Abstract
In higher plants, sugars are required not only to sustain heterotrophic growth but also to regulate the expression of a variety of genes. Environmental stresses, such as pathogen infection and wounding, activate a cascade of defense responses and may also affect carbohydrate metabolism. In this study, the relationship between sugar- and stress-activated signal transduction pathways and the underlying regulatory mechanism was analyzed. Photoautotrophically growing suspension culture cells of Chenopodium rubrum were used as a model system to study the effects of the metabolic regulator D-glucose and of different stress-related stimuli on photosynthesis, sink metabolism, and defense response by analyzing the regulation of mRNAs for representative enzymes of these pathways. Glucose as well as the fungal elicitor chitosan, the phosphatase inhibitor endothall, and benzoic acid were shown to result in a coordinated regulatory mechanism. The mRNAs for phenylalanine ammonia-lyase, a key enzyme of defense response, and for the sink-specific extracellular invertase were induced. In contrast, the mRNA for the Calvin cycle enzyme ribulose bisphosphate carboxylase was repressed. This inverse regulatory pattern was also observed in experiments with wounded leaves of C. rubrum plants. The differential effect of the protein kinase inhibitor staurosporine on mRNA regulation demonstrates that the carbohydrate signal and the stress-related stimuli independently activate different intracellular signaling pathways that ultimately are integrated to coordinately regulate source and sink metabolism and activate defense responses. The various stimuli triggered the transient and rapid activation of protein kinases that phosphorylate the myelin basic protein. The involvement of phosphorylation in signal transduction is further supported by the effect of the protein kinase inhibitor staurosporine on mRNA levels.