Induction of pathogenesis-related proteins in tobacco leaves

Salicylic acid-induced differential resistance to the Tomato yellow leaf curl virus among resistant and susceptible tomato cultivars

BACKGROUND

In higher plants, salicylic acid (SA) plays important roles in inducing resistance to biotic and abiotic stresses. Tomato yellow leaf curl virus (TYLCV) causes a highly devastating viral disease in plants, particularly in tomato. However, the roles of SA in inducing tomato plant resistance to TYLCV remain unclear.

RESULTS

In this study, we investigated whether the exogenous application of SA can improve the resistance of tomato plants to TYLCV in two tomato cultivars, resistant 'Zhefen-702' and susceptible 'Jinpeng-1'. The impacts of SA on the accumulation of ascorbic acid (AsA) and biosynthetic gene expression, the activity of some important reactive oxygen species (ROS)-scavenging enzymes, and the expression patterns of stress-related genes were also determined. Results indicated that SA can effectively regulate the accumulation of AsA, especially in 'Jinpeng-1'. Similarly, the expression patterns of most of the AsA biosynthetic genes showed a negative relationship with AsA accumulation in the resistant and susceptible tomato cultivars. In the two tomato cultivars, the activities of ascorbate peroxidase (APX) and peroxidase (POD) in the SA + TYLCV treated plants were increased during the experiment period except at 14 days (APX in 'Jinpeng-1' was also at 4 days) post infected (dpi) with TYLCV. Simultaneously, the activity of SOD was reduced in 'Jinpeng-1' and increased in 'Zhefen-702' after treatment with SA + TYLCV. SA can substantially induce the expression of ROS-scavenging genes at different extents. From 2 to 10 dpi, the virus content in the SA + TYLCV treated plants was remarkably lower than those in the TYLCV treated plants in 'Jinpeng-1'and Zhefen-702'.

CONCLUSIONS

The above results suggest that SA can enhance tomato plant resistance by modulating the expression of genes encoding for ROS-scavenging players, altering the activity of resistance-related enzymes, and inducing the expression of pathogenesis-related genes to produce systemic acquired resistance. Simultaneously, these results confirm that SA is a resistance-inducing factor against TYLCV infection that can be effectively applied in tomato plants.

Seamless editing of the chloroplast genome in plants

BACKGROUND

Gene editing technologies enable the precise insertion of favourable mutations and performance enhancing trait genes into chromosomes whilst excluding all excess DNA from modified genomes. The technology gives rise to a new class of biotech crops which is likely to have widespread applications in agriculture. Despite progress in the nucleus, the seamless insertions of point mutations and non-selectable foreign genes into the organelle genomes of crops have not been described. The chloroplast genome is an attractive target to improve photosynthesis and crop performance. Current chloroplast genome engineering technologies for introducing point mutations into native chloroplast genes leave DNA scars, such as the target sites for recombination enzymes. Seamless editing methods to modify chloroplast genes need to address reversal of site-directed point mutations by template mediated repair with the vast excess of wild type chloroplast genomes that are present early in the transformation process.

RESULTS

Using tobacco, we developed an efficient two-step method to edit a chloroplast gene by replacing the wild type sequence with a transient intermediate. This was resolved to the final edited gene by recombination between imperfect direct repeats. Six out of 11 transplastomic plants isolated contained the desired intermediate and at the second step this was resolved to the edited chloroplast gene in five of six plants tested. Maintenance of a single base deletion mutation in an imperfect direct repeat of the native chloroplast rbcL gene showed the limited influence of biased repair back to the wild type sequence. The deletion caused a frameshift, which replaced the five C-terminal amino acids of the Rubisco large subunit with 16 alternative residues resulting in a ~30-fold reduction in its accumulation. We monitored the process in vivo by engineering an overlapping gusA gene downstream of the edited rbcL gene. Translational coupling between the overlapping rbcL and gusA genes resulted in relatively high GUS accumulation (~0.5 % of leaf protein).

CONCLUSIONS

Editing chloroplast genomes using transient imperfect direct repeats provides an efficient method for introducing point mutations into chloroplast genes. Moreover, we describe the first synthetic operon allowing expression of a downstream overlapping gene by translational coupling in chloroplasts. Overlapping genes provide a new mechanism for co-ordinating the translation of foreign proteins in chloroplasts.

