  It is shown that, in the absence of matter fields, the coupling of a scalar
field to the non-chiral Plebanski action can be obtained by relaxing the trace
component of the simplicity constraints. This is realized by considering a
subclass of generalized theories, where a potential depending on invariants of
the Lagrange multipliers is added to the Plebanski action. Generically such
theories propagate eight degrees of freedom, but here the (singular) subclass
with the potential only depending on the trace of the Lagrange multiplier is
displayed as the Bergmann-Wagoner-Nordtvedt class of scalar-tensor theories.
