The contribution of fatty acid-derived volatiles to aphid resistance in tomato

Fatty acid-derived volatiles play an important role in defensive functions against insects. In tomato, six-carbon (C6-) aldehydes and alcohols are mainly synthesized from linoleic and linolenic acids through the successive action of the enzymes lipoxygenase (LOX) and hydroperoxide lyase (HPL). Fatty acid desaturase 7 (FAD7) is an omega-3 FAD that regulates levels of linoleic and linolenic acids. Lipoxygenase C (LOXC), an isoform of tomato lipoxygenase, is specifically involved in the generation of a fatty acid-derived C6 volatile compounds. HPL cleaves the deoxygenated linoleic and linolenic acids then generate C6 volatiles. The aphid-resistant tomato mutant spr2 with impaired FAD7 functions emits more hexanal and less (Z)-3-hexenal and (Z)-3-hexenol than wild-type controls. To investigate if variation of fatty acid-derived C6 volatiles contributes to spr2 mutant tomato resistance to potato aphid, this study measured and compared the settling behavior, survival and fecundity of potato aphids in mutant tomato lines with impaired functions of FAD7, antisense suppression of LOXC or HPL-reduced and their respective wild-type controls. The results indicated that on 5-week-old tomato plants loss function of FAD7 significantly inhibited the settling behavior, survival and fecundity of potato aphids; reduction of HPL increased aphid host preference and aphid population growth; suppression of LOXC had no significant effect on aphid performance. All of these three mutant lines had the lower levels of (Z)-3-hexenal, (Z)-3-hexenol and (E)-2-hexenal than their controls; whereas spr2 but not antisense LoxC or HPL-reduced line had a higher level of hexanal than wild-type controls. Therefore, aphid resistance could be impacted by the multiple C6 volatiles.