TESTS OF A MECHANISTIC MODEL OF ONE HORMONE REGULATING BOTH SEXES IN BUCHLOE DACTYLOIDES (POACEAE)

A mechanistic model of one hormone regulating both sexes in flowering plants was tested in buffalograss (Buchloe dactyloides). This model as sumes that one hormone has male and female cell receptors to inhibit o ne sex and induce the other independently. Three components-the normal range of hormone level in the plant and the sensitivity levels of the two receptors-interact to regulate sex expression. The study organism , buffalograss, is usually considered dioecious, but natural populatio ns consist of varying proportions of male, female, and monoecious plan ts. Prior research with growth regulators had shown that only gibberel lin (GA) had consistent and significant effects on sex expression in t his species. To test the model assumption of a hormone with a dual fun ction, GA and a GA inhibitor (paclobutrazol, PAC) were applied to thre e monoecious genotypes; in two of the genotypes the GA treatment yield ed a significantly higher proportion of male inflorescences, and this transition involved both inducing male and inhibiting female. PAC trea tment produced exclusively female inflorescences, illustrating the dua l effects of GA. To test the predictability of the model, GA was appli ed to two dwarf female genotypes. These plants were transformed into n euter and near-neuter plants with normal height and vegetative growth, as predicted by our model for genotypes with a physiologically wide o verlapping of male and female sterile regions. The model also predicts that male or female plants would be induced to produce inflorescences of the other sex if the hormone level could be shifted from one side of the overlapping sterile regions to the other. This was verified by applying high levels of GA to a normal female genotype that resulted i n the production of male inflorescences. However, this is the only nor mal female that has responded to GA application by producing male infl orescences, and males lose vigor and/or die without producing female i nflorescences at high levels of PAC. The model suggests that the const ancy of these males and females is due to the relative location of the sensitivity levels in relation to each other and to the hormone range . We conclude that the one-hormone model can facilitate both applied a nd basic research.