Nitration is exclusive to defense-related PR-1, PR-3 and PR-5 proteins in tobacco leaves

Protein tyrosine nitration is an important post-translational modification. A variety of nitrated proteins are reported in Arabidopsis leaves and seedlings, sunflower hypocotyls, and pea roots. The identities of nitrated proteins are species-/organ-specific, and chloroplast proteins are most nitratable in leaves. However, precise mechanism is unclear. Here, we investigated nitroproteome in tobacco leaves following exposure to nitrogen dioxide. Proteins were extracted, electrophoresed and immunoblotted using an anti-3-nitrotyrosine antibody. Mass spectrometry and FASTA search identified for the first time an exclusive nitration of pathogenesis-related proteins, PR-1, PR-3 and PR-5, which are reportedly located in the apoplast or the vacuole. Furthermore, Tyr(36) of thaumatin-like protein E2 was identfied as a nitration site. The underlying mechanism and physiological relevance are discussed.

Defence gene expression profiling to Ascochyta rabiei aggressiveness in chickpea

Significant differences in defence pathway-related gene expression were observed among chickpea cultivars following A. rabiei infection. Differential gene expression is indicative of diverse resistances, a theoretical tool for selective breeding. A high number of Ascochyta rabiei pathotypes infecting chickpea in Australia has severely hampered efforts towards breeding for sustained quantitative resistance in chickpea. Breeding for sustained resistance will be aided by detailed knowledge of defence responses to isolates with different aggressiveness. As an initial step, the conserved and differential expressions of a suit of previously characterised genes known to be involved in fungal defence mechanisms were assessed among resistant and susceptible host genotypes following inoculation with high or low aggressive A. rabiei isolates. Using quantitative Real-Time PCR (qRT-PCR), 15 defence-related genes, normalised with two reference genes, were temporally differentially expressed (P < 0.005) as early as 2 h post inoculation of Genesis090 (resistant) or Kaniva (susceptible). The highly aggressive isolate, 09KAL09, induced vastly different expression profiles of eight key defence-related genes among resistant and susceptible genotypes. Six of these same genes were differentially expressed among ten host genotypes, inclusive of the best resistance sources within the Australian chickpea breeding program, indicating potential use for discrimination and selection of resistance "type" in future breeding pursuits.

Melatonin as a signal molecule triggering defense responses against pathogen attack in Arabidopsis and tobacco

Melatonin plays pleiotropic roles in both animals and plants. The possible role of melatonin in plant innate immune responses was recently discovered. As an initial study, we employed Arabidopsis to determine whether melatonin is involved in defense against the virulent bacterial pathogen Pseudomonas syringae DC3000. The application of a 10 μM concentration of melatonin on Arabidopsis and tobacco leaves induced various pathogenesis-related (PR) genes, as well as a series of defense genes activated by salicylic acid (SA) and ethylene (ET), two key factors involved in plant defense response, compared to mock-treated leaves. The induction of these defense-related genes in melatonin-treated Arabidopsis matched an increase in resistance against the bacterium by suppressing its multiplication about ten-fold relative to the mock-treated Arabidopsis. Like melatonin, N-acetylserotonin also plays a role in inducing a series of defense genes, although serotonin does not. Furthermore, melatonin-induced PR genes were almost completely or partially suppressed in the npr1, ein2, and mpk6 Arabidopsis mutants, indicative of SA and ET dependency in melatonin-induced plant defense signaling. This suggests that melatonin may be a novel defense signaling molecule in plant-pathogen interactions.

Salicylic acid and gentisic acid induce RNA silencing-related genes and plant resistance to RNA pathogens

We have observed that treatments with salicylic acid (SA) or gentisic acid (GA) induced resistance to RNA pathogens such as ToMV and CEVd in tomato and Gynura auriantiaca, respectively. Accumulation of SA and GA has been found to occur in plants infected by these pathogens, thus pointing out a possible defence role of both molecules. To study the molecular basis of the observed induced resistance to RNA pathogens the induction of silencing-related genes by SA and GA was considered. For that purpose, we searched for tomato genes which were orthologous to those described in Arabidopsis thaliana, such as AtDCL1, AtDCL2, AtDCL4, AtRDR1, AtRDR2 and AtRDR6, and we tracked their induction in tomato along virus and viroid infections. We observed that CEVd significantly induced all these genes in tomato, with the exception of ToRDR6, being the induction of ToDCL4 the most outstanding. Regarding the ToMV asymptomatic infection, with the exception of ToRDR2, we observed a significant induction of all the indicated silencing-related genes, being ToDCL2 the most induced gene. Subsequently, we analyzed their transcriptional activation by SA and at the time when ToMV was inoculated on plants. ToDCL2, ToRDR1 and ToRDR2 were significantly induced by both SA and GA, whereas ToDCL1 was only induced by SA. Such an induction resulted more effective by SA treatment, which is in agreement with the stronger SA-induced resistance observed. Our results suggest that the observed delay in the RNA pathogen accumulation could be due to the pre-induction of RNA silencing-related genes by SA or GA.

