Zooming in on calcium signals in plants - from organism to organelle level

In plants, as in animals, changes in cytosolic free calcium ([Ca²⁺]_cyt) are an early and essential element of signalling networks activated by abiotic stress (such as cold, drought, or salt) and biotic cues (such as herbivory or the perception of pathogen-associated molecular patterns, PAMPs). Those Ca²⁺-dependent signalling networks operate on different levels of scale, from whole-organism down to organelle level, for which examples will be presented in this talk. On organism level, imaging experiments on whole Arabidopsis rosettes revealed that plants respond to localized wounding and herbivory with a systemic [Ca²⁺]_cytelevation in adjacent leaves, which is dependent on a vacuolar Ca²⁺-permeable channel and which impacts systemic defense. On cellular level, imaging of epidermal guard cells identified that [Ca²⁺]_cyt responses to PAMPs are of an oscillatory nature, supporting the concept that kinetics of [Ca²⁺]_cyt elevations determine the activation of downstream responses. On organellar level, stromal free Ca²⁺ in the chloroplast has been shown to be responsive to environmental parameters, and numerous chloroplast proteins are known to be regulated by Ca²⁺, but the molecular mechanisms that determine Ca²⁺ homeostasis in chloroplasts have been unclear. I will present our recent identification of two transport proteins that localize to distinct chloroplast membranes, determine the kinetics of Ca²⁺ signals in the chloroplast stroma, and are vital for chloroplast function. Paralogs of those proteins are present in the Golgi apparatus and thus likely to mediate vesicular cation homeostasis. Evidence for this will be shown in the talk.

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