Silicon alleviates the impairments of iron toxicity on the rice photosynthetic performance via alterations in leaf diffusive conductance with minimal impacts on carbon metabolism

Physiological and molecular mechanisms of silicon and potassium on mitigating iron-toxicity stress in Panax ginseng

Iron plays a crucial role in plant chlorophyll synthesis, respiration, and plant growth. However, excessive iron content can contribute to ginseng poisoning. We previously discovered that the application of silicon (Si) and potassium (K) can mitigate the iron toxicity on ginseng. To elucidate the molecular mechanism of how Si and K alleviate iron toxicity stress in ginseng. We investigated the physiological and transcriptional effects of exogenous Si and K on Panax ginseng. The results suggested that the leaves of ginseng with Si and K addition under iron stress increased antioxidant enzyme activity or secondary metabolite content, such as phenylalanine amino-lyase, polyphenol oxidase, ascorbate peroxidase, total phenols and lignin, by 6.21%-25.94%, 30.12%-309.19%, 32.26%-38.82%, 7.81%-23.66%, and 4.68%-48.42%, respectively. Moreover, Si and K increased the expression of differentially expressed genes (DEGs) associated with resistance to both biotic and abiotic stress, including WRKY (WRKY1, WRKY5, and WRKY65), bHLH (bHLH35, bHLH66, bHLH128, and bHLH149), EREBP, ERF10 and ZIP. Additionally, the amount of DEGs of ginseng by Si and K addition was enriched in metabolic processes, single-organism process pathways, signal transduction, metabolism, synthesis and disease resistance. In conclusion, the utilization of Si and K can potentially reduce the accumulation of iron in ginseng, regulate the expression of iron tolerance genes, and enhance the antioxidant enzyme activity and secondary metabolite production in both leaves and roots, thus alleviating the iron toxicity stress in ginseng.

Comprehensive analysis of silicon impact on defense and metabolic responses in rice exposed to herbivory stress

Silicon (Si) uptake is generally beneficial for plants that need protection from insect herbivores. In pursue of mechanisms involved in Si-mediated defense, we comprehensively explored the impact of Si on several defensive and metabolic traits in rice exposed to simulated and real herbivory of Mythimna loreyi Duponchel larvae. Hydroponic experiments showed that Si-deprived rice supplemented with Si 72 h prior to insect infestation were similarly resistant to larvae as plants continuously grown in Si-containing media. Both Si and herbivory altered primary metabolism in rice, including the levels of several sugars, amino acids, and organic acids. While the accumulation of sugars was generally positively correlated with Si presence, multiple amino acids showed a negative correlation trend with Si supplementation. The levels of secondary metabolites, including isopentylamine, p-coumaroylputrescine and feruloylputrescine, were typically higher in the leaves of Si-supplemented plants exposed to herbivory stress compared to Si-deprived plants. In addition, simulated herbivory treatment in Si-supplemented plants induced more volatile emissions relative to Si-deprived plants, which was consistent with the increased transcripts of key genes involved in volatile biosynthesis. In ecological interactions, Si alone did not affect the oviposition choice of M. loreyi but gravid females showed a significant preference for simulated herbivory-treated/Si-deprived compared to Si-supplemented plants. Our data suggest that apart from mechanical defense, Si may affect rice metabolism in multiple ways that might enhance/modulate defense responses of rice under herbivory stress.

Silicon: A valuable soil element for improving plant growth and CO2 sequestration

BACKGROUND

Silicon (Si), the second most abundant and quasi-essential soil element, is locked as a recalcitrant silicate mineral in the Earth's crust. The physical abundance of silicates can play an essential role in increasing plant productivity. Plants store Si as biogenic silica (phytoliths), which is mobilized through a chemical weathering process in the soil.

AIM OF REVIEW

Although Si is a critical element for plant growth, there is still a considerable need to understand its dissolution, uptake, and translocation in agroecosystems. Here, we show recent progress in understanding the interactome of Si, CO2, the microbiome, and soil chemistry, which can sustainably govern silicate dissolution and cycling in agriculture.

