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.

Accumulation of isoflavones and pterocarpan phytoalexins in cell suspension cultures of different cultivars of chickpea (Cicer arietinum).

Cell suspension cultures of chickpea (Cicer arietinum L.) were established from cultivars ILC 3279 and ILC 1929, resistant and susceptible towards the chickpea pathogenic fungus Ascochyta rabiei. The two cell culture lines possess identical growth properties and show high accumulation of the isoflavones biochanin A and formononetin together with their glucoside and malonylglucoside conjugates. The cultures of the two cultivars, however, significantly differ in their accumulation of the phytoalexins medicarpin and maackiain essentially as previously demonstrated for the plant genotypes. Phytoalexin formation was elicited by using yeast extract as an inducing agent.

Stress responses in alfalfa (Medicago sativa L.) III. Induction of medicarpin and cytochrome P450 enzyme activities in elicitor-treated cell suspension cultures and protoplasts.

Cell suspension cultures of alfalfa (Medicago sativa L.) accumulated phenolic secondary metabolites in a pattern similar to that seen in alfalfa roots. Upon treatment with a crude elicitor preparation from the bean pathogen Colletotrichum lindemuthianum, the pterocarpan phytoalexin medicarpin accumulated in cells and culture medium. The extractable activities of six enzymes involved in medicarpin biosynthesis (including three cytochrome P450 activities) were induced by treatment with elicitor, and their induction kinetics correlated with the rate of medicarpin accumulation. However, protoplasts prepared from these cultures accumulated neither medicarpin nor other secondary products after treatment with elicitor. The cytochrome P450 activities were induced during the preparation of the protoplasts, but could be further induced by treatment with fungal elicitor. The results are discussed in relation to the use of alfalfa protoplasts as a system for functional analysis of cloned defense genes.

Stress Responses in Alfalfa (Medicago sativa L.) : V. Constitutive and Elicitor-Induced Accumulation of Isoflavonoid Conjugates in Cell Suspension Cultures.

The isoflavonoid conjugates medicarpin-3-O-glucoside-6''-O-malonate (MGM), afrormosin-7-O-glucoside (AG), and afrormosin-7-O-glucoside-6''-O-malonate (AGM) were isolated and characterized from cell suspension cultures of alfalfa (Medicago sativa L.), where they were the major constitutive secondary metabolites. They were also found in alfalfa roots but not in other parts of the plant. The phytoalexin medicarpin accumulated rapidly in suspension cultured cells treated with elicitor from Colletotrichum lindemuthianum, and this was subsequently accompanied by an increase in the levels of MGM. In contrast, net accumulation of afrormosin conjugates was not affected by elicitor treatment. Labeling studies with [(14)C]phenylalanine indicated that afrormosin conjugates were the major de novo synthesized isoflavonoid products in unelicited cells. During elicitation, [(14)C]phenylalanine was incorporated predominantly into medicarpin, although a significant proportion of the newly synthesized medicarpin was also conjugated. Treatment of (14)C-labeled, elicited cells with l-alpha-aminooxy-beta-phenylpropionic acid, a potent inhibitor of PAL activity in vivo, resulted in the initial appearance of labeled medicarpin of very low specific activity, suggesting that the phytoalexin could be released from a preformed conjugate under these conditions. Our data draw attention to the involvement of isoflavone hydroxylases during the constitutive and elicitor-induced accumulation of isoflavonoids and their conjugates in alfalfa cell cultures.

Stress responses in alfalfa (Medicago sativa L.) IV. Expression of defense gene constructs in electroporated suspension cell protoplasts.

We have investigated conditions for the uptake and expression of chimeric genes in protoplasts of alfalfa (Medicago sativa L.). Constructs containing the bacterial reporter gene chloramphenicol acetyltransferase (CAT) under the control of either the cauliflower mosaic virus 35S promoter or a bean chalcone synthase (CHS) promoter were introduced into protoplasts by electroporation in the presence of polyethyleneglycol. The extent of expression in the absence of added inducers depended on the conditions for isolation, electroporation and subsequent culture of the protoplasts. Expression of the CHS promoter construct was increased on exposure of the protoplasts to a fungal elicitor or reduced glutathione. The relative levels of induced expression in relation to either basal expression or the type of elicitor used depended on the age of the suspension cultures from which the protoplasts were isolated. Electroporation of protoplasts with a construct from which bean CHS antisense transcripts were synthesized under the control of the 35S promoter resulted in the inhibition of appearance of elicitor-induced endogenous alfalfa CHS activity. The suitability of the alfalfa protoplast system for analysis and potential identification of defense response genes is discussed.

Benzothiadiazole induces disease resistance in Arabidopsis by activation of the systemic acquired resistance signal transduction pathway.

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.

Signal transduction in systemic acquired resistance.

Systemic acquired resistance (SAR) is an important component of plant defense against pathogen infection. Accumulation of salicylic acid (SA) is required for the induction of SAR. However, SA is apparently not the translocated signal but is involved in transducing the signal in target tissues. Interestingly, SA accumulation is not required for production and release of the systemic signal. In addition to playing a pivotal role in SAR signal transduction, SA is important in modulating plant susceptibility to pathogen infection and genetic resistance to disease. It has been proposed that SA inhibition of catalase results in H2O2 accumulation and that therefore H2O2 serves as a second messenger in SAR signaling. We find no accumulation of H2O2 in tissues expressing SAR; thus the role of H2O2 in SAR signaling is questionable.

Benzothiadiazole, a novel class of inducers of systemic acquired resistance, activates gene expression and disease resistance in wheat.

Systemic acquired resistance is an important component of the disease resistance repertoire of plants. In this study, a novel synthetic chemical, benzo(1,2,3)thiadiazole-7-carbothioic acid S-methyl ester (BTH), was shown to induce acquired resistance in wheat. BTH protected wheat systemically against powdery mildew infection by affecting multiple steps in the life cycle of the pathogen. The onset of resistance was accompanied by the induction of a number of newly described wheat chemically induced (WCI) genes, including genes encoding a lipoxygenase and a sulfur-rich protein. With respect to both timing and effectiveness, a tight correlation existed between the onset of resistance and the induction of the WCI genes. Compared with other plant activators, such as 2,6-dichloroisonicotinic acid and salicylic acid, BTH was the most potent inducer of both resistance and gene induction. BTH is being developed commercially as a novel type of plant protection compound that works by inducing the plant's inherent disease resistance mechanisms.