Molecular cloning and characterization of a pea chitinase gene expressed in response to wounding, fungal infection and the elicitor chitosan

Comparison of leaf transcriptome in response to Rhizoctonia solani infection between resistant and susceptible rice cultivars

Background

Sheath blight (SB), caused by Rhizoctonia solani, is a common rice disease worldwide. Currently, rice cultivars with robust resistance to R. solani are still lacking. To provide theoretic basis for molecular breeding of R. solani-resistant rice cultivars, the changes of transcriptome profiles in response to R. solani infection were compared between a moderate resistant cultivar (Yanhui-888, YH) and a susceptible cultivar (Jingang-30, JG).

Results

In the present study, 3085 differentially express genes (DEGs) were detected between the infected leaves and the control in JG, with 2853 DEGs in YH. A total of 4091 unigenes were significantly upregulated in YH than in JG before infection, while 3192 were significantly upregulated after infection. Further analysis revealed that YH and JG showed similar molecular responses to R. solani infection, but the responses were earlier in JG than in YH. Expression levels of trans-cinnamate 4-monooxygenase (C4H), ethylene-insensitive protein 2 (EIN2), transcriptome factor WRKY33 and the KEGG pathway plant-pathogen interaction were significantly affected by R. solani infection. More importantly, these components were all over-represented in YH cultivar than in JG cultivar before and/or after infection.

Conclusions

These genes possibly contribute to the higher resistance of YH to R. solani than JG and were potential target genes to molecularly breed R. solani-resistant rice cultivar.

Function of Hevea brasiliensis NAC1 in dehydration-induced laticifer differentiation and latex biosynthesis

MAIN CONCLUSIONS

HbNAC1 is a transcription factor in rubber plants whose expression is induced by dehydration, leading to latex biosynthesis. Laticifer is a special tissue in Hevea brasiliensis where natural rubber is biosynthesized and accumulated. In young stems of epicormic shoots, the differentiation of secondary laticifers can be induced by wounding, which can be prevented when the wounding site is wrapped. Using this system, differentially expressed genes were screened by suppression subtractive hybridization (SSH) and macroarray analyses. This led to the identification of several dehydration-related genes that could be involved in laticifer differentiation and/or latex biosynthesis, including a NAC transcription factor (termed as HbNAC1). Tissue sections confirmed that local tissue dehydration was a key signal for laticifer differentiation. HbNAC1 was localized at the nucleus and showed strong transcriptional activity in yeast, suggesting that HbNAC1 is a transcription factor. Furthermore, HbNAC1 was found to bind to the cis-element CACG in the promoter region of the gene encoding the small rubber particle protein (SRPP). Transgenic experiments also confirmed that HbNAC1 interacted with the SRPP promoter when co-expressed, and enhanced expression of the reporter gene β-glucuronidase occurred in planta. In addition, overexpression of HbNAC1 in tobacco plants conferred drought tolerance. Together, the data suggest that HbNAC1 might be involved in dehydration-induced laticifer differentiation and latex biosynthesis.

Non-host Resistance: DNA Damage Is Associated with SA Signaling for Induction of PR Genes and Contributes to the Growth Suppression of a Pea Pathogen on Pea Endocarp Tissue

Salicylic acid (SA) has been reported to induce plant defense responses. The transcriptions of defense genes that are responsible for a given plant's resistance to an array of plant pathogens are activated in a process called non-host resistance. Biotic signals capable of carrying out the activation of pathogenesis-related (PR) genes in pea tissue include fungal DNase and chitosan, two components released from Fusarium solani spores that are known to target host DNA. Recent reports indicate that SA also has a physical affinity for DNA. Here, we report that SA-induced reactive oxygen species release results in fragment alterations in pea nuclear DNA and cytologically detectable diameter and structural changes in the pea host nuclei. Additionally, we examine the subsequent SA-related increase of resistance to the true pea pathogen F. solani f.sp. pisi and the accumulation of the phytoalexin pisatin. This is the first report showing that SA-induced PR gene activation may be attributed to the host pea genomic DNA damage and that at certain concentrations, SA can be temporally associated with subsequent increases in the defense response of this legume.

