Complete the letter analogy: STONE : ETONS :: PLANT : ?

The first letter and the last letter change places while the three in the middle stand still. In STONE the S and the E are exchanged, giving ETONS. In PLANT the P and the T are exchanged, giving TLANP. 'TNALP' is the whole word written backwards, which on STONE would have given ENOTS and not ETONS. 'LPANT' exchanges the first two letters instead of the outer ones, and 'PTANL' exchanges the second letter with the last, leaving the first where it stood.