The NlM7 (for noninducible hmunity) gene product is involved in the signal transduction cascade leading to both systemic acquired resistance (SAR) and gene-for-gene disease resistance in Arabidopsis. We have isolated and characterized five new alleles of nim7 that show a range of phenotypes from weakly impaired in chemically induced pathogenesis-related protein-1 gene expression and funga1 resistance to very strongly blocked. We have isolated the NlM7 gene by using a map-based cloning procedure. Interestingly, the NlMl protein shows sequence homology to the mammalian signal transduction factor IKB subclass (Y. NF-KB/IKB signaling pathways are implicated in disease resistance responses in a range of organisms from Drosophila to mammals, suggesting that the SAR signaling pathway in plants is representative of an ancient and ubiquitous defense mechanism in higher organisms.

The NIM1 (for noninducible immunity, also known as NPR1) gene is required for the biological and chemical activation of systemic acquired resistance (SAR) in Arabidopsis. Overexpression of NIM1 in wild-type plants (hereafter referred to as NIM1 plants or lines) results in varying degrees of resistance to different pathogens. Experiments were performed to address the basis of the enhanced disease resistance responses seen in the NIM1 plants. The increased resistance observed in the NIM1 lines correlated with increased NIM1 protein levels and rapid induction of PR1 gene expression, a marker for SAR induction in Arabidopsis, following pathogen inoculation. Levels of salicylic acid (SA), an endogenous signaling molecule required for SAR induction, were not significantly increased compared with wild-type plants. SA was required for the enhanced resistance in NIM1 plants, however, suggesting that the effect of NIM1 overexpression is that plants are more responsive to SA or a SA-dependent signal. This hypothesis is supported by the heightened responsiveness that NIM1 lines exhibited to the SAR-inducing compound benzo(1,2,3)-thiadiazole-7-car-bothioic acid S-methyl ester. Furthermore, the increased efficacy of three fungicides was observed in the NIM1 plants, suggesting that a combination of transgenic and chemical approaches may lead to effective and durable disease-control strategies.

Recent work has demonstrated that plants have endogenous defence mechanisms that can be induced as a response to attack by insects and pathogens. There are two well-studied examples of these induced defence responses. Systemic acquired resistance (SAR) results in increased resistance to a broad spectrum of pathogens throughout a plant in response to localized necrosis caused by pathogen infection. The second example is the systemic induction of proteinase inhibitors to deter feeding by herbivores following an initial event of feeding. In addition, there is now preliminary evidence for other induced defence response pathways. By understanding the breadth of induced defence responses and the mechanisms used to control these pathways, novel plant protection strategies may be developed for use in agronomic settings. Rather than reducing crop losses caused by pests or pathogens by using chemicals that are designed to kill the offending organism, the plant's own defence mechanisms can be used to limit damage due to pests. Novel crop protection strategies based on genetic or chemical regulation of these induced responses show great potential. The first example of a crop protection product that acts by inducing an endogenous defence response pathway is now on the market. Bion reduces the level of pathogen infection in plants by activating SAR.

The NIM1 (for noninducible immunity) gene product is involved in the signal transduction cascade leading to both systemic acquired resistance (SAR) and gene-for-gene disease resistance in Arabidopsis. We have isolated and characterized five new alleles of nim1 that show a range of phenotypes from weakly impaired in chemically induced pathogenesis-related protein-1 gene expression and fungal resistance to very strongly blocked. We have isolated the NIM1 gene by using a map-based cloning procedure. Interestingly, the NIM1 protein shows sequence homology to the mammalian signal transduction factor I kappa B subclass alpha. NF-kappa B/I kappa B signaling pathways are implicated in disease resistance responses in a range of organisms from Drosophila to mammals, suggesting that the SAR signaling pathway in plants is representative of an ancient and ubiquitous defense mechanism in higher organisms.

