Salicylic Acid Is a Reducing Substrate and Not an Effective Inhibitor of Ascorbate Peroxidase

Abstract

This communication describes the interactions of salicylic acid (SA) with plant ascorbate peroxidase (APX).Contrary to a recent report (Durner, J., and Klessig, D. F. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 11312-11316) we show conclusively that ascorbate oxidation by APX is not inhibited by SA (10 mM), but that SA is a slow reducing substrate of this enzyme. The suggestion that SA-dependent inhibition of APX in planta may result in the elevation of H 2 O 2 levels, which in turn acts as a second messenger in systemic acquired resistance signaling, is therefore not tenable. We conclude that APX remains a key antioxidant during systemic acquired resistance following pathogenic infection of plants. The transient products of SA oxidation by APX appear to be SA free radicals that undergo subsequent chemistry. APX-dependent oxidation of SA could be essential for diminishing the detrimental effects of this phenolic acid on plant cells.Salicylic acid (2-hydroxybenzoic acid) is believed to play an important role in plant defense responses against pathogen attack (see Refs. 1 and 2 and references therein). Many studies implicate SA 1 as an essential signal in the development of the systemic acquired resistance (SAR) in several plant species. SAR is an inducible, long lasting, broad-based pathogen defense mechanism following infection of plants, particularly by a necrotizing pathogen. An understanding of the molecular basis of SAR may well lead to the development of both genetically engineered crops with enhanced disease resistance and novel chemicals that stimulate the plants' resistance to pathogens.Although induction of SAR is associated with a significant increase in endogenous SA levels in plants (3, 4), the biochemical basis of this phenomenon is obscure. SA accumulation is essential for pathogenesis-related (PR) gene expression (5, 7). Exogenous application of SA to Arabidopsis or tobacco switches on the same set of SAR genes as pathogen infection and induces protection against the same spectrum of pathogens (5,8). Experiments with transgenic plants that express a bacterial salicylate hydroxylase gene have shown that SA could be a transmissible signal (9). The constitutive expression of salicylate hydroxylase, encoded by the nahG gene from Pseudomonas putida, prevented the accumulation of SA in transgenic tobacco by converting it to catechol, which is inactive in SAR. Treatment of nahG plants with tobacco mosaic virus did not induce SAR (9) or SAR gene expression in systemic leaves (10). Grafting experiments involving nahG and wild-type tobacco have suggested that although SA is probably not a long distance signal, its presence in systemic tissue is absolutely required for PR protein expression and establishment of SAR (10). PR-1a gene expression in infected tissues is dependent on SA accumulation and supports the suggestion that at least local induction of this PR protein operates via salicylate signaling (11).Klessig and co-workers (12-14) have suggested that one mechanism of SA action is to inhibit catalase an...

Post-Transcriptional Suppression of Cytosolic Ascorbate Peroxidase Expression during Pathogen-Induced Programmed Cell Death in Tobacco

“…However, recent reports have indicated that SA may not inhibit these activities during the HR (Kvaratskhelia et al, 1997;Tenhaken and Rubel, 1997). Our findings suggest that in addition to the possible inhibition of cAPX activity by SA, the expression of the cAPX protein is suppressed.…”

“…Thus, no newly synthesized cAPX protein may replace the SA-inactivated cAPX molecules during pathogen-induced PCD. On the other hand, because SA may not inhibit the activity of APX (Kvaratskhelia et al, 1997), because it was found that H 2 O 2 application stimulated the production of SA (Leon et al, 1995), and because SA application in the absence of pathogen infection was found to act as a rather weak signal for the induction of defense mechanisms (Shirasu et al, 1997), it is possible that the suppression of cAPX expression acts before or independent of the accumulation of SA. It is also possible that the reduced capability of cells to remove H 2 O 2 together with the extracellular production of and H 2 O 2 induce the synthesis of SA and enable the rapid and efficient activation of defense mechanisms and PCD.…”

“…As indicated above, it has been suggested that SA, which accumulates during the HR, inhibits the activity of cAPX. However, it was shown recently that SA is not an effective inhibitor of cAPX (Kvaratskhelia et al, 1997;Tenhaken and Rubel, 1997 tivity should be suppressed via a different mechanism that is not dependent on SA. Here, we report that although the level of cAPX mRNA is induced during PCD, the translation of the mRNA encoding cAPX is inhibited.…”

Salicylic Acid Mediated by the Oxidative Burst Is a Key Molecule in Local and Systemic Responses of Cotton Challenged by an Avirulent Race of Xanthomonas campestris pvmalvacearum

“…Previous studies have demonstrated that SA inhibits ascorbate peroxidase and catalase, two key enzymes for scavenging H 2 O 2 , by serving as an electron-donating substrate (Durner and Klessig, 1995), although SA was shown to be a better reducing substrate than an effective inhibitor (Kvaratskhelia et al, 1997). This role is supported by the redox deactivation mechanism by SA of the iron Fenton reaction centers (Cheng et al, 1996).…”

The Effects of Salicylic Acid on Helianthus annuus L. Exposed to Quizalofop-P-Ethyl

“…APX is also an important defence enzyme, which protects plant cells from disease agents via systemic D. Bayram et al 2421 acquired resistance [61]. In this study, as the number of days increased, a marked increase was observed in the APX activity in the groups treated with quizalofop-p-ethyl, whereas a decrease was observed on the 10th and 15th days (Figure 3).…”