Best Buds and Flaky Friends


Breaking down the microbiology world one bite at a time

Best Buds and Flaky Friends

There are vast amounts of bacteria on your skin. These are your friends: they help you to protect yourself against invaders. Plants do something similar! Their leaves are teeming with microbial life. These plants have friends just like we do, who help them to thrive despite pathogens lurking around every corner.

Christine Vogel and her colleagues at ETH Zürich, Switzerland investigated bacteria on the model plant Arabidopsis thaliana (a cousin of cauliflower, mustard and cabbage). According to their research, this poor plant might be caught up in an unfair friendship with many microbes.

Arabidopsis thaliana, or “mouse-ear cress”, grown in the author’s laboratory

In nature, hundreds of bacterial species colonize a plant leaf. In return, these symbiotic bacteria aid in fighting off harmful microorganisms. Such a relationship is called mutualism. The researchers wanted to know if all bacteria actually help, or that maybe some of them are slacking. The team grew plants with only one symbiotic bacterium at a time, and introduced the same pathogen to all plants. They then scored if the plant remained healthy (meaning the bacterium helped) or got sick (the bacterium did not help). After testing 224 (!) symbiotic bacteria in plants, it seemed that the plant had few actual friends: only one out of five bacteria actually helped to fight off the bad guy! Did the majority really not care?

To investigate this further, the researchers created a variant of Arabidopsis with a weakened immune system. They disabled the central regulation of a particular mechanism that senses microbes (both good and bad). This means that the plant cannot detect invaders anymore. Would it mean that more bacteria jump to the rescue? As it turns out, even fewer microorganisms protected their host.

In two cases, disabling the sensing mechanism even exposed turncoat bacteria! The culprits, two species of Xanthomonas, are called “opportunistic pathogens”. They help a healthy Arabidopsis, but whenever they see a chance, such as in the immunocompromised plant, they eat up their host before an ‘honest’ pathogen can reach it. Bacteria with this sneaky lifestyle are called “opportunistic pathogens”, and are also found in our gut and on our skin.

A friend in need is a friend indeed…but a friend who’ll bleed is better! (from: Placebo’s “Perfect Morning”). Created with BioRender.

The scientists suspected that their strategy to disable the immune system also disrupted a communication line between the plant and its friends. To investigate their hunch, they focused their attention on one particular bacterial family, Rhizobium, because this family contained both friends and foes of Arabidopsis. More than half of the genetic differences in this torn family were found in “secretion systems”, which are the bacterial equivalents to communication channels. The researchers disabled a secretion system in a ‘friendly Rhizobium’, and guess what? The bacterium did not help the plant to fight off invaders anymore.

So, there you have it: to be friends, you need to stay in touch! That doesn’t just hold true for humans in COVID-related lockdowns, but also for plants. This discovery helps us to design new technologies to protect our most important plants: crops. Right now, our food production is prone to infections, and farmers use pesticides to control these. These chemicals secure our food supply, which is a great feat, but they also harm our environment (and in some cases even ourselves). This new insight in plant immunology gives plant breeders a fresh idea on how to improve a plant’s immune system, so we can ultimately make pesticides a thing of the past.

Link to the original post: Vogel, C.M., Potthoff, D.B., Schäfer, M. et al. Protective role of the Arabidopsis leaf microbiota against a bacterial pathogen. Nat Microbiol 6, 1537–1548 (2021).

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