La presente invention concerne un procede de protection des plantes contre les attaques pathogenes par le developpement de la resistance synergique a la maladie obtenue par l'application d'un microbicide aux plantes immunomodulees. Ces dernieres sont des plantes dans lesquelles la resistance acquise systemique est activee, et sont par consequent dites plantes a activation de resistance acquise systemique. Les plantes immunomodulees peuvent etre obtenues de trois manieres differentes: en appliquant aux plantes un inducteur chimique de resistance synergique a la maladie comme le BTH, l'INA, ou le SA; par un programme de culture selectif fonde sur l'expression constitutive de genes de resistance acquise systemique et/ou un phenotype resistant a la maladie ou en transformant des plantes avec un ou plusieurs genes de resistance acquise systemique telle qu'une forme fonctionnelle du gene NIM1. En appliquant simultanement un microbicide a une plante immunomodulee, la resistance a la maladie est fortement augmentee de maniere synergique; c'est-a-dire, le niveau de resistance a la maladie est superieur aux niveaux additifs de resistance a la maladie.

Benzothiadiazole (BTH) is a novel chemical activator of disease resistance in tobacco, wheat and other important agricultural plants. In this report, it is shown that BTH works by activating SAR in Arabidopsis thaliana. BTH-treated plants were resistant to infection by turnip crinkle virus, Pseudomonas syringae pv 'tomato' DC3000 and Peronospora parasitica. Chemical treatment induced accumulation of mRNAs from the SAR-associated genes, PR-1, PR-2 and PR-5. BTH treatment induced both PR-1 mRNA accumulation and resistance against P. parasitica in the ethylene response mutants, etr1 and ein2, and in the methyl jasmonate-insensitive mutant, jar1, suggesting that BTH action is independent of these plant hormones. BTH treatment also induced both PR-1 mRNA accumulation and P. parasitica resistance in transgenic Arabidopsis plants expressing the nahG gene, suggesting that BTH action does not require salicylic acid accumulation. However, because BTH-treatment failed to induce either PR-1 mRNA accumulation or P. parasitica resistance in the non-inducible immunity mutant, nim1, it appears that BTH activates the SAR signal transduction pathway.

Little is known about the signal transduction events that lead to the establishment of the broad-spectrum, inducible plant immunity called systemic acquired resistance (SAR). Salicylic acid (SA) accumulation has been shown to be essential for the expression of SAR and plays a key role in SAR signaling. Hydrogen peroxide has been proposed to serve as a second messenger of SA. However, our results do not support such a role in the establishment of SAR. Further elucidation of SAR signal transduction has been facilitated by the identification and characterization of mutants. The lesions simulating disease (lsd). resistance response mutant class exhibits spontaneous lesions similar to those that occur during the hypersensitive response. Interestingly, some lsd mutants lose their lesioned phenotype when SA accumulation is prevented by expression of the nahG gene (encoding salicylate hydroxylase), thereby providing evidence for a feedback loop in SAR signal transduction. Characterization of a mutant non-responsive to SAR activator treatments has provided additional evidence for common signaling components between SAR and gene-for-gene resistance.

SummarySystemic acquired resistance (SAR) appears to be a ubiquitous higher plant defence response. Resistance is dependent on the high‐level expression of SAR genes. Mutations, transgenes or chemicals that cause high‐level SAR gene expression also cause resistance to pathogens. Furthermore, when SAR is eliminated, through the reduction of salicylic acid accumulation or activity, generalized severe disease‐susceptibility occurs. Therefore, SAR appears to be an essential component of the defensive repertoire that ensures plant health in nature. Chemicals that activate SAR in the field, as well as crop varieties with constitutive SAR gene expression, will provide new solutions to disease problems for growers in the near future.

SummarySystemic acquired resistance (SAR) is a pathogen‐induced disease resistance response in plants that is characterized by broad spectrum disease control and an associated coordinate expression of a set of SAR genes. Benzo(1,2,3)‐thiadiazole‐7‐carbothioic acid S‐methyl ester (BTH) is a novel synthetic chemical capable of inducing disease resistance in a number of dicotyledenous and monocotyledenous plant species. In this report, the response of tobacco plants to BTH treatment is characterized and the fact that it controls disease by activating SAR is demonstrated. BTH does not cause an accumulation of salicylic acid (SA), an intermediate in the SAR signal transduction pathway. As BTH also induces disease resistance and gene expression in transgenic plants expressing the nahG gene, it appears to activate the SAR signal transduction pathway at the site of or downstream of SA accumulation. BTH, SA and TMV induce the PR‐1a promoter using similar cis‐acting elements and gene expression is blocked by cycloheximide treatment. Thus, BTH induces SAR based on all of the physiological and biochemical criteria that define SAR in tobacco.

