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

Plant-specific calreticulin is localized in the nuclei of highly specialized cells in the pistil-new observations for an old hypothesis

One of the first cellular locations of the calreticulin (CRT) chaperone in eukaryotic cells, apart from its obvious localization in the endoplasmic reticulum (ER), was the cell nucleus (Opas et al. 1991). The presence of CRT has been detected inside the nucleus and in the nuclear envelope of animal and plant cells, and a putative nuclear localization signal (NLS) in the CRT amino acid sequence has been mapped in several animal and plant species. Over the last 30 years, other localization sites of this protein outside the ER and cell nucleus have also been discovered, suggesting that CRT is a multifunctional Ca2+-binding protein widely found in various cell types. In our previous studies focusing on plant developmental biology, we have demonstrated the presence of CRT inside and outside the ER in highly specialized plant cells, as well as the possibility of CRT localization in the cell nucleus. In this paper, we present a detailed analysis of immunocytochemical localization of CRT inside nuclei of the pistil transmission tract somatic cells before and after pollination. We show a similar pattern of the nuclear CRT localization in relation to exchangeable Ca2+ for two selected species of angiosperms, dicotyledonous Petunia and monocot Haemanthus, that differ in anatomical structure of the pistil and discuss the potential role of CRT in the cell nucleus.

Comprehensive genomic characterization and expression analysis of calreticulin gene family in tomato

Calreticulin (CRT) is a calcium-binding endoplasmic reticulum (ER) protein that has been identified for multiple cellular processes, including protein folding, regulation of gene expression, calcium (Ca2+) storage and signaling, regeneration, and stress responses. However, the lack of information about this protein family in tomato species highlights the importance of functional characterization. In the current study, 21 CRTs were identified in four tomato species using the most recent genomic data and performed comprehensive bioinformatics and SlCRT expression in various tissues and treatments. In the bioinformatics analysis, we described the physiochemical properties, phylogeny, subcellular positions, chromosomal location, promoter analysis, gene structure, motif distribution, protein structure and protein interaction. The phylogenetic analysis classified the CRTs into three groups, consensus with the gene architecture and conserved motif analyses. Protein structure analysis revealed that the calreticulin domain is highly conserved among different tomato species and phylogenetic groups. The cis-acting elements and protein interaction analysis indicate that CRTs are involved in various developmental and stress response mechanisms. The cultivated and wild tomato species exhibited similar gene mapping on chromosomes, and synteny analysis proposed that segmental duplication plays an important role in the evolution of the CRTs family with negative selection pressure. RNA-seq data analysis showed that SlCRTs were differentially expressed in different tissues, signifying the role of calreticulin genes in tomato growth and development. qRT-PCR expression profiling showed that all SlCRTs except SlCRT5 were upregulated under PEG (polyethylene glycol) induced drought stress and abscisic acid (ABA) treatment and SlCRT2 and SlCRT3 were upregulated under salt stress. Overall, the results of the study provide information for further investigation of the functional characterization of the CRT genes in tomato.

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

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.

The Dynamics of the Cell Wall Proteome of Developing Alfalfa Stems

In this study, the cell-wall-enriched subproteomes at three different heights of alfalfa stems were compared. Since these three heights correspond to different states in stem development, a view on the dynamics of the cell wall proteome during cell maturation is obtained. This study of cell wall protein-enriched fractions forms the basis for a description of the development process of the cell wall and the linking cell wall localized proteins with the evolution of cell wall composition and structure. The sequential extraction of cell wall proteins with CaCl2, EGTA, and LiCl-complemented buffers was combined with a gel-based proteome approach and multivariate analysis. Although the highest similarities were observed between the apical and intermediate stem regions, the proteome patterns are characteristic for each region. Proteins that bind carbohydrates and have proteolytic activity, as well as enzymes involved in glycan remobilization, accumulate in the basal stem region. Beta-amylase and ferritin likewise accumulate more in the basal stem segment. Therefore, remobilization of nutrients appears to be an important process in the oldest stem segment. The intermediate and apical regions are sites of cell wall polymer remodeling, as suggested by the high abundance of proteins involved in the remodeling of the cell wall, such as xyloglucan endoglucosylase, beta-galactosidase, or the BURP-domain containing polygalacturonase non-catalytic subunit. However, the most striking change between the different stem parts is the strong accumulation of a DUF642-conserved domain containing protein in the apical region of the stem, which suggests a particular role of this protein during the early development of stem tissues.

Cytosolic calcium localization and dynamics during early endosperm development in the genus Agave (Asparagales, Asparagaceae)

Calcium is a secondary messenger that regulates and coordinates the cellular responses to environmental cues. Despite calcium being a key player during fertilization in plants, little is known about its role during the development of the endosperm. For this reason, the distribution, abundance, and dynamics of cytosolic calcium during the first stages of endosperm development of Agave tequilana and Agave salmiana were analyzed. Cytosolic calcium and actin filaments detected in the embryo sacs of Agave tequilana and A. salmiana revealed that they play an important role during the division and nuclear migration of the endosperm. After fertilization, a relatively high concentration of cytosolic calcium was located in the primary nucleus of the endosperm, as well as around migrating nuclei during the development of the endosperm. Cytosolic calcium participates actively during the first mitosis of the endosperm mother cell and interacts with the actin filaments that generate the motor forces during the migration of the nuclei through the large cytoplasm of the central cell.

Localization of calreticulin and calcium ions in mycorrhizal roots of Medicago truncatula in response to aluminum stress

Aluminum (Al) toxicity limits growth and symbiotic interactions of plants. Calcium plays essential roles in abiotic stresses and legume-Rhizobium symbiosis, but the sites and mechanism of Ca²⁺ mobilization during mycorrhizae have not been analyzed. In this study, the changes of cytoplasmic Ca²⁺ and calreticulin (CRT) in Medicago truncatula mycorrhizal (MR) and non-mycorrizal (NM) roots under short Al stress [50 μM AlCl₃ pH 4.3 for 3 h] were analyzed. Free Ca²⁺ ions were detected cytochemically by their reaction with potassium pyroantimonate and anti-CRT antibody was used to locate this protein in Medicago roots by immunocytochemical methods. In MR and NM roots, Al induced accumulation of CRT and free Ca²⁺. Similar calcium and CRT distribution in the MR were found at the surface of fungal structures (arbuscules and intercellular hyphae), cell wall and in plasmodesmata, and in plant and fungal intracellular compartments. Additionally, degenerated arbuscules were associated with intense Ca²⁺ and CRT accumulation. In NM roots, Ca²⁺ and CRT epitopes were observed in the stele, near wall of cortex and endodermis. The present study provides new insight into Ca²⁺ storage and mobilization in mycorrhizae symbiosis. The colocalization of CRT and Ca²⁺ suggests that CRT is essential for calcium mobilization for normal mycorrhiza development and response to Al stress.

Cell-cell communications and molecular mechanisms in plant sexual reproduction

Sexual reproduction is achieved by precise interactions between male and female reproductive organs. In plant fertilization, sperm cells are carried to ovules by pollen tubes. Signals from the pistil are involved in elongation and control of the direction of the pollen tube. Genetic, reverse genetic, and cell biological analyses using model plants have identified various factors related to the regulation of pollen tube growth and guidance. In this review, I summarize the mechanisms and molecules controlling pollen tube growth to the ovule, micropylar guidance, reception of the guidance signal in the pollen tube, rupture of the pollen tube to release sperm cells, and cessation of the tube guidance signal. I also briefly introduce various techniques used to analyze pollen tube guidance in vitro.