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

Abstract

The identity of the cell files necessary for the leaf-to-leaf transmission of wound signals plants has been debated for decades. In <i>Arabidopsis</i>, 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. <i>glr3.3</i> mutants were also compromised in their defense against herbivores. Since wounding is known to cause increases in cytosolic calcium, we monitored electrical signals and Ca²⁺ transients simultaneously. This revealed that wound-induced membrane depolarizations in the wild-type preceded cytosolic Ca²⁺ maxima. The axial and radial distributions of calcium fluxes were differentially affected in each <i>glr</i> 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.