Transgenic tobacco and Arabidopsis thaliana expressing the bacterial enzyme salicylate hydroxylase cannot accumulate salicylic acid (SA). This defect not only makes the plants unable to induce systemic acquired resistance, but also leads to increased susceptibility to viral, fungal, and bacterial pathogens. The enhanced susceptibility extends even to host-pathogen combinations that would normally result in genetic resistance. Therefore, SA accumulation is essential for expression of multiple modes of plant disease resistance.

https://science.sciencemag.org/content/sci/266/5188/1247.full.pdf

A Central Role of Salicylic Acid in Plant Disease Resistance

The endogenous plant hormones salicylic acid (SA) and jasmonic acid (JA), whose levels increase on pathogen infection, activate separate sets of genes encoding antimicrobial proteins in Arabidopsis thaliana. The pathogen-inducible genes PR-1, PR-2, and PR-5 require SA signaling for activation, whereas the plant defensin gene PDF1.2, along with a PR-3 and PR-4 gene, are induced by pathogens via an SA-independent and JA-dependent pathway. An Arabidopsis mutant, coi1, that is affected in the JA-response pathway shows enhanced susceptibility to infection by the fungal pathogens Alternaria brassicicola and Botrytis cinerea but not to Peronospora parasitica, and vice versa for two Arabidopsis genotypes (npr1 and NahG) with a defect in their SA response. Resistance to P. parasitica was boosted by external application of the SA-mimicking compound 2,6-dichloroisonicotinic acid [Delaney, T., et al. (1994) Science 266, 1247–1250] but not by methyl jasmonate (MeJA), whereas treatment with MeJA but not 2,6-dichloroisonicotinic acid elevated resistance to Alternaria brassicicola. The protective effect of MeJA against A. brassicicola was the result of an endogenous defense response activated in planta and not a direct effect of MeJA on the pathogen, as no protection to A. brassicicola was observed in the coi1 mutant treated with MeJA. These data point to the existence of at least two separate hormone-dependent defense pathways in Arabidopsis that contribute to resistance against distinct microbial pathogens.

/pdf/separate-jasmonate-dependent-and-salicylate-dependent-2jey8k0lcr.pdf

Separate jasmonate-dependent and salicylate-dependent defense-response pathways in Arabidopsis are essential for resistance to distinct microbial pathogens

Acquired resistance is an important component of the complex disease resistance mechanism in plants, which can result from either pathogen infection or treatment with synthetic, resistance-inducing compounds In this study, Arabidopsis, a tractable genetic system, is shown to develop resistance to a bacterial and a fungal pathogen following 2,6-dichloroisonicotinic acid (INA) treatment Three proteins that accumulated to high levels in the apoplast in response to INA treatment were purified and characterized Expression of the genes corresponding to these proteins was induced by INA, pathogen infection, and salicylic acid, the latter being a putative endogenous signal for acquired resistance Arabidopsis should serve as a genetic model for studies of this type of immune response in plants

Acquired resistance in Arabidopsis.

The conditional fluorescent (flu) mutant of Arabidopsis accumulates the photosensitizer protochlorophyllide in the dark. After a dark-to-light shift, the generation of singlet oxygen, a nonradical reactive oxygen species, starts within the first minute of illumination and was shown to be confined to plastids. Immediately after the shift, plants stopped growing and developed necrotic lesions. These early stress responses of the flu mutant do not seem to result merely from physicochemical damage. Peroxidation of chloroplast membrane lipids in these plants started rapidly and led to the transient and selective accumulation of a stereospecific and regiospecific isomer of hydroxyoctadecatrieonic acid, free (13S)-HOTE, that could be attributed almost exclusively to the enzymatic oxidation of linolenic acid. Within the first 15 min of reillumination, distinct sets of genes were activated that were different from those induced by superoxide/hydrogen peroxide. Collectively, these results demonstrate that singlet oxygen does not act primarily as a toxin but rather as a signal that activates several stress-response pathways. Its biological activity in Arabidopsis exhibits a high degree of specificity that seems to be derived from the chemical identity of this reactive oxygen species and/or the intracellular location at which it is generated.

