metabolites in metabolic pathways of amino acids sugars and
lipids. One example of such metabolites is pyruvate that serves as a precursor for a
number of amino acid carbohydrate and lipid molecules. In microorganisms, the
regulatory or rate-limiting roles of such intermediate metabolites can be studied
by feeding experiments. The multicellular and multiorgan nature of higher plants
does not enable proper feeding experiments in all tissues of intact plants and
therefore provides additional levels of complexity that render the dissection of
metabolic fluxes much more difficult to predict and study than in microorganisms.
Understanding the regulation of metabolic fluxes and the importance of
rate-limiting metabolites in different plant organs cannot be easily done by feeding
experiments alone. Hence, such studies will depend strongly on tissue-specific
and/or condition-specific genetic engineering as well as on isotope-labeling
studies.
The identification of regulatory networks of amino acid metabolism as well as
possible complexes of enzymes that may regulate these networks is also needed.
Such studies can be strongly assisted by genetic engineering approaches. For
example, identification of enzyme and complexes can be obtained by expressing
chimeric genes encoding epitope-tagged enzymes in transgenic plants. It is
expected that interdisciplinary approaches, such as that of the ‘‘matrix effect’’
will contribute to unraveling interacting molecular, metabolic, and environmental
signals that regulate the networks of amino acid metabolism.
Understanding the compound regulation of metabolic networks (amino acid
metabolism is included as a part of these metabolic networks) can be aided by detailed analysis of a large number of metabolites as well as by detailed analysis of
the spatial, temporal and developmental patterns of expression of genes encoding
enzymes and regulatory proteins associated with these networks. Thus, modern
approaches such as metabolic profiling, gene expression profiling in microarrays,
and proteomics will be progressively used in these studies. These issues have not
been discussed in this chapter due to space limitation. Yet, several recent publications
(Hunter
et al., 2002; Lee
et al., 2002; Ruuska
et al., 2002) illustrate how
microarray analyses of gene expression in
Arabidopsis and maize seeds uncovered
specific spatial and temporal expression patterns of genes associated with the
metabolism of sugars, lipids, amino acids, and storage proteins during seed
development.
