Stress-responsive a-dioxygenase expression in tomato roots

Alpha-dioxygenase (a-DOX) enzymes catalyse the oxygenationof fatty acids to yield anewly identifiedgroupof oxylipins that play a role in protecting tissues from oxidative damage and cell death. In tomato (Lycopersicon esculentum Mill.) a-DOX was identified as saltregulated by differential display of mRNA, and is represented by a small gene family comprising at least three members: LEa-DOX1, -2, and -3 of which only LEa-DOX1 was salt-responsive. The enhancement of LEa-DOX1 expression in rootsbysalt,woundingandchallengewith Pythium aphanidermatum (Edson) Fitzp. suggests that a-DOX-generatedoxylipinsmaymediate the responseof roots to these environmental stresses. In roots, LEaDOX1 was abscisic acid (ABA)-responsive. However, in the ABA-deficient mutant flacca salt-responsive expression was equivalent to that in the wild type. Similarly, in roots exposed to fluridone (FLU) salt up-regulated expression; however, in this case salt-responsive LEaDOX1 expression was greater than that in roots that were not exposed to FLU. A possible explanation for this is provided by the role of ABA in suppressing ethylene accumulation in osmotically stressed roots. The ethylene-generating agent ethephon and precursor 1-aminocyclopropane-1-carboxylic acid markedly elevated LEa-DOX1 expression, and this enhanced expression was suppressed by ABA. LEa-DOX1 expression in salt-stressed roots was not markedly affected by AVG indicating that ABA may be responsible for enhanced a-DOX expression in salt-treated roots.