GRP-3 and KAPP, encoding interactors of WAK1, negatively affect defense responses induced by oligogalacturonides and local response to wounding

Conserved microbe-associated molecular patterns (MAMPs) and damage-associated molecular patterns (DAMPs) act as danger signals to activate the plant immune response. These molecules are recognized by surface receptors that are referred to as pattern recognition receptors. Oligogalacturonides (OGs), DAMPs released from the plant cell wall homogalacturonan, have also been proposed to act as local signals in the response to wounding. The Arabidopsis Wall-Associated Kinase 1 (WAK1), a receptor of OGs, has been described to form a complex with a cytoplasmic plasma membrane-localized kinase-associated protein phosphatase (KAPP) and a glycine-rich protein (GRP-3) that we find localized mainly in the cell wall and, in a small part, on the plasma membrane. By using Arabidopsis plants overexpressing WAK1, and both grp-3 and kapp null insertional mutant and overexpressing plants, we demonstrate a positive function of WAK1 and a negative function of GRP-3 and KAPP in the OG-triggered expression of defence genes and the production of an oxidative burst. The three proteins also affect the local response to wounding and the basal resistance against the necrotrophic pathogen Botrytis cinerea. GRP-3 and KAPP are likely to function in the phasing out of the plant immune response.

The role of MYB34, MYB51 and MYB122 in the regulation of camalexin biosynthesis in Arabidopsis thaliana

The phytoalexin camalexin and indolic glucosinolates share not only a common evolutionary origin and a tightly interconnected biosynthetic pathway, but regulatory proteins controlling the shared enzymatic steps are also modulated by the same R2R3-MYB transcription factors. The indolic phytoalexin camalexin is a crucial defense metabolite in the model plant Arabidopsis. Indolic phytoalexins and glucosinolates appear to have a common evolutionary origin and are interconnected on the biosynthetic level: a key intermediate in the biosynthesis of camalexin, indole-3-acetaldoxime (IAOx), is also required for the biosynthesis of indolic glucosinolates and is under tight control by the transcription factors MYB34, MYB51, and MYB122. The abundance of camalexin was strongly reduced in myb34/51 and myb51/122 double and in triple myb mutant, suggesting that these transcription factors are important in camalexin biosynthesis. Furthermore, expression of MYB51 and MYB122 was significantly increased by biotic and abiotic camalexin-inducing agents. Feeding of the triple myb34/51/122 mutant with IAOx or indole-3-acetonitrile largely restored camalexin biosynthesis. Conversely, tryptophan could not complement the low camalexin phenotype of this mutant, which supports a role for the three MYB factors in camalexin biosynthesis upstream of IAOx. Consistently expression of the camalexin biosynthesis genes CYP71B15/PAD3 and CYP71A13 was not negatively affected in the triple myb mutant and the MYBs could not activate pCYP71B15::uidA expression in trans-activation assays with cultured Arabidopsis cells. In conclusion, this study reveals the importance of MYB factors regulating the generation of IAOx as precursor of camalexin.

Identification of quantitative trait loci and candidate genes for specific cellular resistance responses against Didymella pinodes in pea

KEY MESSAGE

Phenotyping of specific cellular resistance responses and improvement of previous genetic map allowed the identification of novel genomic regions controlling cellular mechanisms involved in pea resistance to ascochyta blight and provided candidate genes suitable for MAS. Didymella pinodes, causing ascochyta blight, is a major pathogen of the pea crop and is responsible for serious damage and yield losses. Resistance is inherited polygenically and several quantitative trait loci (QTLs) have been already identified. However, the position of these QTLs should be further refined to identify molecular markers more closely linked to the resistance genes. In previous works, resistance was scored visually estimating the final disease symptoms; in this study, we have conducted a more precise phenotyping of resistance evaluating specific cellular resistance responses at the histological level to perform a more accurate QTL analysis. In addition, P665 × Messire genetic map used to identify the QTLs was improved by adding 117 SNP markers located in genes. This combined approach has allowed the identification, for the first time, of genomic regions controlling cellular mechanisms directly involved in pea resistance to ascochyta blight. Furthermore, the inclusion of the gene-based SNP markers has allowed the identification of candidate genes co-located with QTLs and has provided robust markers for marker-assisted selection.

