Volume 34, Issue 6 p. 1142-1157
PERSPECTIVE
Free Access

Is it time to include legumes in plant silicon research?

Rocky Putra

Corresponding Author

Rocky Putra

Hawkesbury Institute for the Environment, Western Sydney University, Penrith, NSW, Australia

Correspondence

Rocky Putra

Email: [email protected]

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Jeff R. Powell

Jeff R. Powell

Hawkesbury Institute for the Environment, Western Sydney University, Penrith, NSW, Australia

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Susan E. Hartley

Susan E. Hartley

York Environmental Sustainability Institute, Department of Biology, University of York, York, UK

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Scott N. Johnson

Scott N. Johnson

Hawkesbury Institute for the Environment, Western Sydney University, Penrith, NSW, Australia

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First published: 07 April 2020
Citations: 30

[Correction added after online publication on 27 April 2020: affiliation and present address updated for Susan E. Hartley]

Abstract

  1. To date, the functional role of plant silicon has mostly been investigated in grasses (Poaceae). This potentially overlooks the importance of silicon in other plant functional groups such as legumes (Fabaceae). Legumes form a symbiotic relationship with nitrogen-fixing bacteria (rhizobia) inside the root nodules for fixing atmospheric nitrogen. A small, but growing number of studies suggest that silicon promotes this symbiotic relationship.
  2. We consider how legumes may take up and deposit silicon relative to what is known about these processes in grasses. We synthesize information about how silicon affects legume growth and function in the context of environmental stresses and the legume–rhizobia symbiosis.
  3. The available literature indicates that silicon is broadly beneficial to legumes, alleviating the effects of stresses including metal toxicity, salinity, alkalinity and pathogens. Crucially, there is also evidence that silicon promotes the legume–rhizobia interaction including increased root nodulation, numbers of bacteroids and nitrogen fixation across several legume species.
  4. We propose a model for how silicon may benefit the legume–rhizobia interaction. We hypothesize that silicification in the tissues may reduce the high metabolic cost of carbon-based compounds in cell wall construction, optimize solute transport and gas exchange in root nodules and/or promote protection against environmental stresses. We therefore propose a hypothetical framework to better understanding the impacts of silicon on legume–rhizobia relationships.
  5. We also suggest potential research priorities that would help us to better understand the functional role of silicon in nitrogen-fixing legumes. These research priorities focus on characterizing how silicon affects the chemical dialogues between the host plant and its rhizobial partner, how silicon is deposited in legume roots and how resources are exchanged by the two. Given the growing importance of legumes at a global scale, silicon could play a vital role in improving legume health and productivity with manifold environmental benefits.

A free Plain Language Summary can be found within the Supporting Information of this article.

1 INTRODUCTION

Over the past decade, the ecological significance of plant silicon research is increasingly recognized and was recently featured as a special issue published in Functional Ecology (Cooke, Degabriel, & Hartley, 2016). It is known that plant silicon alleviates abiotic or biotic stresses (Cooke & Leishman, 2016; Debona, Rodrigues, & Datnoff, 2017). For example, a meta-analytic study of 145 experiments revealed that silicon benefitted plants under abiotic stresses (Cooke & Leishman, 2016). Moreover, plant silicon plays an important role in herbivore defence and is known to be an inducible defence (Johnson, Rowe, & Hall, 2019; Massey, Roland Ennos, & Hartley, 2007). For herbivore defence, at least, it is known that biophysical traits can hinder herbivory via abrasion, toughness and reductions in resource acquisition (Alhousari & Greger, 2018; Massey & Hartley, 2006, 2009; Reynolds, Padula, Zeng, & Gurr, 2016). Silicon may also influence the activities of other biochemical defences following herbivory, for example by affecting phytohormonal pathways (Hall, Waterman, Vandegeer, Hartley, & Johnson, 2019) and herbivore-induced plant volatiles (Reynolds et al., 2016). Silicon has also been shown to affect plant traits including growth, yield and physiology (Detmann et al., 2012; Frew, Weston, Reynolds, & Gurr, 2018). However, more recently it has been suggested that these benefits only occur when plants are under stress: silicon alleviates the negative effects of the stressing agent, thereby allowing plants to reach their full growth potential (Coskun et al., 2018).

Most studies involving plant silicon focus on monocots (i.e. the Poaceae). For example, a search of Web of Science found 422 independent studies published between 1999 and 2019 that investigated the role of silicon in plant biology. Collectively, these contained 641 accounts of how silicon affected plant biology across 159 plant families, but Poaceae accounted for the majority, with 268 (42%) of these observations (Table S1). The second largest group was the Fabaceae (the legumes), which accounted for 7% of the observations. Because certain dicot families are known to accumulate high concentrations of silicon (e.g. Cucurbitaceae, Asteraceae and Fabaceae; Hodson, White, Mead, & Broadley, 2005), this taxonomic bias in research suggests that the role of silicon in plant families other than the Poaceae needs further investigation. Such high levels of accumulation suggest functional roles for silicon in these plant families (Katz, 2014, 2018), although this is not necessarily the case: Arabidopsis thaliana (Brassicaceae) accumulates very little silicon yet is still better defended against a fungal pathogen (powdery mildew) when grown in a silicon-supplemented nutrient solution (Fauteux, Chain, Belzile, Menzies, & Belanger, 2006).

