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?

I guess, you do not say anything to your child in front of your MIL, since you know that she is going to side him. Always call him to the other room and make him understand. Also, you have to understand that your child knows that if you will scold him then he always has his grandmother for a shield, so you will have to build a bond with your child so that he doesn't run to his grandmother if something goes wrong between you too. Things between you and your son should be such that if you are upset with him then instead of thinking of going to his grandmother or for that matter anyone, he should think 'what should i do to make mamma fine?' For this, you have to develop a very open conversational relationship with your son. Have more and more discussions with him. Let him have his viewpoint. Instead of preachy, listen to his opinion as well, and appreciate too.