Examination of the whole genome response in Arabidopsis thaliana to auxinic herbicide 2,4-dichlorophenoxyacetic acid coupled with adjuvant NUL1026

Foliage-applied herbicide formulations often consist of compounds known as surfactants that promote the uptake of herbicide molecules through leaf surfaces. Several lines of research have demonstrated that the efficacy of most herbicides relies on the amount of herbicide molecules successfully entering the plant tissues and reaching the target sites, rather than on increasing the dosage of the herbicides. Therefore, combining the right surfactant to a compatible herbicide formulation may lead to a decrease in the amount to active ingredients being used in weed control. Agrochemical companies are now investing into the development of surfactant products which perform very specific functions that complement herbicide chemistries, with the aim of developing novel herbicide formulations which are effective at lower active ingredients.<br><br>This study took a genomic approach using microarray technology to elucidate the mode of action of an etheramine surfactant, called NUL1026. Knowledge gained from this study will aid in the identification of metabolic and physiological processes that may contribute to herbicide toxicity. The same gene expression profiling approach was also undertaken to investigate the synergistic effect of combining surfactant NUL1026 to the auxinic herbicide 2,4-dichlorophenoxyacetic acid.<br><br><i>Arabidopsis thaliana </i>plants (14 days) grown <i>in vitro </i>were separately sprayed using the Potter spray tower with either of the following solutions: water, 0.2% (v/v) surfactant NUL1026, 1.0 mM 2,4-D and 1.0 mM 2,4-D + 0.2% (v/v) surfactant NUL1026. The total RNA in response to each of the sprayed solutions was extracted and quantified at 1 h and 24 h post-application. The whole genome oligonucleotide microarray (ATH1-121501) manufactured by Affymetrix ® was used to measure gene expression levels of approximately 24,000 genes. <br><br>Expression profiling results in response to surfactant NUL1026 treatment revealed that at both 1 h and 24 h, a number of gene transcripts involved in transcription factor activity, signal transduction pathways, cell wall organisation and biogenesis, disease response, detoxification, senescence and hormone signalling showed altered expression. Of note was the significant regulation of genes involved in jasmonic acid biosynthesis found to be exclusively regulated upon exposure to the surfactant<br>Foliar-application of 1.0 mM 2,4-D resulted in the differential expression of 45 genes, 1 h post-treatment. Functional annotation of these genes showed that they were known auxin-response genes such as <i>IAA1, SAUR </i>and <i>GH3</i>. In contrast, in response to 1.0 mM 2,4-D + 0.2% (v/v) surfactant NUL1026, there was approximately 10 times more genes being regulated for the same time point. This suggests that the surfactant may have promoted the uptake of the herbicide 2,4-D. Moreover, 248 and 354 genes were also exclusive differentially expressed after spraying with 1.0 mM 2,4-D + 0.2% (v/v) surfactant NUL1026, 1 h and 24 h respectively. More in-depth analysis of these exclusively regulated genes showed the increased expression of additional auxin-inducible genes at both time points. Also showing significant up-regulation was genes involved in detoxification and senescence while those transcripts associated with cell wall expansion and photosynthesis was down-regulated. The microarray results of this study highlighted the synergistic influence of surfactant NUL1026 in enhancing 2,4-D toxicity.