Interactions between beneficial fungi and plant silicon: A review

Silicon (Si) accumulation in plants often plays an important functional role by alleviating the adverse impacts of the plant antagonists (e.g. invertebrate herbivores). Mutualistic associations with beneficial fungi, such as below-ground arbuscular mycorrhizal (AM) fungi and above-ground Epichloë endophytes, play similar roles for plant protection. Evidence suggests that beneficial fungi significantly influence Si uptake, often increasing Si in plant tissues while Si supplementation may in turn enhance fungal colonisation. However, this two-way association remains poorly understood due to the fragmented nature of existing literature. We assess the current state of knowledge on interactions between Si and beneficial fungi by extracting and comparing effect sizes (Cohen's d) from peer-reviewed literature for various reported relationships. We aim to understand the (i) interactions between Si and beneficial fungi, (ii) potential mechanisms underlying their interactions and (iii) role of Si-fungal interactions in enhancing plant resilience to environmental stresses. The Glomeraceae AM family was the most frequently investigated and was generally associated with increased Si accumulation in plants (overall effect size, d = 0.7). Additionally, Si supplementation was commonly reported to have a positive effect, with a modest overall increase in AM fungal colonisation (overall effect size, d = 0.3). These observations are often associated with changes in plant growth, morphology, physiology and chemistry by either Si or fungi. The literature covering Epichloë endophytes was limited and showed highly variable, strain-specific trends. Interactions between Si supply and beneficial fungi have mostly been studied in the context of abiotic stresses with limited focus on biotic antagonists (only nine studies). However, the extent to which Si and beneficial fungi increased plant resistance to stresses suggests that their interactions could play a significant role in plant community dynamics, potentially disadvantaging plant species that lack these traits. Synthesis. This review aims to help predict how beneficial fungi may interact with Si supply to enhance resistance to environmental stresses. Overall, Si supply and beneficial fungi seem to work ‘synergistically’ especially when Si availability is low, or when the plant species is a low Si accumulator. Finally, we identified potential avenues for future research based on existing knowledge and key knowledge gaps.