Background

The earth’s surface is covered with 28.8% (dry wt. basis) of silicon (Si) after oxygen, but the existence of Si in its pure form is extremely rare [1,2,3]. Silicon is found as quartz, feldspar, mica and clay minerals in the earth’s crust [2, 4, 5]. In biological systems, Si occurs in various forms of amorphous silica (SiO2nH2O) namely, as phytoliths and in silica rich plants [6, 7]. The biogenic share of silicon is about 1–3% of the total Si pool in the soil [8].

The numerous advantages of Si to crops are now widely acknowledged [3, 9,10,11,12]. The plant-available forms of Si may be limited [1, 13]. The plant biologists/physiologists recognized the significance of Si as early as the nineteenth century [14,15,16]. Silicon is now widely considered as a beneficial element [2, 17], but it is still considered non-essential for plant growth and development [18]. Lewin and Reimann [19] demonstrated that the Si played a major metabolic role in living organisms due to its relatively abundance in nature. According to Epstein [20], silicon is essential for plant growth, acts as a mechanical barrier and provides resistance against plant pathogens and herbivores, and as such drawn the more attention of its role in plant biology. Furthermore, [9] found that the majority of crops contain significant quantity of Si, implying that this is unlikely to be a result of stochastic element absorption, similar to how evolutionary mechanisms have evolved for uptake and accumulation of other nutritional elements.

Furthermore, Si uptake by plant roots can be adaptive in response to changing atmospheric variables namely, stress, whether abiotic or biotic [2, 3, 21, 22], and thus is not always necessary, but arguably important. While the importance of this feature to plants is still argued, considerable progress has been made in our understanding of Si uptake and accumulation in plants. Plants can be classified as hyper-accumulators, accumulators, passive accumulators and non-accumulating varieties. Furthermore, the benefits of application of liquid forms of Si have been demonstrated, especially during plant stress tolerance. Limited studies have been carried out whether Si plays a significant role in plant metabolic activities [2, 10, 22, 23].

The experiments showing the impact of Si on plant tolerance to environmental stresses [3, 24,25,26,27,28] have been carried out at physiological, molecular and ecological levels [12, 29,30,31]. The current research articles published on the mitigation of plant stress by Si demonstrates the interest in this area [3, 11, 22, 32, 33]. However, none of the researchers took into account the possibility of Si interacting with fundamental plant omics. Indeed, most studies have concentrated on species-specific and narrow aspects of Si–plant stress interactions. Some studies have contributed to our understanding of the larger effects of Si on plant growth and defence, including interactions with plant diseases, but the mechanisms underlying these effects are still unknown. The present review, briefly discusses the physiological and molecular basis of amelioration of biotic stress conditions in plants by silicon and the mechanisms involved.

Biotic stress

In natural conditions, plants suffer from various types of stresses caused by living organisms like bacteria, viruses, fungi, parasites, significant and non-significant insects. Like livestock, plants also have a defence system, which provides tolerance against environmental stresses. On invasion by pathogens and herbivorous pests, plants make use of pre-existing physical, chemical and mechanical barriers to protect themselves. The plant defence functions are also activated upon attack by pest; plant protection functions as a unit to decrease negative responses of biotic stress (Table 1). The stress induced defence system is associated with complex interconnected pathways of signal transduction in which phytohormones namely, abscisic acid (ABA), ethylene (ETH), jasmonic acid (JA) and salicylic acid (SA) plays an important role [2, 13, 34,35,36]. The biotic stress may enhance in the coming years because of climate change. The costs linked with stress are potentially enormous, and the effects of the stress may have a great impact on sustainable agriculture and environmental systems [3].

Table 1 The adaptive mechanisms of silicon in crop plants against biotic stress
Full size table

Silicon mitigates biotic stress in plants: physical and mechanical barriers

Plants grown under normal conditions are exposed to environmental stresses such as biotic (caused by viral and bacterial pathogens or fungi and herbivores) and abiotic stresses (saline, high and low temperature, flooding, UV, wind, drought, metal toxicity, light and mineral deficiency or excess). Supplementing plants with Si have been shown to enhance plant tolerance to mammalian, arthropod, and molluscan herbivores, fungal and bacterial pathogens, viruses and nematodes [2, 21, 22, 26, 37, 38]. The physical defence induced by Si deposition in plant parts in the form of phytoliths (largely composed of SiO2) was one of the first theories proposed for studying stress tolerance to pests [39, 40]. Silicon translocated from the soil solution as monosilicic acid into plants. Monosilicic acid polymerizes to form phytoliths, which are accumulated within the plant in an irreversible manner [20, 29]. Deposition of phytoliths enhances plant immunity and physical resilience, and serves as a physical barrier to fungal penetration [41, 42]. Silicon deposition may also wear away the feeding mouthparts, or mandibles of insects [43], decrease plant digestibility for both insect and mammalian herbivores [21, 44, 45], and have an adverse effect on herbivores [46]. Importantly, plant tissue silicification may be induced more in those plants which are highly attacked by various organisms [42, 47]. Silicon also affects the plant metabolites associated with plant defence [10, 48, 49], such as chitinase (CHT), β-1,3-glucanase, phenylalanine ammonia-lyase (PAL), polyphenol oxidase (PPO), in a number of plant–pathogens such as necrotrophic, biotrophic and hemibiotrophic pathogens [37, 50, 51]. Silicon-induced increased production of flavonoids, peroxidases (PODs) and chitinase (CHT) in some necrotrophic pathogens have been reported [37, 52].

