Scott Uknes

Requirement of salicylic acid for the induction of systemic acquired resistance

It has been proposed that salicylic acid acts as an endogenous signal responsible for inducing systemic acquired resistance in plants. The contribution of salicylic acid to systemic acquired resistance was investigated in transgenic tobacco plants harboring a bacterial gene encoding salicylate hydroxylase, which converts salicylic acid to catechol. Transgenic plants that express salicylate hydroxylase accumulated little or no salicylic acid and were defective in their ability to induce acquired resistance against tobacco mosaic virus. Thus, salicylic acid is essential for the development of systemic acquired resistance in tobacco.

A central role of salicylic Acid in plant disease resistance.

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.

Acquired resistance in Arabidopsis.

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.

Arabidopsis mutants simulating disease resistance response

We describe six Arabidopsis mutants, defining at least four loci, that spontaneously form necrotic lesions on leaves. Lesions resemble those resulting from disease, but occur in the absence of pathogen. In five mutants, lesion formation correlates with expression of histochemical and molecular markers of plant disease resistance responses and with expression of genes activated during development of broad disease resistance in plants (systemic acquired resistance [SAR]). We designate this novel mutant class Isd (for lesions simulating disease resistance response). Strikingly, four Isd mutants express substantial resistance to virulent fungal pathogen isolates. Isd mutants vary in cell type preferences for lesion onset and spread. Lesion formation can be conditional and can be induced specifically by biotic and chemical activators of SAR in Isd1 mutants.

Salicylic Acid Is Not the Translocated Signal Responsible for Inducing Systemic Acquired Resistance but Is Required in Signal Transduction.

Infection of plants by necrotizing pathogens can induce broad-spectrum resistance to subsequent pathogen infection. This systemic acquired resistance (SAR) is thought to be triggered by a vascular-mobile signal that moves throughout the plant from the infected leaves. A considerable amount of evidence suggests that salicylic acid (SA) is involved in the induction of SAR. Because SA is found in phloem exudate of infected cucumber and tobacco plants, it has been proposed as a candidate for the translocated signal. To determine if SA is the mobile signal, grafting experiments were performed using transgenic plants that express a bacterial SA-degrading enzyme. We show that transgenic tobacco root-stocks, although unable to accumulate SA, were fully capable of delivering a signal that renders nontransgenic scions resistant to further pathogen infection. This result indicated that the translocating, SAR-inducing signal is not SA. Reciprocal grafts demonstrated that the signal requires the presence of SA in tissues distant from the infection site to induce systemic resistance.

Isolation and Expression of Three Gibberellin 20-Oxidase cDNA Clones from Arabidopsis

Using degenerate oligonucleotide primers based on a pumpkin (Cucurbita maxima) gibberellin (GA) 20-oxidase sequence, six different fragments of dioxygenase genes were amplified by polymerase chain reaction from Arabidopsis thaliana genomic DNA. One of these was used to isolate two different full-length cDNA clones, At2301 and At2353, from shoots of the GA-deficient Arabidopsis mutant ga1–2. A third, related clone, YAP169, was identified in the Database of Expressed Sequence Tags. The cDNA clones were expressed in Escherichia coli as fusion proteins, each of which oxidized GA12 at C-20 to GA15, GA24, and the C19 compound GA9, a precursor of bioactive GAs; the C20 tricarboxylic acid compound GA25 was formed as a minor product. The expression products also oxidized the 13-hydroxylated substrate GA53, but less effectively than GA12. The three cDNAs hybridized to mRNA species with tissue-specific patterns of accumulation, with At2301 being expressed in stems and inflorescences, At2353 in inflorescences and developing siliques, and YAP169 in siliques only. In the floral shoots of the ga1–2 mutant, transcript levels corresponding to each cDNA decreased dramatically after GA3 application, suggesting that GA biosynthesis may be controlled, at least in part, through down-regulation of the expression of the 20-oxidase genes.