Several "pathogenesis-related" proteins in potato are 1,3-beta-glucanases and chitinases

Chitinase {poly[1,4-(N-acetyl-beta-D-glucosaminide)]glycanohydrolase, EC 3.2.1.14} and 1,3-beta-glucanase (1,3-beta-D-glucan 3-glucanohydrolase, EC 3.2.1.6) activities increased rapidly in potato (Solanum tuberosum) leaves inoculated with the pathogenic fungus Phytophthora infestans or treated with fungal elicitor. The enzyme activities were resolved into a total of two distinct 1,3-beta-glucanases and six proteins with chitinase activity. By several criteria, all of these proteins are classified as "pathogenesis-related" proteins whose biochemical functions have so far been unknown. Some of them constitute a major portion of the proteins accumulating in the intercellular space of infected potato leaves and are assumed to play an important role in pathogen defense.

Cytology of Infection of Maize Seedlings by Fusarium moniliforme and Immunolocalization of the Pathogenesis-Related PRms Protein

ABSTRACT We have investigated the histology of infection of maize seedlings by Fusarium moniliforme in association with a biochemical host defense response, the accumulation of the PRms (pathogenesis-related maize seed) protein. Light microscopy of trypan blue-stained sections and scanning electron microscopy revealed direct penetration by F. moniliforme hyphae through the epidermal cells of the seedling and colonization of the host tissue by inter- and intracellular modes of growth. Pathogen ingress into the infected tissue was associated with the induction of defense-related ultrastructural modifications, as exemplified by the formation of appositions on the outer host cell wall surface, the occlusion of intercellular spaces, and the formation of papillae. Cellular and subcellular immunolocalization studies revealed that PRms accumulated at very high levels in those cells types that represent the first barrier for fungal penetration such as the aleurone layer of germinating seeds and the scutellar epithelial cells of isolated germinating embryos. A highly localized accumulation of PRms within papillae of the inner scutellar parenchyma cells also occurred, suggesting that signaling mechanisms that lead to the accumulation of PRms in papillae of cell types that are distant from the invading pathogen must operate in the infected maize tissues. Our study also revealed the presence of a large number of fungal cells with an abnormal shape that showed PRms-specific labeling. PRms was found to accumulate in clusters over the fungal cell wall. Taken together, the occurrence of PRms in cell types that first establish contact with the pathogen, as well as in papillae, and in association with fungal cell walls suggests that PRms may have a function in the plant defense response.

The salicylic acid-inducible alternative oxidase gene aox1 and genes encoding pathogenesis-related proteins share regions of sequence similarity in their promoters

We have isolated and characterized a genomic clone, lambda AOSG11, corresponding to aox1, which encodes the 42 kDa alternative oxidase precursor protein of Sauromatum guttatum Schott. The sequence of lambda AOSG11 revealed that aox1 consists of four exons separated by three short introns. Exon three contains the region of aox1 that (1) is highly conserved in the corresponding genes of potato, rice, and yeast, and (2) encodes a region of the deduced protein that is predicted to form two transmembrane alpha-helices. Southern blot analysis of restriction endonuclease-digested genomic DNA, indicated that aox1 is a single, nuclear-encoded gene in S. guttatum. We have determined the transcriptional start site of aox1 using nuclease protection and primer extension experiments. Comparison of the putative promoter region of aox1 to promoters of PR1a and GRP8 revealed some sequence similarity.

Biological activities of human interferon and 2'-5' oligoadenylates in plants

Exogenous human interferon 2 (IFN) and 2'-5' oligoadenylates (2-5A) have been shown to cause at least a dual physiological effect in tobacco and wheat: (i) increased cytokinin activity and (ii) induced synthesis of numerous proteins, among which members of two groups of stress proteins have been identified, namely pathogenesis-related (PR) and heat shock (HS) proteins. These effects were observed only by low concentrations of these substances: IFN at 0.1-1 u/ml and 2-5A at 1-10 nM.

