High silicon concentrations in grasses are linked to environmental conditions and not associated with C4 photosynthesis

Fluctuating climate and dietary innovation drove ratcheted evolution of proboscidean dental traits

Identification of the selective forces that shaped adaptive phenotypes generally relies on current habitat and function, but these may differ from the context in which adaptations arose. Moreover, the fixation of adaptive change in a fluctuating environment and the mechanisms of long-term trends are still poorly understood, as is the role of behaviour in triggering these processes. Time series of fossils can provide evidence on these questions, but examples of individual lineages with adequate fossil and proxy data over extended periods are rare. Here, we present new data on proboscidean dental evolution in East Africa over the past 26 million years, tracking temporal patterns of morphological change in relation to proxy evidence of diet, vegetation and climate (aridity). We show that behavioural experimentation in diet is correlated with environmental context, and that major adaptive change in dental traits followed the changes in diet and environment but only after acquisition of functional innovations in the masticatory system. We partition traits by selective agent, showing that the acquisition of high, multiridged molars was primarily a response to an increase in open, arid environments with high dust accumulation, whereas enamel folding was more associated with the amount of grass in the diet. We further show that long-term trends in these features proceeded in a ratchet-like mode, alternating between directional change at times of high selective pressure and stasis when the selective regime reversed. This provides an explanation for morphology adapted to more extreme conditions than current usage (Liem's Paradox). Our study illustrates how, in fossil series with adequate stratigraphic control and proxy data, environmental and behavioural factors can be mapped on to time series of morphological change, illuminating the mode of acquisition of an adaptive complex.

Why do plants silicify?

Despite seminal papers that stress the significance of silicon (Si) in plant biology and ecology, most studies focus on manipulations of Si supply and mitigation of stresses. The ecological significance of Si varies with different levels of biological organization, and remains hard to capture. We show that the costs of Si accumulation are greater than is currently acknowledged, and discuss potential links between Si and fitness components (growth, survival, reproduction), environment, and ecosystem functioning. We suggest that Si is more important in trait-based ecology than is currently recognized. Si potentially plays a significant role in many aspects of plant ecology, but knowledge gaps prevent us from understanding its possible contribution to the success of some clades and the expansion of specific biomes.

A review of the extraction methods and advanced applications of lignin-silica hybrids derived from natural sources

The global trend of increasing energy demand along the large volume of wastewater generated annually from the paper pulping and cellulose production industries are considered as serious dilemma that may need to be solved within these current decades. Within this discipline, lignin, silica or lignin-silica hybrids attained from biomass material have been considered as prospective candidates for the synthesis of advanced materials. In this study, the roles and linking mechanism between lignin and silica in plants were studied and evaluated. The effects of the extraction method on the quality of the obtained material were summarized to show that depending on the biomass feedstocks, different retrieval processes should be considered. The combination of alkaline treatment and acidic pH adjustment is proposed as an effective method to recover lignin-silica with high applicability for various types of raw materials. From considerations of the advanced applications of lignin and silica materials in environmental remediation, electronic devices and rubber fillers future valorizations hold potential in conductive materials and electrochemistry. Along with further studies, this research could not only contribute to the development of zero-waste manufacturing processes but also propose a solution for the fully exploiting of by-products from agricultural production.

Climatic Drivers of Silicon Accumulation in a Model Grass Operate in Low- but Not High-Silicon Soils

Grasses are hyper-accumulators of silicon (Si), which is known to alleviate diverse environmental stresses, prompting speculation that Si accumulation evolved in response to unfavourable climatic conditions, including seasonally arid environments. We conducted a common garden experiment using 57 accessions of the model grass Brachypodium distachyon, sourced from different Mediterranean locations, to test relationships between Si accumulation and 19 bioclimatic variables. Plants were grown in soil with either low or high (Si supplemented) levels of bioavailable Si. Si accumulation was negatively correlated with temperature variables (annual mean diurnal temperature range, temperature seasonality, annual temperature range) and precipitation seasonality. Si accumulation was positively correlated with precipitation variables (annual precipitation, precipitation of the driest month and quarter, and precipitation of the warmest quarter). These relationships, however, were only observed in low-Si soils and not in Si-supplemented soils. Our hypothesis that accessions of B. distachyon from seasonally arid conditions have higher Si accumulation was not supported. On the contrary, higher temperatures and lower precipitation regimes were associated with lower Si accumulation. These relationships were decoupled in high-Si soils. These exploratory results suggest that geographical origin and prevailing climatic conditions may play a role in predicting patterns of Si accumulation in grasses.

Silicon mobilisation by root-released carboxylates

Plants have evolved numerous strategies to acquire poorly available nutrients from soil, including the release of carboxylates from their roots. Silicon (Si) release from mineral dissolution increases in the presence of chelating substances, and recent evidence shows that leaf [Si] increases markedly in old phosphorus (P)-depleted soils, where many species exhibit carboxylate-releasing strategies, compared with younger P-richer soils. Here, we propose that root-released carboxylates, and more generally rhizosphere processes, play an overlooked role in plant Si accumulation by increasing soil Si mobilisation from minerals. We suggest that Si mobilisation is costly in terms of carbon but becomes cheaper if those costs are already met to acquire poorly available P. Uptake of the mobilised Si by roots will then depend on whether they express Si transporters.

