Senior AIADMK leader E Madhusudhanan will be the party's candidate for the Radhakrishnan Nagar Assembly seat in Chennai, by-elections for which will be held on December 21. The RK Nagar seat was left vacant by the death of AIADMK chief and former Chief Minister J Jayalalithaa and retaining it is a matter of prestige for Tamil Nadu's ruling party and also its first electoral test since the death of its powerful leader.