Submitted to: Phytochemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/28/2007
Publication Date: 6/27/2007
Citation: Morse, A.M., Tschaplinski, T.J., Dervinis, C., Pijut, P., Schmelz, E.A., Day, W., Davis, J.M. 2007. Salicylate and catechol levels are maintained in nahG transgenic poplar. Phytochemistry. 68:2043-2052. Interpretive Summary: Phytohormones regulate many essential aspects of plant germination, growth, reproduction and defense. Salicylic acid (SA) is key regulator of plant defense responses to bacterial infection. Most healthy plants have extremely low levels of SA yet rapidly elevate SA production during infection with incompatible pathogens. The SA pathway is critical for programmed cell death and hypersensitive responses that rapidly restrict pathogen invasion. Unlike most other plants, poplar trees exhibit unusually high basal SA levels. In a collaborative effort, scientists at the Center for Medical, Agricultural and Veterinary Entomology in Gainesville, FL, have discovered that, when transformed with the nahG transgene to designed to prevent SA accumulation, poplar trees unexpectedly maintain uniform levels of free SA . Levels of the expected SA catabolite catechol also remained unchanged in transgenic nahG poplars. Interestingly, related metabolites in this pathway were dramatically altered including catechol-glycoside conjugates. These results demonstrate nahG transgene activity in poplar and the action of compensatory metabolic changes to maintain uniform levels of free SA and catechol. The homeostasis of SA levels in polar and resilience to attempted genetic manipulation suggests that high basal SA levels have significant if not essential physiological roles in this system.
Technical Abstract: Metabolic profiling was used to investigate the molecular phenotypes of transgenic Populus tremula x P. alba bybrids expressing the nahG transgene, a bacterial gene encoding salicylate hydroxylase that converts salicylic acid to catechol. Despite the efficacy of this transgenic approach to reducing salicylic acid pools in other model systems and thereby elucidating roles for salicylic acid in plant signaling, transgenic poplars exhibited no visible phenotypes and had stable foliar levels of salicylic acid and catechol. To gain a deeper understanding of the cellular networks that might underlie this result, we analyzed metabolic profiles of leaves as influenced by transgene expression. nahG expression resulted in changes in metabolite pools and identified a metabolic grid immediately surrounding salicylic acid biosynthesis. The net effect of this compensatory mechanism is to maintain consistent levels of salicylate and catechol, the substrate and product, respectively, of the nahG enzyme. This suggests an important biological role of elevated constitutive salicylic acid levels in Populus, in contrast to other plant systems in which nahG dramatically reduces salicylic acid levels.