Salicylic acid signal transduction: the initiation of biosynthesis, perception and transcriptional reprogramming

Abstract

The phytohormone salicylic acid (SA) is a small phenolic compound that regulates diverse physiological processes, in particular plant resistance against pathogens. Understanding SA-mediated signaling has been a major focus of plant research. Pathogen-induced SA is mainly synthesized via the isochorismate pathway in chloroplasts, with ICS1 (ISOCHORISMATE SYNTHASE 1) being a critical enzyme. Calcium signaling regulates activities of a subset of transcription factors thereby activating nuclear ICS1 expression. The produced SA triggers extensive transcriptional reprogramming in which NPR1 (NON-EXPRESSOR of PATHOGENESIS-RELATED GENES 1) functions as the central coactivator of TGA transcription factors. Recently, two alternative but not exclusive models for SA perception mechanisms were proposed. The first model is that NPR1 homologs, NPR3 and NPR4, perceive SA thereby regulating NPR1 protein accumulation. The second model describes that NPR1 itself perceives SA, triggering an NPR1 conformational change thereby activating SA-mediated transcription. Besides the direct SA binding, NPR1 is also regulated by SA-mediated redox changes and phosphorylation. Emerging evidence show that pathogen virulence effectors target SA signaling, further strengthening the importance of SA-mediated immunity.

A Role of the FUZZY ONIONS LIKE Gene in Regulating Cell Death and Defense in Arabidopsis

“…Besides its critical role in photosynthesis, the chloroplast has been shown as the primary source of many important defense molecules, such as SA biosynthesis, production of ROS and some secondary compounds4748. Like the mitochondria, the chloroplast is also an important player in PCD49.…”

How does the multifaceted plant hormone salicylic acid combat disease in plants and are similar mechanisms utilized in humans?

“…ETI is activated when plant-encoded resistance (R) proteins, which are generally located within the plant cell, directly or indirectly recognize their cognate pathogen-encoded effectors. Both PTI and ETI are associated with the activation of defenses in the inoculated tissue, including the generation of reactive oxygen species (ROS), increases in intracellular Ca 2+ concentrations, activation of mitogen-activated protein kinases (MAPKs), increased expression of various defense-associated genes, synthesis of antimicrobial compounds and accumulation of SA [26, 27]. Generally, ETI induces these defenses more rapidly and intensely than PTI.…”

Differential Costs of Two Distinct Resistance Mechanisms Induced by Different Herbivore Species in Arabidopsis

“…SA mediates transcriptional reprogramming through the NONEXPRESSOR OF PATHOGENESIS-RELATED GENE1 (NPR1) protein, which is a master regulator that synchronizes the activation of SA-responsive TFs and downstream upregulation of defense-related genes (Després et al, 2003;Vlot et al, 2009;Wu et al, 2012;Seyfferth and Tsuda, 2014). In addition to the widely proposed role as an activator of plant resistance against phloemfeeding herbivores such as aphids, a role of SA as a negative regulator of plant defense against tissuechewing herbivores has also been reported.…”