The present invention provides a method for hybrid seed production comprisingproducing a conditional female sterile plant comprising a female-preferentialpromoter operably linked to a coding sequence which encodes an enzyme whichcatalyzes the conversion of a protoxin to a toxin, interplanting theconditional female sterile plant with a male sterile plant, applying theprotoxin to the conditional female sterile plant, and producing hybrid seed.Viable seed formation is prevented on the conditional female sterile plant asa result of the conversion of the protoxin to the toxin in the femalereproductive structures, and pollen production is prevented on the malesterile plant, thus allowing interplanting of the two parents of the hybridcross in order to provide more efficient pollen transfer. Also provided areexpression cassettes useful in the invention, plants transformed with theexpression cassette, and novel female-preferential promoters.
