Cyanogenesis, a Plant Defence Strategy against Herbivores

Abstract

Plants and phytophagous arthropods have coevolved in a long battle for survival. Plants respond to phytophagous feeders by producing a battery of antiherbivore chemical defences, while herbivores try to adapt to their hosts by attenuating the toxic effect of the defence compounds. Cyanogenic glucosides are a widespread group of defence chemicals that come from cyanogenic plants. Among the non-cyanogenic ones, the Brassicaceae family has evolved an alternative cyanogenic pathway to produce cyanohydrin as a way to expand defences. When a plant tissue is disrupted by an herbivore attack, cyanogenic substrates are brought into contact with degrading enzymes that cause the release of toxic hydrogen cyanide and derived carbonyl compounds. In this review, we focus our attention on the plant metabolic pathways linked to cyanogenesis to generate cyanide. It also highlights the role of cyanogenesis as a key defence mechanism of plants to fight against herbivore arthropods, and we discuss the potential of cyanogenesis-derived molecules as alternative strategies for pest control.

Keywords: cyanide; cyanide detoxification; cyanogenesis; cyanogenic glucosides; cyanohydrins; defence strategy; phytophagous insects and mites.

Figure 1

Enzymatic reaction for cyanohydrin and HCN interconversion catalysed by α-hydroxynitrile lyase.

Figure 2

Scheme of biosynthesis and catabolism of Cyanogenic Glucosides (CNglcs). Enzymes are indicated in red colour.

Figure 3

Scheme of different plant pathways involved in HCN production, dependent on the plant species. Enzymes are indicated in red. Yellow boxes indicate final products of each pathway, and blue boxes indicate common convergence points in the network of pathways. Adapted from Araniz et al. [20].

Figure 4

Cyanogenesis events in plant defence against pests. Insect and acari feeding on plants disrupt cells bringing in contact substrates (CNglcs: cyanogenic glucosides and 4-OH-ICN: 4-OH-Indolecarbonyl-nitrile derivatives) and enzymes (β-glucosidases and hydroxynitrile lyases). Consequently, HCN is released, and aldehyde- or ketone-compounds are produced to combat pests.