Abstract
Main conclusion
Floral primordia-targeted expression of the ethylene biosynthetic gene, ACS , in melon suggests that differential timing and ethylene response thresholds combine to promote carpels, inhibit stamens, and prevent asexual bud formation.
Typical angiosperm flowers produce both male and female reproductive organs. However, numerous species have evolved unisexuality. Melons (Cucumis melo L.) can produce varying combinations of male, female or bisexual flowers. Regardless of final sex, floral development begins with sequential initiation of all four floral whorls; unisexuality results from carpel or stamen primordia arrest regulated by the G and A loci, respectively. Ethylene, which promotes femaleness, is a key factor regulating sex expression. We sought to further understand the location, timing, level, and relationship to sex gene expression required for ethylene to promote carpel development or inhibit stamen development. Andromonoecious melons (GGaa) were transformed with the ethylene biosynthetic enzyme gene, ACS (1-aminocyclopropane-1-carboxylate synthase), targeted for expression in stamen and petal, or carpel and nectary, primordia using Arabidopsis APETALA3 (AP3) or CRABSCLAW (CRC) promoters, respectively. CRC::ACS plants did not exhibit altered sex phenotype. AP3::ACS melons showed increased femaleness manifested by gain of a bisexual-only phase not seen in wild type, decreased male buds and flowers, and loss of the initial male-only phase. In extreme cases, plants became phenotypically hermaphrodite, rather than andromonoecious. A reduced portion of buds progressed beyond initial whorl formation. Both the ACS transgene and exogenous ethylene reduced the expression of the native carpel-suppressing gene, G, while elevating expression of the stamen-suppressing gene, A. These results show ethylene-mediated regulation of key sex expression genes and suggest a mechanism by which temporally regulated ethylene production and differential ethylene response thresholds can promote carpels, inhibit stamens, and prevent the formation of asexual buds.
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Acknowledgments
We thank Dan Raba for assistance in the greenhouse, Dr. Randy Beaudry for assistance with ethylene measurements, Dr. Jeff Landgraff of the MSU Research Technology Support Facility for advice with qRT-PCR analysis, and Drs. John Bowman (UC Riverside) and Vivian Irish (Yale Univ.) for providing the CRC and AP3 promoters. We also thank Drs. Cornelius Barry and Suzanne Hoffman-Benning for their helpful reviews of the manuscript. This project was in part supported by research grant US-3735-05C from BARD, the United States, Israel Binational Agricultural Research and Development Fund.
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Switzenberg, J.A., Little, H.A., Hammar, S.A. et al. Floral primordia-targeted ACS (1-aminocyclopropane-1-carboxylate synthase) expression in transgenic Cucumis melo implicates fine tuning of ethylene production mediating unisexual flower development. Planta 240, 797–808 (2014). https://doi.org/10.1007/s00425-014-2118-y
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DOI: https://doi.org/10.1007/s00425-014-2118-y