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

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.

Calreticulin expression and localization in relation to exchangeable Ca2+ during pollen development in Petunia

“…Previous work from our laboratory provided evidence that CRT may be involved in Ca 2+ homeostasis and molecular chaperoning during the key reproductive events in Petunia pistil, such as pistil transmitting tract maturation, pollen-pistil interactions, double fertilization, and early embryogenesis [ 17 , 18 , 22 ]. We also demonstrated that CRT has a critical role in pollen tube growth in vitro [ 20 ].…”

“…However, localization of this protein outside the ER has also been observed in eukaryotic cells. Plant CRT was detected in many different compartments and structures, including dictyosomes, vesicles, protein bodies, nucleus, plasma membrane, and the cell wall [ 22 ]. Although, some previous work from our lab and others indicated location of CRT in several extracellular regions were somewhat controversial, an elegant work of Luczak et al [ 34 ] provided clear evidence that CRT, similar to several other proteins, is present in the cell walls of few plant species including maize, Lupinus and Arabidopsis .…”

“…Although, some previous work from our lab and others indicated location of CRT in several extracellular regions were somewhat controversial, an elegant work of Luczak et al [ 34 ] provided clear evidence that CRT, similar to several other proteins, is present in the cell walls of few plant species including maize, Lupinus and Arabidopsis . We have previously demonstrated that both CRT and exchangeable Ca 2+ were localized to the extracellular peripheries of highly specialized plant cells, such as pollen tubes and synergids [ 14 , 16 , 18 , 22 ]. In pollen tubes, CRT labeling was detected in the peripheral ER-rich cytoplasm of the tube adjacent to the cell membrane and in the callosic cell wall.…”

“…Thus, we postulate that excess of exchangeable Ca 2+ is translocated from the cell cytosol not only to the ER but also to the callosic cell wall, and bound by CRT present there in order to maintaining Ca 2+ homeostasis in meiocytes during microsporogenesis. We proposed this idea previously when we observed co-localization of CRT, callose and Ca 2+ ppts in the internal cell wall of growing pollen tube [ 14 , 16 , 22 ]. More precise research is required, however, to verify this hypothesis.…”

The Dynamics of the Cell Wall Proteome of Developing Alfalfa Stems

“…A C-terminally truncated calreticulin was observed (CaCl 2 spot 1274). Although regarded as an ER-resident protein, calreticulin was described in other subcellular compartments [43]. Animal calreticulin is linked with the recognition of tumor and apoptotic cells, and wound healing in plants it is linked to growth and stress responses [44].…”