This invention describes a novel method of producing bioactive recombinant proteins from plants. General methods of designing and engineering plants for expression and production of such proteins are also disclosed. Methods for the expression of Transforming Growth Factor-.beta. (TGF-.beta.) proteins, such as Mullerian Inhibiting Substance (MIS), in plants, and methods of producing recombinant proteins from plants are specifically disclosed. Furthermore, the present invention provides methodology for the direct expression and production of a bioactive C-terminal fragment of a TGF-.beta. protein, such as C-terminal MIS. The new method is more cost-effective than other large-scale expression systems, by eliminating the need for costly cell culture and fermentation manufacturing facilities.