Molecular studies of dicamba-resistant Kochia scoparia L

Extensive use of the auxinic herbicide dicamba (3,6-dichloro-2-methoxybenzoic acid) in Montana grain production systems has selected for biotypes of Kochia scoparia that are insensitive to the herbicide. Dicamba is thought to induce the same physiological responses as the natural phytohormone auxin (indole-3-acetic acid), including changes in gene expression that may be involved in growth and developmental responses. Since the mechanism of dicamba resistance in K. scoparia is currently unknown, differential mRNA display techniques were conducted to compare patterns of dicamba-induced gene expression in resistant (R) and susceptible (S) biotypes. Examination of >80,000 mRNA fragments showed that changes in mRNA abundance occurred within minutes after dicamba treatment, and most changes were similar in R and S plants. From 106 cDNAs isolated, sequenced, and used as probes on northern blots, 14 represented mRNAs whose abundance changed after dicamba treatment. Of these 14 cDNAs, four were repressed similarly in R and S, two were similarly induced, and eight responded differentially to dicamba treatment in R and S. Eight cDNAs were assigned putative functions based on DNA or deduced amino acid sequence similarities to known genes, and included enzymes involved in basic carbon metabolism, cellular chloride uptake, photosynthesis, initiation of protein synthesis, synthesis and degradation of cell wall material, and a protein with a chaperone function. A partial cDNA encoding choline monooxygenase (CMO), an enzyme involved in the biosynthesis of the osmoprotectant glycine betaine (GB), was chosen for more detailed study. Characterization of expression patterns indicated that levels of CMO mRNA were increased by osmotic stress but rapidly declined after dicamba treatment. Levels of the CMO enzyme and GB were similarly reduced in R and S plants following dicamba treatment. However, R and S plants prestressed with NaCl showed differential CMO response after dicamba treatment, likely indicating a fundamental difference in dicamba translocation, perception, or signal transduction between the two biotypes. This research demonstrates that differential display is a useful technique for discovering changes in gene expression that may be initially involved in basic plant responses.