In maize and many other plants, F1 hybrids perform better than their inbred parent lines - a phenomenon known as heterosis or hybrid vigor. The causes of heterosis have been investigated for over a century but are still poorly understood: genetic and molecular mechanisms such as dominance, overdominance, and gene dosage can explain some but not all observations of heterosis. The strength of heterosis can be environment-dependent, usually with stronger heterosis in more stressful conditions (e.g., drought or cold). Our recent work shows that soil microbes are another environmental factor that affects heterosis, opening a fresh line of inquiry into the underlying genetic and physiological mechanisms.
In a series of lab, growth chamber, and field experiments we show that perturbation of soil biota differentially affects the vigor of hybrid and inbred maize genotypes, although the direction of the effect is context-dependent. Our follow-up work tests several potential explanations, including (1) superior resistance of hybrids to weakly pathogenic soil biota, (2) immune over-reactions by inbred maize in response to innocuous soil biota, (3) over-reliance of inbred genotypes on microbial mutualists for nutrient provision, and (4) superior ability of hybrids to recruit or benefit from growth-promoting mutualists. To explore the role of immunity in heterosis, we identified multiple resistance and susceptibility alleles with dominant modes of action, but no overall heterosis for disease symptom severity. Reactive oxygen species production in response to a microbe-associated molecular pattern (flg22) appears to show negative heterosis. Relative to inbreds, hybrid plants appear to support a higher microbial load per mg of root than do inbred plants. Early evidence suggests that non-pathogenic and beneficial microbes also differentially impact the growth of hybrids and inbreds. Overall, our results support a complex relationship between hybrid vigor and plant-microbe interactions, although the underlying mechanisms and the direction of causality remain unclear.