Sexual differentiation in higher plants

As in all organogenetic studies, sex differentiation implies three fields of research: the identification of macsomalecular markers specific for stamens or carpels }expression program); the analysis of the signals (such as phytohormones{ inducing them: and finally the knowledge of the regulator genes (sex detsrmination genes).
In dioecious plants, sex determination (male and female genes or combinations of heterochromosomes) occurs at the fertilization stage. These regulators probably act early by means of inducers; their action is perceptible mainly when reproductive organs develop, and the existence of target cells able to respond to the inductive message is obvious. Experimental control of sex expression by phytohormones leads to the repression of the normal organogenetic program (induced by the presence of the sex genes) arrd to the induction of the opposite program (normally incompatible with these genes).
Analogous mechanisms occur in monoecious plants, but here the male program is always expressed before the female one. Experimental control of sex expression suggests that the successive induction of first male and then the female program results from inverse gradients of male and female signals. Sex differentiation studies of monaeciousness are more difficult than studies on dioeciousness since, in all the cells. not only male and female programs co-exist as in dioeciousness, but also male and female sex penes and signals.
The present model has been tested in the dioecious Mercurialisannua (2n = 16) and some other species, and the results that have been established are described.