Arabinogalactan proteins and arabinan pectins abound in the specialized matrices surrounding female gametes of the fern Ceratopteris richardii

Abstract

Both male and female gametes of archegoniates are highly specialized cells surrounded by an extraprotoplasmic matrix rich in AGPs, which are speculated to facilitate development and gamete fusion through Ca 2+) oscillations. An additional layer, the egg envelope, forms around the egg periphery, except at the fertilization pore, and contains arabinose-rich polymers that presumably impart flexibility for the rapidly growing zygote and embryo. The abundant AGPs and arabinan pectins associated with the eggs of C. richardii not only are integral to development, fertilization, and early embryogenesis, but also may be involved in desiccation tolerance important to the survival of the reproductive gametophyte. A defining feature of gametogenesis in archegoniates is the deposition of a special matrix outside of the plasmalemma of both egg and sperm cells that displaces the primary cell wall away from the protoplasm. It is within this matrix that gamete differentiation occurs. In leptosporangiate ferns, maturation of the egg cell involves the deposition of a second specialized wall, the so-called egg envelope that surrounds the cell except at the fertilization pore, a narrow site where gamete fusion takes place. We provide the first conclusive evidence of the macromolecular constituents in the unique structures surrounding fern egg cells before and after fertilization. To test the hypotheses that the egg extracellular matrix contains arabinogalactan proteins (AGPs) as does the sperm cell matrix, and that cell wall polysaccharides, especially pectins, are components of the egg envelope, we examined the expression patterns of AGPs and cell wall constituents during oogenesis in Ceratopteris richardii. Utilizing histochemical stains for callose, cellulose and AGPs coupled with immunogold localizations employing a suite of monoclonal antibodies to cell wall components (JIM13, JIM8, LM2, LM5, LM6, LM19, LM20 and anticallose), we demonstrate that AGPs, but not pectins, are abundant in the matrix around egg cells and degrading neck canal and ventral canal cells during archegonial development. A striking finding is that both AGPs and (1,5)-α-L-arabinan pectin epitopes are principle components of the egg envelope before and after fertilization, suggesting that they are important in both egg maturation and gamete fusion.

Search for evolutionary roots of land plant arabinogalactan‐proteins in charophytes: presence of a rhamnogalactan‐protein in Spirogyra pratensis (Zygnematophyceae)

“…A role of AGPs in sexual reproduction of angiosperms is well documented ( Ma et al, 2018 ) and has also been described for the gymnosperm Larix ( Rafińska et al, 2021 ) and the fern Ceratopteris ( Lopez and Renzaglia, 2014 , 2016 ). An ovular AGP named AMOR with the terminal disaccharide 4- O -methyl-glucuronosyl galactose (possibly also present in Spirogyra RGP) ( Figure 4 ) is involved in pollen tube guidance in Torenia ( Jiao et al, 2017 ).…”

Fern cell walls and the evolution of arabinogalactan-proteins in streptophytes

“…Monoclonal antibodies directed against arabinogalactan epitopes (e.g. JIM4, JIM8, JIM13, KM1, LM2, LM6 or MAC207 (for epitopes and references see Classen et al ., 2019) were used to verify the presence of AGPs in combination with glycan microarrays, ELISA and microscopy to detect AGPs in different monilophytes (Popper, 2006; Moore et al ., 2013; Leroux et al ., 2015; Bartels and Classen, 2017; Eeckhout et al ., 2014; Lopez and Renzaglia, 2014; Lopez and Renzaglia, 2016). However, reliability of these discoveries are sometimes questionable because the exact structures of epitopes of the antibodies are not always known and cross-reactions with other polysaccharides might occur.…”

The Placenta of Physcomitrium patens: Transfer Cell Wall Polymers Compared across the Three Bryophyte Groups

“…Their significance in sexual reproduction has been observed in the process of spermatogenesis in Ceratopteris [ 46 ] and the moss, Aulacomnium palsutre [ 103 ]. The female gametes of Ceratopteris also express AGPs during development [ 55 ].…”