Hybrids account for nearly all commercially planted varieties of maize and many other crop plants because crosses between inbred lines of these species produce first-generation [F₁] offspring that greatly outperform their parents. The mechanisms underlying this phenomenon, called heterosis or hybrid vigor, are not well understood despite over a century of intensive research. The leading hypotheses—which focus on quantitative genetic mechanisms (dominance, overdominance, and epistasis) and molecular mechanisms (gene dosage and transcriptional regulation)—have been able to explain some but not all of the observed patterns of heterosis. Abiotic stressors are known to impact the expression of heterosis; however, the potential role of microbes in heterosis has largely been ignored. Here, we show that heterosis of root biomass and other traits in maize is strongly dependent on the belowground microbial environment. We found that, in some cases, inbred lines perform as well by these criteria as their F₁ offspring under sterile conditions but that heterosis can be restored by inoculation with a simple community of seven bacterial strains. We observed the same pattern for seedlings inoculated with autoclaved versus live soil slurries in a growth chamber and for plants grown in steamed or fumigated versus untreated soil in the field. In a different field site, however, soil steaming increased rather than decreased heterosis, indicating that the direction of the effect depends on community composition, environment, or both. Together, our results demonstrate an ecological phenomenon whereby soil microbes differentially impact the early growth of inbred and hybrid maize.

杂交种几乎涵盖了所有商业种植的玉米品种和许多其他作物植物，因为这些物种的近交系之间的杂交产生了第一代 [F ₁] 大大优于父母的后代。尽管进行了一个多世纪的深入研究，但这种现象背后的机制称为杂种优势或杂种优势，尚未得到很好的理解。主要假设——侧重于定量遗传机制（优势、超优势和上位性）和分子机制（基因剂量和转录调控）——已经能够解释一些但不是所有观察到的杂种优势模式。众所周知，非生物胁迫因素会影响杂种优势的表达；然而，微生物在杂种优势中的潜在作用在很大程度上被忽视了。在这里，我们表明玉米的根生物量和其他性状的杂种优势强烈依赖于地下微生物环境。我们发现，在某些情况下，近交系在这些标准下的表现与其 F _1一样好在不育条件下产生后代，但杂种优势可以通过接种 7 种细菌菌株的简单群落来恢复。我们观察到在生长室中接种高压灭菌与活土壤浆液的幼苗以及在田间蒸过或熏蒸过的土壤与未经处理的土壤中生长的植物的相同模式。然而，在不同的田间地点，土壤蒸腾增加而不是减少杂种优势，表明影响的方向取决于群落组成、环境或两者。总之，我们的结果证明了一种生态现象，即土壤微生物对近交系和杂交玉米的早期生长有不同的影响。