Multiple effects of chitosan on plant systems: solid science or hype

Chitosan, a naturally occurring polymer, became available in the 1980s in industrial quantities enabling it to be tested as an agricultural chemical. A usual procedure for developing agricultural chemicals starts by testing a number of different chemically synthesized molecules on a targeted biological system. Alternately, chitosan has been investigated as a single natural molecule assayed with numerous biological systems. This report describes the unique properties of the molecule and its oligomers, primarily in plant defense, additionally in yield increase, induction of cell death and stomatal closing. The plant plasma membrane and nuclear chromatin have been proposed as targets, though chitosan oligomers enter most regions of the cell. Subsequent changes occur in: cell membranes, chromatin, DNA, calcium, MAP kinase, oxidative burst, reactive oxygen species (ROS), callose, pathogenesis related (PR) genes/proteins, and phytoalexins. Chitosan oligomer mode(s) of action are proposed for different plant systems. Chitosan efficacy was based on documentation from published data. Attention was given to how chitosan, either applied externally or released by fungal inoculum, is transferred into plant cells and its subsequent action upon membrane and/or chromatin components. Within is a proposed scheme describing chitosan generation, signaling routes and mechanisms of defense gene activation. Examples of beneficial chitosan applications to major crop/food plants were included.

Proteomic analysis of embryonic axis of Pisum sativum seeds during germination and identification of proteins associated with loss of desiccation tolerance

Seed germination is an important stage in life cycle of higher plants. The germination processes and its associated loss of desiccation tolerance, however, are still poorly understood. In present study, pea seeds were used to study changes in embryonic axis proteome during germination by 2-DE and mass spectrometry. We identified a total of 139 protein spots showing a significant (>2-fold) change during germination. The results show that seed germination is not only the activation of a series of metabolic processes, but also involves reorganization of cellular structure and activation of protective systems. To uncouple the physiological processes of germination and its associated loss of desiccation tolerance, we used the fact that pea seeds have different desiccation tolerance when imbibed in water, CaCl(2) and methylviologen at the same germination stage. We compared the proteome amongst these seeds to identify the candidate proteins associated with the loss of desiccation tolerance and found a total of seven proteins - tubulin alpha-1 chain, seed biotin-containing protein SBP65, P54 protein, vicilin, vicilin-like antimicrobial peptides 2-3, convicilin and TCP-1/cpn60 chaperonin family protein. The metabolic function of these proteins indicates that seed desiccation tolerance is related to pathogen defense, protein conformation conservation and cell structure stabilization.

Heterologous expression of viral suppressors of RNA silencing complements virulence of the HC-Pro mutant of clover yellow vein virus in pea

Many plant viruses encode suppressors of RNA silencing, including the helper component-proteinase (HC-Pro) of potyviruses. Our previous studies showed that a D-to-Y mutation at amino acid position 193 in HC-Pro (HC-Pro-D193Y) drastically attenuated the virulence of clover yellow vein virus (ClYVV) in legume plants. Furthermore, RNA-silencing suppression (RSS) activity of HC-Pro-D193Y was significantly reduced in Nicotiana benthamiana. Here, we examine the effect of expression of heterologous suppressors of RNA silencing, i.e., tomato bushy stunt virus p19, cucumber mosaic virus 2b, and their mutants, on the virulence of the ClYVV point mutant with D193Y (Cl-D193Y) in pea. P19 and 2b fully and partially complemented Cl-D193Y multiplication and virulence, including lethal systemic HR in pea, respectively, but the P19 and 2b mutants with defects in their RSS activity did not. Our findings strongly suggest that the D193Y mutation exclusively affects RSS activity of HC-Pro and that RSS activity is necessary for ClYVV multiplication and virulence in pea.

Activation of the salicylic acid signaling pathway enhances Clover yellow vein virus virulence in susceptible pea cultivars

