Down to earth defence : how mutualistic fungi augment silicon-based defence against insect pests

Abstract

Grasses typically lack the rich array of defensive secondary metabolites found in other plant families, and thus rely on a rapid turnover of (affected) organs, physical defences (tissue hardness) and multi-symbiotic associations for defence against insect herbivores. In particular, silicon (Si), a major component of soil-plant systems, is a structural defence that can account for as much as 10% of grass dry mass. Further, many temperate grasses associate with symbiotic fungi, such as endophytes (Clavicipitaceae: Epichlo+½) and arbuscular mycorrhizal (AM) fungi (Glomeromycotina), in shoots and roots, respectively. Endophytes and AM fungi often affect insect herbivores by influencing the overall chemistry of their host plant (e.g. defensive-alkaloid production by endophytes). However, little is known about the interplay between structural, Si- based defences and chemical, fungi-based defences. Recent evidence suggests that both AM fungi and endophytes facilitate Si uptake; and Si supply in combination with AM fungi has been found to diminish the growth of a soil-dwelling herbivore pests. However, it is unknown whether interactions between Si, endophytes, and AM fungi further impact aboveground insect herbivores; or whether endophytes and AM fungi interact in this regard. For instance, while Si is primarily a physical defence, it can also affect secondary metabolism; Si supply might therefore modulate the production of Epichlo+½-alkaloids, although this remains untested. This PhD thesis addresses these research gaps, and extends previous ideas by evaluating how interactive effects of Si and mutualistic fungi (AM and Epichlo+½) may interact to either synergistically enhance (or antagonistically compromise) plant defences against aboveground insect stressors. Results of this PhD research provide novel insights regarding the function of interactive defence mechanisms naturally available to grasses. We prove that, for grasses, Si is a more effective defence than endophytes against folivore (e.g. caterpillars), whereas endophyte- alkaloids are more effective against piercing-sucking (e.g. aphids) herbivores. Moreover, although endophyte-alkaloid concentrations, for the most part, remained unaffected by Si supply, AM fungi presence negatively affected their production. We propose that the nature of the protective interaction between Si and mutualist fungi is rather complex, with multiple interactions between different defence mechanisms (e.g. Si or symbiont) and modulated in turn by the presence / absence of stressors (e.g. caterpillar or aphid) and quite specific depending on the genetics of the plant species and symbiont(s) involved. Our results allow envisaging that increases in Si uptake mediated by mutualists could potentially make plants more resilient to a wider diversity of biotic and abiotic stressors, although this avenue remains to be investigated.

Date of Award	2021
Original language	English