Hello Suman,
Adding to what Rohit has said, you could also use
1. A HTTPPost action block with the URL mentioned in Rohit's reply. Make sure pass the credentials by creating a credential alias or by using the link editor in the action block.
2. You could use WSDLGen Service to expose the Plant MII's transaction as webservice and then call the same from Primary MII's server using webservice action.
3. You could also use an XML Query/Xacute Query to call the transaction using its corresponding virtual connectors. these connectors would inturn point to the Plant MII server.
I would suggest using HTTPPost or using WSDLGen services,
Check SAP Library - Content Development for more information on different ways to call a transaction.
Regards
Tufale Ashai