Software Defined Networks (SDN) are considered an efficient and flexible network architecture for managing large scale networks, in particular as it decouple between hardware and software and facilitates global network optimizations. Incorporating SDN concepts into Wireless Sensor Networks (WSN) opens interesting research challenges. This paper proposes an architecture for applying SDN to WSN. The architecture relies on recent extendibility features of OpenFlow to enable reconfiguring sensor behavior and network forwarding tables according to changes such as arrival of new sensors or change in user requests, as well as to apply dynamic in-network aggregation and filtering options. An evaluation of our approach for the specific case of in-network aggregation and its beneficial impact on the number of sent messages is also presented.