Get Tough, Get Toxic, or Get a Bodyguard: Identifying Candidate Traits Conferring Belowground Resistance to Herbivores in Grasses

Abstract

Grasses (Poaceae) are the fifth-largest plant family by species and their uses for crops, forage, fiber, and fuel make them the most economically important. In grasslands, which broadly-defined cover 40% of the Earth's terrestrial surface outside of Greenland and Antarctica, 40–60% of net primary productivity and 70–98% of invertebrate biomass occurs belowground, providing extensive scope for interactions between roots and rhizosphere invertebrates. Grasses invest 50–70% of fixed carbon into root construction, which suggests roots are high value tissues that should be defended from herbivores, but we know relatively little about such defenses. In this article, we identify candidate grass root defenses, including physical (tough) and chemical (toxic) resistance traits, together with indirect defenses involving recruitment of root herbivores' natural enemies. We draw on relevant literature to establish whether these defenses are present in grasses, and specifically in grass roots, and which herbivores of grasses are affected by these defenses. Physical defenses could include structural macro-molecules such as lignin, cellulose, suberin, and callose in addition to silica and calcium oxalate. Root hairs and rhizosheaths, a structural adaptation unique to grasses, might also play defensive roles. To date, only lignin and silica have been shown to negatively affect root herbivores. In terms of chemical resistance traits, nitrate, oxalic acid, terpenoids, alkaloids, amino acids, cyanogenic glycosides, benzoxazinoids, phenolics, and proteinase inhibitors have the potential to negatively affect grass root herbivores. Several good examples demonstrate the existence of indirect defenses in grass roots, including maize, which can recruit entomopathogenic nematodes (EPNs) via emission of (E)-β-caryophyllene, and similar defenses are likely to be common. In producing this review, we aimed to equip researchers with candidate root defenses for further research.

When resistance is futile, tolerate instead: silicon promotes plant compensatory growth when attacked by above- and belowground herbivores

“…This study also contributes information to another knowledge gap in our understanding of how Si defences operate against root herbivores [9,27]. There is substantial Si deposition in the roots (see examples in [28]) and evidence for Si-based defence against root herbivores [29][30][31]. More importantly, the study illustrates that Si may also promote pest tolerance in a key crop which is one of three species (wheat, maize and rice) to provide 42% of the world's calorie intake [32].…”

Elevated carbon dioxide and warming impact silicon and phenolic‐based defences differently in native and exotic grasses

“…Canegrub performance was even positively correlated with phenolic concentrations in sugarcane because of the trade‐off with the more effective silicon defences (Frew et al., ). There are potential examples of grass phenolics being effective against both foliar and root herbivores (Moore & Johnson, ; Vicari & Bazely, ), however, so increases in phenolics might compensate for any overall decreases in silicon uptake. We also found negative correlations between plant dry mass (and implicitly growth rates) and both defences.…”

Plant–Soil Feedback: Bridging Natural and Agricultural Sciences

“…Agricultural practices using pesticides and synthetic fertilizers alter the balance between beneficial and pathogenic rhizosphere organisms with consequences for plant defense [38]. From studies in natural ecosystems, we can learn how plant trait-based approaches might be used to improve crop resistance to soil pests and diseases [39,16]. For example, traits which influence the phenolic profile of roots are important predictors of defense against root herbivores [39,40].…”