Question: wont the bacteria eventually take over the pea plant? and can it spread to other plants where it might not be so helpful

Isabel Webb answered on 18 Mar 2014:

Great question!

The interaction between the pea and the bacteria has evolved to work together happily. The bacteria gets more food, and the plant gets more nitrogen than if they lived separately. Because of this teamwork, the plant suppresses its defences and lets the bacteria invade. Once the bacteria invade though, their pathway through the plant is very carefully controlled, and the plant changes the root to become a home from the bacteria. The plant sends out signals to the bacteria that force it to change from a normal, ‘free-living’ bacteria into a specialised bacteria that only has one purpose – almost like a factory to make nitrates, the nitrogen compounds that plants need.

Once the bacteria has changed into this factory, it is left unable to grow or reproduce – so even if it were to try and break free into the rest of the plant, it couldn’t do much, and the plant could easily stop it.

My bacteria, Rhizobium, sends out special signals to the pea plant which signal the pea plant to let it in. Other plant species do not have the ability to recognise these signals, and so it can’t invade other plants. In fact, many scientists wish it did, and are try to use GM to make important plants like wheat recognise these signals. If they did recognise the signals, they could take up the bacteria and turn them into the same nitrate-making factories – which would help the wheat plants get more food and grow better.