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Review
. 2019 Apr 16;10:423.
doi: 10.3389/fpls.2019.00423. eCollection 2019.

Intra and Extracellular Journey of the Phytohormone Salicylic Acid

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Free PMC article
Review

Intra and Extracellular Journey of the Phytohormone Salicylic Acid

Israel Maruri-López et al. Front Plant Sci. .
Free PMC article

Abstract

Salicylic acid (SA) is a plant hormone that has been described to play an essential role in the activation and regulation of multiple responses to biotic and to abiotic stresses. In particular, during plant-microbe interactions, as part of the defense mechanisms, SA is initially accumulated at the local infected tissue and then spread all over the plant to induce systemic acquired resistance at non-infected distal parts of the plant. SA can be produced by either the phenylalanine or isochorismate biosynthetic pathways. The first, takes place in the cytosol, while the second occurs in the chloroplasts. Once synthesized, free SA levels are regulated by a number of chemical modifications that produce inactive forms, including glycosylation, methylation and hydroxylation to dihydroxybenzoic acids. Glycosylated SA is stored in the vacuole, until required to activate SA-triggered responses. All this information suggests that SA levels are under a strict control, including its intra and extracellular movement that should be coordinated by the action of transporters. However, our knowledge on this matter is still very limited. In this review, we describe the most significant efforts made to date to identify the molecular mechanisms involved in SA transport throughout the plant. Additionally, we propose new alternatives that might help to understand the journey of this important phytohormone in the future.

Keywords: defense response; phytohormone; plant-microbe interactions; salicylic acid; systemic acquired resistance; transport.

Figures

FIGURE 1
FIGURE 1
SA-mediated gene expression regulation for plant defense. (A) SA (silver spheres) is synthesized by either the phenylalanine ammonia-lyase (PAL) or the isochorismate (IC) pathways. At low SA levels, NPR1 occurs in the cytosol as an oligomer bound by disulfide bridges. (B) At high SA intracellular concentration, the increase in SA modifies the cellular reduction potential, which leads to NPR1 structure changes to monomers through the reduction of the intermolecular bridges by a change in redox. This allows NPR1 to enter the nucleus, where it binds to specific TGA transcriptions factors inducing the expression of SA-induced defensive response genes.
FIGURE 2
FIGURE 2
Metabolism of SA. Plants utilize two pathways to produce SA, the phenylalanine ammonia-lyase (PAL) and the isochorismate (IC). Shikimic acid serves as precursor in both routes. The PAL route is carried out in the cytosol, where shikimic acid is transformed into chorismic acid and then into phenylalanine by the action of the chorismic mutase enzyme. Later, the PAL enzyme uses phenylalanine to produce trans-cinnamic acid which in turn is converted into both ortho-coumaric acid and benzaldehyde. Subsequently, benzaldehyde is converted to benzoic (BA) acid by the aldehyde oxidase (AAO) enzyme. BA is transformed to SA through a reaction catalyzed by the benzoic acid hydroxylase (BA2H) enzyme. The IC pathway takes place at the chloroplast – SA is synthetized by the isochorismate synthase to generate isochorismate, which is later transformed into SA through the action of the isochorismate pyruvate lyase enzymes. Salicylic acid glucosides (SAG and SGE) are produced by glucosyltransferases (SAGT); while, methylation of SA is performed by the methyltransferases enzyme. The chemical structures and its subcellular localization of SA substrates and derivatives are shown.
FIGURE 3
FIGURE 3
A proposed dynamic model of free and conjugated SA. (Lower leaf) The SA synthesized by ICS and PAL pathways is shown in gray spheres. At the chloroplast, SA is translocated to the cytosol through EDS5 transporters (red carrier). Once extruded, SA is conjugated with glucose to form SAG or SGE (red and yellow spheres, respectively). Later, SAG is transported into the vacuole by ABC transporter/H+ -antiporter systems (yellow carriers). SA may spread out to the apoplast by a carrier-mediated system (green carriers). SA is converted to volatile MeSA (purple spheres) by carboxyl methyltransferase enzyme (SAMT). (Stem) SA phloem transport may be based on a symplastic outer cell transport, phloem apoplast intake through an ion trap mechanism and an apoplast intake mediated by a carrier system. MeSA also can be found in the phloem. (Upper leaf) After a pathogen attack, SA levels rise in the primary infected tissue. SA is converted to MeSA (purple spheres). The accumulating MeSA is translocated to the uninoculated systemic tissue. MeSA is demethylated to form SA and induces de-novo synthesis of SA at the distal tissue.

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