Silicon enrichment alters functional traits in legumes depending on plant genotype and symbiosis with nitrogen-fixing bacteria

1. 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.

2. 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.

3. 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 was 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).

4.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.

1. 硅（Si）在植物组织中的吸收与沉积（硅化作用，silicification）可缓解逆境胁迫，整体提升植物健康水平。当前该领域的研究多集中于高硅累积植物（如草本植物），而其对豆科植物等其他植物功能群的效应却相对较少被探明。有推测认为硅可能对豆科植物与其共生的固氮细菌（根瘤菌，rhizobia）之间的共生关系产生积极影响，但相关机制仍未得到充分阐释。本研究探讨了富硅处理对与根瘤菌共生的豆科植物的影响，以及硅发挥作用的潜在内在机制。
2. 本研究采用模式根瘤菌株，针对紫花苜蓿（Medicago sativa）与截形苜蓿（Medicago truncatula）开展了温室盆栽试验。共设置6个基因型（每个物种各3个），分别进行富硅补充处理（+Si组）与未处理对照（-Si组）。我们定量测定了16项功能性状，涵盖植物生长、生理、元素化学、根瘤活性及固氮作用五大类别。
3. 两种豆科植物对硅处理的响应存在显著差异。例如，富硅处理使紫花苜蓿的地上部生物量提升超过10%，但对截形苜蓿的生长无显著影响。与之相反，富硅处理使截形苜蓿的固氮酶（nitrogenase）活性提升超过85%，而紫花苜蓿的该活性未受影响。此外，对紫花苜蓿进行富硅处理可使其叶片硅浓度提升超过36%，但根瘤中的硅浓度无显著变化。紫花苜蓿的Sequel与Trifecta基因型中，叶片硅浓度的提升分别与其地上部、根部生物量的增加呈显著正相关。与之相反，截形苜蓿经富硅处理后，其根瘤中的硅浓度提升29%，但叶片组织中的硅浓度无明显变化。截形苜蓿的不同基因型之间，固氮酶活性以及硅化作用发生的部位均存在差异：其中Sephi基因型的固氮酶活性与根瘤硅浓度呈正相关，而非叶片硅浓度；而Hannaford基因型则呈现相反的关联模式。
4. 本研究表明，两种亲缘关系相近的豆科植物对硅处理的响应方式存在显著差异，且该差异取决于植物基因型与共生关系。本研究结果揭示了硅在豆科植物-根瘤菌共生互作中被忽视的功能，该功能有望提升这类重要植物类群的生产性能。