Effect of Carpel Primordia-Targeted Inhibition of Ethylene Perception On Sex Expression and Fruit Ripening in Melon (Cucumis melo L.)

Ethylene is a key factor modulating sex expression in cucurbits.  Our previous studies indicated that ethylene perception by stamen primordia, but not carpel primordia, is essential for production of carpel bearing buds.  Transgenic andromonoecious melon plants (male and bisexual flowers) expressing the dominant negative ethylene perception mutant gene, etr1-1, under control of the carpel- and nectary- directed CRABSCLAW (CRC) promoter showed increased and earlier carpel bearing buds and flowers.  To further investigate this phenomenon, which could be potentially useful for earlier fruit production, we examined: ethylene production by CRC::etr1-1 plants; asked whether increased femaleness also was observed in a monoecious (male and female) genetic background; and observed transgenic andromonoecious and monoecious CRC::etr1-1 plants in the field for sex expression and fruit ripening.  It has been frequently observed that lack of ethylene perception results in increased ethylene production due to lack of feedback inhibition.  Floral buds from CRC::etr1-1 plants showed elevated ethylene production, suggesting that increased femaleness may result from perception of increased ethylene by the stamen primordia.  To test the effect in a monoecious background, gynoecious ‘WI998' (W) melon was crossed with the andromonoecious ‘Hale’s Best Jumbo’ (H) CRC::etr1-1 and wild type (WT) plants.  Greenhouse observations of the transgenic F₁ progeny showed increased femaleness relative to WT F₁ as evidenced by increased number of carpel bearing buds and open flowers and earlier onset of carpel bearing buds on the main stem.  Field studies of the H-WT, H-CRC::etr1-1 T₂ lines M5 and M15, (W x H)-WT F₁, and W x H-CRC::etr1-1 M15 F₁ lines also showed increased number of carpel bearing buds and open flowers, and earlier onset of the first open carpel-bearing flower. An additional phenotype observed in the greenhouse for H-CRC::etr1-1 plants was frequent conversion of bisexual buds to female.  The H-CRC::etr1-1 T₂ M5 and M15 plants also showed approximately 60% conversion of bisexual to female flowers in the field.  The CRC::etr1-1 transgene also influenced fruit ripening in both andromonoecious and monoecious genotypes.  The exocarp remained green while the mesocarp became extremely soft. This observation likely reflects differences in requirement for ethylene for different ripening processes.  Thus, targeting inhibition of ethylene perception to the carpel and nectary led to increased and earlier carpel-bearing buds in the greenhouse and field, however, the transgenic plants exhibited an adverse phenotype of asynchronous ripening between the exocarp and mesocarp.