Traditionally, legumes were not the focus of plant silicon research. This is partly because silicon was not regarded as an essential nutrient for plant growth and development, which applied to all plant taxa (Epstein, 1994) but, more specifically, legumes were considered as silicon-excluding species (Jones & Handreck, 1967) based on experiments conducted in common bean Phaseolus vulgaris (Barber & Shone, 1966; Shone, 1964). The dearth of research addressing the role of silicon in legumes was broken by Miyake and Takahashi (1985) and there has been a small, but growing number of silicon studies in this area. In particular, a major landmark was the discovery that some legumes (e.g. soybean) possess similar transporters for silicon uptake from the soil as hyper-accumulating grasses (Deshmukh et al., 2013). A meta-analysis has shown that some legume species, such as pigeonpea Cajanus cajan, common bean P. vulgaris and soybean Glycine max can accumulate relatively high concentrations of silicon in their foliar tissues (Hodson et al., 2005). Legumes are a plant functional group forming mutualistic symbioses with nitrogen-fixing bacteria (collectively known as rhizobia) in root nodules (Beringer, Brewin, Johnston, & Schulman, 1979). These rhizobial symbionts provide a valuable source of nitrogen for their host plant and consequently the high protein content of legumes makes them globally significant food and forage crops (Stagnari, Maggio, Galieni, & Pisante, 2017). Furthermore, there is interest in increasing global production of legumes to ensure improved human health and future food security (Foyer et al., 2016). Understanding the functional role of silicon in the legume–rhizobia symbiosis could aid these endeavours. A recent study has demonstrated that silicon supplementation promoted root nodulation and biosynthesis of foliar amino acids in lucerne Medicago sativa (Johnson et al., 2017), but how these benefits arise is still unknown.

In this article, we consider how some legumes have the machinery for high levels of silicon uptake, provide examples of silicon affecting legumes in the context of environmental stresses and symbiotic functions (e.g. root nodulation and nitrogen fixation), highlight potential mechanisms that underpin these effects and propose future research directions.

2 LEGUMES AND THEIR NITROGEN-FIXING CAPACITY

The Fabaceae comprise approximately 20,000 species and 700 genera (Lewis, Schrire, MacKinder, & Lock, 2005) and are known collectively as legumes. Functionally, legumes serve as the most valuable source of protein for crops and animal feed; they enrich the soil with nitrogen and provide habitats for beneficial organisms (e.g. natural enemies of insect pests and pollinators; Rodriguez-Saona, Blaauw, & Isaacs, 2012; Stagnari et al., 2017). Legumes form root nodules to accommodate rhizobia that reduce atmospheric nitrogen (N2) into ammonia (NH3) via the enzyme nitrogenase (Vessey, 1994). In return, host legumes potentially provide a variety of resources to their rhizobial partners, including carbon-based compounds (Checcucci, DiCenzo, Bazzicalupo, & Mengoni, 2017). Variation in the degree of mutualism exists, however, and the relationship between legumes and nitrogen-fixing bacteria operates on a continuum from highly mutualistic behaviour to parasitic behaviour (Checcucci et al., 2017; Fujita, Aoki, & Kawaguchi, 2014; West, Kiers, Simms, & Denison, 2002).

3 EVIDENCE FOR SILICON AFFECTING LEGUMES

3.1 Machinery for silicon uptake in legumes

Plants that take up high amounts of silicon (>1% dry mass) often have silicon transporter genes that belong to nodulin 26-like intrinsic proteins (NIPs; Deshmukh & Bélanger, 2016). These NIPs are part of major intrinsic proteins (MIPs) which include the Low silicon 1 (Lsi1), 2 (Lsi2) and 6 (Lsi6) genes, which were initially characterized in rice (Ma et al., 2006; Yamaji, Mitatni, & Ma, 2008). Lsi1 and Lsi2 are expressed in the roots, whereas Lsi6 is expressed in the leaves alone. These transporter genes underpin the cooperative nature of silicon influx and efflux transporters which enables silicon uptake, translocation and distribution (Ma & Yamaji, 2015). Similar silicon transporter genes have also been found in two dicot families: Cucurbitaceae and Fabaceae (Deshmukh et al., 2013; Mitani & Ma, 2005; Mitani, Yamaji, Ago, Iwasaki, & Ma, 2011; Wang et al., 2015). For example, silicon transporter genes in soybean have been identified, namely GmNIP2-1 and GmNIP2-2 (Deshmukh et al., 2013) that are functionally similar to Lsi1 and Lsi2 in rice respectively (Ma et al., 2006).

The nodulin-26 (GmNOD26) localized on the plant-derived symbiosome membrane (SM) (see the glossary in the supplementary information) in the root nodules was firstly characterized in soybean (Deshmukh et al., 2013; Fortin, Morrison, & Verma, 1987) and it also belongs to NIPs (see the review of Pommerrenig, Diehn, & Bienert, 2015). Moreover, the GmNOD26 on the SM is an aquaporin, a protein efflux for water and ammonia (Clarke, Loughlin, Day, & Smith, 2014; Clarke et al., 2015; Fortin et al., 1987; Verma & Hong, 1996), which has similar functions to NIPs on the root plasma membrane, namely the uptake of water and nutrients, including silicon (Ma et al., 2006). To date, however, it is not known whether this specific GmNOD26 protein on the SM is responsible for transport of metalloids such as silicon and boron, considering that the latter element is needed for nodule development (Bolanos et al., 1994). It is interesting therefore to further investigate whether certain protein channels on the SM are also functionally responsible for metalloid uptake (Pommerrenig et al., 2015), including silicon influx.

