A growing population to feed has made the race for higher-yielding varieties of food crops inevitable. To this end, for nearly a hundred years, biologists have been hybridising commercially important crops by crossing two germlines to create varieties that are not only increasingly resilient but are also taller, stronger and bear more grain.
The phenomenon at play here, whereby the offspring of two genetically divergent individuals is fitter than either of the parents, is known as heterosis or hybrid vigour.
A recent study has found that the microbial community that inhabits the soil could be a key driver of hybrid vigour in maize. Maize, first domesticated in Central America, has a global production higher than that of either wheat or rice, with the USA alone devoting 91.7 million acres to corn production.
— NC State Plant and Microbial Biology (@NCState_PMB) July 22, 2021
The new study shows how the microbial biomass could be involved in conferring an advantage to a hybrid crop. In a previously published study, the team had observed that the microbial communities associated with the root biomass of hybrids differed considerably from the inbred (non-hybrid) varieties.
“A lot of focus has been on the genetics of hybrid vigour, which makes sense because hybridisation is a genetic process — but there’s been some evidence the environment is important as well for affecting the strength of hybrid vigour. In this paper, we showed microbes living in the soil are one of the environmental factors that have a really important effect on hybrid vigour,” lead author Maggie Wagner said in a press release.
In order to examine whether their observations held any ground, both hybrid and inbred varieties were grown in laboratory conditions in sterile soil (i.e. devoid of any microbiota) and no difference was observed.
On the other hand, when a set of microbes known to colonise maize roots was added to the soil, an expected difference in hybrid performance was clearly visible. Specifically, the added microbial community had little effect on the root weight of the hybrids but was able to significantly reduce the root biomass of the inbred seedlings.
Superior disease resistance
Similarly, the microbial community impeded the growth rate of inbred lines but not that of the hybrid; whereas both varieties had germinated equally well in a sterile environment. The authors posit that this may be due to a negative effect of the bacteria on the inbred genotypes, rather than due to any kind of positive effect on the hybrid.
This is a notable study, for numerous studies in the past have shown that plants that are genetically modified to have their immune systems suppressed (immunocompromised), tend to suffer from pathogens that otherwise do not affect their ‘normal’ counterparts. Such studies have been done for maize as well, where immunocompromised maize plants could not achieve maturity in non-sterile growing conditions.
One theory that is forwarded is that some microbes are weakly pathogenic to maize, and end up harming the pure, inbred germline than hybrids. A second, albeit related, postulation is that the inbred individuals may end up exhibiting a defensive overreaction to the microbiota, thereby affecting their growth. This, the authors suggest, could be the reason for microbe dependent heterosis (MDH). “If weak pathogens drive MDH, then this implies that superior disease resistance in hybrids is a key mechanism of heterosis,” the paper states.
– The author is a freelance science communicator. (mail[at]ritvikc[dot]com)