Tou Cheu Xiong

Adapting fluorescent iron imaging to living plants enables visualizing labile Fe2+ and Fe3+ pools, revealing the heterogeneous distribution of iron redox status at the tissue and cellular levels.

Plants must adapt to environmental constraints. For this, they are able to perceive several types of stress in isolation or in combination manner. At the cellular level, after the perception of stress, cell signaling is set up to allow the establishment of the specific response. The calcium ion is known to be one of the ubiquitous second messengers which is involved in most of the stresses perceived by the plant. Changes of free cytosolic calcium but also in other cellular compartments are able to activate or inactivate several mechanisms involved in the cell to cope with the changes of environmental conditions. Several calcium reporters have been intensively used to visualize calcium signals in different conditions. In this chapter, we will present only genetically encoded fluorescent reporters for calcium imaging in living plant tissues to measure variations in calcium at several scales. The FRET (fluorescence resonance energy transfer) YC3.60 and the intensiometric GCamP3 sensors will be used in this method chapter. The image analyses will be also detailed for fluorescence quantification of calcium variation. Key words Arabidopsis thaliana Calcium imaging Calcium signature Fluorescent reporters FRET Genetically encoded calcium indicators In vivo calcium imaging Microscopy

Alighting aphids probe a new host plant by intracellular test punctures for suitability. These induce immediate calcium signals that emanate from the punctured sites and might be the first step in plant recognition of aphid feeding and the subsequent elicitation of plant defence responses. Calcium is also involved in the transmission of non-persistent plant viruses that are acquired by aphids during test punctures. Therefore, we wanted to determine whether viral infection alters calcium signalling. For this, calcium signals triggered by aphids were imaged on transgenic Arabidopsis plants expressing the cytosolic FRET-based calcium reporter YC3.6-NES and infected with the non-persistent viruses cauliflower mosaic (CaMV) and turnip mosaic (TuMV), or the persistent virus, turnip yellows (TuYV). Aphids were placed on infected leaves and calcium elevations were recorded by time-lapse fluorescence microscopy. Calcium signal velocities were significantly slower in plants infected with CaMV or TuMV and signal areas were smaller in CaMV-infected plants. Transmission tests using CaMV-infected Arabidopsis mutants impaired in pathogen perception or in the generation of calcium signals revealed no differences in transmission efficiency. A transcriptomic meta-analysis indicated significant changes in expression of receptor-like kinases in the BAK1 pathway as well as of calcium channels in CaMV- and TuMV-infected plants. Taken together, infection with CaMV and TuMV, but not with TuYV, impacts aphid-induced calcium signalling. This suggests that viruses can modify plant responses to aphids from the very first vector/host contact.

Post-translational regulations of Shaker-like voltage-gated K⁺ channels were reported to be essential for rapid responses to environmental stresses in plants. In particular, it has been shown that calcium-dependent protein kinases (CPKs) regulate Shaker channels in plants. Here, the focus was on KAT2, a Shaker channel cloned in the model plant Arabidopsis thaliana, where is it expressed namely in the vascular tissues of leaves. After co-expression of KAT2 with AtCPK6 in Xenopuslaevis oocytes, voltage-clamp recordings demonstrated that AtCPK6 stimulates the activity of KAT2 in a calcium-dependent manner. A physical interaction between these two proteins has also been shown by Förster resonance energy transfer by fluorescence lifetime imaging (FRET-FLIM). Peptide array assays support that AtCPK6 phosphorylates KAT2 at several positions, also in a calcium-dependent manner. Finally, K⁺ fluorescence imaging in planta suggests that K⁺ distribution is impaired in kat2 knock-out mutant leaves. We propose that the AtCPK6/KAT2 couple plays a role in the homeostasis of K⁺ distribution in leaves.

A complex signaling network involving voltage‐gated potassium channels from the Shaker family contributes to the regulation of stomatal aperture. Several kinases and phosphatases have been shown to be crucial for ABA‐dependent regulation of the ion transporters. To date, the Ca²⁺‐dependent regulation of Shaker channels by Ca²⁺‐dependent protein kinases (CPKs) is still elusive. A functional screen in Xenopus oocytes was launched to identify such CPKs able to regulate the three main guard cell Shaker channels KAT1, KAT2, and GORK. Seven guard cell CPKs were tested and multiple CPK/Shaker couples were identified. Further work on CPK33 indicates that GORK activity is enhanced by CPK33 and unaffected by a nonfunctional CPK33 (CPK33‐K102M). Furthermore, Ca²⁺‐induced stomatal closure is impaired in two cpk33 mutant plants.