The phenomenon you're referring to is called bioluminescence, and it is not common for tree roots to exhibit this property. Bioluminescence is the emission of light by living organisms, and it is typically caused by chemical reactions that produce light energy. In the case of tree roots, it is possible for certain types of fungi or other microorganisms that live in association with the roots to produce bioluminescence. These organisms may use bioluminescence as a way to attract insects or other animals that can help disperse their spores or seeds.