3.2 The role of silicon in legumes under abiotic and biotic stresses

Silicon benefits plants by alleviating biotic and abiotic stresses, but the majority of reviews focus on stress alleviation in the grasses (Poaceae). There is, however, ample evidence that silicon may also alleviate stress in legumes. A review by Zhang, Xie, and Lang (2017) considered the role of silicon in alleviating abiotic stresses in legumes, such as drought, metal toxicity, salinity stress and ultraviolet (UV) radiation. For example, silicon can reduce sodium and increase potassium uptake in the roots of some legume species under salinity stress, indicating that silicon is able to influence sodium and potassium homoeostasis through effects on this selective transport mechanism. There have also been reports demonstrating the role of silicon in alleviating other abiotic stresses in legumes, such as acidity stress (Owino-Gerroh, Gascho, & Phatak, 2005), cadmium toxicity (Kabir, Hossain, Khatun, Mandal, & Haider, 2016) and macronutrient deficiency (Miao, Han, & Zhang, 2010).

In addition to abiotic stress, silicon can alleviate the effects of biotic stresses in legumes (Guérin et al., 2014; Nascimento et al., 2014, 2016; Rasoolizadeh et al., 2018) which we summarize in Table 1, though this is mostly studied for pathogens. Broadly speaking, silicon reduces disease severity in legumes following pathogen infection, possibly by hampering the effector recognition of pathogens through silicon deposition at the apoplast (Rasoolizadeh et al., 2018). Very few studies have considered the extent to which silicon alleviates herbivory stress. Ferreira, Moraes, and antunes (2011) reported that silicon supplementation in soybean had a detrimental effect on the silverleaf whitefly Bemisia tabaci. Recently, Johnson et al. (2017), Johnson, Ryalls, Gherlenda, Frew, and Hartley (2018) showed that silicon supplementation in lucerne slightly benefitted the phloem-sucking aphid Acyrthosiphon pisum but it was also reported by Johnson et al. (2019) that silicon effectively reduced relative growth rate of the Australian native bollworm Helicoverpa punctigera. However, the limited number of studies thus far restricts our understanding of how silicon affects legume–herbivore interactions.

