Differential accumulation of phytohormones in wheat seedlings attacked by avirulent and virulent Hessian fly (Diptera: Cecidomyiidae) larvae

We analyzed the accumulations of six phytohormones and fatty acids in a wheat (Triticum aestivium) genotype ‘Molly’ following attacks by avirulent and virulent Hessian fly (Mayetiola destructor) larvae and examined the expression of genes in the jasmonic acid (JA) pathway by Northern blot analysis. Compared to uninfested plants, attacks by the avirulent biotype resulted in increased accumulation of salicylic acid (SA) by 11.3 and 8.2 folds, 12-oxo-phytodienoic acid (OPDA) by 36.4 and 18.7 folds, 18:3 fatty acid by 4.5 and 2.2 folds, and 18:1 fatty acid by 1.8 and 1.9 folds on day one and day three, respectively, but an 20% decrease in JA accumulation on day one in the plant tissue at the attack site. In contrast, attacks by the virulent biotype did not affect the accumulation of SA, OPDA, 18:3 and 18:1 fatty acids, but resulted in a dramatic increase in auxin, with concentration from undetectable in uninfected plants to 385.6 ng/g on day one and 71.0 ng/g on day three in the infested plants. Expression of genes putatively encoding lipoxygenase 2 (LOX2), allene oxide synthase (AOS), and Arabidopsis storage protein 2 (AtVSP2) were up-regulated by the avirulent biotype but down-regulated by the virulent biotype. Our results suggest 1) OPDA and SA are likely to act together in wheat resistance to the Hessian fly, 2) High-level of auxin is linked to the susceptibility of wheat plants, 3) The increased OPDA accumulation caused by the avirulent Hessian fly infestation is at least partially regulated through gene transcription