Bernard Vernooij

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.

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.

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.

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.

Characterization of tobacco plants expressing a bacterial salicylate hydroxylase gene

Transgenic tobacco plants that express the bacterial nahG gene encoding salicylate hydroxylase have been shown to accumulate very little salicylic acid and to be defective in their ability to induce systemic acquired resistance (SAR). In recent experiments using transgenic NahG tobacco and Arabidopsis plants, we have also demonstrated that salicylic acid plays a central role in both disease susceptibility and genetic resistance. In this paper, we further characterize tobacco plants that express the salicylate hydroxylase enzyme. We show that tobacco mosaic virus (TMV) inoculation of NahG tobacco leaves induces the accumulation of the nahG mRNA in the pathogen infected leaves, presumably due to enhanced stabilization of the bacterial mRNA. SAR-associated genes are expressed in the TMV-infected leaves, but this is localized to the area surrounding necrotic lesions. Localized acquired resistance (LAR) is not induced in the TMV-inoculated NahG plants suggesting that LAR, like SAR, is dependent on SA accumulation. When SA is applied to nahG-expressing leaves SAR gene expression does not result. We have confirmed earlier reports that the salicylate hydroxylase enzyme has a narrow substrate specificity and we find that catechol, the breakdown product of salicylic acid, neither induces acquired resistance nor prevents the SA-dependent induction of the SAR genes.

Salicylic acid as a signal molecule in plant-pathogen interactions

Significant insight has been gained in the past year into the roles of salicylic acid (SA) in plant-pathogen interactions. The ability to accumulate SA has been shown to be essential for systemic acquired resistance in tobacco plants. Further experiments have shown that SA is apparently not a systemic, vascular-mobile signal, but rather is required for signal transduction at the local level. Its mode of action may include inhibition of catalase activity, leading to increased levels of hydrogen peroxide.

Transgenic approaches to microbial disease resistance in crop plants.

Recent progress in the genetic dissection of plant disease resistance signaling pathways has opened a number of new avenues towards engineering pathogen resistance in crops. Genes controlling race-specific and broad-spectrum resistance responses have been cloned, and novel induced resistance pathways have been identified in model and crop systems. Advances continue to be made in identification of antifungal proteins with effects inhibitory to either pathogen development or accumulation of associated mycotoxins.