TABLE 1. The effects of silicon on legumes following pathogen infections and insect herbivores. Upward arrow indicates a significant increase in the response; downward arrow indicates a significant decrease in the response; NS = an insignificant response at p > 0.05. Asterisks (*) indicate that while statistically significant effects of silicon supplementation were reported, these depended on specific circumstances (e.g. silicon source, day after inoculation or cultivar types)
Biotic stressors Legume species Observed responses Outcomes References
Pathogens
Biotrophic fungus Soybean (G. max) Shoot silicon (Si) Arsenault-Labrecque, Menzies, and Bélanger (2011)
Phakopsora pachyrhizi   Disease severity
  Leaf incidence (%) of pathogen symptoms NS Nolla, Korndörfer, and Coelho (2006)
  Incubation period (IP) ↑* Cruz, Rodrigues, and Diniz (2014)
  Latent period (LP50) NS
  Number of lesions ↓*
  Number of uredia ↓*
  Area under rust progress curve (AURPC) ↓*
  Final disease severity (FDS) ↓*
Biotrophic oomycete   Leaf incidence (%) of pathogen symptoms ↓* Nolla et al. (2006)
Peronospora manshurica  
Hemibiotrophic oomycete   Phenotypic responses  
Phytophthora sojae   Plant dry biomass Rasoolizadeh et al. (2018)
  Number of differentially expressed genes (DEFGs)
  Defence-related genes
  Secondary metabolism-related genes
  Phytohormone-related genes
  Primary metabolism-related genes
  Pathogen transcriptome
  Expression of pathogen effectors
Soybean (G. max)
Cultivar 1: TaLsi1 Shoot dry biomass Guérin et al. (2014)
Area under disease progress curve (AUDPC)
Shoot silicon
Cultivar 2: EaLsi1 Shoot dry biomass
Area under disease progress curve
Shoot silicon
Cultivar 3: Hikmok sorip Shoot dry biomass
Area under disease progress curve
Shoot silicon
Cultivar 4: Jack Shoot dry biomass
Area under disease progress curve NS
Shoot silicon
Necrotrophic fungus Soybean (G. max) Shoot silicon Nascimento et al. (2014)
Cercospora sojina   Disease severity
  Lipoxygenases (LOX) ↓*
  Phenylalanine ammonia-lyases (PAL)
  β−1,3-glucanases (GLU) ↓*
  Chitinases (CHI) ↓*
  Cell wall peroxidases (POX)
  Polyphenoloxidases (PPO)
  Cellulases (CEL) ↑*
  Xylanases (XYL) ↑*
  Pectin methyl esterases (PME) ↑*
  Polygalacturonases (PG) ↑*
  Total soluble phenolics (TSP) ↑*
  Lignin-thioglycolic acid (LTGA)
  Leaf incidence (%) of pathogen symptoms Nolla et al. (2006)
Soybean (G. max) Nascimento et al. (2016)
Cultivar 1: Bossier (susceptible) Frogeye leaf spot severity
  Foliar superoxide dismutase (SOD)
  Foliar catalase (CAT)
  Foliar peroxidase
  Foliar ascorbate peroxidase (APX)
  Foliar glutathione peroxidase (GPX)
  Foliar glutathione s-transferase (GST)
  Foliar glutathione reductase (GR)
  Foliar ascorbate (AsA) concentrations NS
  Foliar reduced glutathione (GSH) concentrations NS
  Foliar oxidised glutathione (GSSG) concentrations NS
  Superoxide (O2-) concentrations
  Foliar hydrogen peroxide (H2O2) concentrations NS
  Foliar malondialdehyde (MDA) concentrations NS
Cultivar 2: Conquista (resistant) Frogeye leaf spot severity
  Foliar superoxide dismutase NS
  Foliar catalase
  Foliar peroxidase
  Foliar ascorbate peroxidase
  Foliar glutathione peroxidase NS
  Foliar glutathione s-transferase NS
  Foliar glutathione reductase
  Foliar ascorbate (AsA) concentrations NS
  Foliar reduced glutathione (GSH) concentrations
  Foliar oxidised glutathione (GSSG) concentrations
  Superoxide (O2-) concentrations
  Foliar hydrogen peroxide (H2O2) concentrations NS
    Foliar malondialdehyde (MDA) concentrations NS
Viruses (applied singly)       Izaguirre-Mayoral et al. (2017)
Cowpea chlorotic mottle virus (CCMV) Cowpea Shoot dry biomass
V. unguiculata
  Leaf ureide
  Leaf amino acids
  Nodule dry biomass
  Nodule ureide
  Nodule amino acids
Yardlong bean Shoot dry biomass
V. unguiculata subsp. sesquipedalis
  Leaf ureide
  Leaf amino acids
  Nodule dry biomass
  Nodule ureide
  Nodule amino acids
Mung bean Shoot dry biomass
V. radiata
  Leaf ureide
  Leaf amino acids
  Nodule dry biomass
  Nodule ureide
  Nodule amino acids
Cowpea mild mottle virus (CMMV) Cowpea Shoot dry biomass Izaguirre-Mayoral et al. (2017)
V. unguiculata
  Leaf ureide
  Leaf amino acids
  Nodule dry biomass
  Nodule ureide
  Nodule amino acids
Yardlong bean Shoot dry biomass
V. unguiculata subsp. sesquipedalis
  Leaf ureide
  Leaf amino acids
  Nodule dry biomass
  Nodule ureide
  Nodule amino acids
Mung bean Shoot dry biomass
V. radiata
  Leaf ureide
  Leaf amino acids
  Nodule dry biomass
  Nodule ureide
  Nodule amino acids
Insect herbivores
The pea aphid (Acyrthosiphon pisum) Lucerne Plant dry biomass Johnson et al. (2017)
M. sativa cv. Sequel
  Nodule numbers
  Aphid abundance
  Shoot silicon NS
  Dry mass Johnson et al. (2018)
  Nodule numbers
  Foliar silicon NS
  Aphid abundance
The silverleaf whitefly (Bemisia tabaci biotype B) Soybean (G. max)     Ferreira et al. (2011)
Cultivar 1: IAC−19 (moderately resistant) Nymphal survival
  Foliar concentrations of non-protein organic nitrogen
Cultivar 2: MONSOY−8001 (susceptible) Nymphal survival NS
  Foliar concentrations of non-protein organic nitrogen NS
The Australian native bollworm (Helicoverpa punctigera) Cultivar Richmond Foliar K concentrations NS Johnson et al. (2019)
  Foliar Na concentrations NS
  Foliar Ca concentrations NS
  Foliar Si concentrations
  Leaf dry biomass NS
  Insect relative growth rate

Based on the information summarized in Table 1, little is known about the underlying molecular mechanisms of how silicon alleviates stress in legumes. The mechanisms putatively identified at biochemical and physiological levels in legumes may be similar to the mechanisms by which silicon alleviates stresses in other plant taxa, since these seem generic and not specific to particular plant families (Coskun et al., 2018; Frew et al., 2018; Ma, 2004).

3.3 Does silicon alter symbiotic nitrogen fixation?

In contrast to the many studies of the benefits of silicon to legume species in terms of stress alleviation, the functional role of silicon in the context of legume–rhizobia symbioses has been largely overlooked. Nonetheless, we have some information about how silicon affects the symbiotic traits, host growth and chemical composition in legume species (summarised in Table 2).

