Question: can you explain what happens in the plants to sense bacteria?

Jo Hulsmans answered on 10 Jun 2011:

I could easily write about 10 pages here, but I will keep it short and try to explain how plants defend against bad bacteria and cooperate with good bacteria.

Well the first thing the plant has to do is recognise the bacteria. Bacteria have lots of proteins and sugars on the outside that can act as a sort of fingerprint. If a plant recognises the fingerprint of a dangerous bacteria the most important thing it will do is the hypersensitivity response: It will send a blast of H2O2 molecules through the cells around the bacterial infection. This will kill all the cells around the infection and hopefully, the bacteria too.

In the case of symbiosis the story is different. The symbiosis I am interested in happens between legume plants (like peas) and a species of bacteria called rhizobium. The rhizobium has a bunch of proteins, called the NOD factors, which will tell the plant the bacteria are nearby and want to enter. Instead of forming extra roots, the plant will use these cells to form nodules (small nodes on the stem), which act as a house for the bacteria. The bacteria have a nice, sheltered house to live in, while they provide Nitrogen to the plant.