Short-term resistance that persists: Rapidly induced silicon anti-herbivore defence affects carbon-based plant defences

1. Silicon (Si) is known to alleviate diverse biotic and abiotic stresses
including insect herbivory. Si accumulation in plants, notably the
Poaceae, can be induced through stimulation of the jasmonic acid (JA)
pathway (associated with chewing herbivores). Nevertheless, the temporal
dynamics of Si accumulation as a defence response and its consequential
effects on carbon-based defences (e.g. phenolics), particularly in the
short-term, remain unclear. 2. The model grass Brachypodium distachyon was
grown in a hydroponic solution where half the plants were supplemented
with 2 mM potassium silicate and half had no Si supplied. Plants were
treated with methyl jasmonate (MeJA) as a form of standardised simulated
herbivory. We measured Si accumulation, the phytohormones JA and salicylic
acid (SA), and carbon-based defences over 24 hours to determine the
temporal dynamics of Si accumulation and the interplay between Si,
simulated herbivory and plant defence machinery. 3. MeJA-induced Si
accumulation occurred as early as 6 hours after treatment via increased JA
concentrations. Si supplementation decreased SA concentrations, which
could have implications on additional downstream defences. We show a
trade-off between Si and phenolics in untreated plants, but this
relationship was weakened upon MeJA treatment. Further, this trade-off did
not apply to phenolic precursor compounds such as phenylalanine. 4. We
provide evidence for rapidly induced Si accumulation associated with
herbivory, and that increased Si accumulation impacts on phytohormones and
carbon-based defences over a 24-hour period. Additionally, herbivory
modifies the relationship between Si- and carbon-based defences. Thus, in
addition to its well-documented role as a long-term defence against
herbivores, we demonstrate that, over short-term temporal scales, Si
accumulation responds to herbivore signals and impacts on plant defence
machinery.