Investigation of root signaling under heterogeneous salt stress: A case study for Cucumis sativus L.

- In a split-root system, salinity caused changes in the root electrical activity, in both sides, exposed directly and not directly to salt stress.
- Electrical changes in the portion of the root system not directly exposed to salt stress were not because of Na+ diffusion.
- K+ influx observed in the roots not exposed to salt could be explained as a compensation of K+ efflux from the roots exposed to salt.
- Salt marker genes regulated differently in the two sides of a split-root system, indicating plant ability to respond to heterogeneous conditions.
- Different electrical, physiological and genetic responses observed in the root portion not exposed to salt suggest root-to-root communication.

To sense, respond and adapt to the constantly changing environmental conditions, plants have developed sophisticated signaling mechanisms. In this study, plant signaling under heterogeneous salt conditions was investigated. A split root system was established in which one half of the root apparatus (HR1) was treated with salt. The impact of the salt on the electrical signals as the initial response of the plant to exterior stimulus was measured in the other half of the root system (HR2) with a Multi Electrode Array (MEA) system. Duration and amplitude of the recorded electrical signals increased compared to the control. In order to identify the nature of the signal traveling from HR1 to HR2, H+ and K+ fluxes were measured in HR2 by using ion-selective microelectrodes (vibrating probe technique). A net potassium influx was observed after the treatment while no change in proton flux was detected. By corona sodium green and confocal microscopy we confirmed that changes in electrical signals and fluxes are not simply associated with apoplastic diffusion of sodium from HR1 to HR2. The activation of salt stress marker genes was also examined: the modulation of CsGOL-like expression seems to have systemic response, while ERF109 and aspartate oxidase showed localized responses.

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