Regulation of alpha-dioxygenase expression and functional analyses in salt-stressed Arabidopsis thaliana

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(Thesis) Ph.D.
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Alpha-dioxygenase (α-DOX) catalyzes the oxygenation of fatty acids to produce a newly identified group of oxylipins. In Arabidopsis, α-DOX is represented by a small gene family comprised of two members, Atα-DOX1 and Atα-DOX2. Both α-DOX genes were constitutively expressed but in distinct locations. Atα-DOX1 was expressed in roots, stamens and Atα-DOX2 was expressed in shoots, sepals, siliques and developing seeds. The expression of both α-DOX genes was responsive to salt in the roots and shoots and this salt-responsive expression was accompanied by increased α-DOX activity in both root and shoot tissues of salt stressed Arabidopsis. 2-Hydroxylinolenic acid (2HOT) and heptadecatrienal (C17 aldehyde) were the major products detected in in-vitro α-DOX assays using linolenic acid as a substrate. The role of hormones in regulating salt-induced changes in Atα-DOX expression was explored using exogenous application of hormones and hormone mutants. Abscisic acid (ABA) and salicylic acid (SA) were major hormone signals that up-regulated Atα-DOX expression in roots, whereas ABA, SA and ethylene up-regulated Atα-DOX expression in shoots. The functional significance of α-DOX in salt-stressed Arabidopsis plants was explored using lines with altered Atα-DOX1 and/or Atα-DOX2 expression. Knockout lines lacking Atα-DOX1 or Atα-DOX2 expression were more sensitive to the damaging effects of salt than wild type suggesting that α-DOX products contribute to salt tolerance. In the same lines, increased levels of H2O2 were detected in the roots indicating that α-DOX may suppress the accumulation of reactive oxygen species or promote their removal. An unanticipated function for α-DOX in regulating root system architecture was discovered whereby Atα-DOX1 and Atα-DOX2 play a role in checking LR emergence under salt stressed conditions. Such a role is consistent with the spatial expression of Atα-DOX in roots, which occurred in the zone of cell differentiation within which LR primordia are known to develop. The expression of both Atα-DOX genes was ABA and salt-responsive; it is therefore possible that the resulting α-DOX products contribute to the known ability of ABA to check LR emergence in osmotically-stressed plants. The α-DOX products contribute to salt tolerance in Arabidopsis by protecting cells against oxidative stress and checking LR production.
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