Silicon fertilisation affects morphological and immune defences of an insect pest and enhances plant compensatory growth

Herbivorous insects have evolved various anti-predator defences, including morphological, behavioural, and immune defences, which can make biocontrol of herbivorous pests challenging. Silicon (Si) accumulation in plants is a potent physical defence against mandibulate insects. However, it remains uncertain how Si affects the anti-predator defences of insect herbivores and plant defences following herbivory. We grew the model grass, Brachypodium distachyon, hydroponically with (+Si) or without (–Si) Si and investigated the plant-mediated effects of Si on the anti-predator defences of the cotton bollworm, Helicoverpa armigera, integrating morphological (i.e. integument resistance and thickness), behavioural, and immune defences. We also examined the effects of Si on plant compensatory growth and leaf trichome production. Larval growth, leaf consumption, and integument resistance were lower when feeding on +Si plants compared to when feeding on –Si plants. Larval integument thickness, defensive behaviours, haemocyte density, and lysozyme-like activity in the haemolymph were unaffected by Si. Larvae fed on +Si plants had higher haemolymph phenoloxidase (PO) and total-PO activities than larvae fed on –Si plants, although this did not enhance the melanisation response of larvae. Furthermore, Si supplies increased plant compensation for herbivory and constitutive trichome production, whereas herbivory induced trichome production only on –Si plants. We provide the first evidence for plant-mediated effects of Si on anti-predator defences of an insect herbivore. We suggest that the lower integument resistance of larvae when feeding on Si-supplemented plants could contribute to their vulnerability to natural enemies and that high PO activity may impose fitness costs (e.g. delayed development).

Diversity in nonlinear responses to soil moisture shapes evolutionary constraints in Brachypodium

Water availability is perhaps the greatest environmental determinant of plant yield and fitness. However, our understanding of plant-water relations is limited because—like many studies of organism-environment interaction—it is primarily informed by experiments considering performance at two discrete levels—wet and dry—rather than as a continuously varying environmental gradient. Here, we used experimental and statistical methods based on function-valued traits to explore genetic variation in responses to a continuous soil moisture gradient in physiological and morphological traits among 10 genotypes across two species of the model grass genus Brachypodium. We find that most traits exhibit significant genetic variation and nonlinear responses to soil moisture variability. We also observe differences in the shape of these nonlinear responses between traits and genotypes. Emergent phenomena arise from this variation including changes in trait correlations and evolutionary constraints as a function of soil moisture. Our results point to the importance of considering diversity in nonlinear organism-environment relationships to understand plastic and evolutionary responses to changing climates.

Silicon Fertilisation Affects Morphological and Immune Defences of an Insect Pest and Enhances Plant Compensatory Growth

Insect herbivores employ various defences, including morphological, behavioural, and immune responses against their natural enemies (e.g., predators, parasitoids) which can make biocontrol of herbivorous pests challenging. Silicon (Si) accumulation in plants is a potent physical defence against herbivores. However, it remains uncertain how Si affects pest defences to their enemies and plant defences following herbivore attack. We grew the model grass, Brachypodium distachyon, hydroponically with (+Si) or without (–Si) Si and investigated the impacts of Si on morphological (integument resistance and thickness), behavioural (flee, headrear, thrash, and regurgitation), and immune defences of the cotton bollworm, Helicoverpa armigera. We further examined the effects of Si on plant compensatory growth and leaf trichome production. Larval growth, leaf consumption, and integument resistance were lower when feeding on +Si plants compared to when feeding on –Si plants. Larval integument thickness, defensive behaviours, hemocyte density and lysozyme-like activity in the hemolymph were unaffected by Si. Larvae fed on +Si plants had higher hemolymph phenoloxidase (PO) and total-PO activities than larvae fed on –Si plants, although this did not enhance larval melanisation response. Furthermore, Si supply increased plant compensatory growth and constitutive trichome production whereas herbivory induced trichome production only on –Si plants. We provide the first evidence that Si fertilisation affects insect defences in addition to reducing their growth and feeding. Lower integument resistance might enhance larval vulnerability to parasitoids and pathogens and higher PO activities could impose fitness costs (e.g., delayed development), potentially increasing overall insect susceptibility to enemies.

Tracking the Ancestry of Known and 'Ghost' Homeologous Subgenomes in Model Grass Brachypodium Polyploids

Unraveling the evolution of plant polyploids is a challenge when their diploid progenitor species are extinct or unknown or when their progenitor genome sequences are unavailable. The subgenome identification methods cannot adequately retrieve the homeologous genomes that are present in the allopolyploids if they do not take into account the potential existence of unknown progenitors. We addressed this challenge in the widely distributed dysploid grass genus Brachypodium, which is a model genus for temperate cereals and biofuel grasses. We used a transcriptome-based phylogeny and newly designed subgenome detection algorithms coupled with a comparative chromosome barcoding analysis. Our phylogenomic subgenome detection pipeline was validated in Triticum allopolyploids, which have known progenitor genomes, and was used to infer the identities of three extant and four ghost subgenomes in six Brachypodium polyploids (B. mexicanum, B. boissieri, B. retusum, B. phoenicoides, B. rupestre and B. hybridum), of which five contain undescribed homeologous subgenomes. The existence of the seven Brachypodium progenitor genomes in the polyploids was confirmed by their karyotypic barcode profiles. Our results demonstrate that our subgenome detection method is able to uncover the ancestral genomic components of both allo- and autopolyploids.

