Decreased Intercellular Transport Due to Increased Flavonoid Biosynthesis: A Candidate Dicamba Resistance Mechanism

Resistance to dicamba (a synthetic auxin herbicide) has been documented in Kochia scoparia populations since 1994, but the molecular mechanisms of observed resistance cases remain elusive. An RNA-Seq approach was used to identify 386 transcripts with significantly differential transcription responses between inbred lines of dicamba-resistant (9425R) and -susceptible (7710S) K. scoparia in response to dicamba application. Among the significantly differentially expressed transcripts was Flavonol 3’-Hydroxylase (F3’H), a key enzyme in the flavonoid biosynthesis pathway found to be upregulated in 9425R. This enzyme determines the relative abundance of quercetin to kaempferol, known inhibitors of auxin transport. Chalcone Synthase (CHS), the first enzyme and rate-limiting step in the flavonoid biosynthesis pathway, displayed significant upregulation in 9425R. In silico expression patterns of both transcripts was confirmed with qRT-PCR. An F2 population derived from a cross of 9425R x 7710S segregating for the resistance phenotype was assayed for CHS and F3’H expression using qRT-PCR. Dicamba-resistant F2 individuals displayed significantly higher CHS transcription and lower F3’H transcription compared to dicamba-susceptible F2 individuals, associating the resistant phenotype of 9425R with a greater overall flux and higher relative ratio of kaempferols to quercetins. Co-segregation of this expression profile with the resistance phenotype in the F2 suggests that kaempferol and quercetin may play a role in the resistance phenotype by inhibiting auxin transporters. Reducing intercellular transport and vascular loading could substantially reduce dicamba efficacy by reducing movement to sensitive meristematic tissue, thereby conferring the observed resistance phenotype.