24 April 2024

This is how soil microbes fight plant infections


According to University of California Davis research, certain strains of soil microbes act on the roots of sorghum plants, protecting them from external threats

by Matteo Cavallito


Certain soil microbes can suppress infections by striga, a parasitic plant that affects several crops. This ability is shown in the impact of the microorganisms on the roots, which, once changed in structure by their action, become more resistant to infection. This is reported in research from the University of California Davis published in the journal Cell Reports. The study focused on some specific strains of bacteria that, in particular, could be applied as probiotics in sorghum crops.

“These microbes have great promise as soil additives that can help farmers grow sorghum successfully in sub-Saharan Africa,” said Siobhan Brady, professor and co-author of the study in a statement released by the U.S. university.

Parasites attacking sorghum

“Sorghum bicolor is among the most important cereals globally and a staple crop for smallholder farmers in sub-Saharan Africa,” the study explains. “Approximately 20% of sorghum yield is lost annually in Africa due to infestation with the root parasitic weed Striga hermonthica.” In short: when in phosphate-poor soil, the statement recalls, sorghum plants release chemicals from their roots. In this way they attract fungi that join them to help acquire the phosphate.

This release functions as an actual signaling to the parasitic plant, which responds by sprouting and latching onto the sorghum roots.

“Existing Striga management strategies are not singularly effective and integrated approaches are needed” the study further explains. The researchers’ initial hypothesis is that microbes may contribute to infection control. Previous studies, they explain, had shown that a soil fungus, Fusarium, suppresses parasitic plant germination. But it was unknown whether soil bacteria also have this ability.

Microbes are a valuable asset against parasitic plant

Using genetic survey techniques, microscopy and in vitro experiments, researchers have shown that microbes modify both the chemical signaling that helps the parasitic plant attach to its host and the anatomy of sorghum roots, thus making attachment more difficult. The scientists, the statement details, observed that the bacteria activate certain genes that result in a thicker layer of suberin. This is a waxy substance that can act as a barrier for the parasite, as well as more gaps that can also prevent attachment to the plant.

Through genetic sequencing, scientists have identified more than 100 strains of bacteria associated with sorghum’s resistance to external attack. The in vitro analysis particularly highlighted the crucial role of two of them: Pseudomonas, which is responsible for the degradation of chemical signals in the soil, and an Arthrobacter strain that increases root suberification.

New solutions

“The ultimate goal is to identify microbial solutions that farmers can treat the soil or seeds with to help prevent Striga infection,” Brady said. “The intention is that this should be part of an integrated package of solutions to farmers—another tool in the toolkit.” Scientists are currently looking for the microbes responsible for conferring the other resistance traits, such as the formation of aerenchyma, a particular spongy tissue.

At the same time, the same researchers are trying to identify soil microbes involved in these mechanisms in various regions to see if they can confer resistance to plants affected by the pest. In summary, the authors conclude, “this work lays the foundation for designing a multi-member microbial consortium that suppresses haustorium formation and induces diverse structural barriers in roots to collectively reduce Striga parasitism.”