Monitoring calcium handling by the plant endoplasmic reticulum with a low-Ca2+ -affinity targeted aequorin reporter

Nonbinary fungal signals and calcium-mediated transduction in plant immunity and symbiosis

Chitin oligomers (COs) are among the most common and active fungal elicitors of plant responses. Short-chain COs from symbiotic arbuscular mycorrhizal fungi activate accommodation responses in the host root, while long-chain COs from pathogenic fungi are acknowledged to trigger defence responses. The modulation of intracellular calcium concentration - a common second messenger in a wide variety of plant signal transduction processes - plays a central role in both signalling pathways with distinct signature features. Nevertheless, mounting evidence suggests that plant immunity and symbiosis signalling partially overlap at multiple levels. Here, we elaborate on recent findings on this topic, highlighting the nonbinary nature of chitin-based fungal signals, their perception and their interpretation through Ca2+ -mediated intracellular signals. Based on this, we propose that plant perception of symbiotic and pathogenic fungi is less clear-cut than previously described and involves a more complex scenario in which partially overlapping and blurred signalling mechanisms act upstream of the unambiguous regulation of gene expression driving accommodation or defence responses.

Spatially and temporally distinct Ca2+ changes in Lotus japonicus roots orient fungal-triggered signalling pathways towards symbiosis or immunity

Plants activate an immune or symbiotic response depending on the detection of distinct signals from root-interacting microbes. Both signalling cascades involve Ca2+ as a central mediator of early signal transduction. In this study, we combined aequorin- and cameleon-based methods to dissect the changes in cytosolic and nuclear Ca2+ concentration caused by different chitin-derived fungal elicitors in Lotus japonicus roots. Our quantitative analyses highlighted the dual character of the evoked Ca2+ responses taking advantage of the comparison between different genetic backgrounds: an initial Ca2+ influx, dependent on the LysM receptor CERK6 and independent of the common symbiotic signalling pathway (CSSP), is followed by a second CSSP-dependent and CERK6-independent phase, that corresponds to the well-known perinuclear/nuclear Ca2+ spiking. We show that the expression of immunity marker genes correlates with the amplitude of the first Ca2+ change, depends on elicitor concentration, and is controlled by Ca2+ storage in the vacuole. Our findings provide an insight into the Ca2+-mediated signalling mechanisms discriminating plant immunity- and symbiosis-related pathways in the context of their simultaneous activation by single fungal elicitors.

WeiTsing: a new face of Ca2+-permeable channels in plant immunity

Plants employ pattern- and effector-triggered immunity (PTI and ETI) to synergistically defend invading pathogens and insect herbivores. Both PTI and ETI can induce cytosolic Ca2+ spikes, despite in different spatiotemporal patterns, to activate downstream Ca2+-dependent immune signaling cascades. While multiple families of Ca2+-permeable channels at the plasma membrane have been uncovered, the counterparts responsible for Ca2+ release from intracellular stores remain poorly understood. In a groundbreaking paper published recently by Cell, the authors reported that WeiTsing, an Arabidopsis endoplasmic reticulum (ER)-resident protein that was specifically expressed in the pericycle upon Plasmodiophora brassicae (Pb) infection, could form resistosome-like Ca2+-conducting channel and protect the stele of Brassica crops from Pb colonization. As the channel activity of WeiTsing was indispensable for its immune function, the findings highlight a previously underappreciated role of Ca2+ release from intracellular repertoire in promoting plant disease resistance.

Quantitative Analysis of Plant Cytosolic Calcium Signals in Response to Water Activated by Low-Power Non-Thermal Plasma

Non-thermal plasma technology is increasingly being applied in the plant biology field. Despite the variety of beneficial effects of plasma-activated water (PAW) on plants, information about the mechanisms of PAW sensing by plants is still limited. In this study, in order to link PAW perception to the positive downstream responses of plants, transgenic Arabidopsis thaliana seedlings expressing the Ca²⁺-sensitive photoprotein aequorin in the cytosol were challenged with water activated by low-power non-thermal plasma generated by a dielectric barrier discharge (DBD) source. PAW sensing by plants resulted in the occurrence of cytosolic Ca²⁺ signals, whose kinetic parameters were found to strictly depend on the operational conditions of the plasma device and thus on the corresponding mixture of chemical species contained in the PAW. In particular, we highlighted the effect on the intracellular Ca²⁺ signals of low doses of DBD-PAW chemicals and also presented the effects of consecutive plant treatments. The results were discussed in terms of the possibility of using PAW-triggered Ca²⁺ signatures as benchmarks to accurately modulate the chemical composition of PAW in order to induce environmental stress resilience in plants, thus paving the way for further applications in agriculture.