509 A Glycine-rich Protein from Tomato Involved in Pollen Wall Formation

The sporophytic tissue of the anther and, in particular, the tapetum, a cell layer surrounding the pollen sac, is know to be essential for the production of pollen. The isolation and characterization of the gene 92B from tomato that encodes an extracellular glycine rich protein (GRP) has been used to further elucidate the role of the tapetum in pollen development. RNA from the 92B gene accumulates exclusively in the tapetum. Polyclonal antibodies raised against the 92B GRP detect four proteins in stamens with microspores beginning meiosis. In pollen extracts, the antibodies detect a single protein. Expression of the tomato 92B gene in transgenic tobacco indicates that the four protein products are derived from only the 92B gene. The 92B GRP is localized to the tapetum, the callose wall of microspore mother cells, the exine (outer wall) of mature pollen, and orbicules. Orbicules are globular bodies derived from tapetal material that form on the tapetum wall and line the exterior of the pollen sac. Expression of 92B antisense RNA resulted in a significant decrease of 92B RNA and protein levels in transgenic tomatoes. This reduction was correlated with a decrease in pollen germination and an abnormal exine morphology. The function of the 92B protein in pollen development and function will be discussed.

Retarded embryo growth and early degeneration of sporophytic tissue are associated with embryo abortion in the interspecific cross Alstroemeria pelegrina × Alstroemeria aurea

Histological studies on crosses between distantly related Alstroemeria species revealed post-fertilization crossing barriers. Developmental events between normal developing seeds (after self-fertilization) and aborting seeds (after cross-fertilization) during 0–32 days after pollination were compared. In both cases the pollen tube remained as a swollen structure inside the micropyle during eight days, and thereafter degenerated. The first division of the zygote occurred within four days after pollination and resulted in a polar two-celled proembryo with a large basal cell and a small apical cell. The self-fertilized embryos developed a polar structure with suspensor and a globular embryo eight days after pollination according to the monocotyledonous type. The cross-fertilized zygotes showed retardation and deviation in plane of the cell plate in the second and third divisions, resulting respectively in linear three-celled and eight-celled proembryos 4–6 days after pollination. Differentiation in suspensor and globular stage embryos proper was not found in the cross-fertilized ovules but in stead aberrant undifferentiated embryos were observed, which formed two groups of cells in later stages. Endosperm development was quite similar in both cases till 12 days after pollination, when the coenocytic endosperm of the self-fertilized ovules formed cell walls around the nuclei, whereas cell wall formation was completely absent in the cross-fertilized ovules. The chalazal nucellus of the cross-fertilized ovules started to degenerate six days after pollination, and presumably restricted the flow of carbohydrates to the developing embryo and endosperm. Subsequently, a loss of contact between the endospermic transfer wall and the chalazal nucellus occurred after twelve days of development. Finally the cross-fertilized embryos aborted after they had lost their contact with the micropylar side of the former embryo sac. Thus, a number of events occurred from the second division of the hybrid zygote onwards, all leading to the abortion of the embryo 18–22 days after pollination. It seemed that during the early cell divisions of the zygote the fate of the embryo was already determined. Embryo abortion was associated with retarded embryo growth and early degeneration of sporophytic tissue. Key words: Alstroemeria spp., interspecific hybridization, embryogenesis, embryo abortion, endosperm degeneration, chalazal nucellus degeneration.