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Background: Light and carbon are two important interacting signals affecting plant growth and development. The mechanism(s) and/or genes involved in sensing and/or mediating the signaling pathways involving these interactions are unknown. This study integrates genetic, genomic and systems approaches to identify a genetically perturbed gene network that is regulated by the interaction of carbon and light signaling in Arabidopsis.

Mechanisms of cAMP signal transduction have been thoroughly investigated for more than 40 years. From the binding of hormonal ligands to their receptors on the outer surface of the plasma membrane to the cytoplasmic activation of effectors, the ensuing cAMP signaling cascades and the nuclear gene regulatory functions, coupled with the structural elucidation of the cAMP-dependent protein kinase (PKA) and in vivo functional characterizations of each of the components of PKA by homologous recombination gene targeting, our understanding of cAMP-mediated signal transduction has reached its pinnacle. Despite this trove of knowledge, some recent findings have emerged that suggest hitherto novel and alternative mechanisms of cAMP action that could increase the signaling bandwidth of cAMP and PKA in cell growth and transcriptional regulation. This article attempts to review some of these novel and unconventional mechanisms of cAMP and PKA signaling, and to generate further enthusiasm in investigating and validating these new frontiers of the cAMP signal transduction pathway.

We are using molecular, biochemical, and genetic approaches to study the structural and regulatory genes controlling the assimilation of inorganic nitrogen into the amino acids glutamine, glutamate, aspartate and asparagine. These amino acids serve as the principal nitrogentransport amino acids in most crop and higher plants including Arabidopsis thaliana. We have begun to investigate the regulatory mechanisms controlling nitrogen assimilation into these amino acids in plants using molecular and genetic approaches in Arabidopsis. The synthesis of the amide amino acids glutamine and asparagine is subject to tight regulation in response to environmental factors such as light and to metabolic factors such as sucrose and amino acids. For instance, light induces the expression of glutamine synthetase (GLN2) and represses expression of asparagine synthetase (ASN1) genes. This reciprocal regulation of GLN2 and ASN1 genes by light is reflected at the level of transcription and at the level of glutamine and asparagine biosynthesis. Moreover, we have shown that the regulation of these genes is also reciprocally controlled by both organic nitrogen and carbon metabolites. We have recently used a reverse genetic approach to study putative components of such metabolic sensing mechanisms in plants that may be conserved in evolution. These components include an Arabidopsis homolog for a glutamate receptor gene originally found in animal systems and a plant PII gene, which is a homolog of a component of the bacterial Ntr system. Based on our observations on the biology of both structural and regulatory genes of the nitrogen assimilatory pathway, we have developed a model for metabolic control of the genes involved in the nitrogen assimilatory pathway in plants. Key words Light and metabolic regulation of genes involved in nitrogen metabolismIn non-legume plants, nitrogen assimilation begins with the uptake of inorganic nitrogen, nitrate or ammonium, from the soil.Nitrate is subsequently reduced to ammonium (NH 4 + ) by the sequential action of nitrate reductase and nitrite reductase (1,2). The resulting ammonium is then assimilated into an organic form as glutamate and glutamine. These amino acids are the nitrogen donors in the biosynthesis of essentially all