Plant-herbivore interactions: Combined effect of groundwater level, root vole grazing, and sedge silicification

Accumulation of silica (Si) by plants can be driven by (1) herbivory pressure (and therefore plant-herbivore interactions), (2) geohydrological cycles, or (3) a combination of (1) and (2), with (1-3) possibly affecting Si concentration with a 1-year delay.To identify the relative significance of (1-3), we analyzed the concentration of Si in fibrous tussock sedge (Carex appropinquata), the population density of the root vole (Microtus oeconomus), and the groundwater level, over 11 years.The largest influence of autumn Si concentration in leaves (Sileaf) was on the level of the current-year groundwater table, which was positive and accounted for 13.3% of its variance. The previous year's vole population density was weakly positively correlated with Sileaf, and it alone explained 9.5% of its variance.The only variable found to have a positive, significant effect on autumn Si concentration in rhizomes (Sirhiz) was the current-year spring water level, explaining as much as 60.9% of its variance.We conclude that the changes in Si concentration in fibrous tussock sedge are predominantly driven by hydrology, with vole population dynamics being secondary.Our results provide only partial support for the existence of plant-herbivore interactions, as we did not detect the significant effects of Si tussock concentration on the vole density dynamics. This was mainly due to the low level of silicification of sedges, which was insufficient to impinge herbivores.Future studies on plant-herbivore interactions should therefore aim at disentangling whether anti-herbivore protection is dependent on threshold values of herbivore population dynamics. Furthermore, studies on Si accumulation should focus on the effect of water-mediated Si availability.

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

Silicon (Si) has an important role in mitigating diverse biotic and abiotic stresses in plants, mainly via the silicification of plant tissues. Environmental changes such as atmospheric CO₂ concentrations may affect grass Si concentrations which, in turn, can alter herbivore performance. We recently demonstrated that pre-industrial atmospheric CO₂ increased Si accumulation in Brachypodium distachyon grass, yet the patterns of Si deposition in leaves and whether this affects insect herbivore performance remains unknown. Moreover, it is unclear whether CO₂ -driven changes in Si accumulation are linked to changes in gas exchange (e.g. transpiration rates). We therefore investigated how pre-industrial (reduced; rCO₂ , 200 ppm), ambient (aCO₂ , 410 ppm) and elevated (eCO₂ , 640 ppm) CO₂ concentrations, in combination with Si-treatment (Si+ or Si-), affected Si accumulation in B. distachyon and its subsequent effect on the performance of the global insect pest, Helicoverpa armigera. rCO₂ increased Si concentrations by 29% and 36% compared to aCO₂ and eCO₂ respectively. These changes were not related to observed changes in gas exchange under different CO₂ regimes, however. The increased Si accumulation under rCO₂ decreased herbivore relative growth rate (RGR) by 120% relative to eCO_2, whereas rCO₂ caused herbivore RGR to decrease by 26% compared to eCO₂ . Si supplementation also increased the density of macrohairs, silica and prickle cells, which was associated with reduced herbivore performance. There was a negative correlation among macrohair density, silica cell density, prickle cell density and herbivore RGR under rCO₂ suggesting that these changes in leaf surface morphology were linked to reduced performance under this CO₂ regime. To our knowledge, this is the first study to demonstrate that increased Si accumulation under pre-industrial CO₂ reduces insect herbivore performance. Contrastingly, we found reduced Si accumulation under higher CO₂ , which suggests that some grasses may become more susceptible to insect herbivores under projected climate change scenarios.

Silicon in the Soil-Plant Continuum: Intricate Feedback Mechanisms within Ecosystems

Plants' ability to take up silicon from the soil, accumulate it within their tissues and then reincorporate it into the soil through litter creates an intricate network of feedback mechanisms in ecosystems. Here, we provide a concise review of silicon's roles in soil chemistry and physics and in plant physiology and ecology, focusing on the processes that form these feedback mechanisms. Through this review and analysis, we demonstrate how this feedback network drives ecosystem processes and affects ecosystem functioning. Consequently, we show that Si uptake and accumulation by plants is involved in several ecosystem services like soil appropriation, biomass supply, and carbon sequestration. Considering the demand for food of an increasing global population and the challenges of climate change, a detailed understanding of the underlying processes of these ecosystem services is of prime importance. Silicon and its role in ecosystem functioning and services thus should be the main focus of future research.

Contrasting effects of Miocene and Anthropocene levels of atmospheric CO2 on silicon accumulation in a model grass

Grasses are hyper-accumulators of silicon (Si), which they acquire from the soil and deposit in tissues to resist environmental stresses. Given the high metabolic costs of herbivore defensive chemicals and structural constituents (e.g. cellulose), grasses may substitute Si for these components when carbon is limited. Indeed, high Si uptake grasses evolved in the Miocene when atmospheric CO₂ concentration was much lower than present levels. It is, however, unknown how pre-industrial CO₂ concentrations affect Si accumulation in grasses. Using Brachypodium distachyon, we hydroponically manipulated Si-supply (0.0, 0.5, 1, 1.5, 2 mM) and grew plants under Miocene (200 ppm) and Anthropocene levels of CO₂ comprising ambient (410 ppm) and elevated (640 ppm) CO₂ concentrations. We showed that regardless of Si treatments, the Miocene CO₂ levels increased foliar Si concentrations by 47% and 56% relative to plants grown under ambient and elevated CO₂, respectively. This is owing to higher accumulation overall, but also the reallocation of Si from the roots into the shoots. Our results suggest that grasses may accumulate high Si concentrations in foliage when carbon is less available (i.e. pre-industrial CO₂ levels) but this is likely to decline under future climate change scenarios, potentially leaving grasses more susceptible to environmental stresses.