KEY SCIENTIFIC CONCEPTS OF THIS REVIEW

Si cycling is directly related to carbon cycling, and the resulting climate stability can be enhanced by negative feedback between atmospheric CO2 and the silicate uptake process. Improved Si mobilization in the rhizosphere by the presence of reactive elements (for example, Ca, Na, Al, Zn, and Fe) and Si uptake through genetic transporters in plants are crucial to achieving the dual objectives of (i) enhancing crop productivity and (ii) abiotic stress tolerance. Furthermore, the microbiome is a symbiotic partner of plants. Bacterial and fungal microbiomes can solubilize silicate minerals through intriguingly complex bioweathering mechanisms by producing beneficial metabolites and enzymes. However, the interaction of Si with CO2 and the microbiome's function in mobilization have been understudied. This review shows that enhancing our understanding of Si, CO2, the microbiome, and soil chemistry can help in sustainable crop production during climatic stress events.

Photoprotective mechanisms and higher photorespiration are key points for iron stress tolerance under heatwaves in rice

Considering the current climate change scenario, the development of heat-tolerant rice cultivars (Oryza sativa L.) is paramount for cultivation in waterlogged systems affected by iron (Fe) excess. The objective of this work was to investigate the physiological basis of tolerance to excess Fe in rice cultivars that would maintain photosynthetic efficiency at higher temperatures. In an experimental approach, two rice cultivars (IRGA424 - tolerant and IRGA417- susceptible to Fe toxicity) were exposed to two concentrations of FeSO4-EDTA, control (0.019 mM) and excess Fe (7 mM) and subsequent exposition to heatwaves at different temperatures (25 °C - control, 35, 40, 45, 50, and 55 °C). The increase in temperatures resulted in a higher Fe concentration in shoots accompanied by a lower Rubisco carboxylation rate in both cultivars, but with lower damage in the tolerant one. Stomatal limitation only occurred as a late response to Fe toxicity, especially in the sensitive cultivar. The activation of photorespiration as electron sink under Fe excess with increasing temperature during heatwaves appear as a major mechanism to alleviate oxidative stress in cultivars tolerant to excess Fe. The tolerance to iron toxicity and heat stress is associated with increased photoprotective mechanisms driving non-photochemical dissipation.

Modulation of plant photosynthetic processes during metal and metalloid stress, and strategies for manipulating photosynthesis-related traits

Metals constitute vital elements for plant metabolism and survival, acting as essential co-factors in cellular processes which are indispensable for plant growth and survival. Excess or deficient provision of metal/metalloids puts plant's life and survival at risk, thus considered a potent stress for plants. Chloroplasts as an organelle with a high metal demand form a pivotal site within the metal homeostasis network. Therefore, the metal-mediated electron transport chain (ETC) in chloroplasts is a primary target site of metal/metalloid-induced stresses. Both excess and deficient availability of metal/metalloids threatens plant's photosynthesis in several ways. Energy demands from the photosynthetic carbon reactions should be in balance with energy output of ETC. Malfunctioning of ETC components as a result of metal/metalloid stress initiates photoinhiition. A feedback inhibition from carbon fixation process also impedes the ETC. Metal stress impairs antioxidant enzyme activity, pigment biosynthesis, and stomatal function. However, genetic manipulations, nutrient management, keeping photostasis, and application of phytohormones are among strategies for coping with metal stress. Consequently, a comprehensive understanding of the underlying mechanisms of metal/metalloid stress, as well as the exploration of potential strategies to mitigate its impact on plants are imperative. This review offers a mechanistic insight into the disruption of photosynthesis regulation by metal/metalloids and highlights adaptive approaches to ameliorate their effects on plants. Focus was made on photostasis, nutrient interactions, phytohormones, and genetic interventions for mitigating metal/metalloid stresses.

