Acquired resistance in Arabidopsis

Osmotin overexpression in potato delays development of disease symptoms

Transgenic potato and tobacco plants carrying the osmotin gene under the control of the cauliflower mosaic virus 35S promoter constitutively overexpressed osmotin to a level of approximately 2% of total cellular protein. Leaves of transgenic potato plants exhibited delayed development of disease symptoms after inoculation with spore suspensions of Phytophthora infestans, which is the cause of late blight disease of potato. In contrast, transgenic tobacco plants did not display any change in the development of disease symptoms when challenged with either spore suspensions or fungal mycelia of Phytophthora parasitica var. nicotianae. Using in vitro assays, purified osmotin was found to be more effective against P. infestans. Some inhibition of P. parasitica also was observed in vitro even though no in vivo effect could be established.

[beta]-Aminobutyric Acid Induces the Accumulation of Pathogenesis-Related Proteins in Tomato (Lycopersicon esculentum L.) Plants and Resistance to Late Blight Infection Caused by Phytophthora infestans

Tomato (Lycopersicon esculentum L.) plants were sprayed with aqueous solutions of isomers of aminobutyric acid and were either analyzed for the accumulation of pathogenesis-related (PR) proteins or challenged with the late blight fungal agent Phytophthora infestans. The [beta] isomer of aminobutyric acid induced the accumulation of high levels of three proteins: P14a, [beta]-1,3 glucanase, and chitinase. These proteins either did not accumulate or accumulated to a much lower level in [alpha]- or [gamma]-aminobutyric acid-treated plants. Plants pretreated with [alpha]-, [beta]-, and [gamma]-aminobutyric acid were protected up to 11 d to an extent of 35, 92, and 6%, respectively, against a challenge infection with P. infestans. Protection by [beta]-aminobutyric acid was afforded against the blight even when the chemical was applied 1 d postinoculation. Examination of ethylene evolution showed that [alpha]-aminobutyric acid induced the production of 3-fold higher levels of ethylene compared with [beta]-aminobutyric acid, whereas [gamma]-aminobutyric acid induced no ethylene production. In addition, silver thiosulfate, a potent inhibitor of ethylene action, did not abolish the resistance induced by [beta]-aminobutyric acid. The results are consistent with the possibility that [beta]-aminobutyric acid protects tomato foliage against the late blight disease by a mechanism that is not mediated by ethylene and that PR proteins can be involved in induced resistance.

Plant pathology. Resistance crumbles?

A plant disease resistance gene, of the kind involved in "gene-for-gene" interactions with pathogens, has been cloned and found to encode a putative serine/threonine kinase.

Cytochrome and Alternative Pathway Respiration in Tobacco (Effects of Salicylic Acid)

In suspension cultures of NT1 tobacco (Nicotiana tabacum L. cv Bright Yellow) cells the cytochrome pathway capacity increased between d 3 and d 4 following subculturing and reached the highest level observed on d 7. The capacity decreased significantly by d 10 and was at the same level on d 14. Both alternative pathway capacity and the amount of the 35-kD alternative oxidase protein increased significantly between d 5 and d 6, reached the highest point observed on d 7, remained constant until d 10, and decreased by d 14. The highest capacities of the alternative and cytochrome pathways and the highest amount of the 35-kD protein were attained on the day that cell cultures reached a stationary phase of growth. Addition of salicylic acid to cell cultures on d 4 caused a significant increase in alternative pathway capacity and a dramatic accumulation of the 35-kD protein by 12 h. The alternative pathway capacity and the protein level reached the highest level observed by 16 h after salicylic acid addition, and the cytochrome pathway capacity was at about the same level at each time point. The accumulation of the 35-kD alternative oxidase protein was significantly decreased by addition of actinomycin D 1 h before salicylic acid and was blocked by addition of cycloheximide. These results indicate that de novo transcription and translation were necessary for salicylic acid to cause the maximum accumulation of the 35-kD protein.

Arabidopsis as a model host for studying plant-pathogen interactions

Because the molecular biology and genetics of Arabidopsis thaliana are so well defined, it is potentially a superb subject for research on plant-pathogen interactions. Viruses, bacteria and fungi that infect Arabidopsis and are representative pathogens of economically important plants have recently been described. The search now is for a pathogenic fungus with tractable genetics to combine with a direct analysis of plant resistance genes.

A cDNA clone for a pathogenesis-related protein 1 from barley

A barley cDNA clone (PRb-1) corresponding to an mRNA differentially induced in resistant compared to susceptible barley cultivars by powdery mildew infection was isolated and characterised. The deduced amino acid sequence revealed 24 amino acids comprising the signal peptide and 140 amino acids of the mature peptide (15 kDa). This showed close homology to PR-1-like proteins, which have been isolated from maize, tobacco, tomato and Arabidopsis thaliana. Northern blot analysis showed accumulation of the corresponding mRNA 12 h after inoculation of resistant barley cultivars with Erysiphe graminis. Increased expression of the PRb-1 gene was also observed in resistant compared with near-isogenic susceptible barley plants following treatment with ethylene, salicylic acid, methyl jasmonate and 2,6-dichloro-isonicotinic acid.

