An asset administration shell (AAS), as a key concept of Industry 4.0 (I4.0), provides a machine-accessible interface to any kind of asset. To enable interoperability and smooth integration of the devices into the I4.0 middleware, an application implementing the device's functionality should be able to interact with different AASs. In this work, we investigate the integration of the Programmable Logic Controller (PLC) runtime systems into the I4.0 middleware. For doing this, we specify the function blocks (FBs) for connecting the PLCs with AASs and other I4.0 components, such as registry and discovery server. We analyze the requirements of such FBs while focusing on REST/HTTP- and OPC UA-based AASs, and provide interface specification for IEC 61499- and IEC 61131-3-based FBs. Furthermore, we implemented an FB library that enables communication with an AAS from the respective control applications. Those FBs allow accessing properties and invoking operations of remote AASs, as well as hosting AASs submodels. Common functionalities, such as registering the runtime system at the registry component, or finding AASs, are also supported. The results obtained in this paper will ease interaction with the complex AAS structure from the low-level devices.