Choline monooxygenase (CMO) is the first regulatory enzyme in the biosynthetic pathway for glycine betaine, an effective osmoprotectant in Kochia scoparia, a highly drought-and salt-tolerant species. In seedlings, CMO transcript levels are rapidly increased in response to both NaCl and osmotic stress treatments. The mRNA level in shoots was substantially higher than in roots. The rapid induction seen in whole plants was in contrast to the apparent downregulation observed in suspension-cultured K. scoparia cells in response to the same salt stress. Treatment with exogenous abscisic acid (ABA) or fluridone shows that CMO induction proceeds via an ABA-independent signal transduction pathway. Examination of the CMO upstream regulatory region reveals a number of stress response-related elements, some of which may be involved in the stress tolerance shown by this species.
In an ongoing effort to investigate the mechanism of auxinic herbicide resistance in Kochia scoparia (kochia), polymerase chain reaction-based cDNA suppression subtractive hybridization was used to identify genes that are differentially expressed between dicamba-resistant (HRd) and dicamba-susceptible (S1) kochia biotypes in response to herbicide treatment. Both the HRd and S1 adaptor-ligated cDNAs were used in separate hybridizations in order to generate biotype-specific clones. A total of 710 cDNAs, representing putative differentially expressed mRNAs, were isolated and subjected to further screening. The false-positive cDNAs were removed by conducting two colony hybridizations and at least one Northern hybridization. Differential or biotype-specific expression was confirmed for six clones each from the HRd and S1 plants.The S1-related genes were constitutively expressed at higher levels than in the HRd plants, but none had significant sequence similarity to known genes. Among the HRd-related genes, HRd-88 had 42% amino acid sequence identity to a conserved domain within thiol peptidases, which might be involved in auxin-regulated gene expression. The constitutively expressed and inducible (by the dicamba treatment) HRd-39 had 40% identity and 60% similarity to a domain from the Fe(II)/a-ketoglutarate-dependent hydroxylase superfamily. The HRd-39 gene product had the characteristics of an enzyme that is able to detoxify dicamba via oxidative hydroxylation and thus its overexpression might confer the dicamba resistance phenotype.
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