Metabolomic Variation Aligns with Two Geographically Distinct Subpopulations of Brachypodium Distachyon before and after Drought Stress

Brachypodium distachyon (Brachypodium) is a non-domesticated model grass that has been used to assess population level genomic variation. We have previously established a collection of 55 Brachypodium accessions that were sampled to reflect five different climatic regions of Turkey; designated 1a, 1c, 2, 3 and 4. Genomic and methylomic variation differentiated the collection into two subpopulations designated as coastal and central (respectively from regions 1a, 1c and the other from 2, 3 and 4) which were linked to environmental variables such as relative precipitation. Here, we assessed how far genomic variation would be reflected in the metabolomes and if this could be linked to an adaptive trait. Metabolites were extracted from eight-week-old seedlings from each accession and assessed using flow infusion high-resolution mass spectrometry (FIE-HRMS). Principal Component Analysis (PCA) of the derived metabolomes differentiated between samples from coastal and central subpopulations. The major sources of variation between seedling from the coastal and central subpopulations were identified. The central subpopulation was typified by significant increases in alanine, aspartate and glutamate metabolism and the tricarboxylic acid (TCA) cycle. Coastal subpopulation exhibited elevated levels of the auxin, indolacetic acid and rhamnose. The metabolomes of the seedling were also determined following the imposition of drought stress for seven days. The central subpopulation exhibited a metabolomic shift in response to drought, but no significant changes were seen in the coastal one. The drought responses in the central subpopulation were typified by changes in amino acids, increasing the glutamine that could be functioning as a stress signal. There were also changes in sugars that were likely to be an osmotic counter to drought, and changes in bioenergetic metabolism. These data indicate that genomic variation in our Turkish Brachypodium collection is largely reflected as distinctive metabolomes (“metabolotypes”) through which drought tolerance might be mediated.

Is silicon a double-edged sword against insect herbivores?

<p>In recent years, silicon (Si) has been increasingly linked to biotic stress management in plants including insect herbivory. The effectiveness of Si against chewing insects is now well recognized. Silicification of plant tissues makes them abrasive and tougher, reducing their masticability and digestibility to insect herbivores. This can cause mandibular wearing of chewers and affect their growth and feeding. Although there has been extensive research on the effects of Si on plant defences (i.e. antixenosis and antibiosis), it remains unclear how feeding on silicified plants affects insect defences to their natural enemies. Insect herbivores show morphological and behavioural defences when encountering predators and parasitoids. For example, lepidopteran larvae can regurgitate, twist the body, or even drop off the plants when attacked by natural enemies. Moreover, insects possess innate immunity (physiological defence) against the attackers, demonstrating cellular and humoral responses upon attack. Notably, there could be potential trade-offs between different defence and immunity traits. Given that feeding on Si-rich plants affects insect growth rates, this could impact their relative investment in different defences, thereby making insects more susceptible to their enemies. We are investigating the effects of Si on plant resistance and tolerance to herbivory and its cascading effects on insect defences to their enemies. We have been growing the model grass, <em>Brachypodium distachyon</em>, a high Si-accumulator, hydroponically with or without Si and examining the effects of Si against the global insect herbivore, <em>Helicoverpa armigera</em>. Our preliminary results suggest that Si supplementation enhances plant antixenotic and antibiotic traits and increases plant tolerance to herbivory. We are currently exploring insect defence and immunity traits when fed on silicified versus non-silicified plants. Our study would shed light on the impacts of Si on insects’ susceptibility to biocontrol agents and provide a better understanding of the effects of Si on insect-plant-natural enemy interactions.</p>

Anti-herbivore silicon defences in a model grass are greatest under Miocene levels of atmospheric CO2

<p>Silicon (Si) has important role in mitigating diverse biotic and abiotic stresses, mainly via silicification of plant tissues. However, environmental changes such as reduced atmospheric CO<sub>2</sub> concentrations may affect grass Si concentration which, in turn, can alter herbivore performance. Recently, we demonstrated that pre-industrial atmospheric CO<sub>2</sub> increased Si accumulation in a grass, however, how Si is deposited and whether this affects insect herbivores performance is unknown. We, therefore, investigated how pre-industrial (reduced) (rCO<sub>2</sub>, 200 ppm), ambient (aCO<sub>2</sub>, 410 ppm) and elevated (eCO<sub>2</sub>, 640 ppm) CO<sub>2</sub> concentrations and Si-treatments (Si+ or Si-) affect Si accumulation in the model grass, <em>Brachypodium distachyon</em> and its subsequent effects on the performance of the global insect, <em>Helicoverpa armigera</em>. rCO<sub>2</sub> caused Si concentrations to increase by 29% and 36% compared to aCO<sub>2</sub> and eCO<sub>2</sub>, respectively. Furthermore, increased Si accumulation under rCO<sub>2</sub> decreased herbivore relative growth rate (RGR) by 120% relative to eCO<sub>2, </sub>whereas<sub></sub> rCO<sub>2</sub> caused herbivore RGR to decrease by 26% compared to eCO<sub>2</sub>. Moreover, Si supplementation increased the density of trichomes, silica and prickle cells, and these changes in leaf surface morphology reduced larval feeding performance. The observed negative correlation between macrohair density, silica cell density, prickle cell density and herbivore RGR supports this. To our knowledge, this is the first study to demonstrate that increased Si accumulation under pre-industrial CO<sub>2</sub> environment reduced the performance of this generalist insect herbivore performance.<strong> </strong>Contrastingly, we found  reduced Si accumulation under higher CO<sub>2</sub>, which suggests  that some grasses might become more susceptible to insect herbivore under the projected climate change scenarios.</p>