Sequestration and activation of plant toxins protect the western corn rootworm from enemies at multiple trophic levels

Abstract

Highly adapted herbivores can phenocopy two-component systems by stabilizing, sequestering and reactivating plant toxins. However, whether these traits protect herbivores against their enemies is poorly understood. We demonstrate that the western corn rootworm Diabrotica virgifera virgifera, the most damaging maize pest on the planet, specifically accumulates the root-derived benzoxazinoid glucosides HDMBOA-Glc and MBOA-Glc. MBOA-Glc is produced by D. virgifera through stabilization of the benzoxazinoid breakdown product MBOA by N-glycosylation. The larvae can hydrolyze HDMBOA-Glc, but not MBOA-Glc, to produce toxic MBOA upon predator attack. Accumulation of benzoxazinoids renders D. virgifera highly resistant to nematodes which inject and feed on entomopathogenic symbiotic bacteria. While HDMBOA-Glc and MBOA reduce the growth and infectivity of both the nematodes and the bacteria, MBOA-Glc repels infective juvenile nematodes. Our results illustrate how herbivores combine stabilized and reactivated plant toxins to defend themselves against a deadly symbiosis between the third and the fourth trophic level enemies.

Plant defense resistance in natural enemies of a specialist insect herbivore

“…H. bacteriophora has a broad host range, and the strains may also have infected other root herbivores occurring in maize fields, including wireworms and other rootworm species. Benzoxazinoids and their breakdown products can be found in the midgut of a wide range of insect herbivores (31, 32), but the western corn rootworm is the only herbivore known to selectively accumulate benzoxazinoids in its hemolymph (28). Nematode strains from other parts of the world never encountered the western corn rootworm, as they came from regions where the rootworm is not present, or they were isolated before the western corn rootworm invaded these regions (table S1).…”

“…In contrast to the western corn rootworm, the banded cucumber beetle feeds on many different plant species apart from maize and does not sequester benzoxazinoids (Fig. 1B and S1) (28). The banded cucumber beetle occurs mainly in Central America, Mexico and the Southern US, outside of our collection range.…”

“…They are used as biological control agents against many different root pests, including the western corn rootworm (29, 30). Benzoxazinoid sequestration by the western corn rootworm reduces the capacity of a commercial H. bacteriophora strain to infect and kill the herbivore, suggesting that the corn rootworm co-opts these plant defense metabolites for self-protection against entomopathogenic nematodes (28). Using natural variation and forward evolution, we investigated whether adapted H. bacteriophora nematodes are able to overcome this defenses strategy of the western corn rootworm, and whether their capacity to resist benzoxazinoids is associated with increased infectivity towards the western corn rootworm.…”

Resistance and Tolerance to Root Herbivory in Maize were Mediated by Domestication, Spread, and Breeding

“…500 YBP) and northern (ca. 180 YBP) USA statesmay have led to an evolutionary compromise, with maize gaining tolerance and WCR becoming a specialist (Robert et al, 2012a;Lombaert et al, 2017;Robert et al, 2017).…”

“…WCR prefers maize over other hosts, which may be due to the crop plant's comparatively weakened resistance against herbivory and greater nutritional value (de Lange et al, 2014;Bernal and Medina, 2018). Additionally, maize tolerance to WCR may have evolved as the crop faced less competition and non-native herbivory after its spread (Buckler and Stevens, 2006;Hahn and Maron, 2016;Robert et al, 2017). Currently, WCR is found in northern Mexico, USA, and Europe (Branson and Krysan, 1981;Gerdes et al, 1993;Gray et al, 2009).…”