Glucosinolates in oilseed rape: secondary metabolites that influence interactions with herbivores and their natural enemies

Glucosinolates are sulphur-containing secondary metabolites characteristic of Brassicaceous plants. Glucosinolate breakdown products, which include isothiocyanates, are released following tissue damage when hydrolytic enzymes act on them. The isothiocyanates have toxic effects on generalist herbivores when they attempt to feed on oilseed rape, Brassica napus, and also function as repellents. However, specialist herbivores such as Brevicoryne brassicae aphids, flea beetles, Psylliodes chrysocephala and the Lepidopteran pest, Pieris rapae, are adapted to the presence of glucosinolates and thrive on plants containing them. They may do this by avoiding tissue damage to prevent the formation of isothiocyanates or by metabolising or tolerating glucosinolates. For many specialist herbivores, the isothiocyanates function as attractants and glucosinolates can even be sequestered for defence against predatory insects. Thus, these herbivores have evolved resistance to host-plant secondary metabolites and this type of evolutionary history may have given some insects an enhanced ability to adapt to xenobiotics. In an agricultural context, this may make pests better able to evolve resistance to artificially applied pesticides. The effect of increased glucosinolate content in making oilseed rape cultivars more susceptible to specialist pests was highlighted in a seminal article in the Annals of Applied Biology in 1995. This review of the literature considers developments in this area since then.

硫代葡萄糖苷是十字花科植物特有的含硫次生代谢产物。当水解酶作用于硫代葡萄糖苷时，其分解产物（包括异硫氰酸酯）会在组织受损后释放出来。当广食性草食动物试图取食油菜（Brassica napus）时，异硫氰酸酯对它们具有毒性作用，并且还起到驱避作用。然而，专食性草食动物，如甘蓝蚜（Brevicoryne brassicae）、跳甲、油菜蓝跳甲（Psylliodes chrysocephala）以及鳞翅目害虫菜粉蝶（Pieris rapae），适应了硫代葡萄糖苷的存在，并在含有硫代葡萄糖苷的植物上茁壮成长。它们可能通过避免组织损伤以防止异硫氰酸酯的形成，或者通过代谢或耐受硫代葡萄糖苷来做到这一点。对于许多专食性草食动物来说，异硫氰酸酯起到引诱剂的作用，硫代葡萄糖苷甚至可以被隔离储存用于防御捕食性昆虫。因此，这些草食动物已经进化出对寄主植物次生代谢产物的抗性，这种进化历程可能使一些昆虫增强了适应外源物质的能力。在农业环境中，这可能使害虫更有能力进化出对人工施用的农药的抗性。1995年发表在《应用生物学年鉴》上的一篇重要文章强调了硫代葡萄糖苷含量增加使油菜品种更容易受到专食性害虫侵害的影响。本文献综述考虑了自那以后该领域的发展情况。