Stress-responsive alpha-dioxygenase expression in tomato roots.

Alpha-dioxygenase (alpha-DOX) enzymes catalyse the oxygenation of fatty acids to yield a newly identified group of oxylipins that play a role in protecting tissues from oxidative damage and cell death. In tomato (Lycopersicon esculentum Mill.) alpha-DOX was identified as salt-regulated by differential display of mRNA, and is represented by a small gene family comprising at least three members: LEalpha-DOX1, -2, and -3 of which only LEalpha-DOX1 was salt-responsive. The enhancement of LEalpha-DOX1 expression in roots by salt, wounding and challenge with Pythium aphanidermatum (Edson) Fitzp. suggests that alpha-DOX-generated oxylipins may mediate the response of roots to these environmental stresses. In roots, LEalpha-DOX1 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 LEalpha-DOX1 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 LEalpha-DOX1 expression, and this enhanced expression was suppressed by ABA. LEalpha-DOX1 expression in salt-stressed roots was not markedly affected by AVG indicating that ABA may be responsible for enhanced alpha-DOX expression in salt-treated roots.