The wild-type strain (Cl-WT) of Clover yellow vein virus (ClYVV) systemically induces cell death in pea cv. Plant introduction (PI) 118501 but not in PI 226564. A single incompletely dominant gene, Cyn1, controls systemic cell death in PI 118501. Here, we show that activation of the salicylic acid (SA) signaling pathway enhances ClYVV virulence in susceptible pea cultivars. The kinetics of virus accumulation was not significantly different between PI 118501 (Cyn1) and PI 226564 (cyn1); however, the SA-responsive chitinase gene (SA-CHI) and the hypersensitive response (HR)-related gene homologous to tobacco HSR203J were induced only in PI 118501 (Cyn1). Two mutant viruses with mutations in P1/HCPro, which is an RNA-silencing suppressor, reduced the ability to induce cell death and SA-CHI expression. The application of SA and of its analog benzo (1,2,3) thiadiazole-7-carbothioic acid S-methyl ester (BTH) partially complemented the reduced virulence of mutant viruses. These results suggest that high activation of the SA signaling pathway is required for ClYVV virulence. Interestingly, BTH could enhance Cl-WT symptoms in PI 226564 (cyn1). However, it could not enhance symptoms induced by White clover mosaic virus and Bean yellow mosaic virus. Our report suggests that the SA signaling pathway has opposing functions in compatible interactions, depending on the virus-host combination.

Wounding of Arabidopsis leaves causes a powerful but transient protection against Botrytis infection

Physical injury inflicted on living tissue makes it vulnerable to invasion by pathogens. Wounding of Arabidopsis thaliana leaves, however, does not conform to this concept and leads to immunity to Botrytis cinerea, the causal agent of grey mould. In wounded leaves, hyphal growth was strongly inhibited compared to unwounded controls. Wound-induced resistance was not associated with salicylic acid-, jasmonic acid- or ethylene-dependent defence responses. The phytoalexin camalexin was found to be involved in this defence response as camalexin-deficient mutants were not protected after wounding and the B. cinerea strains used here were sensitive to this compound. Wounding alone did not lead to camalexin production but primed its accumulation after inoculation with B. cinerea, further supporting the role of camalexin in wound-induced resistance. In parallel with increased camalexin production, genes involved in the biosynthesis of camalexin were induced faster in wounded and infected plants in comparison with unwounded and infected plants. Glutathione was also found to be required for resistance, as mutants deficient in gamma-glutamylcysteine synthetase showed susceptibility to B. cinerea after wounding, indicating that wild-type basal levels of glutathione are required for the wound-induced resistance. Furthermore, expression of the gene encoding glutathione-S-transferase 1 was primed by wounding in leaves inoculated with B. cinerea. In addition, the priming of MAP kinase activity was observed after inoculation of wounded leaves with B. cinerea compared to unwounded inoculated controls. Our results demonstrate how abiotic stress can induce immunity to virulent strains of B. cinerea, a process that involves camalexin and glutathione.

Pea-Fusarium solani interactions contributions of a system toward understanding disease resistance

This mini-review points to the usefulness of the pea-Fusarium solani interaction in researching the biochemical and molecular aspects of the nonhost resistance components of peas. This interaction has been researched to evaluate the resistance roles of the phytoalexin, pisatin, the cuticle barrier, and the activation of the nonhost resistance response. Concurrently, evaluations of associated signaling processes and the tools possessed by the pathogen to contend with host obstacles were included. The properties of some pathogenesis-related genes of pea and their regulation and contribution to resistance are discussed. A proposed action of two biotic elicitors on both chromatin conformation and the architectural transcription factor, HMG A, is presented and includes time lines of events within the host immune response.

Candidate genes for quantitative resistance to Mycosphaerella pinodes in pea (Pisum sativum L.)

Partial resistance to Mycosphaerella pinodes in pea is quantitatively inherited. Genomic regions involved in resistance (QTLs) have been previously identified in the pea genome, but the molecular basis of the resistance is still unknown. The objective of this study was to map resistance gene analogs (RGA) and defense-related (DR) genes in the JI296 x DP RIL population that has been used for mapping QTLs for resistance to M. pinodes, and identify co-localizations between candidate genes and QTLs. Using degenerate oligonucleotide primers designed on the conserved motifs P-loop and GLPL of cloned resistance genes, we isolated and cloned 16 NBS-LRR sequences, corresponding to five distinct classes of RGAs. Specific second-generation primers were designed for each class. RGAs from two classes were located on the linkage group (LG) VII. Another set of PCR-based markers was designed for four RGA sequences previously isolated in pea and 12 previously cloned DR gene sequences available in databases. Out of the 16 sequences studied, the two RGAs RGA-G3A and RGA2.97 were located on LG VII, PsPRP4A was located on LG II, Peachi21, PsMnSOD, DRR230-b and PsDof1 were mapped on LG III and peabetaglu and DRR49a were located on LG VI. Two co-localizations between candidate genes and QTLs for resistance to M. pinodes were observed on LG III, between the putative transcription factor PsDof1 and the QTL mpIII-1 and between the pea defensin DRR230-b gene and the QTL mpIII-4. Another co-localization was observed on LG VII between a cluster of RGAs and the QTL mpVII-1. The three co-localizations appear to be located in chromosomal regions containing other disease resistance or DR genes, suggesting an important role of these genomic regions in defense responses against pathogens in pea.

