Glycine Betaine Biosynthesis Is Induced by Salt Stress but Repressed by Auxinic Herbicides in <Emphasis Type="Italic">Kochia scoparia</Emphasis>.

Kochia scoparia biotypes that are susceptible or resistant to the auxinic herbicide dicamba were used to characterize expression levels of choline monooxygenase (CMO) and glycine betaine accumulation in response to salt stress and herbicide treatment. A 1180-bp cDNA was isolated using differential display and 3 RACE with a deduced amino acid sequence that was more than 90% similar to the carboxy terminal 290 residues of CMOs from four related plant species. Salt stress led to a substantial increase in CMO mRNA and enzyme levels in K. scoparia biotypes, and the accumulation of up to 80 mol g^–1 fresh weight glycine betaine. In contrast, dicamba treatment was followed by the rapid attenuation of CMO message and protein levels, with a recovery of expression in the resistant but not the susceptible biotype. CMO mRNA and enzyme levels similarly declined, and recovered in the resistant biotype, after dicamba treatment of plants that were previously salt stressed for 4 days. The opposing effects of these two stresses may represent a regulatory scheme in which competition for the substrate choline leads to a repression of glycine betaine biosynthesis to make sufficient choline available for auxin-mediated growth processes.