TABLE 2. The effects of silicon on root nodulation, nitrogen fixation, host chemistry and growth in legumes. Upward arrow indicates a significant increase in the response; downward arrow indicates a significant decrease in the response; NS = an insignificant response at p > 0.05. Asterisks (*) indicate that while statistically significant effects of silicon supplementation were reported, these depended on specific circumstances (e.g. silicon concentration, silicon source or growing media). The number in parentheses indicates references: [1] Nelwamondo and Dakora (1999); [2] Nelwamondo et al. (2001); [3] Dakora and Nelwamondo (2003); [4] Mali and Aery (2009); [5] Izaguirre-Mayoral et al. (2017); [6] Kurdali et al. (2018); [7] Johnson et al. (2017); [8] Johnson et al. (2018); [9] Garg and Singh (2018); [10] Steiner et al. (2018); [11] Miyake and Takahashi (1985)
Observed responses Legume species
Cowpea Dhaincha Lucerne Mung bean

Pigeonpea

cv. PUSA 2002

Pigeonpea

cv. PUSA 991

Soybean cv. BRS 1074 IPRO Soybean cv. BRS 800A Soybean Isogenic line A62-1 Yardlong bean
Root nodulation
Nodule numbers [1]*, [4]   [7], [8]   NS[9] NS[9] NS[10] [10]    
Nodule dry biomass [1]*, [5]; NS[4]     [5] [9] NS[9] [10] [10]   [5]
Nodule ureide [5]     NS[5]           [5]
Nodule amino acids [5]     NS[5]           [5]
Nodule silicon         [9] [9]        
Nodule nitrogen         [9] NS[9]        
Nodule phosphorus         NS[9] NS[9]        
Nitrogenase activity         NS[9] NS[9]        
Leghaemoglobin concentration         [9] [9]        
Number of bacteroids [2]                  
Size of intercellular spaces [2]*                  
Nodule cell wall thickness [2]*                  
Number of symbiosomes [2]*                  
Size of bacteroids [2]                  
Size of symbiosomes [2]                  
Size of peribacteroid spaces [2]                  
Nitrogen fixation
Nitrogen fixed (total nitrogen-seed nitrogen) [1]*                  
Specific nitrogen-fixing activity (nodule nitrogen) [1]*                  
Root nitrogen derived from atmosphere (%Ndfa)   [6]                
Shoot nitrogen derived from atmosphere (%Ndfa)   NS[6]                
Total plant nitrogen derived from atmosphere (%Ndfa)   NS[6]                
Root nitrogen use efficiency (%NUE)   [6]                
Shoot nitrogen use efficiency (%NUE)   NS[6]                
Total plant nitrogen use efficiency (%NUE)   NS[6]                
Plant chemistry
Total shoot nitrogen content [4]* NS[6]                
Total root nitrogen content [4]* [6]             [11]  
Total foliar nitrogen content             [10] [10] [11]  
Total stem nitrogen content                 [11]  
Total plant nitrogen   NS[6]                
Foliar ureide [5]     [5]           [5]
Foliar amino acids [5]   [7] [5]           [5]
Relative chlorophyll index (RCI)             [10] [10]    
Abscisic acid [3]*                  
Cytokinins zeatin ribose [3]*                  
Plant growth
Root dry biomass [3]*, [4] [6]         [10] [10]    
Shoot dry biomass NS[3]; ↑[4]*, [5] [6]   [5]     [10] NS[10]   [5]
Root: shoot ratio [3]* NS[6]                
Total plant dry biomass   [6] [7], [8]   [9] NS[9]        
Plant height             [10] NS[10]    
Stem diameter             [10] NS[10]    
Number of trifoliate leaves per plant             [10] [10]    
Leaf area             [10] [10]    
Specific leaf area             NS[10] NS[10]    
Leaf area ratio             [10] [10]    
Leaf weight ratio             [10] [10]    

Studies investigating the role of silicon on root nodulation and nitrogen fixation originated around 20 years ago. Amongst the first findings was that silicon promoted root nodulation and nitrogen fixation in cowpea Vigna unguiculata (Nelwamondo & Dakora, 1999). Moreover, they found a significant increase of nitrogen fixed from the nodules depending on silicon concentrations, types of silicon and growing substrates. Dakora and Nelwamondo (2003) subsequently reported that silicon supplementation increased the production of endogenous abscisic acid (ABA), which can play a role during nodulation (Ferguson & Mathesius, 2003).

Silicon not only affects nodule numbers and nitrogen fixation but also the internal structure of the nodule. In particular, Nelwamondo, Jaffer, and Dakora (2001) showed that at moderate concentrations, silicon significantly increased the number of bacteroids and symbiosomes (see the glossary in the supplementary information) but decreased the size of peribacteroid space. At high concentrations, silicon significantly increased cell wall thickness of the nodules. Increases in cell wall thickness are characteristic of silicon integration in the cell wall in most of high silicon-accumulating plants (Kumar, Soukup, & Elbaum, 2017; Marschner, Oberle, Cakmak, & Römheld, 1990). Moreover, a recent study by Garg and Singh (2018) reported the presence of silicon in the root nodules which may explain the thicker cell walls of the nodules observed by Nelwamondo et al. (2001).