/pdf/rapid-induction-of-distinct-stress-responses-after-the-13z4n0s0pb.pdf

Rapid Induction of Distinct Stress Responses after the Release of Singlet Oxygen in Arabidopsis

Infection of a plant by a pathogen induces a variety of defense responses that imply the action of several signaling molecules, including salicylic acid (SA), jasmonic acid (JA) and ethylene (E). Here we describe the role of ETHYLENE-RESPONSE-FACTOR1 (ERF1) as a regulator of ethylene responses after pathogen attack in Arabidopsis. The ERF1 transcript is induced on infection by Botrytis cinerea, and overexpression of ERF1 in Arabidopsis is sufficient to confer resistance to necrotrophic fungi such as B. cinerea and Plectosphaerella cucumerina. A positive co-operation between E and SA pathways was observed in the plant response to P. cucumerina. Infection by Pseudomonas syringae tomato DC3000, however, does not affect ERF1 expression, and activation of ethylene responses by ERF1 overexpression in Arabidopsis plants reduces tolerance against this pathogen, suggesting negative crosstalk between E and SA signaling pathways, and demonstrating that positive and negative interactions between both pathways can be established depending on the type of pathogen.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-313x.2002.01191.x

Constitutive expression of ETHYLENE-RESPONSE-FACTOR1 in Arabidopsis confers resistance to several necrotrophic fungi

Although cuticular and epidermal penetration by the rust and death of the penetrated cell were similar in two cultivars, subsequent hyphal branching and growth was decreased and retention of trypan blue stain was increased in the resistant cultivar. No uredinia formed in the resistant cultivar. Phytoalexin formation began earlier in the resistant cultivar and at a time when many cells had changed in their physiology as revealed by stain retention. A product of germinating urediniospores caused more rapid browning and phytoalexin formation in pods of the resistant cultivar.

Comparison of histological and physiological responses to Phakopsora pachyrhizi in resistant and susceptible soybean

Germ-tubes of Phakopsora pachyrhizi penetrate cuticles of leaves of soybeans and grow through the underlying epidermal cells before giving rise to intercellular hyphae which pass between palisade cells and develop uredinial initials in the spongy mesophyll. The rust causes the early death of the penetrated epidermal cells and alters the ability of palisade cells to retain trypan blue stain. Emerging uredinia are surrounded by areas of dead tissue. Phytoalexins form at an early stage in rust development and accumulate to substantial amounts. The phytoalexins comprise at least two fractions probably containing related pterocarpanoid compounds. Germinating urediniospores release a thermostable toxic substance into water. This substance may be significant in the host-parasite interaction because it causes necrosis in soybean pods.

Responses of soybean to infection by, and to germination fluids from, urediniospores of Phakopsora pachyrhizi

Bioassay methods were developed using radial extension of hyphae from mycelial disks of Gaeumannomyces graminis (Sacc.) Arx & Olivier var. tritici Walker mounted in droplets of dilute nutrient solution on slides or on agar overlays on silica gel thin layer chromatography plates. These methods revealed the sensitivity of hyphae to the phytoalexin phaseollin, to the pre-formed inhibitors which leached from coloured onion scales and to phytoalexins leached from rust-infected soybean leaves. They failed to detect comparable antifungal activity in ethanolic extracts of segments of uninoculated wheat roots and of wheat roots infected with G. graminis var. tritici or G. graminis var. graminis. Low activities were found in roots succumbing to take-all and also in those cross-protected by inclusion of G. graminis var. graminis in the inoculum. Antifungal substances found in leachates of wheat roots in benzene and in macerates in water were considered to be artefacts of extraction procedures.

Stella McLeod

Papers

Comparison of histological and physiological responses to Phakopsora pachyrhizi in resistant and susceptible soybean

Responses of soybean to infection by, and to germination fluids from, urediniospores of Phakopsora pachyrhizi

Failure to implicate antifungal substances in cross-protection of wheat against take-all