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.

Aims Silicon (Si) uptake and accumulation improves plant resilience to environmental stresses, but most studies examining this functional role of Si have focussed on grasses (Poaceae) and neglected other important plant groups, such as legumes (Fabaceae). Legumes have evolved a symbiotic relationship with nitrogen-fixing bacteria (rhizobia) housed in root nodules. Our study determined the impacts of silicon (Si) supplementation on Medicago truncatula inoculated with Ensifer meliloti rhizobial strains that differed in their capacity for nitrogen fixation: Sm1021 (‘low-efficiency’) or Sm1022 (‘high-efficiency’). Methods We examined how Si and rhizobial efficacy influence nodule and plant functional traits, including their chemical aspects. These combinations were supplied with or without Si in a glasshouse experiment, where we quantified nodule flavonoids and foliar chemistry (free amino acids, soluble protein, elemental C, N and Si). Results Si supply increased nodule number per plant, specific nodule flavonoid concentrations, contents of foliar nitrogenous compounds and foliar C, but not foliar Si. We also demonstrated that rhizobial efficacy altered the magnitude of Si effects on certain traits. For example, Si significantly promoted concentrations of foliar N and soluble protein in the plants associated with the ‘low-efficiency’ strain only, and this was not the case with the ‘high-efficiency’ one. Conclusions Collectively, our study indicates that Si generates positive effects on M. truncatula, particularly when the association with rhizobia is relatively inefficient, and may play a more prominent role in rhizobial functionality than previously thought.

Silicon (Si) uptake and deposition (silicification) in tissues is known to alleviate stresses and generally improve plant health. This is mostly studied in Si‐high accumulators, such as grasses, with comparatively less known about its effects on other plant functional groups, such as legumes. There is speculation that Si may positively impact the symbiosis between legumes and the nitrogen‐fixing bacteria (rhizobia) they associate with, but this is poorly understood. This study examined the effects of Si enrichment on legume species associated with rhizobia and the potential underlying mechanism of Si impacts. We conducted a glasshouse experiment with lucerne Medicago sativa and barrel medic M. truncatula associated with a model rhizobial strain. Six genotypes (three per species) were either supplemented with Si (+Si) or untreated (−Si). We quantified 16 functional traits which could be classified as plant growth, physiology, elemental chemistry, nodule activity and nitrogen fixation. The two legume species responded to Si distinctively. For example, Si supplementation increased shoot biomass by more than 10% in lucerne but growth was unaffected in barrel medic. Conversely, nitrogen‐fixing enzyme (nitrogenase) activity was promoted by more than 85% in +Si barrel medic plants but not in lucerne. Moreover, Si supplementation of lucerne increased the concentrations of Si in leaves by more than 36% but not in root nodules. Increased foliar concentrations of Si in lucerne were positively associated with increased shoot and root biomass in Sequel and Trifecta genotypes, respectively. Conversely, Si supplementation of barrel medic increased the concentration of Si in root nodules by 29% but not that in foliar tissues. Nitrogenase activity and where silicification occurred, differed between genotypes in barrel medic; nitrogenase activity was correlated with concentrations of Si in root nodules rather than that in foliar tissues in one genotype (Sephi) but the reverse was true in another (Hannaford). This study demonstrates that two closely related legume species can respond to Si in distinct ways, depending on plant genotype and symbiosis. These results present the overlooked function of Si in legume–rhizobia interactions, which could potentially enhance productivity of this important group of plants. A free Plain Language Summary can be found within the Supporting Information of this article.

Plants are involved in intimate interactions with microbes throughout their entire life cycle, and these interactions are essential for the growth and health of the plants. Endophytes are nonpathogenic microorganisms that inhabit plants without causing them any harm (Hardoim, Hardoim, Overbeek, & Elsas, 2012; Rosenblueth & Martinez-Romero, 2006). Endophytic microorganisms live in an intimate relationship with their host throughout many generations (Johnston-Monje & Raizada, 2011). Many endophytic bacteria are known for their growth-promoting effect on plants and for priming plant immunity by triggering induced systemic resistance (ISR) and/ or induced systemic tolerance (IST) (Hardoim, Overbeek, & Elsas

Resource availability during development shapes not only adult phenotype but also the phenotype of subsequent offspring. When resources are absent and periods of starvation occur in early life, such developmental stress often influences key life-history traits in a way that benefits individuals and their offspring when facing further bouts of starvation. Here we investigated the impacts of different starvation regimes during larval development on life-history traits and measures of consumption in the turnip sawfly, Athalia rosae (Hymenoptera: Tenthredinidae). We then assessed whether offspring of starved and non-starved parents differed in their own life-history if reared in conditions that either matched that of their parents or were a mismatch. Early life starvation effects were more pronounced within than across generations in A. rosae, with negative impacts on adult body mass and increases in developmental time, but no effects on adult longevity in either generation. We found some evidence of higher growth rates in larvae having experienced starvation, although this did not ameliorate the overall negative effect of larval starvation on adult size. However, further work is necessary to disentangle the effects of larval size and instar from those of starvation treatment. Finally, we found weak evidence for transgenerational effects on larval growth, with intra-generational larval starvation experience being more decisive for life-history traits. Our study demonstrates that intra-generational effects of starvation are stronger than transgenerational effects on life-history traits and consumption measures in A. rosae.