Identification of methyl jasmonate and salicylic acid response elements from the nopaline synthase (nos) promoter

Transgenic tobacco plants carrying a fusion between the nopaline synthase (nos) promoter and chloramphenicol acetyltransferase (CAT) reporter gene (cat) were studied for their inducibility by salicylic acid (SA) or methyl jasmonate (MJ) treatments. Either chemical significantly increased CAT activity to a level much higher than that achieved by wounding. Northern blot analysis showed a corresponding increase in mRNA levels. After 20 h of induction of flowering plants, the response to MJ treatment was weaker in old leaves compared with young leaves, whereas the SA response was stronger in old leaves. Kinetic experiments showed that the SA response was much faster than the MJ response, suggesting that the induction mechanism of the nos promoter by these chemicals may differ. Deletion analysis showed that both SA and MJ responses require the DNA sequence between -119 and -112 from the transcription initiation site. This region contains the hexamer sequence (TGACGT) that has been found to be an important regulatory element for several promoters. The MJ response was also reduced by deletions of the CAAT box region or the sequence between -112 and -101, whereas the SA response was not significantly affected by these deletions. This suggests that the nos upstream region containing the hexamer motif is essential for the SA or MJ response and that the CAAT box region and the sequence immediately downstream from the hexamer motif are required for maximum induction by MJ.

Salicylic acid: a systemic signal in induced plant disease resistance

Some plants respond to infection by pathogens with both localized and systemic resistance responses. These prevent the spread of the disease-causing organism and reduce the severity of a subsequent infection. Recent evidence suggests that systemic increases in the host's salicylic acid levels act as a signal for the activation of at least some of these induced defenses.

Emerging strategies for enhancing crop resistance to microbial pathogens

There are marked differences in the pattern of host gene expression in incompatible plant:microbial pathogen interactions compared with compatible interactions, associated with the elaboration of inducible defenses. Constitutive expression of genes encoding a chitinase or a ribosome-inactivating protein in transgenic plants confers partial protection against fungal attack, and a large repertoire of such antimicrobial genes has been identified for further manipulation. In addition, strategies are emerging for the manipulation of multigenic defenses such as lignin deposition and synthesis of phytoalexin antibiotics by overexpression of genes encoding rate determining steps, modification of transcription factors or other regulatory genes, and engineering production of novel phytoalexins by interspecies transfer of biosynthetic genes. The imminent cloning of disease resistance genes, further molecular dissection of stress signal perception and transduction mechanisms, and identification of genes that affect symptom development will provide attractive new opportunities for enhancing crop protection. Combinatorial integration of these novel strategies into ongoing breeding programs should make an important contribution to effective, durable field resistance.

Plant 'pathogenesis-related' proteins and their role in defense against pathogens

The hypersensitive reaction to a pathogen is one of the most efficient defense mechanisms in nature and leads to the induction of numerous plant genes encoding defense proteins. These proteins include: 1) structural proteins that are incorporated into the extracellular matrix and participate in the confinement of the pathogen; 2) enzymes of secondary metabolism, for instance those of the biosynthesis of plant antibiotics; 3) pathogenesis-related (PR) proteins which represent major quantitative changes in soluble protein during the defense response. The PRs have typical physicochemical properties that enable them to resist to acidic pH and proteolytic cleavage and thus survive in the harsh environments where they occur: vacuolar compartment or cell wall or intercellular spaces. Since the discovery of the first PRs in tobacco many other similar proteins have been isolated from tobacco but also from other plant species, including dicots and monocots, the widest range being characterized from hypersensitively reacting tobacco. Based first on serological properties and later on sequence data, the tobacco PRs have been classified in five major groups. Group PR-1 contains the first discovered PRs of 15-17 kDa molecular mass, whose biological activity is still unknown, but some members have been shown recently to have antifungal activity. Group PR-2 contains three structurally distinct classes of 1,3-beta-glucanases, with acidic and basic counterparts, with dramatically different specific activity towards linear 1,3-beta-glucans and with different substrate specificity. Group PR-3 consists of various chitinases-lysozymes that belong to three distinct classes, are vacuolar or extracellular, and exhibit differential chitinase and lysozyme activities. Some of them, either alone or in combination with 1,3-beta-glucanases, have been shown to be antifungal in vitro and in vivo (transgenic plants), probably by hydrolysing their substrates as structural components in the fungal cell wall. Group PR-4 is the less studied, and in tobacco contains four members of 13-14.5 kDa of unknown activity and function. Group PR-5 contains acidic-neutral and very basic members with extracellular and vacuolar localization, respectively, and all members show sequence similarity to the sweet-tasting protein thaumatin. Several members of the PR-5 group from tobacco and other plant species were shown to display significant in vitro activity of inhibiting hyphal growth or spore germination of various fungi probably by a membrane permeabilizing mechanism.(ABSTRACT TRUNCATED AT 400 WORDS)

Functional homologs of the Arabidopsis RPM1 disease resistance gene in bean and pea

We showed that a bacterial avirulence (avr) gene function, avrPpiA1, from the pea pathogen Pseudomonas syringae pv pisi, is recognized by some, but not all, genotypes of Arabidopsis. Thus, an avr gene functionally defined on a crop species is also an avr gene on Arabidopsis. The activity of avrPpiA1 on a series of Arabidopsis genotypes is identical to that of the avrRpm1 gene from P.s. pv maculicola previously defined using Arabidopsis. The two avr genes are homologous and encode nearly identical predicted products. Moreover, this conserved avr function is also recognized by some bean and pea cultivars in what has been shown to be a gene-for-gene manner. We further demonstrated that the Arabidopsis disease resistance locus, RPM1, conditioning resistance to avrRpm1, also conditions resistance to bacterial strains carrying avrPpiA1. Therefore, bean, pea, and conceivably other crop species contain functional and potentially molecular homologs of RPM1.