HHMI researchers identify the mechanisms that pathogenic bacteria use to waterlog the space between plant cells in the leaves, allowing the bacteria to reproduce and spread infection.
Pathogenic bacteria can take advantage of environmental humidity levels and manipulate water content inside plant leaves to reproduce and spread infection, a new study shows. Normally, plants tightly control the water levels in the apoplast – the space between plant cells in the leaves – to maximize carbon dioxide intake and photosynthesis. A report in this week’s issue of Nature describes mechanisms that pathogenic bacteria use to waterlog the apoplast, allowing the bacteria to reproduce and infect the plant.
Farmers have long known that high humidity conditions increase the risk of crops becoming infected with pathogenic bacteria. About 30 percent of crop yields are lost to pathogens each year, but how plant diseases occur is still incompletely understood. Previous research showed that pathogenic bacteria produce virulence proteins and inject them into plant cells to suppress the plant’s immune response. It was not clear if that was sufficient for infection, though.
HHMI-Gordon and Betty Moore Foundation Investigator Sheng Yang He and his postdoctoral fellow Xiu-Fang Xin of Michigan State University reasoned that, if suppressing the plant immune system was the key to infection, non-pathogenic bacteria without virulence proteins should be able to infect immune-compromised plants. He and Xin challenged mutant Arabidopsis plants lacking immune defenses with nonpathogenic bacteria. To their surprise, they found that the bacteria were unable to infect the plants, and inferred that there could be another process that pathogens can disrupt to infect the plant.
He and Xin noticed that infections were often associated with an accumulation of water in the apoplast. But they didn’t know whether water accumulation was a passive process or one that was driven by the pathogen itself. Using genetic methods to test each of the 28 virulence proteins of the pathogen individually, the researchers found two pathogen virulence proteins that were required to cause waterlogging of the apoplast. This suggested that the pathogen had evolved specific proteins to actively promote the accumulation of water into the plant in a way that facilitates infection.
To fully understand the process of infection, He’s dream for the last several years had been to identify all the host targets of these virulence proteins and eliminate the targets one by one, to reconstitute the essential elements of plant infection. Undaunted by this complex and challenging experiment, He and Xin used CRISPR methods to generate a quadruple mutant Arabidopsis plant, with three mutations affecting plant immunity and one affecting a gene involved in apoplastic water control. When challenged with nonpathogenic bacteria, this mutant became infected and colonized.
“That is a dream come true,” said He. “We really can say that we now understand the basic pathogenesis mechanism, because we can recreate the plant infection without bacterial virulence factors,” he explained. “It seems like the pathogen and plant are fighting to control water content in the apoplast. This is a new battle that we discovered.”
The researchers also found that the quadruple mutant plants were unable to regulate water levels in the apoplast, which led to a proliferation of commensal bacteria. In other words, the bacteria that were part of the plant’s normal microbiome started to grow out of control. “So the plant immune system actually monitors the microbiome population, too,” explained He.
The researchers’ next line of investigation is to figure out what about the water inside the plant facilitates infection. It’s clear that bacteria need water to reproduce and spread throughout the leaves. The water could also be transporting nutrients or diluting the plant’s immune response, the researchers suggest.