Identification of cell populations necessary for leaf-to-leaf electrical signaling in a wounded plant.

The identity of the cell files necessary for the leaf-to-leaf transmission of wound signals plants has been debated for decades. In , wounding initiates the glutamate receptor-like (GLR)-dependent propagation of membrane depolarizations that lead to defense gene activation. Using a vein extraction procedure we found pools of GLR-fusion proteins in endomembranes in phloem sieve elements and/or in xylem contact cells. Strikingly, only double mutants that eliminated GLRs from both of these spatially separated cell types strongly attenuated leaf-to-leaf electrical signaling. mutants were also compromised in their defense against herbivores. Since wounding is known to cause increases in cytosolic calcium, we monitored electrical signals and Ca transients simultaneously. This revealed that wound-induced membrane depolarizations in the wild-type preceded cytosolic Ca maxima. The axial and radial distributions of calcium fluxes were differentially affected in each mutant. Resolving a debate over which cell types are necessary for electrical signaling between leaves, we show that phloem sieve elements and xylem contact cells function together in this process.