Systemic Acquired Resistance and Salicylic Acid: Past, Present, and Future

Abstract

This article is part of the Distinguished Review Article Series in Conceptual and Methodological Breakthroughs in Molecular Plant-Microbe Interactions. Salicylic acid (SA) is a critical plant hormone that regulates numerous aspects of plant growth and development as well as the activation of defenses against biotic and abiotic stress. Here, we present a historical overview of the progress that has been made to date in elucidating the role of SA in signaling plant immune responses. The ability of plants to develop acquired immunity after pathogen infection was first proposed in 1933. However, most of our knowledge about plant immune signaling was generated over the last three decades, following the discovery that SA is an endogenous defense signal. During this timeframe, researchers have identified i) two pathways through which SA can be synthesized, ii) numerous proteins that regulate SA synthesis and metabolism, and iii) some of the signaling components that function downstream of SA, including a large number of SA targets or receptors. In addition, it has become increasingly evident that SA does not signal immune responses by itself but, rather, as part of an intricate network that involves many other plant hormones. Future efforts to develop a comprehensive understanding of SA-mediated immune signaling will therefore need to close knowledge gaps that exist within the SA pathway itself as well as clarify how crosstalk among the different hormone signaling pathways leads to an immune response that is both robust and optimized for maximal efficacy, depending on the identity of the attacking pathogen.

Novel crosstalk between ethylene‐ and jasmonic acid‐pathway responses to a piercing–sucking insect in rice

“…Phytohormones, including jasmonic acid (JA), salicylic acid (SA), and ethylene (ET), form an intricate network that underlies many resistance pathways to various biotic and abiotic stresses (Cheng et al, 2013;Berens et al, 2017;Guo et al, 2018;Klessig et al, 2018;Olate et al, 2018). The brown planthopper (BPH; Nilaparvata lugens) is one of the most destructive insect pests of rice (Oryza sativa) throughout Asia.…”

Berberine induces resistance against tobacco mosaic virus in tobacco

“…SA biosynthesis includes the PAL and SCI pathways. In Arabidopsis , SA accumulation was reduced by 90–95% in ICS mutants, but only a 50% decrease in PAL mutants . The ICS approach is considered the major source of SA after fungal and bacterial infection .…”

“…In Arabidopsis, SA accumulation was reduced by 90-95% in ICS mutants, but only a 50% decrease in PAL mutants. 50 The ICS approach is considered the major source of SA after fungal and bacterial infection. 51 However, PAL-suppressed tobacco plants showed decreased SA and increased susceptibility to pathogens, and were unable to induce SAR, indicating that PAL has important roles in pathogen-induced SA biosynthesis.…”

Conservation of N-hydroxy-pipecolic acid-mediated systemic acquired resistance in crop plants

“…Several molecules shown to be involved in long-distance signal transduction for SAR, including SA (Klessig et al, 2018), methyl salicylate (MeSA) (Park et al, 2007), azelaic acid (AzA) (Jung et al, 2009), glycerol-3-phosphate (G3P) (Chanda et al, 2011), dehydroabietinal (DA) (Chaturvedi et al, 2012), and pipecolic acid (Pip) (Navarova et al, 2012). Among these metabolites, N-hydroxy pipecolic acid (NHP) has recently emerged as a key player that is required to initiate SAR (Chen et al, 2018;.…”