Molecular basis and genetic characterisation of evolved resistance to ALS-inhibitors in Papver rhoeas

ALS gene from susceptible and field-selected Papaver rhoeas populations resistant to ALS-inhibitor herbicides was studied. The full-length cDNA and genomic sequence coding for acetolactate synthase (ALS) of a susceptible population of poppy was cloned and sequenced. Some peculiarities in poppy ALS gene were identified aligning the sequences in GenBank. Southern analysis using a 800 bp fragment of ALS showed that poppy possesses a single copy of the gene. Partial ALS genomic DNAs from nine poppy populations resistant to ALS-inhibitor herbicides and four susceptible populations, collected in central and southern Italy where resistance to ALS-inhibitors is increasing due to repeated use of sulfonylureas, were amplified and sequenced. Comparison of the coding sequences identified three independent point mutations leading to different amino acid substitutions in the deduced polypeptide sequence. The three point mutations, all at proline 197 (CCT) (based on Arabidopsis numbering) in the conserved domain A of the gene, included a change of Pro to His (CAT), to Thr (ACT) or to Ser (TCT). These mutations cause similar cross-resistance patterns. Analysis of the progeny of two crosses between resistant (R) and susceptible (S) biotypes indicated that resistance is inherited as a dominant monogenic trait, although seed dormancy may interfere with a correct segregation of R and S biotypes in the progeny. These are the first mutations completely characterised in poppy ALS gene that confer resistance to ALS-inhibitor herbicides.