| Cucurbitaceae species display a range of heritable sex patterns that are subject to hormonal control. Ethylene, which is able to promote pistillate flowers and suppress male flower formation, appears to be the main hormone influencing sex determination. Application or inhibition of ethylene increases or decreases femaleness (pistil development) respectively; higher levels of endogenous ethylene and expression of genes for ethylene biosynthesis or perception, have been correlated with female flower production. I sought to investigate the effect of modified endogenous ethylene production and perception on sex expression by producing transgenic melons constitutively expressing the ACS (1-aminocyclopropane-1-carboxylic acid synthase) gene, encoding the enzyme that catalyzes the first step in ethylene biosynthesis, or the mutant Arabidopsis etr1-1 (ethylene resistant ) gene that causes ethylene insensitivity. The ACS melons showed increased ethylene evolution by leaves and flower buds and also exhibited increased femaleness as measured by early appearance of the first hermaphroditic flower bud, increased number of total hermaphroditic buds, and increased number of hermaphroditic buds that reached anthesis. The heterologous etr1-1 gene was able to confer melon ethylene insensitivity as evidenced by higher ethylene production by male buds, decreased rooting, and decreased rate of flower pedicel abscission, even in presence of ethylene.; Expression of etr1-1 gene also prevented hermaphroditic flower formation in both andromonoecious and gynoecious melon lines. The phenotypic observations indicate that the ability to perceive ethylene is required for promotion of femaleness at the time of sex determination. I also investigated the role of the maize sex determination gene TASSELSEED2 (TS2) and the effect of exogenous brassinosteroids on sex expression pattern of andromonoecious melons. Presence of the TS2 gene did not affect either earliness or the number of hermaphrodite buds formed. Application of the hormone brassinosteroid increased femaleness in cucumber by inducing early formation and higher number of female flowers, likely via induction of increased ethylene production. These results presented in this dissertation provide direct demonstration of the importance of endogenous ethylene synthesis and perception in pistillate flower formation in melon. |
