Transcriptomic response of Pseudomonas nicosulfuronedens LAM1902 to the sulfonylurea herbicide nicosulfuron
Résumé
The overuse of the herbicide nicosulfuron has become a global environmental concern. As a potential bioremediation technology, the microbial degradation of nicosulfuron shows much promise; however, the mechanism by which microorganisms respond to nicosulfuron exposure requires further study. An isolated soil-borne bacteria Pseudomonas nicosulfuronedens LAM1902 displaying nicosulfuron, chlorimuron-ethyl, and cinosulfuron degradabilities in the presence of glucose, was used to determine the transcriptional responses to nicosulfuron exposure. RNA-Seq results indicated that 1102 differentially expressed genes (DEGs) were up-regulated and 702 down-regulated under nicosulfuron stress. DEGs were significantly enriched in "ABC transporters", "sulfur metabolism", and "ribosome" pathways (p ≤ 0.05). Several pathways (glycolysis and pentose phosphate pathways, a two-component regulation system, as well as in bacterial chemotaxis metabolisms) were affected by nicosulfuron exposure. Surprisingly, nicosulfuron exposure showed positive effects on the production of oxalic acid that is synthesized by genes encoding glycolate oxidase through the glyoxylate cycle pathway. The results suggest that P. nicosulfuronedens LAM1902 adopt acid metabolites production strategies in response to nicosulfuron, with concomitant nicosulfuron degradation. Data indicates that glucose metabolism is required during the degradation and adaptation of strain LAM1902 to nicosulfuron stress. The present studies provide a glimpse at the molecular response of microorganisms to sulfonylurea pesticide toxicity and a potential framework for future mechanistic studies. China is a large agricultural country that annually produces and consumes hundreds of different pesticides 1. Nicosulfuron, a typical sulfonylurea herbicide, has been widely used to eradicate different types of broadleaf grasses and weeds in agriculture 2-4. Nicosulfuron inhibits plant acetolactate synthase, a key feedback enzyme associated with the synthesis of branched-chain amino acids such as leucine, valine, and isoleucine 5-7. Inhibited synthesis of these amino acids can lead to a rapid decrease in cell division and plant growth. Due to the extensive use of nicosulfuron in agriculture and the persistence of residuals in farmland soil, nicosulfuron can be acutely toxic to susceptible crops and a decrease in biodiversity 8-12. Preliminary studies showed that nicosulfuron decreased wheat seed germination and emergence as low as 2 mg/kg (Li, pers. communication). Residual herbicides or their intermediates can pose severe ecotoxicological risks, such as altering the soil microbial community, and lethality to some aquatic organisms (e.g., fish and crayfish) 13-19. Furthermore, the use of multiple herbicides is a common practice in the real environment, increasing the risk of herbicides to the ecological environment 18. It is therefore important to understand and evaluate the potential environmental risks of nicosulfuron exposure and gain greater insight into its degradation mechanisms.
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