Suberized transport barriers in plant roots: the effect of silicon

Silicon-mediated alleviation of cadmium toxicity in soil-plant system: historical review

The contamination of crops by Cd is a worldwide problem that needs to be addressed for minimizing risk for human health. Today, numerous investigations have demonstrated that Si plays a role in reducing Cd toxicity and accumulation in cultivated plants. The evolution of scientific understanding - the Cd behavior in soil and in plant is discussed for the first time. Our analysis evidences that the research on Si-Cd interactions in the soil-plant system has quickened only in recent years, although basic interactions between silicic acid and Cd cations in aqueous systems were studied over 40-50 years ago. Today, numerous direct and indirect mechanisms of the Si impact on mobility and translocation of Cd in soil and in plants are reported. More productive studies in this area are those that considered the soil-plant system as a whole. Analysis of the development of the Cd-Si-related ideas suggests the prospects of further studies aimed at finding synergetic action of Si and other substances on Cd behavior in the soil-plant system.

Hydrogen peroxide modulates silica deposits in sorghum roots

Hydrated silica (SiO2·nH2O) aggregates in the root endodermis of grasses. Application of soluble silicates (Si) to roots is associated with variations in the balance of reactive oxygen species (ROS), increased tolerance to a broad range of stresses affecting ROS concentrations, and early lignin deposition. In sorghum (Sorghum bicolor L.), silica aggregation is patterned in an active silicification zone (ASZ) by a special type of aromatic material forming a spotted pattern. The deposition has a signature typical of lignin. Since lignin polymerization is mediated by ROS, we studied the formation of root lignin and silica controlled by ROS via modulating hydrogen peroxide (H2O2) concentrations in the growth medium. Sorghum seedlings were grown hydroponically and supplemented with Si, H2O2, and KI, an ionic compound that catalyses H2O2 decomposition. Lignin and silica deposits in the endodermis were studied by histology, scanning electron and Raman microscopies. Cell wall composition was quantified by thermal gravimetric analysis. Endodermal H2O2 concentration correlated to the extent of lignin-like deposition along the root, but did not affect its patterning in spots. Our results show that the ASZ spots were necessary for root silica aggregation, and suggest that silicification is intensified under oxidative stress as a result of increased ASZ lignin-like deposition.

Hydroponic cultivation conditions allowing the reproducible investigation of poplar root suberization and water transport

BACKGROUND

With increasing joint research cooperation on national and international levels, there is a high need for harmonized and reproducible cultivation conditions and experimental protocols in order to ensure the best comparability and reliability of acquired data. As a result, not only comparisons of findings of different laboratories working with the same species but also of entirely different species would be facilitated. As Populus is becoming an increasingly important genus in modern science and agroforestry, the integration of findings with previously gained knowledge of other crop species is of high significance.

RESULTS

To ease and ensure the comparability of investigations of root suberization and water transport, on a high degree of methodological reproducibility, we set up a hydroponics-based experimental pipeline. This includes plant cultivation, root histochemistry, analytical investigation, and root water transport measurement. A 5-week-long hydroponic cultivation period including an optional final week of stress application resulted in a highly consistent poplar root development. The poplar roots were of conical geometry and exhibited a typical Casparian band development with subsequent continuously increasing suberization of the endodermis. Poplar root suberin was composed of the most frequently described suberin substance classes, but also high amounts of benzoic acid derivatives could be identified. Root transport physiology experiments revealed that poplar roots in this developmental stage have a two- to tenfold higher hydrostatic than osmotic hydraulic conductivity. Lastly, the hydroponic cultivation allowed the application of gradually defined osmotic stress conditions illustrating the precise adjustability of hydroponic experiments as well as the previously reported sensitivity of poplar plants to water deficits.

CONCLUSIONS

By maintaining a high degree of harmonization, we were able to compare our results to previously published data on root suberization and water transport of barley and other crop species. Regarding hydroponic poplar cultivation, we enabled high reliability, reproducibility, and comparability for future experiments. In contrast to abiotic stress conditions applied during axenic tissue culture cultivation, this experimental pipeline offers great advantages including the growth of roots in the dark, easy access to root systems before, during, and after stress conditions, and the more accurate definition of the developmental stages of the roots.

