Symplasmic isolation marks cell fate changes during somatic embryogenesis.

Anneke Horstman,Ewa U Kurczynska,Kim Boutilier,Karol Malota,Mengfan Li,Kamila Godel-Jedrychowska,Mercedes Soriano,Katarzyna Kulinska-Lukaszek,Gerhard Leubner

doi:10.1093/jxb/eraa041

Abstract

Cell-to-cell signalling is a major mechanism controlling plant morphogenesis. Transport of signalling molecules through plasmodesmata is one way in which plants promote or restrict intercellular signalling over short distances. Plasmodesmata are membrane-lined pores between cells that regulate the intercellular flow of signalling molecules through changes in their size, creating symplasmic fields of connected cells. Here we examine the role of plasmodesmata and symplasmic communication in the establishment of plant cell totipotency, using somatic embryo induction from Arabidopsis explants as a model system. Cell-to-cell communication was evaluated using fluorescent tracers, supplemented with histological and ultrastructural analysis, and correlated with expression of a WOX2 embryo reporter. We showed that embryogenic cells are isolated symplasmically from non-embryogenic cells regardless of the explant type (immature zygotic embryos or seedlings) and inducer system (2,4-dichlorophenoxyacetic acid or the BABY BOOM (BBM) transcription factor), but that the symplasmic domains in different explants differ with respect to the maximum size of molecule capable of moving through the plasmodesmata. Callose deposition in plasmodesmata preceded WOX2 expression in future sites of somatic embryo development, but later was greatly reduced in WOX2-expressing domains. Callose deposition was also associated with a decrease DR5 auxin response in embryogenic tissue. Treatment of explants with the callose biosynthesis inhibitor 2-deoxy-D-glucose supressed somatic embryo formation in all three systems studied, and also blocked the observed decrease in DR5 expression. Together these data suggest that callose deposition at plasmodesmata is required for symplasmic isolation and establishment of cell totipotency in Arabidopsis.

Highlights

Intercellular communication between plant cells takes place by the apoplastic pathway, in the shared space within cell walls, or by the symplasmic pathway through plasmodesmata (PD) that traverse the walls of adjacent cells and connect their cytoplasm
We showed that embryogenic cells are isolated symplasmically from non-embryogenic cells regardless of the explant type and inducer system (2,4-dichlorophenoxyacetic acid or the BABY BOOM (BBM) transcription factor), but that the symplasmic domains in different explants differ with respect to the maximum size of molecule capable of moving through the plasmodesmata
Symplasmic domains are established during 2,4-D-induced somatic embryogenesis that coincide with the establishment of embryogenic cells

Summary

Introduction

Intercellular communication between plant cells takes place by the apoplastic pathway, in the shared space within cell walls, or by the symplasmic pathway through plasmodesmata (PD) that traverse the walls of adjacent cells and connect their cytoplasm. PD control the cell-to-cell flow of molecules, and can be reduced or closed through deposition of callose (β-1,3-glucan) in the neck region (Benitez-Alfonso et al, 2013; Han et al, 2014).The level of callose deposition is regulated by a balance between callose synthase and β-1,3-glucanase activity (Zavaliev et al, 2011; de Storme and Geelen, 2014; Sevilem et al, 2015). Callose turnover at PD is an important mechanism regulating movement of signalling molecules during development (Sevilem et al, 2015; Saatian et al, 2018), including shoot apical meristem development (Rinne et al, 2011), lateral root formation (Benitez-Alfonso et al, 2013), stomata patterning (Guseman et al, 2010), root nodulation (Gaudioso-Pedraza et al, 2018), and pollen development (Li et al, 2003)

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