Silicon deposition (silicification) within the root nodules (Garg & Singh, 2018) may change the internal structures of the nodules (as previously reported by Nelwamondo et al., 2001), potentially affecting nodule permeability and diffusion resistance (Sheehy, Minchin, & Witty, 1985). Most likely, this might be related to the hydrophilic property of silicon (Soukup et al., 2017). For example, silicon supplementation improved water retention capacity in sorghum roots, leading to improved drought tolerance in these plants (Hattori et al., 2005). Changes in nodule permeability may potentially affect solute transport and gaseous (e.g. oxygen and nitrogen) diffusion (Dakora & Atkins, 1989; Sheehy et al., 1985). An increase of oxygen and nitrogen fluxes can accelerate bacteroid metabolism inside the root nodules, such as bacterial respiration and nitrogen fixation respectively (Dakora & Atkins, 1989).

Other studies addressing the impacts of silicon on legumes (listed in Table 2) also report how silicon supplementation increased nodule dry biomass in two different cultivars of soybean (Steiner, Zuffo, Bush, & Santos, 2018) and in three different Vigna plants (Izaguirre-Mayoral, Brito, Baral, & Garrido, 2017). In addition, Kurdali, Al-Chammaa, and Al-Ain (2018) showed that the effects of silicon on root nodulation and nitrogen fixation were more profound under salinity and/or water stress in Sesbania legume but relatively minor in the absence of such stress.

A more recent study also reported that silicon promoted nodule activity via the production of leghaemoglobin (see the glossary in the supplementary information) in two different genotypes of pigeonpea (Garg & Singh, 2018). This symbiotic leghaemoglobin is crucial to buffer oxygen level in the nodule environment and to stabilize the oxygen-sensitive nitrogenase while maintaining oxygen supply for the respiration of bacteroids (Ott et al., 2005). However, the mechanism by which silicon increases leghaemoglobin in the root nodules remains unknown.

Taken together, it is therefore clear that silicon may employ dual functionality for legume–rhizobia symbioses: not only does it ameliorate its effects on the stress but it also has beneficial impacts on the symbiotic traits (e.g. increased root nodulation and nitrogen fixation).

4 POSSIBLE MECHANISMS BY WHICH SILICON BENEFITS SYMBIOTIC FUNCTIONS IN LEGUMES

4.1 Silicon and cell wall components: Is this key to resource liberation?

Over the last decade or so it has been increasingly reported that silicon interacts with cell wall components, such as lignin, cellulose, hemicelluloses, callose and ferulic acid (see the glossary in the supplementary information) by forming a silicon-organic complex (He, Ma, & Wang, 2015; He et al., 2013; Kulich et al., 2018; Kumar, Milstein, Brami, Elbaum, & Elbaum, 2017; Kumar, Soukup, et al., 2017; Soukup et al., 2017). As a consequence, some plants preferentially incorporate silicon as part of the cell wall (Schoelynck et al., 2010; Schoelynck & Struyf, 2016).

Yamanaka et al. (2012) revealed the absence of lignin in the silica-rich outer internodes of horsetail Equisetum hyemale. Furthermore, Yamamoto et al. (2012) reported that silicon supplementation significantly decreased the concentrations of lignin in rice. Confirmatory studies reported that there is a strong negative correlation between the concentrations (% of plant dry matter) of silicon and lignin in rice (Klotzbucher et al., 2018; Suzuki et al., 2012) and between silicon and carbon in wheat straw Triticum aestivum (Neu, Schaller, & Dudel, 2017). Structural carbon might be substituted with silicon to a certain degree, hypothesized to be a cheaper mechanism for providing plant structural support with plants calculated to incur 20-fold lower metabolic costs when they incorporated silicon rather than lignin into the cell wall (Raven, 1983). It was suggested that silicon resembled lignin in terms of structural strength and that the physicochemical properties of silicon potentially allows it to form a silicon-organic complex with other cell wall components (Weiss & Herzog, 1978). To date, however, little is understood of how silicification may occur in legume species specifically (but see Rasoolizadeh et al., 2018) and what this means for legume–rhizobia relationships.

4.2 A hypothetical model

Silicification (Figure 1) in leguminous roots might occur in the cell walls of (a) endodermal cells which are in between the pericycle (see the glossary in the supplementary information) and cortical cells, similar to what has been found in sorghum (Kumar, Soukup, et al., 2017; Soukup et al., 2017); (b) epidermal cells, similar to what has been found in rice and horsetail (Fleck et al., 2011; Law & Exley, 2011; Yamanaka et al., 2012) and (c) root nodules based on observed increases in the cell wall thickness in silicon-treated cowpea (Nelwamondo et al., 2001) and the presence of silicon in the root nodules of pigeonpea (Garg & Singh, 2018; Figure 1B).

