Calcium Signaling during Reproduction and Biotrophic Fungal Interactions in Plants  Junyi Chen, Caroline Gutjahr, Andrea Bleckmann, Thomas Dresselhaus  Molecular Plant  Volume 8, Issue 4, Pages 595-611 (April 2015) DOI: 10.1016/j.molp.2015.01.023 Copyright © 2015 The Author Terms and Conditions

Figure 1 Generalized Model of Ca2+ Signaling Occurring in Cells Involved in Plant Reproduction and Fungal Invasion. (A) Model showing the molecular players involved in Ca2+signaling of a reproductive plant cell (e.g. pollen tube, papilla cell, synergid cell, female gametes). Only protein classes are indicated. See the text for individual members of protein classes that have been functionally characterized. [Ca2+]cyt spikes are induced by external ligands perceived by cell surface receptors (shown on the left), which induce opening of [Ca2+]cyt channels, partly via ROS production, resulting in Ca2+spikes after influx from external stores (e.g. cell wall and burst cells). The involvement of Ca2+ release from internal stores (shown on the right: vacuole, ER, or nuclear envelope) is unclear. A battery of Ca2+ binding proteins (e.g. CaM/CML, CBLs, CDPKs, ABPs) decode the spiking pattern and activate fast responses (e.g. ROS production, vesicle secretion, actin depolymerization, Ca2+ efflux, and inhibition of Ca2+ influx) or induce slow responses in the nucleus via modulation of the gene expression pattern. (B) Model showing the molecular players in Ca2+ signaling discussed in the main text during plant cell invasion by pathogens (top) and symbionts (bottom). See text for a detailed description and references. Many signaling events shown in (A) likely occur also during infection of plant cells by symbiotic fungi and during infection by pathogenic filamentous microbes (fungi and oomycetes; see also Seybold et al., 2014). Top: during defense, microbe associated molecular patterns (MAMPS) are perceived by receptor-like kinases, their co-receptors, and extracellular receptor proteins, which leads to the activation of Ca2+channels and a transient increase in [Ca2+]cyt. The increase in [Ca2+]cyt activates CDPKs, which activate NADPH oxidases by phosphorylation. The resulting ROS burst can also trigger Ca2+channels, which again leads to [Ca2+]cyt elevations. Bottom: the molecular players required for generation and interpretation of nuclear Ca2+ spiking during cell infection by symbiotic arbuscular mycorrhiza (AM) fungi. Fungus-released chitin oligomers are perceived by receptor-like kinases that probably trigger the generation of a yet unknown secondary messenger that induces nuclear Ca2+ spiking. This requires currently unknown Ca2+ channels and potassium-conductive counter ion channels. Ca2+ spiking is interpreted by nuclear-localized Ca2+ and CaM-dependent kinases (CCamK), which phosphorylate transcription factors. Molecular Plant 2015 8, 595-611DOI: (10.1016/j.molp.2015.01.023) Copyright © 2015 The Author Terms and Conditions

Figure 2 Comparison of [Ca2+]cyt Signatures in Gametophytic Cells during the Process of Double Fertilization and in Sporophytic Cells during Fungal Infection. (A–D) False color ratio metric images of relative [Ca2+]cyt in reproductive cells expressing the calcium sensor CerTN-L15 using cell type-specific promoters (MYB98, pollen; EC1.1, egg cell; DD65, central cell; and ARO1, pollen). Scale bars, 20 μm. (A) Pollen grains and a Growing pollen tube, (B) synergid cells, (C) egg cell, and (D) central cell of Arabidopsis thaliana. For marker line details, please see Denninger et al., 2014. (E) [Ca2+]cyt signatures in a growing pollen tube, synergid cells, egg cell, and central cell during the process of double fertilization. Displayed are normalized ratio metric changes in CerTN-L15 in respect of the event of pollen tube burst (0 min). Arrow, first physical contact of pollen tube and synergid cells; star, pollen tube burst. In Arabidopsis the growing pollen tube does not display significant [Ca2+]cyt oscillation. In synergids cells, [Ca2+]cyt starts oscillation after physical pollen tube contact. Shortly before pollen tube burst, the high [Ca2+]cyt level is maintained in the cell followed by a [Ca2+]cyt spike, which overlaps with pollen tube burst (Supplemental Movie 1). In the egg cell, two distinct [Ca2+]cyt elevations can be monitored. The first narrow and sharp spike coincides with pollen tube burst/sperm cell arrival (Supplemental Movie 2), whereas the second peak is broader and is correlated to gamete fusion. Please note that deformation of egg cells by the growing pollen tube leads to a focal plane shift associated with a signal increase (arrowhead) but no spiking. The central cell shows only one [Ca2+]cyt elevation in response to pollen tube burst (Supplemental Movie 3). (F and G) Redrawn measurements of [Ca2+]nucl spiking occurring in root tissue during arbuscular mycorrhizal fungal infection of M. truncatula (after Sieberer et al., 2012). Penetration of epidermis cells is associated with a high frequency of Ca2+cyto spikes (cell e1). Low-frequency [Ca2+]nucl spiking is observed in the underlying cortical cell c1, in which the nucleus has migrated to the future site of hyphal entry. Neighboring cells (c2) do not show any increase in [Ca2+]nucl level. Molecular Plant 2015 8, 595-611DOI: (10.1016/j.molp.2015.01.023) Copyright © 2015 The Author Terms and Conditions