Apostart, a Gene Involved in Embryo Development and Parthenogenesis

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.

In Poa pratensis we have isolated a gene, termed APOSTART (Albertini et al. 2005, Plant Phys 138:2185-2199). Our previous results demonstrate that some APOSTART members are expressed exclusively in inflorescences and, overall, ore results 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.

In addition, to strengthen the hypothesis of an involvement of APOSTART in apomixis, we have isolated APOSTART members from two other aposporic species. Our results, showing that at least one APOSTART member/allele is expressed differentially in all species, shed light on the possible role of APOSTART in apomixis. To better understand this function we are characterizing Arabidopsis thaliana APOSTART members. In particular, PpAPO1 shares high homology with the Arabidopsis protein At5G45560, thus renamed AtAPOSTART1 (AtAPO1), and with with EDR2 (Enhanced Disease Resistance 2). Tang and co-workers proved that EDR2 disruption enhances Arabidopsis capacities to resist to E. cichoracearum infections (Tang et al. 2005; Vorwerk et al., 2010), edr2 homozygous plants does not show any developmental defects and are indistinguishable from the wild type ones, whilst AtAPO1 down-regulation affects negatively seed germination. In order to verify if AtAPO1 and EDR2 have additive or redundant roles we generated and analyzed the atapo1-2/edr2 double mutants. Double mutants plants appear smaller than the two parental lines, interestingly also the double mutant developing siliques are smaller. Manual dissection of double mutant silique show that also seed development is compromised in atapo1-2/edr2. Microscopic analyses show that all the seeds contain embryos, 30% of which show a delayed or arrested development. Moreover, by using a computational approach we have determined the tri-dimensional structure of the START domain with the aim of investigating the possible interactions between the START domain and one or more phytosterols (i.e. stigmasterol, campesterol, brassicasterol) which are believed to be important during the embryogenesis. These data will showed and critically discussed.