Amino acid metabolic pathways are connected with each other as well as to other metabolic pathways, such as nitrogen and sulfur assimilation, photosynthesis, and carbon/nitrogen balance. Essentially, AAAM provides the core of these metabolic networks and is in itself regulated by many signals, such as a number of light signals (different wavelengths), various metabolites (such as nitrogen and sugars), and photosynthesis. However, little is known about the networking of AAAMwith other pathways of amino acid metabolism, and how the networks are concertedly regulated by the large number of dynamically changing signals that exert a ‘‘matrix effect’’ (Coruzzi and Zhou, 2001). For example, it is unknown how dynamically changing light signals of different wavelengths and intensities operate in concert with sugar and nitrogen signals to regulate amino acid metabolism in different tissues during plant development and in response to stress conditions. Do these signals either operate independently or do at least some of them operate in concert? Can some signals override others? This complex ‘‘matrix effect’’ has only recently been addressed, using new combinatorial tools (Thum et al., 2003), on three Arabidopsis genes (GLN2, ASN1, and ASN2) encoding, respectively, glutamine synthetase and two asparagine synthetase enzymes. The GLN2 and ASN1 genes are reciprocally regulated by light as well as by sucrose that mimics the light effect (Lam et al., 1995, 1996; Oliveira et al., 2001), while expression of ASN2 is reciprocally regulated with that of the ASN1 gene being stimulated by light and sucrose like the GLN2 gene (Lam et al., 1995, 1998). To study the regulatory effects of different light signals and sucrose on the expression of the GLN2, ASN1, and ASN2 genes, Thum et al. (2003) used Arabidopsis seeds germinated either in the dark or in the light (germination in the light was followed by 2 days of dark adaptation) in media containing 0% or 1% sucrose. Each of these groups was then exposed to treatments with red, blue, or far-red lights at two different intensities (2 or 100 mE/m2s) or to white light (70 mE/m2s) for 3 h. Sucrose attenuated the blue-light induction of the GLN2 gene in etiolated seedlings and the white-, blue-, and red-light induction of the GLN2 and ASN2 genes in light grown plants. Sucrose also strengthened the far-red light induction of GLN2 and ASN2 in light grown plants. Depending on the intensity of the far-red light, sucrose was able to either attenuate or strengthen light repression of the ASN1 gene in light plants. On a more general basis, sucrose exceeded light as a major regulator of ASN1 and GLN2 gene expression in etiolated seedlings, whereas, oppositely, light exceeded carbon as a major regulator of GLN2 and ASN2 gene expression in light grown plants. These results illustrate the complex interaction of light and carbon signals and apparently expose a complex interaction between signal transduction cascades that translate these signals into gene expression.
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