Primary metabolism and transcriptional regulation in higher plants

Abstract

Metabolism, which is a flow of metabolites, is orchestrated by gene expression, and it is in turn regulated by metabolites. Some of the metabolites are known to act as signaling molecules that regulate metabolism and also developmental processes in plants. Here we summarize findings about metabolites-directed gene regulation, focusing on primary metabolites including sugars and organic acids. Abundant soluble sugars such as sucrose and glucose are known to induce genes involved in starch storage, synthesis and degradation. These sugars also activate anthocyanin biosynthesis and suppress photosynthetic genes, both of which promote photoprotection responses. Several kinases, like hexokinase, sucrose non-fermenting1-related kinase 1 and target of rapamycin kinase have been identified, which are the major regulators of transcriptional regulation by these sugars. Although less studied than sugars, metabolites constituting the TCA cycle are good candidates of signaling molecules, because they are located in the primary metabolism adjacent to the respiratory electron transport system and thus are reflected by both primary metabolism and redox status of the electron transport system. Some organic acids as citrate, malate and 2-oxoglutarate have also been known to be implicated in the regulation of gene expression. Deficiency or over accumulation of some primary metabolites not only modulate gene expression of the local metabolic enzymes to supplement distorted metabolism but produce signals to balance between bigger metabolic blocks like carbon and nitrogen metabolisms. Status of primary metabolites can also produce signals of overall energy levels of plants. Relationship of sugar signaling to reactive oxygen species signaling is also suggested.