Genome-wide identification and functional analysis of silicon transporter family genes in moso bamboo (Phyllostachys edulis)

Silicon (Si) has crucial effects on plant development and stress resistance. Silicon transporters regulate Si absorption, transport, and distribution in plants. In this study, we identified and characterized the Si transporter gene family of moso bamboo (Phyllostachys edulis) and cloned seven putative Si transporter genes. Moso bamboo Si transporters contain conserved functional domains that mediate the accumulation of considerable amounts of Si. The analysis of gene duplication patterns and divergence times suggested that the expansion of the moso bamboo Si transporter family was mainly due to segmental duplications. The expression of moso bamboo Si transporter genes, which varied among organs, was significantly modulated by Si treatments. The subcellular localization analysis showed that Si transporters are plasma membrane proteins. The Si content increased in transgenic Arabidopsis overexpressing PeLsi1-1 or PeLsi1-2, which affected vegetative and reproductive growth. Our single-particle tracking analysis revealed the four diffusion modes of PeLsi1-1 on the plasma membrane. Moreover, the particle velocity, dwell time, and motion range of PeLsi1-1 decreased in response to Si treatments. The results of this study will further clarify the molecular mechanisms underlying Si absorption and accumulation in bamboo plants.

Biochar-based fertiliser enhances nutrient uptake and transport in rice seedlings

Biochar-based compound fertilisers (BCF) are gaining increasing attention as they are cost-effectiveness and improve soil fertility and crop yield. However, little is known about the mechanisms by which micron-size BCF particles enhance crop growth. In the present study, Wuyunjing7 rice seedlings were exposed to micron-size particles of wheat straw-based BCF (mBCF) diffused through a 25-μm nylon mesh. The control was fertilised with urea, diammonium phosphate, and potassium chloride to ensure that both treatments received comparables level of N, P, and K. The effects of mBCF on rice seedling growth were evaluated by determining the changes in nitrogen uptake and utilisation via nitrogen content measurements, short-term ¹⁵N-NH₄⁺ influx assays, and analyses of transcript-level nutrient transporter gene expression. The shoot biomass of rice seedling treated with mBCF at the rate of 5 mg/ g soil was 33% greater than that for the control. Root and shoot ¹⁵N accumulation rates were 44% and 14% higher, respectively, in the mBCF-treated than the control. The mBCF-treated rice seedlings had higher phosphorus, potassium, and iron content than the control. Moreover, the treatments significantly differed in terms of their nutrient transporter gene expression levels. Spectroscopy and microscopy were used to visualise nutrient distributions across transverse root sections. There were relatively higher iron oxide nanoparticle and silicon-based compound concentrations in the roots of the mBCF-treated rice seedlings than in those of the control. The foregoing difference might account for the fact that the growth of the mBCF-treated rice was superior to that of the control. We demonstrated that the mBCF treatment created a more negative electrical potential at the root epidermal cell layer (~ - 160 mV) than the root surface. This potential difference may have been the driving force for mineral nutrient absorption.

Iron toxicity increases oxidative stress and impairs mineral accumulation and leaf gas exchange in soybean plants during hypoxia

Iron toxicity is a major challenge faced by plants in hypoxic soils; however, the consequences of such combined stress for soybean (Glycine max) remain to be determined. Here we assessed the physiological responses of soybean plants exposed to hypoxia and a high concentration of iron. Soil-grown plants cultivated in a greenhouse until the vegetative stage were transferred to a hydroponic system containing nutrient solution and subjected to two oxygen conditions (normoxia (6.2 mg L-1) and hypoxia (0.33 mg L-1)) and two iron concentrations (Fe-EDTA) (0.09 and 1.8 mM) for 72 h. During hypoxia, high concentrations of iron in the nutrient solution resulted in increased iron accumulation in roots and leaves. Under this condition, the concentrations of zinc, nitrogen, potassium, and calcium decreased in the roots, while the concentration of nitrogen and magnesium decreased in the leaves. Additionally, during hypoxia, the higher concentration of iron led to an increase in the activity of the antioxidant enzymes in roots and leaves, while decreased the levels of the photosynthetic pigments, leaf gas exchange, and plant growth. In conclusion, high iron concentration in the root medium results in a considerably more severe damage condition to soybean plants under hypoxia compared to plants grown under low iron availability.

