ORIGINAL PAPER Effect of CRC::etr1-1 transgene expression on ethylene production, sex expression, fruit set and fruit ripening in transgenic melon (Cucumis melo L.) Jessica A. Switzenberg • Randy M. Beaudry • Rebecca Grumet Received: 22 April 2014 / Accepted: 18 November 2014 Ó Springer International Publishing Switzerland 2014 Abstract Ethylene is a key factor regulating sex expression in cucurbits. Commercial melons (Cucum- is melo L.) are typically andromonoecious, producing male and bisexual flowers. Our prior greenhouse studies of transgenic melon plants expressing the dominant negative ethylene perception mutant gene, etr1-1, under control of the carpel- and nectary- primordia targeted CRAB’S CLAW (CRC) promoter showed increased number and earlier appearance of carpel-bearing flowers. To further investigate this phenomenon which could be potentially useful for earlier fruit production, we observed CRC::etr1-1 plants in the field for sex expression, fruit set, fruit development, and ripening. CRC::etr1-1 melon plants showed increased number of carpel-bearing open flowers on the main stem and earlier onset by 7–10 nodes. Additional phenotypes observed in the green- house and field were conversion of approximately 50 % of bisexual buds to female, and elongated ovaries and fruits. Earlier and greater fruit set occurred on the transgenic plants. However, CRC::etr1-1 plants had greater abscission of young fruit, and smaller fruit, so that final yield (kg/plot) was equivalent to wild type. Earlier fruit set in line M5 was accompanied by earlier appearance of ripe fruit. Fruit from line M15 frequently did not exhibit external ripening processes of rind color change and abscission, but when cut open, the majority showed a ripe or overripe interior accompanied by elevated internal ethylene. The non- ripening external phenotype in M15 fruit corre- sponded with elevated etr1-1 transgene expression in the exocarp. These results provide insight into the role of ethylene perception in carpel-bearing flower pro- duction, fruit set, and ripening. Keywords Sex determination  Fruit development  Muskmelon  Monoecy  Fruit softening Introduction The Cucurbitaceae family contains species that exhibit a variety of heritable sex phenotypes (Grumet and Taft 2011; Roy and Saran 1990). The predominant sex expression pattern in commercial melon (Cucumis melo L.) is andromonoecious, producing a combina- tion of male and bisexual flowers. As the plant develops, vegetative nodes are produced first, fol- lowed by nodes bearing only male flowers, and then by nodes with a mixture of male and bisexual flowers (Grumet et al. 2007). Depending on the variety, the period of exclusively male flower production can vary extensively, leading to delayed fruit set. J. A. Switzenberg  R. Grumet (&) Graduate Program in Genetics, Michigan State University, 1066 Bogue Street, East Lansing, MI 48824, USA e-mail: grumet@msu.edu R. M. Beaudry  R. Grumet Department of Horticulture, Michigan State University, 1066 Bogue Street, East Lansing, MI 48824, USA 123 Transgenic Res DOI 10.1007/s11248-014-9853-5