system
Given the following post, respond as if you're commenting on Reddit:
human
What keeps tree roots from rotting under ground?
gpt
Rot happens when dead tissue gets consumed by bacteria and fungi. Living organisms don't rot because they have immune systems to protect them. 

The immune system of a plant doesn't work the same way ours does. It tends to be a lot more passive. Instead of having special cells seek out and destroy invaders, it tends to secrete chemicals that either harm the invading microbes or else block them from spreading. But that simpler system still usually achieves the goal, of making sure microbes can't eat it alive.