My MIL always interferes whenever I discipline my son. As a result, my son always runs to her whenever I am disciplining him and this has made him defiant. How to tackle this situation?

Tell your MIL when your kid is not around that her shielding of him is causing problem in his behaviour and this way your son will become ill mannered and will not take you seriously. Tell her that she can love him or do whatever and if there are any suggestions that she wants to make, she can tell you later but not in front of the kid. I think if you will have a word with her over a cup of tea and nicely explain it to her, she will understand your point. Sometimes, just because the woman is MIL, we underestimate their understanding capability.