Is it time to include legumes in plant silicon research?

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.

Silicon in the Soil–Plant Continuum: Intricate Feedback Mechanisms within Ecosystems

“…Several transporters and genes that are involved in Si uptake and accumulation have been studied so far. Although the study of Si transporters focuses on rice and other grasses (as is commonly the case in plant Si research [ 5 , 18 ]), the first plant gene to regulate Si accumulation was discovered in the gourd Cucurbita (Cucurbitaceae), regulating Si and phytolith formation in the fruit rind [ 108 ]. Shortly after, a surge of discoveries of the physiology and genetics of Si uptake in grasses has arisen, revolving around the four Lsi transporters, all belonging to the NIP aquaporin family.…”

“…Second, although the variability of Si content has a clear taxonomic signal, with orders accounting for 67% of the variation [ 172 ], phylogenetic analyses indicate great variations below the order level [ 23 ]. For example, most literature cites the grass family Poaceae and more generally the order Poales as being the most Si-rich, at least among angiosperms [ 18 , 172 , 173 , 174 ], which is in part why a large portion of plant Si research focuses on this family [ 5 , 18 ]. However, Poales is also the only commelinid order to date in which Si-poor families were observed (e.g., Typhaceae) [ 23 ].…”

“…These positive effects of Si and plant nutrition are not only shown for grasses [ 85 , 207 , 388 ] but also for legumes with Si increasing N-fixation by rhizobacteria [ 389 ]. However, as N fixation being increased by better P nutrition [ 5 , 390 ] and Si increasing plants’ P nutrition (as shown for grasses, see above), the positive effect of Si on N fixation may be indirectly due to Si increasing plants P nutrition. For sugarcane, no positive effect of Si on P nutrition was found [ 391 ].…”

“…Silicon (Si) uptake and accumulation is a functional trait with multiple implications for plant biology and ecology [ 1 , 2 ]. Silicon’s manifold functions in plant biology include protection from a myriad of abiotic and biotic stresses and confers many benefits to plants that are capable of taking up and accumulating large amounts of it, ranging from practically zero in some taxa to 5% dry weight (and in extreme cases even more) in grasses [ 1 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 ], and probably even to those plants that take up considerably smaller amounts [ 5 , 18 ]. Moreover, its uptake from the soil and eventual reincorporation into the soil through plant litter and herbivore feces also affects soil properties and Si cycling [ 19 , 20 , 21 , 22 , 23 ].…”

The Regulatory Role of Silicon in Mitigating Plant Nutritional Stresses

“…For this purpose, the different responses of Si-accumulating and non-accumulating species must be carefully considered when exposed to individual mineral stress, to build a general conclusion. Notably, as proposed by Putra et al [ 95 ] an extensive investigation needs to be considered on the cross-talk between Si and legumes to decipher how Si promotes the symbiotic relationship with nitrogen-fixing bacteria. This body of Si is crucial in the future works as numerous studies showed the positive interactive effect of Si and plant growth promoting rhizobacteria on plant growth particularly under unfavorable environmental conditions [ 96 ].…”