Colleters in Bathysa nicholsonii K. Schum. (Rubiaceae): ultrastructure, secretion protein composition, and antifungal activity

Colleters are secretory structures well distributed in many organs of Angiosperms. Ultrastructurally, the colleters secretory cell presents an enhanced endoplasmic reticulum, Golgi apparatus, and mitochondria. Secretion synthesis, transportation, and passage through outer cell wall is poorly characterized. This study characterized the anatomy and ultrastructure of BATHYSA NICHOLSONII (Rubiaceae) colleters and evaluated the presence of protein in the secretion and its antifungal property. Samples were collected and prepared according to usual techniques in light and electron microscopy, electrophoresis, and fungal growth inhibition assay. Colleters are of a standard type, cylindrical and elongated, formed by one secretory epidermal palisade layer, and a central axis formed by parenchymatic cells and a vascular trace. Epidermal cells have dense cytoplasm with abundant ribosome, a nucleus, enhanced endoplasmic reticulum and Golgi apparatus. The outer cell wall presented morphologically distinct layers. The presence of secretory cavities was noted in all outer cell wall extents. Secretion preparations analyzed by SDS-PAGE showed that B. NICHOLSONII secretion is a mixture of proteins with molecular masses covering a range of approximately 66 to 24 kDa. This preparation presented an inhibitory effect on the fungi spore growth.

A novel lipoxygenase in pea roots. Its function in wounding and biotic stress

The genome of pea (Pisum sativum) contains genes encoding a family of distinct lipoxygenases (LOX). Among these, LOXN2 showed eight exons encoding a 93.7-kD enzyme, harboring two C-terminal deletions and an unusual arginine/threonine-tyrosine motif in the domain considered to control the substrate specificity. LOXN2, when overexpressed in yeast, exhibited normal enzyme activity with an optimum at pH 4.5, and a dual positional specificity by releasing a 3:1 ratio of C-9 and C-13 oxidized products. The predicted LOXN2 structure lacked a loop present in soybean (Glycine max) LOX1, in a position consistent with control of the degree of substrate access to the catalytic site and for LOXN2's dual positional specificity. The LOXN2 gene was tightly conserved in the Progress 9 and MG103738 genotypes, respectively, susceptible and resistant to the root cyst nematode Heterodera goettingiana. LOXN2 transcription was monitored in roots after mechanical injury and during nematode infection. The message peaked at 3 and 24 h after wounding in both genotypes and was more abundant in the resistant than in the susceptible pea. In nematode-infected roots, transcription of several LOX genes was triggered except LOXN2, which was repressed in both genotypes. In situ hybridization revealed that LOXN2 message was widespread in the cortex and endodermis of healthy roots, but specifically localized at high level in the cells bordering the nematode-induced syncytia of infected roots. However, LOXN2 transcript signal was particularly intense in collapsing syncytia of MG103738 roots, suggesting LOXN2 involvement in late mechanisms of host resistance.

Tissue-specific localization of pea root infection by Nectria haematococca. Mechanisms and consequences

Root infection in susceptible host species is initiated predominantly in the zone of elongation, whereas the remainder of the root is resistant. Nectria haematococca infection of pea (Pisum sativum) was used as a model to explore possible mechanisms influencing the localization of root infection. The failure to infect the root tip was not due to a failure to induce spore germination at this site, suppression of pathogenicity genes in the fungus, or increased expression of plant defense genes. Instead, exudates from the root tip induce rapid spore germination by a pathway that is independent of nutrient-induced germination. Subsequently, a factor produced during fungal infection and death of border cells at the root apex appears to selectively suppress fungal growth and prevent sporulation. Host-specific mantle formation in response to border cells appears to represent a previously unrecognized form of host-parasite relationship common to diverse species. The dynamics of signal exchange leading to mantle development may play a key role in fostering plant health, by protecting root meristems from pathogenic invasion.