The effect of silicon supply on photosynthesis and carbohydrate metabolism in two wheat (Triticum aestivum L.) cultivars contrasting in response to phosphorus nutrition

Phosphorus (P) deficiency affects agricultural systems by limiting crop quality and yield. Studies have suggested that silicon (Si) improves P uptake in plants grown under P deficiency. However, the effects of Si on photosynthesis and carbohydrate metabolism under P stress remain unclear. We performed a hydroponic study using two wheat cultivars with contrasting sensitivity to P deficiency (Púrpura, sensitive; Fritz, semi-tolerant) that were exposed to P (0, 0.01, or 0.1 mM) and Si (0 or 2 mM), and we evaluated the photosynthetic performance and metabolic alterations. In plants from the sensitive cultivar undergoing P deficiency, Si application increased sucrose levels, starch breakdown and length of shoots, and also improved plant dry weight. In Fritz (the semi-tolerant cultivar), Si exposure reduced P concentration, and increased shoot length and P use efficiency (PUE) under P shortage. Interestingly, Si application altered cell wall composition, which was associated with higher mesophyll conductance and net CO₂ assimilation in Fritz plants grown under P stress. Together, our results indicate that under P deficiency, Si nutrition positively affects photosynthesis and carbohydrate levels in a genotype-dependent manner. Furthermore, these results suggest that Si plays an important role in maintaining high photosynthetic rates in wheat plants undergoing P deficiency.

Interactions of Silicon With Essential and Beneficial Elements in Plants

Silicon (Si) is not classified as an essential element for plants, but numerous studies have demonstrated its beneficial effects in a variety of species and environmental conditions, including low nutrient availability. Application of Si shows the potential to increase nutrient availability in the rhizosphere and root uptake through complex mechanisms, which still remain unclear. Silicon-mediated transcriptional regulation of element transporters for both root acquisition and tissue homeostasis has recently been suggested as an important strategy, varying in detail depending on plant species and nutritional status. Here, we summarize evidence of Si-mediated acquisition, uptake and translocation of nutrients: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), iron (Fe), zinc (Zn), manganese (Mn), copper (Cu), boron (B), chlorine (Cl), and nickel (Ni) under both deficiency and excess conditions. In addition, we discuss interactions of Si-with beneficial elements: aluminum (Al), sodium (Na), and selenium (Se). This review also highlights further research needed to improve understanding of Si-mediated acquisition and utilization of nutrients and vice versa nutrient status-mediated Si acquisition and transport, both processes which are of high importance for agronomic practice (e.g., reduced use of fertilizers and pesticides).

Regulation of Agronomic Traits, Nutrient Uptake, Osmolytes and Antioxidants of Maize as Influenced by Exogenous Potassium Silicate under Deficit Irrigation and Semiarid Conditions

Understanding the link between the protective role of potassium silicate (K2SiO3) against water shortage and the eventual grain yield of maize plants is still limited under semiarid conditions. Therefore, in this study, we provide insights into the underlying metabolic responses, mineral nutrients uptake and some nonenzymatic and enzymatic antioxidants that may differ in maize plants as influenced by the foliar application of K2SiO3 (0, 1 and 2 mM) under three drip irrigation regimes (100, 75 and 50% of water requirements). Our results indicated that, generally, plants were affected by both moderate and severe deficit irrigation levels. Deficit irrigation decreased shoot dry weight, root dry weight, leaf area index (LAI), relative water content (RWC), N, P, K, Ca, Fe, Zn, carotenoids, grain yield and its parameters, while root/shoot ratio, malondialdehyde (MDA), proline, soluble sugars, ascorbic acid, soluble phenols, peroxidase (POD), catalase (CAT), polyphenol oxidase (PPO), and ascorbate peroxidase (APX) were improved. The foliar applications of K2SiO3 relatively alleviated water stress-induced damage. In this respect, the treatment of 2 mM K2SiO3 was more effective than others and could be recommended to mitigate the effect of deficit irrigation on maize plants. Moreover, correlation analysis revealed a close link between yield and the most studied traits.