Reciprocal Effects of Silicon Supply and Endophytes on Silicon Accumulation and Epichloë Colonization in Grasses

Abstract

Cool season grasses associate asymptomatically with foliar Epichloë endophytic fungi in a symbiosis where Epichloë spp. protects the plant from a number of biotic and abiotic stresses. Furthermore, many grass species can accumulate large quantities of silicon (Si), which also alleviates a similar range of stresses. While Epichloë endophytes may improve uptake of minerals and nutrients, their impact on Si is largely unknown. Likewise, the effect of Si availability on Epichloë colonization remains untested. To assess the bidirectional relationship, we grew tall fescue (Festuca arundinacea) and perennial ryegrass (Lolium perenne) hydroponically with or without Si. Grasses were associated with five different Epichloë endophyte strains [tall fescue: AR584 or wild type (WT); perennial ryegrass: AR37, AR1, or WT] or as Epichloë-free controls. Reciprocally beneficial effects were observed for tall fescue associations. Specifically, Epichloë presence increased Si concentration in the foliage of tall fescue by at least 31%, regardless of endophyte strain. In perennial ryegrass, an increase in foliar Si was observed only for plants associated with the AR37. Epichloë promotion of Si was (i) independent of responses in plant growth, and (ii) positively correlated with endophyte colonization, which lends support to an endophyte effect independent of their impacts on root growth. Moreover, Epichloë colonization in tall fescue increased by more than 60% in the presence of silicon; however, this was not observed in perennial ryegrass. The reciprocal benefits of Epichloë-endophytes and foliar Si accumulation reported here, especially for tall fescue, might further increase grass tolerance to stress.

Interactions between silicon and alkaloid defences in endophyte‐infected grasses and the consequences for a folivore

“…More recently, using hydroponics to completely exclude Si, reciprocal beneficial effects between Si uptake and Epichloë-endophyte colonisation in tall fescue F. arundinacea were reported. Specifically, endophytes increased foliar Si concentrations by at least 31% in tall fescue, when compared with endophyte-free plants, while Epichloë colonisation increased by more than 60% in the presence of Si supply (Cibils-Stewart et al, 2020).…”

“…For this, we determined (a) how Si supply, Epichloë-endophyte presence and herbivory by the global pest Helicoverpa armigera altered concentrations of foliar Si and endophyte-alkaloids; and (b) whether Si supply and Epichloëendophyte presence synergistically, additively or antagonistically altered grass resistance to this pest. In a previous study, Cibils-Stewart et al (2020) showed that these responses might be endophytestrain/host-species specific. Therefore, two economically important pasture grasses, and several endophyte strains were assessed.…”

“…active Si uptake) or internal morphology (i.e. increased vascular bundles); these could act singly or in combination, and are discussed in detail in Cibils-Stewart et al (2020).…”

“…Therefore, we propose that with plentiful Si supply, grass-endophyte associations enable an optimised defensive response, whereby grasses achieve higher resistance to folivore-herbivores using 'low cost' Si rather than endophyte-derived alkaloids . We anticipate that these effects will differ in magnitude depending on Si supplementation treatment and endophyte-strain/hostspecies utilised (Cibils-Stewart et al, 2020). Moreover, based on previous research, we predict that endophytes and Si will have negligible impacts on plant growth, although some aspects of plant primary chemistry (e.g.…”

“…antiherbivore defences, Epichloë-endophytes, Helicoverpa armigera, perennial ryegrass (Lolium perenne), tall fescue (Festuca arundinacea) Despite Si and Epichloë-endophytes being ubiquitously present in temperate grasses and conferring antiherbivore properties, their impacts have been mostly studied separately. In fact, very few studies have addressed how they interact (Cibils-Stewart et al, 2020;Huitu et al, 2014); and none have investigated the consequences of such interactions for insect herbivores. To date, one field study reported that meadow fescue Schedonorus pratensis associated with Epichloë-symbionts had a > 18% increase in foliar Si concentration compared to their endophyte-free counterpart (Huitu et al, 2014).…”