Parthenocarpy and fruit malformations are common among independent transgenic tomato lines, expressing genes encoding different pathogenesis-related (PR) protein and antimicrobal peptides. Abnormal phenotype developed independently of the expression and type of target genes, but distinctive features during flower and fruit development were detected in each transgenic line. We analyzed the morphology, anatomy, and cytoembryology of abnormal flowers and fruits from these transgenic tomato lines and compared them with flowers and fruits of wild tomatoes, line YaLF used for transformation, and transgenic plants with normal phenotype. We confirmed that the main cause of abnormal flower and fruit development was the alterations of determinate growth of generative meristem. These alterations triggered different types of anomalous growth, affecting the number of growing ectopic shoots and formation of new flowers. Investigation of the ovule ontogenesis did not show anomalies in embryo sac development, but fertilization did not occur and embryo sac degenerated. Nevertheless, the ovule continued to differentiate due to proliferation of endothelium cells. The latter substituted embryo sac and formed pseudoembryonic tissue. This process imitated embryogenesis and stimulated ovary growth, leading to the development of parthenocarpic fruit. We demonstrated that failed fertilization occurred due to defective male gametophyte formation, which was manifested in blocked division of the nucleus in the microspore and arrest of vegetative and generative cell formation. Maturing pollen grains were overgrown microspores, not competent for fertilization but capable to induce proliferation of endothelium and development of parthenocarpic ovary. Thus, our study provided new data on the structural transformations of reproductive organs during development of parthenocarpic fruits in transgenic tomato.
Keywords: Endothelium; Floral meristem identity; Male gametophyte; Parthenocarpy; Tomato.