Variability of aliphatic glucosinolates in Arabidopsis and their influence on insect resistance

Due to the sessile nature of plants they have to cope up dynamically with diverse biotic and abiotic stressors. Plants developed diverse, complex defense mechanisms for dealing with their enemies, including the glucosinolate(GS)-myrosinase system of the Brassicaceae and other families of the order Brassicales. GS are classified by their precursor amino acid and aliphatic, aromatic, and indolyl GS are distinguished. Indolyl GS are widely distributed in Arabidopsis thaliana (L.) ecotypes and the Brassicaceae family, but the presence of aliphatic GS is variable and under strong genetic control. There are only few studies paying attention to the impact of certain GS on insect resistance. Due to this, we have investigated the plant resistance of A. thaliana ecotypes with different aliphatic profiles against two specialized insects. For the experiments we selected 20 ecotypes, divided into three groups after HPLC analysis: containing 1) methylsulfinyl, 2) 3-hydroxypropyl, and 3) allyl GS. As herbivore insects the generalist Spodoptera exigua Hübner and the specialist Pieris brassicae L. were selected. The selected A. thaliana ecotype groups were different suitable for consumption, but similar for both insect species. In general, the percentage weight gain of larvae on A. thaliana plants containing 3-hydroxypropyl GS and allyl GS was significantly higher for both insect species, the specialist and the generalist, compared to methylsulfinyl GS containing ecotypes. But the tendency was stronger for P. brassicae than for S. exigua. Additionally, we used simple correlation to examine the relationship between insect feeding and the GS contents in the ecotypes. It can be concluded that 3-hydroxypropyl GS containing ecotypes were less resistant than ecotypes with methyl-sulfinyl GS as main compounds. Weight gain by S. exigua was statistically significant negatively related to constitutive GS levels of methylsulfinyl GS containing ecotypes. There was also a negative relation to constitutive GS levels of methylsulfinyl GS containing ecotypes for P. brassicae but less strong and not statistically significant. A reason for a better host plant suitability of ecotypes containing 3-hydroxypropyl GS might be the short chemical structure and/or different reactivity of this compound compared to ecotypes containing methylsulfinyl GS.