Spatial and temporal localization of homogalacturonans in Hyacinthus orientalis L. ovule cells before and after fertilization.

The composition of homogalacturonans (HGs) in the ovule and the female gametophyte cell walls was shown to be rearranged dynamically during sexual reproduction of H. orientalis. In angiosperms, homogalacturonans (HGs) play an important role in the interaction between the male gametophyte and the pistil transmitting tract, but little is known about the participation of these molecules at the final stage of the progamic phase and fertilization. The aim of our study was to perform immunocytochemical localization of highly (JIM7 MAb) and weakly (JIM5 MAb) methyl esterified and Ca(2+)-associated HG (2F4 MAb) in the ovule and female gametophyte cells of Hyacinthus orientalis before and after fertilization. It was found that pollination induced the rearrangement of HG in (1) the micropylar canal of the ovule, (2) the filiform apparatus of the synergids, and (3) the region of fusion between sperm cells and their target cells. Fertilization led to further changes in pectin composition of these three regions of the ovule. A new cell wall was synthesized around the zygote with a characteristic pattern of localization of all examined HG fractions, which we called "sporoderm-like". The developing endosperm prepared for cellularization by synthesizing highly methyl-esterified HG, which was stored in the cytoplasm. Pollination- and fertilization-induced changes in the composition of the HG in the micropyle of the ovule and the apoplast of female gametophyte cells are discussed in the context of: (1) micropylar pollen tube guidance, (2) preparation of the egg cell and the central cells for fusion with sperm cells, and (3) the polyspermy block.