Expression of the gene encoding the PR-like protein PRms in germinating maize embryos

The PRms protein is a pathogenesis-related (PR)-like protein whose mRNA accumulates during germination of maize seeds. Expression of the PRms gene is induced after infection of maize seeds with the fungus Fusarium moniliforme. To further our investigations on the expression of the PRms gene we examined the accumulation of PRms mRNA in different tissues of maize seedlings infected with F. moniliforme and studied the effect of fungal elicitors, the mycotoxin moniliformin, the hormone gibberellic acid, and specific chemical agents. Our results indicate that fungal infection, and treatment either with fungal elicitors or with moniliformin, a mycotoxin produced by F. moniliforme, increase the steady-state level of PRms mRNA. PRms mRNA accumulation is also stimulated by the application of the hormone gibberellic acid or by treatment with silver nitrate, whereas acetylsalicylic acid has no effect. In situ RNA hybridization in isolated germinating embryo sections demonstrates that the PRms gene is expressed in the scutellum, particularly in a group of inner cells, and in the epithelium lying at the interface of the scutellum and the endosperm. The pattern of expression of the PRms gene closely resembles that found for hydrolytic enzymes, being confined to the scutellum and the aleurone layer of the germinating maize seed. Our results suggest that the PRms protein has a function during the normal process of seed germination that has become adapted to serve among the defence mechanisms induced in response to pathogens during maize seed germination.

A gene coding for a basic pathogenesis-related (PR-like) protein from Zea mays. Molecular cloning and induction by a fungus (Fusarium moniliforme) in germinating maize seeds

Pathogenesis-related proteins (PRs) are plant proteins produced in leaves in response to infection by pathogens including viruses, viroids, fungi and bacteria. Information on the presence and/or expression of PRs in monocotyledonous plants is scare. Here we report the identification of cDNA and genomic clones coding for a basic form of a protein from germinating maize seeds having a high homology with the group of PR-1 from tobacco. A cDNA library enriched in aleurone-specific sequences was prepared from maize seeds two days after germination. One clone was found to contain an open reading frame encoding a protein homologous to PR proteins from tomato (p14) and tobacco (PR-1 group). Sequence analysis of the corresponding genomic clone revealed that it was encoded by a single exon. Besides, DNA blot hybridization indicates that this PR-like protein is encoded by a single-copy gene in maize. The accumulation of its mRNA increases after rehydration of desiccated seeds. Furthermore, a relationship was found between its expression and infection by a natural pathogen of maize, the fungus Fusarium moniliforme. The possible role of this protein as a response mechanism following fungal infection in cereal seeds is discussed.

Analysis of stress-induced or salicylic acid-induced expression of the pathogenesis-related 1a protein gene in transgenic tobacco

The cis-acting elements for regulating gene expression of the tobacco pathogenesis-related 1a protein gene were analyzed in transgenic plants. The 5'-flanking 2.4-kilobase fragment from the pathogenesis-related 1a protein gene was joined to the bacterial beta-glucuronidase gene and introduced into tobacco cells by Agrobacterium-mediated gene transfer. Promoter activity was monitored by quantitative and histochemical assay of beta-glucuronidase activity in leaves of regenerated transgenic plants. The level of beta-glucuronidase activity was clearly increased by treatment with salicylic acid, by cutting stress, and by local lesion formation caused by tobacco mosaic virus infection. Cytochemical studies of the induced beta-glucuronidase activity revealed tissue-specific and developmentally regulated expression of the pathogenesis-related 1a gene after stress or chemical treatment and after pathogen attack. To identify the cis-acting element more precisely, a series of 5'-deleted chimeric genes was constructed and transformed into tobacco plants. Transgenic plants with a 0.3-kilobase fragment of the 5'-flanking region of the pathogenesis-related 1a gene had the same qualitative response as those with the 2.4-kilobase fragment upon treatment with salicylic acid or infection with TMV. Thus, the 0.3-kilobase DNA sequence fragment was sufficient to allow the regulated expression of the pathogenesis-related 1a gene.

Degradation of tomato pathogenesis-related proteins by an endogenous 37-kDa aspartyl endoproteinase

As a response to the stress induced by different afflicting agents, tomato plants (Lycopersicon esculentum) produce the so-called 'pathogenesis-related' proteins. Here we report the degradation of some of these proteins by a constitutive endoproteolytic activity that co-distributes with pathogenesis-related proteins in the intercellular spaces of tomato leaves infected with citrus exocortis viroid. This endoproteinase was purified, showing a pH optimum of 2.5-3.5, a Mr of 37,000 and selective inhibition by pepstatin. In crude homogenates, the enzyme does not seem to degrade other cellular proteins. This specificity indicates that the proteinase might be involved in the extracellular degradative pathway of pathogenesis-related proteins and in the regulation of their biological function.

