Methyl Jasmonate Changes the Composition and Distribution Rather than the Concentration of Defence Compounds: a Study on Pyrrolizidine Alkaloids.

In this study we investigated the effect of methyl jasmonate (MeJA) application on pyrrolizidine alkaloid (PA) concentration and composition of two closely related Jacobaea species. In addition, we examined whether MeJA application affected herbivory of the polyphagous leaf feeding herbivore Spodoptera exigua. A range of concentrations of MeJA was added to the medium of Jacobaea vulgaris and J. aquatica tissue culture plants grown under axenic conditions. PA concentrations were measured in roots and shoots using LC-MS/MS. In neither species MeJA application did affect the total PA concentration at the whole plant level. In J. vulgaris the total PA concentration decreased in roots but increased in shoots. In J. aquatica a similar non-significant trend was observed. In both Jacobaea species MeJA application induced a strong shift from senecionine- to erucifoline-like PAs, while the jacobine- and otosenine-like PAs remained largely unaffected. The results show that MeJA application does not necessarily elicits de novo synthesis, but rather leads to PA conversion combined with reallocation of certain PAs from roots to shoots. S. exigua preferred feeding on control leaves of J. aquatica over MeJA treated leaves, while for J. vulgaris both the control and MeJA treated leaves were hardly eaten. This suggests that the MeJA-induced increase of erucifoline-like PAs can play a role in resistance of J. aquatica to S. exigua. In J. vulgaris resistance to S. exigua may already be high due to the presence of jacobine-like PAs or other resistance factors.

Application of methyl jasmonate and salicylic acid lead to contrasting effects on the plant's metabolome and herbivory.

Phytohormone applications are used to mimic herbivory and can induce plant defences. This study investigated (i) metabolomic changes in leaf tissues of Jacobaea vulgaris and J. aquatica after methyl jasmonate (MeJA) and salicylic acid (SA) applications and (ii) the effects on a leaf-chewing, a leaf-mining and a piercing-sucking herbivore. MeJA treated leaves showed clearly different metabolomic profiles than control leaves, while the differences in metabolomic profiles between SA treated leaves and control leaves were less clear. More NMR peaks increased than decreased after MeJA treatment while this pattern was reversed after SA treatment. The leaf-chewing (Mamestra brassicae) and the leaf-mining herbivores (Liriomyza trifolii) fed less on MeJA-treated leaves compared to control and SA-treated leaves while they fed equally on the latter two. In J. aquatica but not in J. vulgaris, SA treatment reduced feeding damage by the piercing-sucking herbivore (Frankliniella occidentalis). Based on the herbivory and metabolomic data after phytohormone application, we made speculations as follows: For all three herbivore species, plants with high levels of threonine and citric acid showed less herbivory while plants with high levels of glucose showed more herbivory. Herbivory by thrips was lower on plants with high levels of alanine while it was higher on plants with high levels of 3,5-dicaffeoylquinic acid. The plant compounds that related to feeding of piercing-sucking herbivore were further verified with previous independent experiments.