Ultraviolet radiation (UV) is an important modulator of plant defenses against biotic stresses. We have recently described that different supplemental UV exposure times and irradiance intensities enhanced tomato (Solanum lycopersicum) resistance to Western flower thrips (Frankliniella occidentalis). UV increased jasmonic acid-isoleucine (JA-Ile) and salicylic acid (SA) levels, as well as the expression of JA- and SA-responsive genes, before thrips herbivory. Here we report how UV affects tomato defense responses upon thrips infestation, and resistance to pathogens that are susceptible to the activation of SA-associated defenses. Our experiments reveal that, at 7 days after thrips infestation, UV did not enhance the levels of jasmonates, auxin or abscisic acid. UV also did not affect the expression of JA-responsive genes in the cultivar Moneymaker, the jasmonate deficient mutant def-1, the type-VI trichome deficient mutant od-2, or their wild-type Castlemart. However, UV strongly activated SA-associated defense responses in def-1 after thrips infestation. Further bioassays showed that UV increased def-1 resistance to the hemi-biotrophic bacterial pathogen Pseudomonas syringae pv. tomato DC3000, which is susceptible to SA-mediated defenses. Our results suggest that UV might enhance tomato resistance to this pathogen in the JA deficient genotype through the activation of SA defenses.
Keywords: Abscisic acid; auxin; jasmonates; light; salicylic acid; ultraviolet radiation; western flower thrips.