Salicylic Acid-Induced Local and Long-Distance Signaling Models in Plants

Abstract

Salicylic acid (SA) is one of the key hormonal factors determining the fate of plants exposed to stressful conditions, which is naturally found in plants and shown to be involved in the plant defense-related actions against infection by various pathogens. Recently, intracellular SA receptors were finally identified after a long survey of SA-binding proteins. In this chapter, the modes of both the short- and long-distance signaling events involving the actions of SA, a defense-related key signaling molecule, are compared by covering both the biochemical and electrophysiological views. Here, two distinct models for local SA perception and signaling mechanisms involved in the extracellular and intracellular paths (referred to as models i and ii), and the three different models for long-distance signaling mediated by SA are reviewed (referred to as models iii–v). The local SA signaling events can be attributed to (i) the extracellular SA perception model in which reactions between SA and apoplastic proteins result in acute oxidative burst followed by internalization of the derived signals via activation of calcium channels, and/or (ii) intracellular SA perception mechanism by which the action and life cycle of NPR1 protein are determined depending on the concentration of SA in both the infected cells and neighboring cells. On the other hand, the long-distance SA action could be attributed to three different modes, namely, (iii) local increase in SA followed by phloem transport of SA, (iv) systemic propagation of SA-derived mobile signals with both electrical and chemical natures without direct movement of SA, and (v) synergistic propagation of both SA and derived signals through the tissues and phloem. We view here that the long-distance SA signaling events (models iii–v) inevitably involve the mechanisms described in the local signaling models (models i and ii) as the key pieces of the puzzle.