The transcriptome of Arabidopsis thaliana during systemic acquired resistance.

Infected plants undergo transcriptional reprogramming during initiation of both local defence and systemic acquired resistance (SAR). We monitored gene-expression changes in Arabidopsis thaliana under 14 different SAR-inducing or SAR-repressing conditions using a DNA microarray representing approximately 25-30% of all A. thaliana genes. We derived groups of genes with common regulation patterns, or regulons. The regulon containing PR-1, a reliable marker gene for SAR in A. thaliana, contains known PR genes and novel genes likely to function during SAR and disease resistance. We identified a common promoter element in genes of this regulon that binds members of a plant-specific transcription factor family. Our results extend expression profiling to definition of regulatory networks and gene discovery in plants.

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.

Regulation of cucumber class III chitinase gene expression.

The chromosomal region encoding the acidic class III chitinase from cucumber has been isolated and characterized. As a result of an apparent gene triplication, the pathogen-induced gene (CHI2) is flanked by two closely related genes with complete open reading frames (ORF). The high level of conservation within the three ORFs suggests an essential role for each encoded protein in plant growth and development. The developmental and tissue-specific expression of RNA from each gene was analyzed using both gene-specific probes and RNA-PCR. The expression of each gene in response to various inducing treatments was also characterized. Only transcripts corresponding to CHI2 were detected. Chitinase mRNA abundance increased slightly following cycloheximide application; however, its potent induction by salicylic acid was inhibited by cycloheximide treatment.

Recent advances in systemic acquired resistance research--a review.

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.

Monoclonal antibodies identify a strain-specific epitope on the HN glycoprotein of Newcastle disease virus strain Australia-Victoria.

A panel of monoclonal antibodies raised against the hemagglutinin-neuraminidase glycoprotein (HN) of the Australia-Victoria strain of Newcastle disease virus has been used to compare that strain and eight other strains of the virus. The ability of the antibodies to neutralize infectivity, inhibit hemagglutination and neuraminidase, and bind to purified virions in solid-phase radioimmunoassays was determined for each strain. Of the four antigenic sites delineated by these antibodies on the HN of the homologous strain, site 1 (that with the greatest neutralizing susceptibility), is apparently conserved in all the strains tested as revealed by neutralization assays. The least neutralizing site, number 4, is also conserved in most of the strains tested. Site 2, which lies at or near the neuraminidase site, appears to be conserved in the avirulent strains but not in the virulent strains. An antibody to site 3 is unable to bind to a significant extent to any of the heterologous strains tested, and thus recognizes a strain-specific epitope. Inhibition of hemagglutination and neuraminidase by antibodies to each site were also examined and the results suggest that antibodies to sites 1 and 2 may distinguish virulent and avirulent strains at least with respect to these functions.

Ethylene-induced gene expression in carnation petals : relationship to autocatalytic ethylene production and senescence.

Exposure of carnation (Dianthus caryophyllus L.) flowers to ethylene evokes the developmental program of petal senescence. The temporal relationship of several aspects of this developmental program following treatment with ethylene was investigated. Exposure of mature, presenescent flowers to 7.5 microliters per liter ethylene for at least 6 hours induced petal in-rolling and premature senescence. Autocatalytic ethylene production was induced in petals following treatment with ethylene for 12 or more hours. A number of changes in mRNA populations were noted in response to ethylene, as determined by in vitro translation of petal polyadenylated RNA. At least 6 mRNAs accumulated following ethylene exposure. The molecular weights of their in vitro translation products were 81, 58, 42, 38, 35, and 25 kilodaltons. Significant increases in abundance of most mRNAs were observed 3 hours following ethylene exposure. Ethylene exposure resulted in decreased abundance of another group of mRNAs. Treatment of flowers with competitive inhibitors of ethylene action largely prevented the induction of these ethylene responses in petals. An increase in flower age was accompanied by an increase in the capacity for ethylene to induce petal in-rolling, autocatalytic ethylene production, and changes in mRNA populations suggesting that these responses are regulated by both sensitivity to ethylene and ethylene concentration. These results indicate that changes in petal physiology resulting from exposure to ethylene may be the result of rapid changes in gene expression.

