Calreticulin localizes to plant intra/extracellular peripheries of highly specialized cells involved in pollen-pistil interactions

Piotr Wasąg,Robert Lenartowski,Przemysław Zakrzewski,Anna Suwińska,Marta Lenartowska,Jakub Walczewski

doi:10.1007/s00709-017-1134-8

Abstract

Calcium (Ca2+) plays essential roles in generative reproduction of angiosperms, but the sites and mechanisms of Ca2+ storage and mobilization during pollen-pistil interactions have not been fully defined. Both external and internal Ca2+ stores are likely important during male gametophyte communication with the sporophytic and gametophytic cells within the pistil. Given that calreticulin (CRT), a Ca2+-buffering protein, is able to bind Ca2+ reversibly, it can serve as a mobile store of easily releasable Ca2+ (so called an exchangeable Ca2+) in eukaryotic cells. CRT has typical endoplasmic reticulum (ER) targeting and retention signals and resides primarily in the ER. However, localization of this protein outside the ER has also been revealed in both animal and plant cells, including Golgi/dictyosomes, nucleus, plasma membrane/cell surface, plasmodesmata, and even extracellular matrix. These findings indicate that CRT may function in a variety of different cell compartments and specialized structures. We have recently shown that CRT is highly expressed and accumulated in the ER of plant cells involved in pollen-pistil interactions in Petunia, and we proposed an essential role for CRT in intracellular Ca2+ storage and mobilization during the key reproductive events. Here, we demonstrate that both CRT and exchangeable Ca2+ are localized in the intra/extracellular peripheries of highly specialized plant cells, such as the pistil transmitting tract cells, pollen tubes, nucellus cells surrounding the embryo sac, and synergids. Based on our present results, we propose that extracellularly located CRT is also involved in Ca2+ storage and mobilization during sexual reproduction of angiosperms.

Highlights

CRT is a Ca2+-binding/buffering protein implicated in many cellular functions, including lectin-like chaperoning, Ca2+ storage and signaling, regulation of gene expression, cell adhesion, regeneration, immunity, and apoptosis
We have found that CRT is located on the cell membrane/surface and in the apoplast of highly specialized plant cells involved in pollen-pistil interactions (Lenartowska et al 2002, 2009; Lenartowski et al 2015)
Localization of CRT outside the protoplast in different plant cells has been confirmed by other authors (Borisjuk et al 1998; Navazio et al 2002; Šamaj et al 2008; Luczak et al 2015; Niedojadło et al 2015). Since both internal and external Ca2+ stores are likely important during communication of the male gametophyte and the female sporophyte/ gametophyte cells, in this report we focus on CRT located in intra/extracellular peripheries in the context of its probable role/s in mobile Ca2+ storage during pollen-pistil interactions in angiosperms

Summary

Introduction

CRT is a Ca2+-binding/buffering protein implicated in many cellular functions, including lectin-like chaperoning, Ca2+ storage and signaling, regulation of gene expression, cell adhesion, regeneration, immunity, and apoptosis (see reviews by Michalak et al 2009; Jia et al 2009; Thelin et al 2011). Plant CRT was found in many different compartments and structures, including dictyosomes/vesicles (Borisjuk et al 1998; Navazio et al 2002; Lenartowska et al 2002, 2009; Hsieh and Huang 2005; Nardi et al 2006; Lenartowski et al 2015; Niedojadło et al 2015), the cytosol (Lenartowska et al 2002; Jia et al 2008), protein bodies (Torres et al 2001; Šamaj et al 2008), nucleus (Denecke et al 1995; Napier et al 1995; Lenartowska et al 2002; Lenartowski et al 2015), plasma membrane/cell surface (Borisjuk et al 1998; Lenartowska et al 2002; Navazio et al 2002; Šamaj et al 2008), plasmodesmata (Baluška et al 1999; Laporte et al 2003; Bayer et al 2004; Chen et al 2005; Lenartowska et al 2009; Bilska and Sowiński 2010; Christensen et al 2010), and the cell wall (Lenartowska et al 2002, 2009; Lenartowski et al 2015; Luczak et al 2015; Niedojadło et al 2015). These varied locations suggest that CRT may function in different plant cell compartments and specialized structures, including extracellular regions

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