Details are in the caption following the image
Schematic representation of how silicon may promote root nodulation and nitrogen fixation in legumes. (A) Silicon is taken up from the soil by the roots in the form of orthosilicic acid (Si (OH)4), distributed and translocated into above- and below-ground compartments. In nature, plants are commonly exposed to biotic and abiotic stresses (red arrows) - (B) Nodule development (①–④) initially starts when roots emit chemical cues (isoflavonoids) to attract free-living rhizobia and in turn these rhizobia respond to the chemicals by secreting Nod factors (see the glossary in the supplementary information) to the root hairs, allowing them to be recognized by the host. Following the recognition, root hairs begin to curl and rhizobia start to induce the formation of infection thread by injecting certain proteins, allowing them to penetrate into the root cortex. Subsequently, the nodulins (the induced plant proteins) trigger the division of root cortical cells to initiate further nodule development. At this stage, rhizobia start to transform themselves into nitrogen-fixing bacteroids. Finally, mature nodules are formed and nutrient exchange occurs via the nodule vascular tissues. Simultaneously, silicon might interfere with the secretion of chemical cues to free-living rhizobia, affecting legume–rhizobium communication and nodule development. For instance, silicon may be deposited (solid red lines) at these three potential sites of the cell walls, namely of (a) the epidermal cells; (b) the endodermal cells and (c) the nodules. (C) Silicification at these potential sites may cause several downstream consequences, such as 1. Substitution of structural carbon with silicon, 2. Increased nodule permeability and 3. Environmental stresses alleviation. Taken together, we predict that these downstream consequences may result in: (1) increased provision of carbon-derived compounds from the host legume to its associated rhizobia, (2) rapid transport of solute and gasses between nodules, root tissues and the soil and/or (3) increased host tolerance and resistance to stress (red arrows). Finally, these proposed hypotheses could explain the underpinning mechanisms by which silicon promotes root nodulation and nitrogen fixation in legumes (yellow arrow)

Silicification in legume species could generate multiple consequences, three of which seem feasible based on the evidence discussed so far. First, silicification in the root nodules may increase nodule permeability to facilitate rapid solute and gas exchange, accelerating the metabolism of bacteroids and nitrogen fixation and/or facilitating nitrogen transfer to the host plant. Second, potential substitution of structural organic carbon with silicon might liberate carbon that could be allocated by the host legume to bacteroids inside root nodules, thus maintaining the nutrient supply between the host legume and its rhizobial partner. Finally, silicification in legume species may protect the plant more generally from various environmental (abiotic and biotic) stresses, for example by increasing protective chemical and biomechanical traits. These proposed mechanisms and their potential consequences may help explain previous accounts of silicon promoting symbiotic functions in legume–rhizobia relationships (see Table 2).

Other factors associated with silicon supplementation could also affect root nodulation. Firstly, silicon supplementation can increase soil pH (Haynes & Zhou, 2018; Li et al., 2019) which could, theoretically, promote nodulation under some circumstances since acidity stress can impair root nodulation (Owino-Gerroh et al., 2005). Nonetheless, the studies outlined in Table 2 use a variety of growing media ranging from hydroponic solutions, where the pH is tightly controlled, to different soil types thus it seems unlikely that pH changes are solely responsible for the beneficial effects of silicon on root nodulation. Furthermore, the pH effect of silicon on root nodulation may be masked by soil pH buffering capacity as this effect is highly dependent on soil properties, constituents and types (Li et al., 2019; Nelson & Su, 2010). Secondly, silicon supplementation might be able to increase essential nutrient uptake from the soil (e.g. ammonium), possibly reducing the need for legumes to form symbioses with rhizobia when inorganic nitrogen is highly present in the soil (Richardson, Jordan, & Garrard, 1957; Xia, Ma, Dong, Xu, & Gong, 2017). An experiment conducted by Sheng, Ma, Pu, and Wang (2018) demonstrated that silicon supplementation improved cell wall rigidity and stabilized the plasma membrane in rice at the single-cell level. As a result, the cell wall took up more ammonium without upregulation of certain transmembrane transporter proteins associated with ammonium uptake. However, another recent study reported that silicon supplementation only showed a neutral effect on nitrogen concentration in wheat straw and grain (Neu et al., 2017). Key to this argument is that silicon supplementation increases nitrogen uptake by host plants to the extent that nitrogen is no longer limiting for growth, but this is unlikely given the high concentrations of nitrogen required and relatively low concentrations available in soil.

On the other hand, it might be possible that silicon supplementation in legumes may increase water uptake (Chen, Wang, Yin, & Deng, 2018), for instance via increased root-to-shoot ratio (Dakora & Nelwamondo, 2003). An increase of water uptake may promote host tolerance against soil water deficit (Chen et al., 2018), potentially maintaining the legume–rhizobia relationship under periods of drought. It might also be possible that silicon supplementation increases the uptake of important trace elements from the soil, such as molybdenum, cobalt, copper, iron and boron. If this additional mechanism is true then increases in these elements may be beneficial for enhancing nitrogen fixation and nodule formation (Bolanos et al., 1994; Ott et al., 2005).

An important knowledge gap is how silicon affects the mechanics of root nodule formation. Nodule formation starts with the initiation of an infection thread in the root hairs, which then extends into the stele and finally forms mature nodules (Figure 1B). We currently do not fully understand whether silicification on the root cell wall in legumes will alter the chemical communication between host legumes and rhizobia (e.g. exudation of isoflavonoids). So far, it is unknown whether silicon might affect the production of the key symbiotic isoflavonoid signals in nitrogen-fixing legumes. However, this might be possible since Fawe, Abou-Zaid, Menzies, and Belanger (1998) demonstrated that silicon supplementation promoted the production of antifungal flavonoid in cucumber Cucumis sativus to powdery mildew. Considering the multiple role of isoflavonoids in root–rhizosphere interactions (Dakora & Phillips, 1996; Hassan & Mathesius, 2012), a fruitful area of research would be to investigate whether silicon supplementation alters the production of this key symbiotic cue and how this subsequently impacts the attraction of free-living rhizobia in the root–soil interface as suggested by Nelwamondo and Dakora (1999) and Johnson et al. (2017). Silicon may also affect the formation of the infection thread, and the exchange of solutes between nodule vascular tissues and plant vascular tissues. These issues are crucial to address because silicification on root cell walls might be a ‘double-edged sword’, for example, hampering the recognition of Nod-factors during the establishment of the infection thread (as discussed in Nelwamondo & Dakora, 1999; Nelwamondo et al., 2001).

