Expression Analysis of Sex Genes and Ethylene Biosynthesis Genes in Different Sex Genotypes of Melon, Cucumis melo

Melons, like other cucurbit species, are capable of production of unisexual flowers.  Sex expression is genetically controlled by two major loci, G and A, which are responsible for suppression of carpel or stamen primordia, respectively, and in different combinations produce four sex types (monoecious, andromonoecious, gynoecious and hermaphrodite).  Inheritance studies, including those presented here, also implicate involvement of modifier loci.  The hormone ethylene plays a predominant role in sex determination, promoting carpel development and inhibiting stamens.  The A locus encodes a key ethylene biosynthetic enzyme, 1-aminocyclopropane-1-carboxylic acid synthase (ACS), that suppresses stamens.  In cucumber, but not melon, the F locus promoting carpels also encodes an ACS.  In melon, carpel suppression is regulated by a WIP1 transcription factor.  Here we show that the WIP1 promoter has several putative ethylene responsive elements and that WIP1 expression is suppressed by exogenous ethylene or elevated endogenous ethylene in transgenic melons expressing a stamen primordia targeted ACS transgene.  We therefore searched the melon genome for additional ethylene biosynthetic enzyme genes (ACS and ACC oxidase (ACO) family members); a total of eight  ACS and five ACO genes were predicted.  Three of the seven tested ACS genes (CmACS2, 5 and 10) and all three tested ACO genes (CmACO 1,2,and 3) were expressed in apices of young melon plants, the site of floral bud development.  All of those expressed in apices were highest in sex types that do not suppress carpels, i.e., gynoecious and hermaphrodite.