ARYLEX MODE AND SITE OF ACTION CHARACTERIZATION. J. L. Bell*, P. R. Schmitzer, A. E. Robinson; Dow AgroSciences LLC, Indianapolis, IN (389)


ArylexTM Active is the trade name for the new herbicidal compound halauxifen-methyl. The chemical structure is novel and comprises of an amino-pyridine ring linked at position 6 to a substituted aryl group otherwise referred to as a 6-arylpicolinate (6-AP, 4-amino-3-chloro-6-(4-chloro-2-fluoro-3-methoxyphenyl)-5-fluoro-pyridine-2-methyl ester). Halauxifen-methyl is structurally similar to the previously reported experimental herbicide molecule DAS534. When foliarly applied, halauxifen methyl induces growth phenotypes similar to that of other auxin-like herbicides and growth regulators. In sensitive species symptoms arise within hours and include wilting, epinasty, leaf malformation, swelling, stunted growth with necrosis and death occurring in days to several weeks. To begin to understand the physiological mode of action and the molecular site of action of halauxifen-methyl, studies were carried out utilizing both wild type (WT) and mutant lines of Arabidopsis thaliana (ARBTH). Mutants harbored amino acid missense changes in putative auxin binding proteins TIR1 or AFB5. Mutations in TIR1 confer resistance to phenoxy herbicides such as 2, 4-D while mutations in AFB5 impart resistance to picolinate compounds including picloram and DAS534. Plants were either treated foliarly as rosettes prior to bolting or seeded in agar media supplemented with herbicide. In foliar applications of halauxifen-methyl, there was < 0.1 and 4.5 fold resistance over WT plants for TIR1 and AFB5 mutants respectively based on fresh weight. The halauxifen-methyl application rate required to cause 80% growth reduction (GR80) was 16.4 g ae/ha in AFB5 mutants whereas the GR80 of WT and TIR1 were both < 1.1 g ae/ha. AFB5 mutants and WT were equally sensitive to 2, 4-D with GR80 values of 15.8 g ae/h and 15.6 g ae/ha respectively. Phenotypic symptoms suggest that the mode of action of halauxifen-methyl is an auxin-like response. The susceptibility of AFB5 mutants to 2, 4-D suggests that the molecular recognition site of halauxifen-methyl is alternate to TIR1 and has more affinity to the AFB5 protein.