The Role of Silicon in Antiherbivore Phytohormonal Signalling

Abstract

The role of plant silicon (Si) in the alleviation of abiotic and biotic stress is now widely recognised and researched. Amongst the biotic stresses, Si is known to increase resistance to herbivores through biomechanical and chemical mechanisms, although the latter are indirect and remain poorly characterised. Chemical defences are principally regulated by several antiherbivore phytohormones. The jasmonic acid (JA) signalling pathway is particularly important and has been linked to Si supplementation, albeit with some contradictory findings. In this Perspectives article, we summarise existing knowledge of how Si affects JA in the context of herbivory and present a conceptual model for the interactions between Si and JA signalling in wounded plants. Further, we use novel information from the model grass Brachypodium distachyon to underpin aspects of this model. We show that Si reduces JA concentrations in plants subjected to chemical induction (methyl jasmonate) and herbivory (Helicoverpa armigera) by 34% and 32%, respectively. Moreover, +Si plants had 13% more leaf macrohairs than −Si plants. From this study and previous work, our model proposes that Si acts as a physical stimulus in the plant, which causes a small, transient increase in JA. When +Si plants are subsequently attacked by herbivores, they potentially show a faster induction of JA due to this priming. +Si plants that have already invested in biomechanical defences (e.g. macrohairs), however, have less utility for JA-induced defences and show lower levels of JA induction overall.

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

“…Consequently, the resulting spike in JA mediated by Epichloë endophyte symbiosis establishment, may lead to enhanced Si-uptake. However, JA-mediated increases in Si, while significant, have been shown to be relatively small, e.g., 12 and 10% as reported in Hall et al (2020Hall et al ( , 2019, compared to > 30% increased Si concentration found in this study. This suggests that the stimulation of JA pathway alone is unlikely to be the sole mechanism for increased foliar Si observed in endophyte-symbiotic plants.…”

“…This induction has been reported for obligate symbionts including AM-fungi (Jung et al, 2012), rhizobial bacteria (Dean et al, 2014), as well as, Epichloë endophytes (Bastias et al, 2017). Triggering the JA-pathway, either using chemical stimulation (methyl-jasmonate) or authentic herbivory, has been shown to promote Si-uptake (Ye et al, 2013;Kim et al, 2014;Hall et al, 2019Hall et al, , 2020. Consequently, the resulting spike in JA mediated by Epichloë endophyte symbiosis establishment, may lead to enhanced Si-uptake.…”

“…Stresses alleviated by Si include herbivory (Hartley et al, 2015;Reynolds et al, 2016), pathogens (Vivancos et al, 2015;Rasoolizadeh et al, 2018), low temperatures (Zhang et al, 2008), UV radiation (Tripathi et al, 2017), and nutrient deficiency (Miao et al, 2010). The mechanisms underpinning this stress alleviation remain controversial but silicification of plant tissues may provide physical resistance (Coskun et al, 2019) or indirectly cause changes in plant chemistry (Hall et al, 2019). Moreover, Si supplementation has also been reported to increase plant growth (Frew et al, 2018) and enhance uptake of major essential nutrients (Eneji et al, 2008), although these effects mostly take place during stressful conditions (Coskun et al, 2019).…”

Is it time to include legumes in plant silicon research?

“…For herbivore defence, at least, it is known that biophysical traits can hinder herbivory via abrasion, toughness and reductions in resource acquisition (Alhousari & Greger, 2018; Massey & Hartley, 2006, 2009; Reynolds, Padula, Zeng, & Gurr, 2016). Silicon may also influence the activities of other biochemical defences following herbivory, for example by affecting phytohormonal pathways (Hall, Waterman, Vandegeer, Hartley, & Johnson, 2019) and herbivore‐induced plant volatiles (Reynolds et al, 2016). Silicon has also been shown to affect plant traits including growth, yield and physiology (Detmann et al, 2012; Frew, Weston, Reynolds, & Gurr, 2018).…”

Combined effects of soil silicon and drought stress on host plant chemical and ultrastructural quality for leaf‐chewing and sap‐sucking insects

“…However, Si can be especially useful when plants are under environmental stresses, and the benefits of Si fertilization may be minimal unless the plant is under some form of stress (Epstein, ; Fauteux, Rémus‐Borel, Menzies, & Bélanger, ). Beneficial effects of Si fertilization on drought‐stressed plants primarily result from Si mediation of many alterations in plant biochemistry and physiology (Haynes, ; Rizwan et al, ; Sacala, ; Zhu & Gong, ), although the underlying mechanisms that account for the manifold effects of Si in plant biology remain undefined (Frew, Weston, Reynolds, & Gurr, ; Hall, Waterman, Vandegeer, Hartley, & Johnson, ). Because Si ultimately increases photosynthesis, plant growth, biomass, and crop yield and quality during drought conditions (Rizwan et al, ), the effects of drought stress (Blum, ) are mitigated by Si at the whole plant level and crop production.…”