Beneficial Effect of Root or Foliar Silicon Applied to Cucumber Plants under Different Zinc Nutritional Statuses

Zinc (Zn) is an essential micronutrient involved in a large variety of physiological processes, and its deficiency causes mainly growth and development disturbances, as well as oxidative stress, which results in the overproduction and accumulation of reactive oxygen species (ROS). A possible environmentally friendly solution is the application of silicon (Si), an element that has shown beneficial effects under abiotic and biotic stresses on many crops. Si could be applied through the roots or leaves. The aim of this work is to study the effect of Si applied to the root or shoot in cucumber plants under different Zn statuses (sufficiency, deficiency, and re-fertilization). Cucumber plants were grown in hydroponics, with 1.5 mM Si applied at the nutrient solution or sprayed on the leaves. During the different Zn statuses, SPAD index, fresh weight, ROS, and Si, Zn, P, Cu and B mineral concentration were determined. The results suggested that Si application had no effect during sufficiency and deficiency periods, however, during re-fertilization foliar application of Si, it showed faster improvement in SPAD index, better increment of fresh weight, and a decrease in ROS quantity, probably due to a memory effect promoted by Si previous application during the growing period. In summary, Si application to cucumber plants could be used to prepare plants to cope with a future stress situation, such as Zn deficiency, due to its prompt recovery after overcoming the stress period.

Silicon alleviates antimony phytotoxicity in giant reed (Arundo donax L.)

Silicon enhances photosynthetic efficiency, biomass, and lignification of root structures possibly limiting antimony translocation and mitigating phytotoxicity in giant reed plants. Antimony (Sb) is a non-essential metalloid causing toxic effects in plants. Silicon has been reported to impart tolerance against biotic and abiotic stress in plants. Fast-growing plant, giant reed (Arundo donax L.) is a promising energy crop, can be a suitable plant for phytoremediation. However, information regarding the tolerance capacity with respect to Sb toxicity and potential of Si to mitigate the Sb phytotoxicity in giant reed are very scarce. Rhizomes of giant reed were grown for ten weeks in hydroponics exposed to Sb, Si, and their combination wherein treatment without Sb/Si served as control. Effect of these treatments on rate of net photosynthesis and photosynthetic pigments, phytoextraction ability of Sb, Si and Sb uptake, plant biomass, and lignification and suberization of roots along with localization of Sb and Si were analysed. We found that Si considerably improved the growth and biomass of giant reed under Sb toxicity. Antimony reduced the photosynthesis and decreased the content of photosynthetic pigments, which was completely alleviated by Si. Silicon amendment to Sb treated plants enhanced root lignification. Silicon enhanced lignification of root structures probably restricted the Sb translocation. However, co-localization of Sb with Si has not been observed neither at the shoot nor at the root levels. Similarly, Sb was also not detected in leaf phytoliths. These findings suggest that Si treatment promotes overall plant growth by improving photosynthetic parameters and decreasing Sb translocation from root to shoot in giant reed by improving root lignification.

Sustainable management of cadmium-contaminated soils as affected by exogenous application of nutrients: A review

Cadmium (Cd) pollution in arable land is of great concern as it impairs plant growth and further threats human health via food-chain. Exogenous supplementation of nutrients is an environmentally-friendly, cost-effective, convenient and feasible strategy for regulating Cd uptake, transport and accumulation in plants. To sustain Cd-contaminated soils management, on the one hand, a low level of the Cd-contaminated soil is expected to cultivate crops with decreased Cd accumulation as affected by exogenous nutrients application, on another hand, a high level of the Cd-contaminated soil is suggested to cultivate phytoextraction plants with increased Cd accumulation as affected by exogenous nutrients application. Nevertheless, effects of nutrients on Cd accumulation in plants are still ambiguous. Thus, data of Cd accumulation in shoots of plants as affected by exogenous application of nutrients were collected from previously published articles between 2005 and 2021 in the present study. According to the data, exogenous supply of calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn) and silicon (Si) to a larger extent decrease Cd amounts in shoots of plants. By contrast, exogenous nitrogen (N), and deficient Ca, Mg and Fe supply have a great possibility to increase Cd amounts in shoots of plants. Although exogenous application of phosphorus (P), sulfur (S), potassium (K), zinc (Zn) and selenium (Se) have a great opportunity to increase biomass, they show different effects on Cd concentrations. As a result, the odds are even for increasing and decreasing Cd amounts in shoots of plants. Taken together, exogenous application of Ca, Mg, Fe, Mn and Si might decrease Cd accumulation in plants that are recommended for crops production. Exogenous N and deficient Ca, Mg and Fe supply might increase Cd accumulation in plants that are recommended for phytoextraction plants. Exogenous application of P, S, K, Zn and Se have half a chance to increase or decrease Cd accumulation in plants. Therefore, dosages, forms and species should be taken into account when exogenous P, S, K, Zn and Se are added.

