Differences in glucosinolate degradation products related to aphid and Brassicacae host plant myrosinases

Studies on chemical ecology of host plant selection by herbivores were largely carried out to explain the diversity of secondary plant chemicals and host specialisation of herbivores. The glucosinolate – isothiocyanate system in Brassicaceae plants was considered as a plant defence and was known to influence the feeding behaviour of phytophagous insects such as the cabbage aphid, Brevicoryne brassicae. Purified myrosinases from the latter and from white mustard, Sinapis alba, were used to hydrolyse two glucosinolate compounds. Identification of the degradation products, mainly isothiocyanates, was performed by gas chromatography – mass spectrometry. While sinigrin hydrolysis gave identical isothiocyanates for insect and plant enzymes, B. brassicae myrosinase showed a particular activity toward sinalbin. A lost of an hydroxy group was observed for the two latter substrates related products when compared to the S. alba enzymatic specificity. Aphid and plant myrosinases have different properties and specificities which can be discussed in relation to a co-evolution approach.