Department of Ecology and Evolutionary Biology, University of Kansas, Lawrence, KS 66045;
Kansas Biological Survey & Center for Ecological Research, University of Kansas, Lawrence, KS 66045.
Proc Natl Acad Sci U S A. 2021 Jul 27;118(30). doi: 10.1073/pnas.2021965118.
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 [F] 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 F 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.
杂种占所有商业化种植的玉米和许多其他作物品种的近全部，因为这些物种的自交系之间的杂交产生了第一代 [F] 后代，其表现大大优于其亲本。尽管经过一个多世纪的密集研究，但这种被称为杂种优势或杂种优势的现象的背后机制仍未得到很好的理解。主导假说——侧重于定量遗传机制（显性、超显性和上位性）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。已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，自交系在无菌条件下通过这些标准的表现与它们的 F 后代一样好，但通过接种简单的七株细菌群落可以恢复杂种优势。我们在生长室中用灭菌和活土浆接种的幼苗以及在田间用蒸汽或烟熏处理与未处理的土壤种植的植物中观察到了相同的模式。然而，在另一个不同的田间地点，土壤蒸汽处理增加而不是减少了杂种优势，表明效应的方向取决于群落组成、环境或两者兼而有之。总之，我们的结果表明了一种生态现象，即土壤微生物对自交系和杂交系玉米的早期生长有不同的影响。