Current research has looked into the interactions between Si and plant defence signaling transduction pathways, specifically the main plant hormone signaling pathways. Plants develop a complex and unique blend of SA (generally linked with pathogens of (hemi)biotrophic), JA (generally linked with pathogens of necrotrophic and insect herbivores), and ETH (which is usually regarded as ‘fine-tuning' the JA defence action) in response to attack or infection [53, 54]. Plant hormone signaling has been shown to be important for Si-mediated plant tolerance to disease stress [50]. Ye et al. [22, 28, 83, 97]. According to Brunings et al. [83], Si application decreased the expression of essential housekee** genes in rice under control conditions, but increased the expression of housekee** genes to preserve cellular functions throughout pathogen infection. Silicon-mediated up-regulation of housekee** genes such as actin (ACT), alpha-tubulin (TUB), and phosphoglycerate kinase (PGK) in Ralstonia solanacearum-infected in tomatoes [98]. According to Jarosch et al. [99], the actin cytoskeleton provided the basal resistance during infection in R. solanacearum. As a result, the host resistance was induced by the Si-dependent upregulation of actin in tomato (Solanum lycopersicum) plants [98]. Due to lack of a high-density Si transporter, tomato is classified as a low-level Si accumulator (about 0.2% dry weight) [100]. Furthermore, the application of Si in low-accumulating plants, i.e. Solanum lycopersicum [101], Capsicum annuum [102], and Rosa spp. [103] has resulted in overcoming stress resistance. Despite the fact that housekee** genes have a constant expression level, variation in expression levels in response to Si treatment and pathogen infection can trigger the host plant's basal defence mechanism to protect it from the pathogen (Table 3).

Silicon is associated with the metabolic mechanisms of plant–pathogen interactions, triggering host plant defence genes via a sequence of physiological and biochemical reactions and signal transductions, as well as inducing disease resistance in plants [24, 58]. Silicon could play a role in the primary response, modulating the behavior of post-elicitation intracellular signaling pathways that control the expression of defence genes involved in cell wall structural modifications, hypersensitivity responses, synthesis of hormones, antimicrobial compound synthesis, and in formation of PR proteins [24]. To demonstrate the mechanism of protection of Si in various pathological systems, transcriptomic and proteomic experiments have been carried out [2, 21,22,23, 28, 98, 104].

The expression of genes encoding a novel proline-rich protein (PRP1) was increased under the induction of system acquired resistance in Cucumis sativus mediated by Si, which led to cell-wall reinforcement at the site of penetration of fungi into epidermal cells [105]. The expressions of CHI-II, GLU, PGIP, and POD, which are due to virulence factors released by the pathogen to inhibit host resistance and promote host invasion, were reduced by treatment with Si during pathogen interactions in tomato plants (R. solanacearum) [98]. Twenty six proteins were significantly altered by Si treatment in tomato plants, implying that Si-mediated disease resistance is linked to protein changes [106]. For example, Arabidopsis infected with the fungus, Erysiphe cichoracearum showed changes in the expression of about 4000 genes. The number and/or expression level of defence related genes enhanced in Si treated plants [75]. The expression of around 900 genes reacting to pathogen infection were modified in wheat plant leaves infected with Blumeria graminis f. sp. tritici, while the pathogen modified a few genes in silicon treated plants, implying that Si almost removed the stress due to pathogen invasion [76]. Brunings et al. [83] inoculated in the rice transcriptome, Magnaporthe oryzae, and riceblast fungus. Treatment of the plant with silicon appears to eradicate the effect of pathogen invasion on the transcriptome of host plants, rather than inducing resistance through transcriptional reprogramming of defence-related genes.

Silicon as a sustainable alternative

Silicon has been shown to enhance crop resistance to a variety of biotic stresses and can be seen as an alternative to adaptive strategies [3, 10, 21, 22, 32]. Due to the special physical and chemical properties of Si have useful application in various sectors, including promising applications in the agri-sectors, they can easily enter into plant cells and affect the plant development by affecting their metabolism through diverse interactions, thereby triggering the potential to combat stress conditions. Thus, Si has the potential to be used as a fertilizer alone for specific crops and can be used to deliver herbicides and fertilizers in plants. The application of Si in agriculture may also lead to worldwide food security and safety by hel** in the development of advanced varieties of crops with maximum yield. Silicon can provide green and eco-environment friendly alternatives to different synthetic fertilizers without environmental pollution. Simultaneously, the well-known positive impact on crop productivity and quality has a tremendous potential to enhance farmers’ profit margin through the utilization of the alternative approach.

Conclusion and future prospects

Nowadays, there has been a lot of research which focuses on the role of Si in ameliorating plant tolerance to biotic stress, as well as in the regulation of signaling transduction pathways, and also in activating transcription factors in response to stress. Based on the present review, we concluded that Si increases plant resistance capacity to biotic stress, through a complex pathway associated with the plant defence system by activating transcription factors. In this review, we have discussed various aspects of Si and its regulatory functions during unfavorable conditions, and used key points from various relevant studies to explain how Si enhances stress resistance. While Si is associated with thousands of plant genes, it is not clear which other transcription factors and signaling proteins interact with Si to increase stress resistance. It will be very interesting to explore the role of Si signaling pathway, interactions with phytohormones, and crosstalk at the level of plant tissues, and at the cellular level to better understand how plants respond to environmental stresses, especially biotic stress. Overall, future research should concentrate on collecting more evidence to unravel the molecular mechanisms and the role of Si in plant tolerance to biotic stress, as well as the regulation of signal transduction pathways, and gene expressions associated in the biosynthesis of key compounds relevant to plant development.