Modification of sex expression in transgenic melon via altered ethylene production and perception

Cucurbit species are noted for their diversity of sexual phenotypes which are subject to control by developmental, genetic, and hormonal factors. Understanding the basis of sex expression is of basic interest to elucidate underlying mechanisms of sex determination and is of applied interest as pistillate flowers are necessary for fruit production. Studies with exogenous hormones have indicated that ethylene is the primary hormone influencing sex determination in cucurbits, with increased ethylene causing increased femaleness. In this dissertation, I sought to: examine the role of ethylene perception by the developing flower in sex determination, test the ability to increase femaleness in the field through increased endogenous ethylene production, and examine possible ecological impacts of altered ethylene sensitivity or production. In the first study, melon (Cucumis melo L.) plants were transformed with the Arabidopsis dominant mutant ethylene receptor etr1-1, under the control of the petal/stamen targeted Arabidopsis Apetela3 (AP3) promoter, or carpel/nectary targeted Arabidopsis Crab's Claw (CRC) promoter, to evaluate the effects of blocked ethylene perception in different regions of the developing flower. CRC::etr1-1 melon plants showed increased femaleness as measured by earlier and increased pistillate bud production, and the formation of female rather than hermaphrodite buds. AP3::etr1-1 melon plants showed increased maleness as measured by almost exclusive staminate flower production and the presence of poorly developed carpels in the few pistillate buds that were produced. The results of these experiments suggest a model for sex determination in melon whereby ethylene perception by the stamen is required for carpel development while ethylene perception by the carpel is required for stamen development. In the second study, field grown transgenic melon constitutively overexpressing an ACS gene for ethylene biosynthesis showed earlier and increased production of mature pistillate flowers as well as earlier fruit set and increased sequential fruit set on the main stem. These results indicate that ethylene is important for pistillate bud development and also may suggest an additional role in signaling associated with patterns of fruit set. Lastly, published literature was examined to conduct a case study on modified ethylene production and perception in order to provide insight into possible secondary effects and ecological impacts of altered signaling pathways.