Insights into the <scp>ALS</scp> inhibitor resistance in <i>Amaranthus retroflexus</i> : A matter of concern for <scp>CONVISO</scp> ® <scp>SMART</scp> sugar beet cultivation

Abstract Amaranthus retroflexus L. has recently become a problematic weed in CONVISO® SMART sugar beet fields in the Czech Republic, largely due to the rapid adoption of this system and widespread application of ALS inhibiting herbicides in almost all crops, including other systems using ALS herbicide tolerant varieties. In this study, two A. retroflexus biotypes, suspected to be resistant to ALS inhibitors, were collected from CONVISO® SMART sugar beet field. Studies were conducted to confirm their resistance status and investigate the underlying mechanisms. The findings reveal that these biotypes have developed resistance to foramsulfuron + thiencarbazone‐methyl, with cross‐resistance to imazamox and tribenuron‐methyl. The dose–response studies revealed that the resistant (R) biotypes AMARE1 and AMARE2 are 351 and 12 times less sensitive to foramsulfuron + thiencarbazone‐methyl than the susceptible (S) biotype. Upon pretreatment with malathion, the resistance index decreased 2X in AMARE1, indicating the potential involvement of cytochrome P450 in the resistance mechanism. However, no involvement of GSTs was detected following NBD‐Cl pretreatment. Furthermore, a higher ALS enzyme activity was observed in both R biotypes compared to the S biotype. Finally, molecular analysis revealed that a single nucleotide substitution had occurred in the ALS gene, conferring a tryptophan (Trp) to Leucine (Leu) substitution at position 574 of the ALS enzyme involved as a probable target‐site resistance mechanism in both R biotypes. In summary, our study identified point mutation and enhanced metabolism by cytochrome P450s are likely mechanisms of resistance to foramsulfuron + thiencarbazone‐methyl in the AMARE1 biotype, while point mutation was found the sole responsible mechanism of resistance involved in AMARE2. The current study reports the first instance of resistance in A. retroflexus against foramsulfuron + thiencarbazone‐methyl. This evolution highlights the dynamic response of weed populations to herbicide pressure and underscores the need for alternative weed control strategies in sugar beet including rotating ALS‐tolerant crops with the traditional (non‐tolerant) crops, to limit ALS inhibitor use and resistance evolution.