While the systemic acquired resistance (SAR) response has been well characterized in tobacco and cucumber, the signal transduction pathway leading to the resistant state is not understood. The small crucifer, Arabidopsis thaliana (Arabidopsis) is a useful model system for molecular genetic experiments aimed at understanding signal transduction pathways. Therefore, we have developed Arabidopsis as a model for SAR.The SAR hallmarks of broad-spectrum disease resistance, coordinate expression of pathogenesis-related (PR) genes and accumulation of salicylic acid (SA) are induced in Arabidopsis following inoculation with a necrogenic pathogen. Treatment with SA or the chemical activators INA and BTH also induces PR-gene expression and disease resistance. Transgenic Arabidopsis that are unable to accumulate Sq do not establish an SAR response following inoculation with a biological inducer and exhibit enhanced susceptibility to pathogens. Thus, SA is critical in SAR signal transduction and may play a role in restriction of disease symptom development.Mutants have been isolated that constitutively express PR-1 mRNA. These plants are resistant to pathogens and are called cim for constitutive Immunity. Some of these constitutive immune plants also show lesions simulating disease (lsd). Another class of mutants exhibit the noninducible immunity (nim) phenotype. SA, INA or BTH treatment does not induce SAR in these mutants, indicating that these molecules act upon the same signal transduction pathway.

Elicitins are proteinaceous elicitors which are secreted by all Phytophthora species analyzed so far, When applied on tobacco they induce necrosis and systemic acquired resistance (SAR). Elicitin-treated tobacco plants that express the bacterial nahG gene coding for salicylate hydroxylase did not show SAR against infection with the black shank fungus, Phytophthora parasitica var. nicotianae. However, nahG-expressing plants still responded to a treatment with the basic elicitin cryptogein by formation of necrosis, Elicitin-induced expression of genes encoding PR-la, PR-2 PR-5 and basic class III chitinase was suppressed in NahG tobacco, while the induced mRNA accumulation of basic PR-1, PR-3, str-246 and str-319 appeared to be unchanged. NahG plants showed enhanced susceptibility to several Phytophthora species and isolates that produce elicitins, but were not affected in the necrosis response during interaction. While SA is involved in SAR and disease resistance to Phytophthora, our results suggest that it does not mediate the hypersensitive-like necrosis response.

Plants, when locally infected with a necrotizing pathogen or a non-pathogen, often develop a long lasting, broad-spectrum ''immunity'' against subsequent infection. This natural phenomenon - called ''systemic activated (or acquired) resistance'' (SAR) - has been known for almost a century. However, naturally induced SAR is not predictable in timing and level of expression and therefore it could not be used for agricultural practice. Using special screening procedures, we were able to discover small molecules which activate the SAR response (''plant activators''). CGA 245704 (benzo[1,2,3]thiadiazole-7-carbothioic acid S-methyl ester) was developed for commercial use in a wide range of crops.Plant activators protect the plant against the same spectrum of diseases as the natural response after localized infections. They do not exhibit direct activity against pathogens but instead cause the same biochemical changes in the plant as observed after biological activation.Salicylic acid plays a central role in the SAR signalling pathway upon biological activation. Plant activators like CGA 245704 activate the SAR response by acting as functional analogues of salicylic acid in the pathway leading to SAR.

Systemic acquired resistance (SAR) is a pathogen-inducible defense mechanism effective against a wide range of pathogens. Salicylic acid (SA) is an essential component of this pathway, as transgenic plants unable to accumulate salicylic acid are incapable of developing SAR. Here, we show that the synthetic chemical 2,6-dichloroisonicotinic acid (INA) acts via the SAR signal transduction pathway. However, it does not induce SA accumulation during the time required for the induction of SAR gene expression or resistance to TMV. Furthermore, INA can induce both resistance and SAR gene expression in transgenic tobacco and Arabidopsis plants that cannot accumulate SA. Thus, INA apparently activates a component of the SAR signaling pathway downstream of SA accumulation.