Floral primordia-targeted ACS (1-aminocyclopropane-1-carboxylate synthase) expression in transgenic Cucumis melo implicates fine tuning of ethylene production mediating unisexual flower development

Abstract

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.

An ACC Oxidase Gene Essential for Cucumber Carpel Development

“…Previous studies have shown that ethylene could inhibit the CmWIP1 expression (Switzenberg et al, 2014). Combining the potential direct inhibitory effect of CsWIP1 on CsACO2 reported here, there could be reciprocal inhibitory effects between CmWIP1/CsWIP1 and ethylene.…”

Draft genome sequence of bitter gourd (Momordica charantia), a vegetable and medicinal plant in tropical and subtropical regions

“…Expression of the genes for MOMC52_27g (encoding CmWip1-like protein) was confirmed in flower buds, but significant differential expression among analyzed tissues was not observed, which was inconsistent with its male flower-specific expression in melon or cucumber 45 . As shown by Switzenberg et al 46 in melon, specific expression of ACS gene in petal and stamen induced alternation of sex phenotype by ethylene production. Therefore, to elucidate functions in the sex determination of candidate genes in bitter gourd, their spatiotemporal expression patterns should be analyzed in flower buds.…”

Overexpression of a flower‐specific aerolysin‐like protein from the dioecious plant Rumex acetosa alters flower development and induces male sterility in transgenic tobacco

“…The PCR fragment was first cloned into the pGEM-T easy vector system (Promega). The AP3 promoter was derived from a construct described by Switzenberg et al (2014), containing the construct AP3::ACS in the binary vector pCambia2300. The ACS gene in pCambia2300-AP3::ACS was removed by BamHI digestion and replaced with the BamHI fragment of FEM32, previously cloned by the pGEM-T easy vector (Figure 9b).…”