Abstract
Key messageCalreticulin expression is upregulated during sexual reproduction ofHyacinthus orientalis, and the protein is localized both in the cytoplasm and a highly specialized cell wall within the female gametophyte.Several evidences indicate calreticulin (CRT) as an important calcium (Ca2+)-binding protein that is involved in the generative reproduction of higher plants, including both pre-fertilization and post-fertilization events. Because CRT is able to bind and sequester exchangeable Ca2+, it can serve as a mobile intracellular store of easily releasable Ca2+ and control its local cytosolic concentrations in the embryo sac. This phenomenon seems to be essential during the late progamic phase, gamete fusion, and early embryogenesis. In this report, we demonstrate the differential expression of CRT within Hyacinthus female gametophyte cells before and during anthesis, during the late progamic phase when the pollen tube enters the embryo sac, and at the moment of fertilization and zygote/early endosperm activation. CRT mRNA and the protein localize mainly to the endoplasmic reticulum (ER) and Golgi compartments of the cells, which are involved in sexual reproduction events, such as those in sister synergids, the egg cell, the central cell, zygote and the developing endosperm. Additionally, immunogold research demonstrates selective CRT distribution in the filiform apparatus (FA), a highly specific component of the synergid cell wall. In the light of our previous data showing the total transcriptional activity of the Hyacinthus female gametophyte and the results presented here, we discuss the possible functions of CRT with respect to the critical role of Ca2+ homeostasis during key events of sexual plant reproduction. Moreover, we propose that the elevated expression of CRT within the female gametophyte is a universal phenomenon in the cells involved in double fertilization in higher plants.
Highlights
CRT isoforms (CRTs) is an abundant Ca2?-binding/buffering protein that is predominantly localized in the endoplasmic reticulum (ER) of eukaryotic cells, where it acts as a lectin-like chaperone that is involved inPlant Cell Rep (2015) 34:2201–2215 the proper folding and quality control of de novo-synthesized proteins and is a modulator of Ca2? homeostasis and the signaling network
The results of our study indicate that prior to anthesis, all cells from the Hyacinthus embryo sac exhibit an accumulation of CRT mRNA, but significant differences in distribution pattern of investigated transcripts appear after flower opening, followed by the fusion of gametes
The observed changes in the expression of CRT in the cells that were involved in double fertilization are correlated in time with result previously reported by our group regarding the total transcriptional activity of Hyacinthus female gametophyte, whose level is comparable prior to anthesis but changes in response to flower opening, gamete fusion and early embryogenesis (Niedojadło et al 2012a)
Summary
CRT is an abundant Ca2?-binding/buffering protein that is predominantly localized in the ER of eukaryotic cells, where it acts as a lectin-like chaperone that is involved inPlant Cell Rep (2015) 34:2201–2215 the proper folding and quality control of de novo-synthesized proteins and is a modulator of Ca2? homeostasis and the signaling network (see reviews by Gelebart et al 2005; Jia et al 2009; Michalak et al 2009; Thelin et al 2011; Li and Yang 2012; Wang et al 2012). With a high capacity/low affinity that ends with a specific K/HDEL signal that is required for protein retention in the ER lumen (see reviews by Gelebart et al 2005; Jia et al 2009; Michalak et al 2009; Thelin et al 2011; Li and Yang 2012; Wang et al 2012). These findings suggest diverse roles for plant CRT in multiple cellular processes
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