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Sieve Element Ca²⁺ Channels as Relay Stations between Remote Stimuli and Sieve Tube Occlusion in Vicia faba^[W]

^a Plant Cell Biology Research Group, Institute of General Botany, Justus-Liebig-University, D-35390 Giessen, Germany
^b Department of Plant Sciences, University of Oxford, Oxford, OX1 3RB, United Kingdom
^c Institute of General Botany, Justus-Liebig-University, D-35390 Giessen, Germany

¹ Address correspondence to jens.hafke{at}bot1.bio.uni-giessen.de.

Damage induces remote occlusion of sieve tubes in Vicia faba by forisome dispersion, triggered during the passage of an electropotential wave (EPW). This study addresses the role of Ca²⁺ channels and cytosolic Ca²⁺ elevation as a link between EPWs and forisome dispersion. Ca²⁺ channel antagonists affect the initial phase of the EPW as well as the prolonged plateau phase. Resting levels of sieve tube Ca²⁺ of 50 nM were independently estimated using Ca²⁺-selective electrodes and a Ca²⁺-sensitive dye. Transient changes in cytosolic Ca²⁺ were observed in phloem tissue in response to remote stimuli and showed profiles similar to those of EPWs. The measured elevation of Ca²⁺ in sieve tubes was below the threshold necessary for forisome dispersion. Therefore, forisomes need to be associated with Ca²⁺ release sites. We found an association between forisomes and endoplasmic reticulum (ER) at sieve plates and pore-plasmodesma units where high-affinity binding of a fluorescent Ca²⁺ channel blocker mapped an increased density of Ca²⁺ channels. In conclusion, propagation of EPWs in response to remote stimuli is linked to forisome dispersion through transiently high levels of parietal Ca²⁺, release of which depends on both plasma membrane and ER Ca²⁺ channels.