Investigating the 2,4-D Resistance Mechanism in Indian Hedge Mustard (Sisymbrium orientale) from Australia Using RNA-Seq

Indian Hedge Mustard (Sisymbrium orientale) is considered a major weed in Australia because it competes with crops, its wiry stems block cereal harvesters and it produces large numbers of seed which cause grain contamination. In 2013 a population from Port Broughton, South Australia was reported to be resistant to 2,4-D and MCPA, requiring 22 times more herbicide for equivalent control when compared to a susceptible population from Roseworthy, South Australia. The Port Broughton population was also resistant to the sulfonylurea and sulfonamide herbicide groups for which the mechanism of resistance was shown to be a Pro to Ser substitution at position 197 in the protein. However, the mechanism of resistance to the auxinic herbicides remains unknown. Metabolism, absorption, and translocation studies were conducted by applying 14C-labelled 2,4-D on two expanded leaves of plants at the 4-5 true leaf stage. Radioactivity was quantified for translocation and 2,4-D metabolites were measured in the treated leaf and other tissues at 3, 6, 12, 24, 48, 96 and 192h after treatment. No differences between susceptible and resistant individuals were identified for absorption, translocation, or metabolism. An RNA-Seq transcriptome analysis was performed on homozygous resistant and susceptible individuals from a F3 and F4 family to identify candidate genes for 2,4-D resistance.