Systemic Acquired Resistance

In the spectrum of plant-microbe interactions disease is a rare outcome. In many interactions complex, integrated defense mechanisms prevent infection and disease. These defensive systems include preformed physical and chemical barriers as well as inducible defenses such as the strengthening of cell walls or synthesis of antimicrobial compounds (i.e., phytoalexins) and proteins.1,2 In certain cases plants react to pathogen attack by developing long-lasting, broad-spectrum systemic resistance to later attacks by pathogens. This phenomenon, termed systemic acquired resistance (SAR), has been observed in many species and may be ubiquitous among higher plants. In the last five years progress has been made toward understanding the molecular basis of SAR. In this review we first provide a brief history of SAR research, then describe our present knowledge of the manifestation and induction of SAR. We discuss recent findings that indicate a central role for the SAR pathway in plant health and finally present our current working model of SAR induction.

Systemic acquired resistance in Arabidopsis requires salicylic acid but not ethylene.

Systemic acquired resistance (SAR) is an inducible plant response to infection by a necrotizing pathogen. In the induced plant, SAR provides broad-spectrum protection against not only the inducing pathogen, but also against other, unrelated pathogens. Both salicylic acid (SA) and SAR-gene expression have been implicated as playing important roles in the initiation and maintenance of SAR. Here, we describe the characterization of transgenic Arabidopsis plants that express the bacterial nahG gene encoding salicylate hydroxylase, an enzyme that can metabolize SA. Strong, constitutive expression of this gene prevents pathogen-induced accumulation of SA and the activation of SAR by exogenous SA. We show that SAR in Arabidopsis can be induced by inoculation with Pseudomonas syringe pv. tomato against infection by a challenge inoculation with Peronospora parasitica. This response is abolished in transgenic, nahG-expressing Arabidopsis, but not in ethylene-insensitive mutants. These experiments support the critical role of SA in SAR and show that ethylene sensitivity is not required for SAR induction. The NahG Arabidopsis plants will be important for future studies aimed at understanding the role of SA in plant disease resistance mechanisms.

Acquired Resistance Signal Transduction in Arabidopsis Is Ethylene Independent.

To clarify the role of ethylene in systemic acquired resistance (SAR), we conducted experiments using Arabidopsis ethylene response mutants. Plants that are nonresponsive to ethylene (i.e., [theta]tr1 and [theta]in2) showed normal sensitivity to the SAR-inducing chemicals salicylic acid (SA) and 2,6-dichloroisonicotinic acid with respect to SAR gene induction and pathogen resistance. This indicated that chemically induced SAR is not an ethylene-dependent process in Arabidopsis. Ethephon, an ethylene-releasing chemical, induced SAR gene expression in both the wild type and ethylene mutants, whereas ethylene alone did not, suggesting that induction of these genes by ethephon is not due to the action of ethylene. Furthermore, transgenic plants expressing salicylate hydroxylase, a bacterial enzyme that degrades SA to catechol, did not accumulate SAR mRNAs in response to ethephon. Thus, SAR gene induction by ethephon appears to be mediated through SA. Other experiments suggested that ethylene may play a role in SAR by enhancing tissue sensitivity to the action of SA.

Induced Resistance Responses in Maize

Systemic acquired resistance (SAR) is a widely distributed plant defense system that confers broad-spectrum disease resistance and is accompanied by coordinate expression of the so-called SAR genes. This type of resistance and SAR gene expression can be mimicked with chemical inducers of resistance. Here, we report that chemical inducers of resistance are active in maize. Chemical induction increases resistance to downy mildew and activates expression of the maize PR-1 and PR-5 genes. These genes are also coordinately activated by pathogen infection and function as indicators of the defense reaction. Specifically, after pathogen infection, the PR-1 and PR-5 genes are induced more rapidly and more strongly in an incompatible than in a compatible interaction. In addition, we show that monocot lesion mimic plants also express these defense-related genes and that they have increased levels of salicylic acid after lesions develop, similar to pathogeninfected maize plants. The existence of chemically inducible disease resistance and PR-1 and PR-5 gene expression in maize indicates that maize is similar to dicots in many aspects of induced resistance. This reinforces the notion of an ancient plant-inducible defense pathway against pathogen attack that is shared between monocots and dicots.