New biotypes of herbicide-resistant weeds: the case of C4 plants in summer crops

The exponential increase in herbicide resistance among C4 weed species poses a major threat to the sustainability of summer cropping systems in Spain. This doctoral thesis aimed to characterize the levels and mechanisms of resistance to multiple herbicide modes of action (MoAs) in weed biotypes collected from summer field crops and fruit orchards with intensive herbicide use. Dose–response assays complemented by molecular and biochemical analyses revealed diverse and complex resistance mechanisms. In Bassia scoparia (Chapter 2), the first European case of triple resistance to 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), acetolactate synthase (ALS) inhibitors, and synthetic auxins was confirmed. Resistance was conferred by EPSPS gene amplification (average 4.9 copies) and multiple target-site mutations in the ALS gene (Pro197Ser, Pro197Leu, and Trp574Leu), which can confer cross-resistance to imidazolinones. Additionally, resistance to 2,4-D and MCPA was observed, and a non–target-site resistance (NTSR) mechanism to auxin mimics was inferred, likely associated with enhanced metabolism mediated by cytochrome P450 enzymes (P450s). In Panicum dichotomiflorum (Chapter 3), resistance to the ALS inhibitor nicosulfuron was associated with enhanced metabolism involving P450s and glutathione S-transferase (GST) enzymes, representing the first global report of NTSR in this species. In Chloris truncata (Chapter 4), glyphosate resistance was attributed to EPSPS copy number amplification (7.4–7.6-fold). Catalan populations exhibited higher basal sensitivity than the wild-type (WT) reference population. Finally, field studies on Echinochloa crus-galli (Chapter 5) demonstrated that crop rotation and diversified herbicide programs were the most effective strategies for reducing infestations of Trp574Leu-resistant biotypes, thereby improving the long-term performance of integrated weed management (IWM) programs. Overall, this research provides novel evidence of co-occurring TSR and NTSR mechanisms in C4 weeds, highlighting their adaptive significance and the need for diversified management strategies to sustain herbicide efficacy in Mediterranean cropping systems.