Abstract
Salicylic acid (SA) is a critical plant hormone that is involved in many processes, including seed germination, root initiation, stomatal closure, floral induction, thermogenesis, and response to abiotic and biotic stresses. Its central role in plant immunity, although extensively studied, is still only partially understood. Classical biochemical approaches and, more recently, genome-wide high-throughput screens have identified more than two dozen plant SA-binding proteins (SABPs), as well as multiple candidates that have yet to be characterized. Some of these proteins bind SA with high affinity, while the affinity of others exhibit is low. Given that SA levels vary greatly even within a particular plant species depending on subcellular location, tissue type, developmental stage, and with respect to both time and location after an environmental stimulus such as infection, the presence of SABPs exhibiting a wide range of affinities for SA may provide great flexibility and multiple mechanisms through which SA can act. SA and its derivatives, both natural and synthetic, also have multiple targets in animals/humans. Interestingly, many of these proteins, like their plant counterparts, are associated with immunity or disease development. Two recently identified SABPs, high mobility group box protein and glyceraldehyde 3-phosphate dehydrogenase, are critical proteins that not only serve key structural or metabolic functions but also play prominent roles in disease responses in both kingdoms.
Highlights
In plants, salicylic acid (SA) was viewed as a relatively unimportant secondary metabolite until the late twentieth century, when Raskin and coworkers revealed its involvement in signaling thermogenesis [1] and our group [2], together with Métraux and colleagues [3], demonstrated its importance in activating disease resistance
The presence of SA-binding proteins (SABPs) exhibiting a wide range of affinities for SA, combined with the varying SA levels found in specific subcellular compartments, in different tissues, at different developmental stages, or during responses to environmental cues, provides tremendous flexibility and multiple mechanisms through which SA can exert its effects
SA binding to human glyceraldehyde 3-phosphate dehydrogenase (GAPDH) suppresses its ability to bind the poly (U) tract of the 3′ non-coding region of the genome of hepatitis C virus (HCV), which is required for efficient replication and/ or translation (Tian and Klessig, unpublished results)
Summary
Salicylic acid (SA) was viewed as a relatively unimportant secondary metabolite until the late twentieth century, when Raskin and coworkers revealed its involvement in signaling thermogenesis [1] and our group [2], together with Métraux and colleagues [3], demonstrated its importance in activating disease resistance. While the identification of NPR proteins as SA targets is a major step toward elucidating SA’s mechanisms of action in defense against microbial pathogens, the upregulation of some plant immune responses, including expression of a subset of defense-related genes, is mediated via a pathway(s) that is dependent on SA, but independent of NPR1 [25, 26]. Of the various SABPs whose SA-binding affinities have been determined, their Kd values span from 0.046 to 15.5 μM Consistent with this 300-fold range, SA levels in plants can vary dramatically (Table 3). SA levels increase dramatically in the inoculated leaves, where much of it is converted to biologically inactive MeSA by SA/benzoic acid methyl transferase; once the SA concentration becomes sufficiently high, it binds in the active site of SABP2 and inhibits SABP2’s ability to convert MeSA
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