Molecular Cloning and Functional Analysis of GmLACS2-3 Reveals Its Involvement in Cutin and Suberin Biosynthesis along with Abiotic Stress Tolerance

Cutin and wax are the main precursors of the cuticle that covers the aerial parts of plants and provide protection against biotic and abiotic stresses. Long-chain acyl-CoA synthetases (LACSs) play diversified roles in the synthesis of cutin, wax, and triacylglycerol (TAG). Most of the information concerned with LACS functions is obtained from model plants, whereas the roles of LACS genes in Glycine max are less known. Here, we have identified 19 LACS genes in Glycine max, an important crop plant, and further focused our attention on 4 LACS2 genes (named as GmLACS2-1, 2, 3, 4, respectively). These GmLACS2 genes display different expression patterns in various organs and also show different responses to abiotic stresses, implying that these genes might play diversified functions during plant growth and against stresses. To further identify the role of GmLACS2-3, greatly induced by abiotic stresses, we transformed a construct containing its full length of coding sequence into Arabidopsis. The expression of GmLACS2-3 in an Arabidopsis atlacs2 mutant greatly suppressed its phenotype, suggesting it plays conserved roles with that of AtLACS2. The overexpression of GmLACS2-3 in wild-type plants significantly increased the amounts of cutin and suberin but had little effect on wax amounts, indicating the specific role of GmLACS2-3 in the synthesis of cutin and suberin. In addition, these GmLACS2-3 overexpressing plants showed enhanced drought tolerance. Taken together, our study deepens our understanding of the functions of LACS genes in different plants and also provides a clue for cultivating crops with strong drought resistance.

Multiple effects of silicon on alleviation of nickel toxicity in young maize roots

Although beneficial metalloid silicon (Si) has been shown to alleviate the toxicity of various heavy metals, there is a lack of knowledge about the role of Si in possible alleviation of phytotoxicity caused by excess of essential nickel (Ni). In the present study we investigated the growth and biomass production, reactive oxygen species (ROS) formation and activities of selected antioxidants, as well as combined effect of Ni and Si on the integrity of cell membranes and electrolyte leakage in young maize roots treated for 24, 48 and 72 h with excess of Ni and/or Si. By histochemical methods we also visualized Ni distribution in root tissues and compared the uptake of Ni and Si with the development of root apoplasmic barriers. Ni enhanced the root lignification and suberization and shifted the development of apoplasmic barriers towards the root tip. Similarly, localization of Ni correlated with lignin and suberin deposition in root endodermis, further supporting the barrier role of this tissue in Ni uptake. Si reversed the negative impact of Ni on root anatomy. Additionally, improved cell membrane integrity, and enhanced ascorbate-based antioxidant system might be the mechanisms how Si partially mitigates the deleterious effects of Ni excess in maize plants.

Alleviation mechanisms of metal(loid) stress in plants by silicon: a review

Silicon (Si), although not considered as an essential element for plants in general, can ameliorate the phytotoxicity induced by excess metal(loid)s whether non-essential (e.g. Cd, Pb, Cr, Al, As, and Sb) or essential (e.g. Cu, Ni, and Zn). The Si-enhanced resistance allowing plants to cope with this type of abiotic stress has been developed at multiple levels in plants. Restriction of root uptake and immobilization of metal(loid)s in the rhizosphere by Si is probably one of the first defence mechanism. Further, retention of elements in the root apoplasm might enhance the resistance and vigour of plants. At the cellular level, the formation of insoluble complexes between Si and metal(loid)s and their storage within cell walls help plants to decrease available element concentration and restrict symplasmic uptake. Moreover, Si influences the oxidative status of plants by modifying the activity of various antioxidants, improves membrane stability, and acts on gene expression, although its exact role in these processes is still not well understood. This review focuses on all currently known plant-based mechanisms related to Si supply and involved in amelioration of stress caused by excess metal(loid)s.