Summary Some plant species tolerate and accumulate high levels of metals or metalloids in their tissues. The elemental defence hypothesis posits that metal(loid) hyperaccumulation by these plants can serve as protection against antagonists. Numerous studies support this hypothesis. In addition, as other plant species, hyperaccumulators synthesise specialised metabolites that can act as organic defences. In principle, the composition and concentration of plant‐specialised metabolites vary pronouncedly not only among species, but also within species and within individuals. This variation is called chemodiversity. Surprisingly, the role of chemodiversity has received little attention in elemental defence. Thus, we advocate that the concept of the elemental defence hypothesis should be extended and linked to the multifunctionality of plant chemodiversity to better understand the eco‐evolutionary dynamics and maintenance of metal(loid) hyperaccumulation. Comprehensive literature studies revealed that both metal(loid)s and specialised metabolites acting as defences are highly diverse in some hyperaccumulators and the biosynthetic pathways of these two types of defences are partly intertwined. Several edaphic‐, population‐, temporal‐ and spatial‐related factors were found to influence metal(loid) diversity, which should be considered in the elemental defence hypothesis. We thus present a novel synthesis and outlook to extend the elemental defence hypothesis in the light of chemodiversity.

<p>Plants associate with bacteria over the course of evolution. For example, leguminous plants (Leguminosae/Fabaceae) have evolved a distinct symbiosis with nitrogen-fixing bacteria (rhizobia) about 60 million years ago. Rhizobia are housed in specialised root structures, the nodules, and provide the host plants with available nitrogen. In exchange, the host plant rewards rhizobia with carbon-based compounds. The legume-rhizobia symbiosis differs from being mutualistic to somewhat parasitic. One of the driving factors of that is soil nutrients, e.g. silicon (Si). Yet, the functional role of Si in legumes is largely overlooked.</p><p>Previous studies suggest that Si has positive impacts on the legume-rhizobia symbiosis. For example, existing literature demonstrates that Si alleviates a broad range of environmental stresses. Crucially, there is a growing number of studies reporting that Si promotes symbiotic traits, such as increased root nodulation and nitrogen fixation across several leguminous species. To better understand this, a conceptual framework was recently proposed. It is hypothesised that Si uptake and accumulation (silicification) in plant tissues may compensate the high metabolic expenditure of carbon in cell wall formation, accelerate solute transport and gas exchange in the nodules, and protect the plants against stresses.</p><p>To investigate the impacts of Si enrichment on functional traits in legumes, a glasshouse experiment was conducted with a model legume, barrel medic (<em>Medicago truncatula</em>) associated with a rhizobial (<em>Ensifer meliloti</em>) strain SM1021. Three plant genotypes were either enriched with Si (+Si) or untreated (-Si). Furthermore, a suite of key functional traits broadly grouped as plant growth, physiology, elemental chemistry, nodule activity and nitrogen fixation were quantified using several analytical/chemical techniques. Si enrichment altered several traits depending on plant genotype and symbiosis with rhizobia. For example, nodule activity was generally promoted in +Si relative to -Si plants, but with a more profound impact in one specific genotype (Sephi). This promotion was correlated positively with silicification either in the foliar or nodule depending on plant genotype.</p><p>To examine a context dependency of Si impacts in legumes, a full-factorial experiment in a glasshouse was undertaken with the same model legume (two genotypes) and two rhizobial strains, i.e. SM1021 and SM1022, which the former strain is less effective than the latter. Each host-rhizobial association was supplemented with and without Si and challenged with the foliar-chewing cotton bollworm (<em>Helicoverpa armigera</em>) for a 5-day larval infestation (+herbivore and -herbivore). At 30-day post infestation, plants were harvested and further analysed for nodule traits and plant chemistry. Silicon enrichment strongly increased nodule numbers in both rhizobial strains but only in -herbivore plants and this impact was wiped out in +herbivore plants. However, foliar Si was induced only in +Si relative to -Si in +herbivore plants and the reverse was true for foliar C that might indicate a trade-off between Si and C following herbivory. In addition, Si enrichment generally promoted total soluble protein. Finally, when foliar amino acids (AAs) were clustered into essential, non-essential and total compounds, Si enrichment consistently promoted AAs only when herbivory was absent and shifted to a lesser extent when herbivory was present.</p>