Differential Expression In Sexual and Apomictic Genotypes Of Three Model Species Strengthens a Crucial Role Of APOSTART In The Formation Of Seeds By Apomixis

Seed is the key factor of crop productivity. The commercial success of a newly selected cultivar depends not only on its vegetative attributes but also on its ability to produce seeds. Breeding for seed yield and quality requires new sophisticated technologies, such as apomixis, that will allow overcoming the conventional breeding limits. Apomixis is a naturally occurring mode of asexual reproduction in flowering plants, resulting in embryo formation without meiosis or fertilization of the egg. Seed derived progenies are genetically identical to the maternal parent. In crop species, apomixis would enable the instantaneous fixation of the complete genome of the best plants. The introduction of apomixis in crop species is one of the key desirable traits since it would allow the fixation of heterosis in F1 hybrids leading to efficient and consistent production of high-quality seeds, fruits, and vegetables. The development of Apomixis Technology (AT) is expected to have a revolutionary impact on food and agriculture production by reducing cost and breeding time, avoiding some complications of sexual reproduction (incompatibility barriers) and of vegetative propagation (viral transfer). It is estimated that AT in the production of hybrid rice alone could provide benefits exceeding €1800 million per annum. The development of AT in agriculture require a deeper knowledge of reproductive development in plants. We have isolated one gene, which was termed APOSTART because of its START domain and its putative involvement in apomixis (Albertini et al. 2005, Plant Phys 138:2185-2199). Our previous results demonstrate that some APOSTART members are expressed exclusively in inflorescences of Poa pratensis. In situ hybridization analyses revealed that APOSTART is expressed during both male and female meiosis in all micro- and megaspores. Strong signals were recorded up to the mature embryo sac stage. The overall data suggest that APOSTART may be related to the programmed cell death that is involved in the non-functional megaspore and nucellar cell degeneration events that permit enlargement of maturing embryo sacs. Functional characterization of the Arabidopsis thaliana APOSTART1 gene (AtAPO1), shows that AtAPO1 is expressed in mature embryo sacs and developing embryos. To confirm the involvement of APOSTART in apomixis in P. pratensis, we have then created 3 genomic and 3 cDNA libraries using 2 apomictic and 1 sexual genotypes with three aims: i) to isolate all possible APOSTART members/alleles; ii) to identify DNA regions containing APOSTART members/alleles; and iii) to identify genes linked to APOSTART. Moreover, to strengthen the hypothesis of an involvement of APOSTART in apomixis, we have isolated APOSTART members from two other aposporic species: Pennisetum squamulatum and Hypericum perforatum. In each species several members/alleles have been identified and characterized. Our results, demonstrating that APOSTART is expressed differentially in all 3 species, will be reported and critically discussed.