Bacteria Sink in Their TALons to Control Their Host

Endosymbiosis, a phenomenon in which one organism lives inside of another, exists across several species. “It’s basically the basis of life,” said Ingrid Richter, a microbiologist at the Leibniz Institute for Natural Product Research and Infection Biology. She and her team study the relationship between the plant and human pathogenic fungus, Rhizopus microsporus, and its endosymbiotic bacterium, Mycetohabitans rhizoxinica. 

         

In a paper published in mBio, the team showed that transcription activator-like effector (TAL) proteins from M. rhizoxinica controlled the sporulation of R. microsporus, which is advantageous to the fungi.¹ Understanding these endosymbiotic mechanisms could improve treatments against these fungal infections. 

         

TAL in other plant pathogens promote their survival in the host, so the team investigated Mycetohabitans TAL (MTAL) as potential mediators that control R. microsporus sporulation.² They identified three of these proteins in the M. rhizoxinica genome and generated bacterial mutants, each with one MTAL deleted. 

When the researchers replaced the endosymbiotic bacteria from the fungus with the mutant versions, they found significantly reduced sporulation compared to fungi with nonmutant M. rhizoxinica. “It’s basically hijacking the reproduction of the fungus,” Richter said. Confocal microscopy revealed that the bacteria resided primarily within the fungal hyphae, validating that MTAL mutations did not impact the bacteria’s infection abilities.  

“This particular symbiotic association is very unusual because the fungal host is addicted to its bacterial endosymbionts for reproduction,” explained Teresa Pawlowska, a mycologist studying endosymbiosis at Cornell University who was not involved in the study. She found the study exciting because scientists don’t know the mechanisms of this manipulation. However, she pointed out that there are many MTAL in these endosymbionts beyond those studied in the paper. “It would be great to probe further and figure out what are the functionalities of these other TAL effectors,” she said.