Studies on double fertilisation and early embryogenesis of Arabidopsis thaliana Heynh. and Zea mays L.

Sexual reproduction of angiosperms is a multistage process involving two sporophytic generations (parents and embryo) and one gamteophytic generation (male and female gametophyte). The central hallmark of this process is double fertilization. In order to achieve the unification of male and female gametes, sperm cells have to be delivered to the female gametophyte. The structure which delivers the sperm cells is the pollen tube. Pollen tube development is also summarised as progamic development of the male gametophyte. In Chapter 2 it is shown that progamic pollen tube development in maize is mainly governed by the sporophyte. Five phases of progamic pollen tube development were defined in homology to Arabidopsis. The influence of the female gametophyte appears to be restricted to the micropylar region, similar to the area controlled by EGG APPARATUS 1 (EA1) dependent micropylar pollen tube guidance. Intra-specific hybridisation barriers related to GAMTOPHYTE FACTOR 1 and inter-specific barriers between maize and Tripsacum dactyloides point at a specific interaction between pollen tubes and transmitting tracts. From that it is concluded, that targeting of the transmitting tracts by the pollen tubes is an actively regulated process and not exclusively governed by the tissue architecture as formerly assumed. Not only cell-cell communication, also intracellular signalling is an important aspect of fertilization. Calcium is a major signalling compound which has already been shown to be involved in double fertilization of plants. Pollen tube growth has been shown to depend on calcium in vitro and in vivo. In Chapter 3 the construction of marker lines, comprising tools for studying calcium signaling during double fertilization is discussed. A plasma membrane marker for gametophytic cells was supposed to highlight gametophyte cell boundaries during fertilization. The recombinant calcium probe CerTN-L15 was expressed in vegetative tissues and gametophytic cell types (egg cell, synergids and pollen). A different type of signaling molecules are peptides. A class of peptides which is only partially characterized is the EA1-box family, named after ZmEA1 from maize. As shown in Chapter 4, EA1-LIKE 1 (EAL1) is expressed in egg cells and after fertilisation until the transition stage. eal1-RNAi-ovules exhibit developmental defects including degenerated embryo sacs, supernumerary polar nuclei and central cell like structures as well as autonomous endosperm development. The LATERAL ORGAN BOUNDARY transcription factor gene INDETERMINED GAMETOPHYTE 1 was shown to be down-regulated in eal1-RNAi ovules. EAL1 expression itself was found to be independent of auxin. The peptide appears to accumulate at the chalazal pole of the egg cell. In Chapter 5 the additional role of EAL1 during early embryogenesis is discussed. The expression domain partially overlaps with expression of PINFORMED 1a (PIN1a) and ABERRANT PHYLOTAXY 1 (ABPHYL1) but not with DR5. In BMS cells, an additional potential target, a pentatricopeptide repeat protein, was identified. In eal1-RNAi-lines early development of endosperm and embryo seem to be delayed. Finally, a yeast-2-hybrid screen delivered a homologe of yeast SEC18 as putative interaction partner of EAL1.