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Background: Pollen development in the anther in angiosperms depends on complicated cellular interactions associated with the expression of gametophytic and sporophytic genes which control fundamental processes during microsporo/gametogenesis, such as exo/endocytosis, intracellular transport, cell signaling, chromatin remodeling, and cell division. Most if not all of these cellular processes depend of local concentration of calcium ions (Ca2+). Work from our laboratory and others provide evidence that calreticulin (CRT), a prominent Ca2+-binding/buffering protein in the endoplasmic reticulum (ER) of eukaryotic cells, may be involved in pollen formation and function. Here, we show for the first time the expression pattern of the PhCRT1 gene and CRT accumulation in relation to exchangeable Ca2+ in Petunia hybrida developing anther, and discuss probable roles for this protein in the male gametophyte development. Results: Using northern hybridization, western blot analysis, fluorescent in situ hybridization (FISH), immunocytochemistry, and potassium antimonate precipitation, we report that PhCRT1 is highly expressed in the anther and localization pattern of the CRT protein correlates with loosely bound (exchangeable) Ca2+ during the successive stages of microsporo/gametogenesis. We confirmed a permanent presence of both CRT and exchangeable Ca2+ in the germ line and tapetal cells, where these factors preferentially localized to the ER which is known to be the most effective intracellular Ca2+ store in eukaryotic cells. In addition, our immunoblots revealed a gradual increase in CRT level from the microsporocyte stage through the meiosis and the highest CRT level at the microspore stage, when both microspores and tapetal cells show extremely high secretory activity correlated with the biogenesis of the sporoderm. Conclusion: Our present data provide support for a key role of CRT in developing anther of angiosperms – regulation of Ca2+ homeostasis during pollen grains formation. This Ca2+-buffering chaperone seems to be essential for pollen development and maturation since a high rate of protein synthesis and protein folding within the ER as well as intracellular Ca2+ homeostasis are strictly required during the multi-step process of pollen development. [ABSTRACT FROM AUTHOR]

Calcium (Ca) plays essential roles in generative reproduction of angiosperms, but the sites and mechanisms of Ca storage and mobilization during pollen-pistil interactions have not been fully defined. Both external and internal Ca stores are likely important during male gametophyte communication with the sporophytic and gametophytic cells within the pistil. Given that calreticulin (CRT), a Ca-buffering protein, is able to bind Ca reversibly, it can serve as a mobile store of easily releasable Ca (so called an exchangeable Ca) 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 Ca storage and mobilization during the key reproductive events. Here, we demonstrate that both CRT and exchangeable Ca 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 Ca storage and mobilization during sexual reproduction of angiosperms. [ABSTRACT FROM AUTHOR]

The article focuses on the function of calreticulin in protein folding and quality control in the endoplasmic reticulum (ER) and in regulation of calcium homeostatis in eukaryotic cells, and also mentions the role of calcium in plant sexual reproduction.