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The bacterial flagellum, the tail some bacteria use to get around, is made of a protein that does not turn up in anything useful. When that protein suddenly appears, it works as a signature of attack, and some plants have evolved a mechanism that raises their defenses in response.
Early infections need the flagellum to travel, so those bacteria have one. Once they are inside the plant, they produce new bacterial cells that have no flagellum, because a tail would make the plant attack them. Bacteria have no brains, so evolution did the work.
DNA sequencing made this visible. Before it, most bacteria and viruses could only be told apart by what they did. Now researchers extract the DNA or the RNA, sequence it, and see what happens when they inject it, which has opened a whole new field of complicated goings-on between plants and microbes.
How do plants detect a bacterial tail?
Plant cells carry a receptor called FLS2 on their surface that recognizes a short stretch of flagellin, the flagellum's protein. That stretch is a 22-amino-acid piece known as flg22. Thomas Boller's group in Switzerland identified it in the late 1990s, and the receptor was later found in the model plant Arabidopsis. When the piece binds, the cell floods with calcium, releases a burst of reactive oxygen, and switches on defense genes.
Why can't bacteria simply change that protein?
Flagellin has to keep its shape to build a working motor, so the parts that matter most for movement change very little. The flg22 stretch sits in one of those conserved regions, and altering it can cost the bacterium its ability to swim. Some species, including Agrobacterium tumefaciens, carry a variant that the plant receptor barely recognizes. Biologists have documented that this variant was one of the early examples of a microbe sidestepping plant detection.
How did sequencing change plant and microbe research?
Before sequencing, a microbe had to be grown in a dish to be studied, and most microbes cannot be grown in a lab. Reading DNA directly from soil, leaves, or sap, a method called metagenomics, revealed whole communities that nobody had seen. The plant Arabidopsis became the first flowering plant with a fully sequenced genome in 2000. Having both genomes in hand let researchers match plant receptors to the microbe molecules they detect.
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