5 RESEARCH QUESTIONS AND FUTURE PROSPECTS

Our conclusion is that silicon plays a more important functional role in legumes than is currently appreciated and we suggest these functions should be more extensively investigated. Considering our proposed hypotheses (Figure 1), we suggest working with tractable study systems to answer research questions about how silicon promotes root nodulation and nitrogen fixation and how universal these effects are. For example, a number of fully sequenced genomes of host legumes and their rhizobial symbionts (e.g. G. max and its rhizobial symbiont Bradyrhizobium japonicum or M. truncatula and its associated symbiont Ensifer meliloti) would be excellent study systems to investigate the extent to which silicon influences legume–rhizobia symbioses (Galibert et al., 2001; Schmutz et al., 2010; Terpolilli et al., 2013; Terpolilli, O'Hara, & Tiwari, 2008).

We propose four research priorities. First, to characterize how silicon may alter the chemical dialogues between host legumes and rhizobia, for example, the production of isoflavonoids (Dakora, Joseph, & Phillips, 1993). Second, to determine how the corresponding genes that regulate nodulation and nitrogen fixation are affected by silicon addition (Fischer, 1994; Gottfert, 1993). Third, to determine how silicon may alter the anatomical structure of root and/or nodules and how this may affect nodule permeability (Dakora & Atkins, 1989) by combining some techniques in microscopy (Haynes et al., 2004; Kumar & Elbaum, 2018) and other applications. NanoSIMS (Nanoscale Secondary Ion Mass Spectrometry), for example, may be particularly useful for detecting silicon quantitatively (Nuñez, Renslow, Cliff, & Anderton, 2017) in legume tissues, including measuring silicon deposition inside root nodules. Fourth, to understand whether silicon influences the carbon and nitrogen exchange in legume–rhizobia symbioses, exogenous addition of isotopic labelling of 15N and 13C could be applied to measure how much of the externally supplied labelled nitrogen and carbon end up in the host and in the nodules respectively (see the technique in Wilkinson, Ferrari, Hartley, & Hodge, 2019).

The agronomic benefits of silicon in legumes should be viewed within an ecological context, however, changes in nitrogen budgets may be beneficial to herbivorous pests which are usually nitrogen-limited (Mattson, 1980). Silicon supplementation in lucerne M. sativa, for example, slightly increased plant susceptibility to the pea aphid A. pisum possibly via an increase of foliar essential amino acids (Johnson et al., 2017). Furthermore, silicon promotion of root nodulation might increase attraction of nodule-feeding insects (such as Sitona spp. weevils) in soil (Johnson et al., 2006). However, silicon supplementation could equally reverse these negative consequences by augmenting plant resistance to herbivores (Frew, Powell, Allsopp, Sallam, & Johnson, 2017; Johnson et al., 2019).

We propose it is timely to realize the significance and ample benefits of silicon in legumes in various ecological contexts. Several studies have indicated that silicon affects the legume–rhizobia interaction, including root nodulation and nitrogen fixation. We therefore have proposed a hypothetical model by which silicon may promote root nodulation and nitrogen fixation via silicification at three potential sites of the roots including the one inside root nodules. Silicification at these sites may allow the host legume to invest and allocate more carbon to its rhizobial partners in root nodules. In particular, silicification in the nodules may optimize nodule permeability to support oxygen and nitrogen fluxes, potentially enhancing both respiration of bacteroids and nitrogen fixation. Finally, the ameliorative effects of silicon on environmental stresses may also be beneficial in the context of legume–rhizobia relationships, for example increases in host resistance against pathogens and insect antagonists, thereby maintaining legume health. Investigations into how silicon supplementation promotes these symbiotic functions will enable a better understanding of how to maximize legume health, yield and productivity. This functional group includes some of the most important food and forage crops in the world, which improve soil health and fertility and have high nutritional value; hence this research could provide significant benefits for future food security.

ACKNOWLEDGEMENTS

We thank the editors and reviewers, Casey Hall, Jamie Waterman, Rebecca Vandegeer, Tarikul Islam and Ximena Cibils Stewart for their valuable feedback on this manuscript. We also thank Dominika Anggraeni Purwaningsih for her illustration in Figure 1. R.P. is the holder of a scholarship as part of an Australian Research Council Future Fellowship (FT170100342) awarded to S.N.J., the Australian Steel Mill Services (ASMS) and the University of York in the United Kingdom.

    AUTHORS' CONTRIBUTIONS

    R.P., J.R.P., S.E.H. and S.N.J. conceived the idea; R.P. led the writing of the manuscript and all authors critically contributed to the drafts and gave final approval for publication.

    DATA AVAILABILITY STATEMENT

    This manuscript does not use data.