(Questions)n on phloem biology. 1. Electropotential waves, Ca2+ fluxes and cellular cascades along the propagation pathway

This review explores the relationships between electrical long-distance signalling, Ca2+ influx coincident with propagation of electropotential waves, and cellular responses to Ca2+ influx including the consequences for sieve-tube conductivity and mass flow. Ca2+ influx is inherent to electropotential waves and appears to constitute the key link between rapid physical signals and resultant chemical cascades in sieve tubes and adjacent cells. Members of several channel groups are likely involved the regulation of Ca2+ levels in sieve elements. Among them are hyperpolarization-activated, depolarization-activated, and mechanosensitive Ca2+ channels located in the plasma membrane and Ca2+ dependent Ca2+ channels that reside in ER-membranes of sieve elements. These channels collectively determine intracellular Ca2+ levels in sieve elements and their neighbour cells. The latter cells react to Ca2+ elevation by inducing diverse functional responses dependent on the cell type. If the Ca2+ concentration in sieve elements surpasses a threshold level, dual sieve-plate occlusion by proteins and callose deposition is triggered. Occlusion is reversed when Ca2+ levels subside. Electrical messages may regulate the degree of sieve plate hydraulic conductivity in intact plants by partial sieve-plate occlusion that has a major impact on volume flow through sieve tubes. Furthermore, complete but temporary occlusion of sieve tubes may modify mass flow patterns in intact plants.

► Action potentials and variation potentials have an entirely different origin.
► Electropotentials coincide with release of ions, mainly Ca2+, into distant cells.
► Electropotentials make use of all sorts of Ca2+-permeable ion channels.
► Sieve-plate occlusion is a Ca2+-dependent dual event.
► Partial/temporary full sieve-plate occlusion may affect transport in intact plants.