The life cycle of plants alternates between diploid sporophyte and haploid gametophyte generations. Most Angiosperms are monosporic; one haploid megaspore per ovule primordium is selected as the functional megaspore, which gives rise to the megagametophyte, and the remaining three megaspores perish. The molecular mechanisms that determine the fate of megaspores are poorly understood. Based on the finding that megagametophyte formation is accompanied by changes in the distribution of arabinogalactan protein (AGP) epitopes, AGPs were proposed to be involved in the transition from the sporophyte to gametophyte generation (Pennell and Roberts, 1990). Recently, AGP18 was shown to be required for the initiation of megagametogenesis in Arabidopsis thaliana (Acosta-García and Vielle-Calzada, 2004).
In a follow-up study on the role of AGP18 in megagametogenesis, Demesa-Arévalo and Vielle-Calzada (pages 1274–1287) tracked the endogenous localization of AGP18 in developing Arabidopsis ovules using stable transgenic lines that harbored a construct in which cMyc-tagged AGP18 expression was driven by regions of the AGP18 promoter. Immunolocalization analysis using an anti-cMyc antibody revealed that, before meiosis, AGP18 was uniformly localized in the ovule primordium but was absent from the megaspore mother cell. Directly following meiosis, AGP18 localized to the functional megaspore and the neighboring sporophytic nucellar cells, with expression focused toward the functional megaspore. This finding suggests that AGP18 has a role in functional megaspore selection.
To investigate this possibility, the authors generated transgenic Arabidopsis plants that overexpressed AGP18 and analyzed ovule development in these lines. Many of the overexpression lines exhibited reduced fertility, and cytological analysis revealed that up to 24% of the developing ovules contained one to three supernumerary cells during ovule development, which the authors hypothesized to be abnormally surviving megaspores (see figure). Based on the expression of DISRUPTION OF MEIOTIC CONTROL1, a marker of meiosis, in transgenic AGP18-overexpressing transgenic lines harboring the ProAtDMC1-GUS reporter, the authors demonstrated that the additional cells are indeed the products of meiosis. Furthermore, they showed that the supernumerary cells expressed a functional megaspore reporter, ProFM2-GUS. Thus, AGP18 promotes megaspore survival after meiosis.
Overexpression of AGP18 results in the aberrant survival of megaspores in fully differentiated ovules. The dashed lines indicate four surviving megaspores. Bar = 10 μm. (Adapted from Demesa-Arévalo and Vielle-Calzada [2013], Figure 3O.)
The authors conclude that AGP18 is actively involved in selecting the surviving megaspore and suggest that classical arabinogalactan proteins may have had an evolutionary role in the acquisition of monospory.
References
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