Molecular cloning and characterization of senescence-related genes from carnation flower petals.

The senescence of carnation (Dianthus caryophyllus L.) flower petals is associated with increased production of ethylene which plays an important role in regulating this developmental event. Three senescence-related cDNA clones were isolated from a cDNA library prepared from mRNA isolated from senescing petals. These cDNAs are representative of two classes of mRNAs which increase in abundance in senescing petal tissue. The mRNA for one class is present at low levels during the early stages of development and begins to accumulate in mature petals prior to the increase in ethylene production. The accumulation of this mRNA is reduced, but not eliminated, in petals treated with aminooxyacetic acid, an inhibitor of ethylene biosynthesis, or silver thiosulfate, an ethylene action inhibitor. In contrast, expression of the second class of mRNAs appears to be highly regulated by ethylene. These mRNAs are not detectable prior to the rise in ethylene production and increase in abundance in parallel with the ethylene climacteric. Furthermore, expression of these mRNAs is significantly inhibited by both aminooxyacetic acid and silver thiosulfate. Expression of these mRNAs in vegetative and floral organs was limited to floral tissue, and predominantly to senescing petals.

Characterization of an ethylene-regulated flower senescence-related gene from carnation.

The programmed senescence of carnation (Dianthus caryophyllus L.) petals requires active gene expression and is associated with the expression of several senescence-related mRNAs. Expression of the mRNA represented by the cDNA clone pSR12 has previously been shown to be transcriptionally activated by ethylene specifically in senescing flowers. We report in this paper the structural analysis of this cDNA and its corresponding gene. One cloned genomic DNA fragment, SR12-B, contained the entire transcription unit in 17 exons, interrupted by 16 introns. A second gene, SR12-A, was highly homologous to SR12-B with several nucleotide substitutions and a 489 bp deletion in the 5' flanking DNA sequence. The SR12 transcript has an open reading frame of 2193 bp sufficient to encode a protein of 82.8 kDa. No significant homology at the DNA or protein levels was found with other known genes. We have identified a DNA-binding factor which specifically interacts with two upstream fragments (-149 to -337 and -688 to -1055) of SR12-B. Both fragments apparently compete for the same binding factor. The DNA-binding activity was present in nuclear extracts from both presenescent and senescing carnation petals. The upstream DNA fragments that bind this factor have sequence homology with promoter sequences of other ethylene-regulated genes.

Regulation of senescence-related gene expression in carnation flower petals by ethylene.

Ethylene plays a regulatory role in carnation (Dianthus caryophyllus L.) flower senescence. Petal senescence coincides with a burst of ethylene production, is induced prematurely in response to exogenous ethylene, and is delayed by inhibitors of ethylene biosynthesis or action. We have investigated the role of ethylene in the regulation of three senescence-related cDNA clones isolated from a senescent carnation petal library (KA Lawton et al. [1989] Plant Physiol 90: 690-696). Expression of two of the cloned mRNAs in response to ethylene is floral specific, while the expression of another mRNA can be induced in both leaves and flowers exposed to ethylene. Although ethylene induces expression of these mRNAs in petals, message abundance decreases when flowers are removed from ethylene unless an autoenhancement of ethylene production is induced. This indicates continued perception of ethylene is required for their expression. Interruption of ethylene action following the onset of natural senescence results in a substantial decrease in transcript abundance of two of these mRNAs. However, the abundance of another mRNA remains unaffected, indicating this gene responds to temporal cues as well as to ethylene. As flowers age the dosage of exogenous ethylene required to induce expression of the cloned mRNAs decreases, indicating sensitivity to ethylene changes as the tissue matures. Nuclear run-on transcription experiments indicate that relative transcription rates of cloned mRNAs increase in response to exogenous ethylene.

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.