Role of Silicon in Mediating Salt Tolerance in Plants: A Review

Abstract

Salt stress is a major threat for plant growth worldwide. The regulatory mechanisms of silicon in alleviating salt stress have been widely studied using physiological, molecular genetics, and genomic approaches. Recently, progresses have been made in elucidating the alleviative effects of silicon in salt-induced osmotic stress, Na toxicity, and oxidative stress. In this review, we highlight recent development on the impact of silicon application on salt stress responses. Emphasis will be given to the following aspects. (1) Silicon transporters have been experimentally identified in different plant species and their structure feature could be an important molecular basis for silicon permeability. (2) Silicon could mediate salt-induced ion imbalance by (i) regulating Na⁺ uptake, transport, and distribution and (ii) regulating polyamine levels. (3) Si-mediated upregulation of aquaporin gene expression and osmotic adjustment play important roles in alleviating salinity-induced osmotic stress. (4) Silicon application direct/indirectly mitigates oxidative stress via regulating the antioxidant defense and polyamine metabolism. (5) Omics studies reveal that silicon could regulate plants' response to salt stress by modulating the expression of various genes including transcription factors and hormone-related genes. Finally, research areas that require further investigation to provide a deeper understanding of the role of silicon in plants are highlighted.

Keywords: antioxidant enzymes; polyamine; salt stress; silicon; water balance.

Figure 1

Beneficial effects of silicon (Si) on abiotic stress tolerance and main aspects related to the alleviative effect of Si under salt stress. (A) Number of articles related to Si and the alleviative effect of Si on abiotic and biotic stress tolerance published in plant sciences from 1990 to 2018. Articles were mainly collected from Science Direct (https://www.sciencedirect.com/), Spring Link (https://link.springer.com/), PubMed (https://www.ncbi.nlm.nih.gov/pubmed), and Google Scholar (https://scholar.google.com/). (B) The network analysis on key words in articles. Articles related to the alleviative effect of Si on salt stress tolerance were selected and the key words of those articles were used to draw this network by Cytoscape (v3.6.0). In the network, those collected key words indicated the aspects (species, substances, physiological processes, alleviation mechanisms, etc.) involved in the alleviative effects of silicon under salt stress. The network was centered with node ‘Salt stress’, which represent ‘articles related to the alleviative effect of Si on salt stress’, and other nodes represent key words in those articles. Number of lines connecting nodes and ‘Salt stress’ represent the number of articles using the words in certain node as key word. As shown in the bottom legend, colored nodes located in the outer ring of the network are grouped into 10 main mechanisms related to the alleviative effect of silicon under salt stress, for example, the blue nodes, including polyamine, spermidine, and arginine, are grouped into polyamine.

Figure 2

Phylogenetic analysis of plant silicon (Si) transporters identified in Cucumis sativus (Cs), Cucurbita moschata (Cm), Equisetum arvense (Ea), Glycine max (Gm), Hordeum vulgare (Hv), Oryza sativa (Os), Triticum aestivum (Ta), Zea mays (Zm). Amino acid sequences from 8 members of the Si transporters were aligned using ClustalW2, and a phylogenetic tree was then constructed using MEGA7 program with the maximum likelihood method (Bootstrap value: 1000).

Figure 3

A schematic model for the beneficial impact of silicon on plant under salt stress. Six main strategies are involved in Si’s alleviation of salt stress: For strategy I, Si could enhance photosynthesis by maintaining the integrity of photosynthetic organs, increasing the CO₂ utilization rate in plants and increasing the openness and activity of the PSII reaction center. For strategy II, Si regulates ion homeostasis through mediating Na⁺ uptake, transport, and compartmentalization, and corresponding gene expression (e.g., NHX and HKT). For strategy III, Si can regulate the activity/concentration of enzymatic and/or nonenzymatic antioxidants and endogenous polyamine accumulation to alleviate oxidative damage caused by salinity stress. For strategy IV and V, Si enhances the root hydraulic conductance through regulating aquaporin activities and improving osmoregulatory capacities, which contributes to an increase in water uptake and transport. For strategy VI, Si may mediate ion homeostasis and decrease oxidative damage through regulating polyamine metabolism. Single solid black line ended with bar: process of mediating. Single dash black line: speculated mechanisms that need to be experimentally proved. Red arrow: increase (up) or decrease (down). ‘?’ represents mechanisms that are different between species. Chloroplast and mitochondrion component in this schematic model are modified from Yamori [60].