Abstract
Key message 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 Ca2+-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.
Highlights
The plant cell wall is a highly complex and dynamic structure composed of polysaccharides, structural proteins and phenolic compounds (Somerville et al 2004; Cosgrove 2005; Wolf and Greiner 2012)
Polysaccharides are often grouped into three functional categories: celluloses, hemicelluloses, which mainly consist of xyloglucan (XG) and small amounts of glucuronoarabinoxylan, and pectins, which are a family of galacturonic acid-rich polysaccharides including homogalacturonan (HG), rhamnogalacturonan I (RG-I), rhamnogalacturonan II (RG-II) and xylogalacturonan (XGA)
The immunocytochemical localization of HGs revealed different distribution of the epitopes recognized by JIM7, JIM5 and 2F4 antibodies in somatic cells of H. orientalis ovules and in embryo sac cells before and after fertilization
Summary
The plant cell wall is a highly complex and dynamic structure composed of polysaccharides, structural proteins and phenolic compounds (Somerville et al 2004; Cosgrove 2005; Wolf and Greiner 2012). Plant Cell Rep (2015) 34:97–109 model of the cell wall depicts a cellulose–hemicellulose network embedded in a pectin matrix (Mohnen 2008; DickPerez et al 2011) The interactions between these polysaccharides control important aspects of plant development including cell adhesion, wall extensibility, wall porosity and the mediation of defense responses (Krupkova et al 2007; Mohnen 2008; Caffall and Mohnen 2009; Wallace and Anderson 2012). The removal of methyl groups dramatically alters physical properties of the polymers During this process, free carboxylic acid groups are created, and methanol and protons are released (Wolf et al 2009; Wolf and Greiner 2012). The methyl esterification status of HGs can have dramatic consequences on cell wall texture and mechanical properties, thereby regulating cellular growth and shape. Pectic oligogalacturonides (OGAs), small breakdown products of HGs, have been shown to act as signaling molecules, both as elicitors during pathogen attack and as hormone-like compounds that counteract the effects of auxin during plant development (Ridley et al 2001; Wolf et al 2009)
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