Identification and characterization of maize pathogenesis-related proteins. Four maize PR proteins are chitinases

Eight pathogenesis-related proteins extractable at pH 2.8 were found to accumulate in maize leaves after mercuric chloride treatment or brome mosaic virus infection. These proteins were called PRm (pathogenesis-related maize) proteins. Seven PRm proteins were purified to homogeneity by preparative polyacrylamide gel electrophoresis and their amino acid compositions determined. Estimated molecular weights in SDS-containing gels were: PRm 1 14.2 kDa; Prm 2 16.5 kDa; PRm 3 and PRm 4 25 kDa; PRm 6b 30.5 kDa; PRm 6a 32 kDa; PRm 7 34.5 kDa. Antisera raised against either PRm 3 or PRm 4 reacted specifically each with PRm 3 or PRm 4. Antisera raised against PRm 6b reacted with PRm 6b as well as with PRm 6a and antisera against PRm 7 reacted with PRm 7 and PRm 5. Tobacco anti-PR 1b antisera reacted with maize PRm 2.Chitinase (poly[1,4-(N-acetyl-β-D-glucosamide)]glycanhydrolase, EC 3.2.1.14) activity was found for PRm 3, PRm 4, PRm 5, and PRm 7.

Pathogenesis-related proteins of tomato : p-69 as an alkaline endoproteinase

An endoproteinase induced by citrus exocortis viroid has been purified from tomato (Lycopersicon esculentum Mill, cv "Rutgers") leaves. The proteinase corresponds to one of the major pathogenesis-related proteins of tomato plants and was designated proteinase P-69 as it has a molecular weight of 69,000 to 70,000. The proteinase was purified in four steps: (NH(4))(2)SO(4) fractionation, chromatography on Bio-Gel P-60, DEAE-Sepharose chromatography, and casein-Sepharose affinity chromatography. The proteinase had a pH optimum of 8.5 to 9.0 when assayed with either fluorescein thiocarbamoyl derivative (FTC)-casein or FTC-ribulose 1,5-bisphosphate carboxylase/oxygenase as substrates. The proteinase activity was inhibited by pCMB and strongly activated by calcium and magnesium ions as well as by DTT. When analyzed by electrofocusing, the activity showed a pI around 9.0.

In vitro synthesis and processing of a bean pathogenesis-related (PR4) protein

When bean plants (Phaseolus vulgaris var. Saxa) are treated with mercuric chloride or infected with alfalfa mosaic virus, they produce pathogenesis-related (PR) proteins. We report here that functional mRNA encoding bean PR4 protein is only present when synthesis of this protein has been induced. Treatment with mercuric chloride results in a rapid induction of functional bean PR4 mRNA (within 2-3 h), whereas in virus-infected plants this mRNA can only be detected the second day following the infection. Bean PR4 protein is synthesized in vitro, using this mRNA in a rabbit reticulocyte lysate system, as a precursor of 35 kDa. This precursor can be processed into a polypeptide having the same molecular mass (33.5 kDa) as the in vivo PR4 protein by the addition to the cell-free translation system of canine pancreatic microsomal membranes.

Isolation and characterization of cDNA clones encoding pathogenesis-related proteins from tobacco mosaic virus infected tobacco plants

Infection of the tobacco cultivar Samsun NN by tobacco mosaic virus (TMV) results in a hypersensitive response. During this defense reaction several host encoded proteins, known as pathogenesis-related proteins (PR-proteins), are induced. Poly(A)+ RNA from TMV infected tobacco plants was used to construct a cDNA library. Thirty two cDNA clones were isolated and after digestion with different restriction endonucleases, twenty clones were found to code for PR-1a, six clones for PR-1b, and four clones for PR-1c. Two independent cDNA clones of each class were further characterized by DNA sequence analysis. All clones analyzed contained the 138 amino acid coding regions of their respective mature proteins, but only partial sequences of the signal peptides. Minor differences between the nucleotide sequences for clones belonging to the same class were detected. Comparison of the amino acid sequence for PR-1a deduced from its nucleotide sequence with published data obtained by Edman degradation of the protein showed four differences. Analysis of the 3' ends of the cDNA clones indicates that various alternate poly(dA) addition sites are used. Southern blot analysis using these cDNAs as probes suggests the presence of multiple PR-protein genes in the genomes of tobacco and tomato plants.