Plant sex determination: functional characterization of the monoecy and gynoecy genes in Cucurbitaceae. – MELODY

The proposed strategy relies on complementary expertise of 6 partners and integrative analyses of datasets that will be obtained from the combination of different “omics” tools. The hypotheses and the concepts will be validated using the genetic approach.

We aim to decipher (i) the networks of genes involved in sex determination and the molecular mechanisms that lead to unisexual flowers; (ii) the extent of the use of CmACS-7 and CmWIP1 in sex determination in the Cucurbitaceae family; (iii) the mechanisms by which the sex determinism gene CmACS-7 controls fruit shape development and last (iv) the development of plant prototypes to improve Cucurbitaceae breeding.

The ultimate goal of the MELODY project is to produce stable melon plant prototypes of different sexual morphs and to test them for agronomic importance. The finding and the concepts that will be gathered on the melon model system will be transferred to other Cucurbitaceae at later stage of the project.

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How the gender of a flower or plant is determined is an important issue in plant developmental biology. Understanding this process also has practical applications in agriculture and plant breeding, as the gender of a flower or plant often limits how it is bred and cultivated.
Sex determination is a process that leads to the physical separation of male and female gamete producing structures in separate flowers on the same plant (monoecious species) or on separate individuals (dioecious species). Several species in the Cucurbitaceae, including cucumber (Cucumis sativus) and melon (Cucumis melo), have bisexual floral primordia, but often have flowers limited to a single sex. Sex determination occurs by the selective arrest of either the male stamen or female carpel during development.

In melon, sex determination is governed by the andromonoecious a gene and the gynoecious g gene, and the interplay of alleles of these two genes results in a range of sexual types. Monoecious (A-G-) and andromonoecious (aaG-) individuals bear male flowers on the main stem and, respectively, few female or hermaphrodite flowers on axillary branches immediately followed by male flowers. In contrast, gynoecious (AAgg) and hermaphrodite (aagg) individuals only bear female and hermaphrodite flowers, respectively. In addition, cucurbit sex expression patterns can be modified by hormones, such as ethylene, and by environmental factors.

Cloning of the a gene revealed that andromonoecy results from a mutation in the active site of 1-aminocyclopropane-1-carboxylic acid synthase, CmACS-7. Likewise we cloned the g gene and showed that the transition from male to female flowers in gynoecious lines results from epigenetic changes in the promoter of a transcription factor, CmWIP1. Moreover, we showed that CmWIP1 represses indirectly CmACS-7 expression to allow stamen development. Together our data suggest a model where the two genes interact to control the development of male, female and hermaphrodite flowers.

The goal of this proposal is to use the information from the sequence analysis of CmACS-7 and CmWIP1 as a starting point to understand how these two genes determine the sex of the plant. The proposed strategy relies on complementary expertise of 6 partners and integrative analyses of datasets that will be obtained from the combination of different “omics” tools. The hypotheses and the concepts will be validated using the genetic approach. The ultimate goal of the MELODY project is to produce stable melon plant prototypes of different sexual morphs and to test them for agronomic importance. The finding and the concepts that will be gathered on the melon model system will be transferred to other Cucurbitaceae at later stage of the project.