Cell wall remodeling and callose deposition during the embryogenic transition in the tree fern Cyathea delgadii Sternb.

Abstract

With the discovery in 2015 of the ability to induce somatic embryos in Cyathea delgadii, learning more about the relationship between the structure of apoplast and cell differentiation has become possible among ferns. In this study, the distribution of arabinogalactan proteins, pectins, extensins, and callose with specific epitopes recognized by monoclonal antibodies was investigated during direct somatic embryogenesis (SE) of C. delgadii. Eight antigens against the arabinogalactan proteins (JIM8, JIM13, LM2), pectins (JIM5, JIM7), extensins (JIM11, JIM12), and callose (anti-1 → 3-β-glucan) were selected. Two types of explants were analyzed, i.e. stipe fragments and internodes, which give rise to embryos of unicellular and multicellular origin, respectively. The study showed that embryogenic transition in C. delgadii is preceded by cell wall remodeling of initial explants. Dynamic changes in JIM13, JIM12, and anti-1 → 3-β-glucan localization were observed. The differences in the distribution of studied epitopes were observed between the cell walls of the epidermis and those located in the other layers of the explant. Moreover, within the somatic embryos, a stronger fluorescence of the examined antibodies was observed, mainly those reacting with arabinogalactan proteins, extensins, and callose. These results also implicated that, with the exception of the earlier appearance of callose in the stipe explants, the uni- and multicellular pathways of somatic embryo differentiation do not differ in the quality of cell wall components. The presented studies document the first time that SE in ferns can be regulated by changes in apoplast structure and they provide a basis for more detailed research.