Spread the news: systemic dissemination and local impact of Ca2 signals along the phloem pathway

We explored the idea of whether electropotential waves (EPWs) primarily act as vehicles for systemic spread of Ca-2 signals. EPW-associated Ca-2 influx may trigger generation and amplification of countless long-distance signals along the phloem pathway given the fact that gating of Ca-2-permeable channels is a universal response to biotic and abiotic challenges. Despite fundamental differences, both action and variation potentials are associated with a sudden Ca-2 influx. Both EPWs probably disperse in the lateral direction, which could be of essential functional significance. A vast set of Ca-2-permeable channels, some of which have been localized, is required for Ca-2-modulated events in sieve elements. There, Ca-2-permeable channels are clustered and create so-called Ca-2 hotspots, which play a pivotal role in sieve element occlusion. Occlusion mechanisms play a central part in the interaction between plants and phytopathogens (e.g. aphids or phytoplasmas) and in transient re-organization of the vascular symplasm. It is argued that Ca-2-triggered systemic signalling occurs in partly overlapping waves. The forefront of EPWs may be accompanied by a burst of free Ca-2 ions and Ca-2-binding proteins in the sieve tube sap, with a far-reaching impact on target cells. Lateral dispersion of EPWs may induce diverse Ca-2 influx and handling patterns (Ca-2 signatures) in various cell types lining the sieve tubes. As a result, a variety of cascades may trigger the fabrication of signals such as phytohormones, proteins, or RNA species released into the sap stream after product-related lag times. Moreover, transient reorganization of the vascular symplasm could modify cascades in disjunct vascular cells.