Enrichment of chitinolytic microorganisms: isolation and characterization of a chitinase exhibiting antifungal activity against phytopathogenic fungi from a novel Streptomyces strain

Thirteen different chitin-degrading bacteria were isolated from soil and sediment samples. Five of these strains (SGE2, SGE4, SSL3, MG1, and MG3) exhibited antifungal activity against phytopathogenic fungi. Analyses of the 16S rRNA genes and the substrate spectra revealed that the isolates belong to the genera Bacillus or Streptomyces. The closest relatives were Bacillus chitinolyticus (SGE2, SGE4, and SSL3), B. ehimensis (MG1), and Streptomyces griseus (MG3). The chitinases present in the culture supernatants of the five isolates revealed optimal activity between 45 degrees C and 50 degrees C and at pH values of 4 (SSL3), 5 (SGE2 and MG1), 6 (SGE4), and 5-7 (MG3). The crude chitinase preparations of all five strains possessed antifungal activity. The chitinase of MG3 (ChiIS) was studied further, since the crude enzyme conferred strong growth suppression of all fungi tested and was very active over the entire pH range tested. The chiIS gene was cloned and the gene product was purified. The deduced protein consisted of 303 amino acids with a predicted molecular mass of 31,836 Da. Sequence analysis revealed that ChiIS of MG3 is similar to chitinases of Streptomyces species, which belong to family 19 of glycosyl hydrolases. Purified ChiIS showed remarkable antifungal activity and stability.

Wound-induced gene expression of putrescine N-methyltransferase in leaves of Nicotiana tabacum

Putrescine N-methyltransferase (PMT) catalyzes the first committed step in the biosynthesis of pyrrolinium ring-containing alkaloids. Earlier studies have indicated that PMT gene expression is restricted to root tissue in Solanaceus plant species. During the analysis to further elucidate factors that govern the regulation of alkaloid synthesis, evidence was found for a novel expression pattern dictated by the 5'-flanking region of at least two members of the PMT-gene family. A 627-bp DNA fragment upstream of the NtPMT3 gene was fused to the beta-glucuronidase (GUS) reporter gene and used to produce stable transgenic lines of Nicotiana tabacum. Fluorometric and histochemical assays conducted on transgenic plants indicated high expression levels in root tissue and, in agreement with previous studies, no expression was detected in leaves. However, expression was observed in leaves when they were mechanically wounded. This expression was highly localized around the wound site and showed little evidence of long distance signaling, including lack of responsiveness to jasmonic acid. Expression was transient, with maximum levels immediately after wounding and diminishing after approximately 2-4 h. RT-PCR analysis of mRNA isolated from wild-type plants also indicated upregulation of PMT expression in leaves upon wounding as well as very low transcript levels in unwounded leaves. Low levels of PMT activity were detected in leaf tissue, which did not increase significantly upon wounding.

Tissue specific localization of root infection by fungal pathogens: role of root border cells

When roots of pea seedlings were inoculated uniformly with spores of Nectria haematocca or other pea pathogenic fungi, more than 90% developed lesions in the region of elongation within 3 days. More mature regions of most roots as well as the tip showed no visible signs of infection. Yet, microscopic observation revealed that 'mantles,' comprised of fungal hyphae intermeshed with populations of border cells, covered the tips of most roots. After physical detachment of the mantle, the underlying tip of most roots was found to be free of infection. Mantle-covered root tips did not respond to invasion of their border cells by activation of known defense genes unless there was invasion of the tip itself, as revealed by the presence of a lesion. Concomitant with the activation of defense genes was the induction of a cell-wall degrading enzyme whose expression is a marker for renewed production of border cells. Mantle formation did not occur in response to nonpathogens. The data are consistent with the hypothesis that border cells serve as a host-specific 'decoy' that protects root meristems by inhibiting fungal infection of the root tip.

A class I chitinase from soybean seed coat

Protein extracts from soybean (Glycine max [L.] Merr) seed hulls were fractionated by isoelectric focusing and SDS-PAGE analysis and components identified by peptide microsequencing. An abundant 32 kDa protein possessed an N-terminal cysteine-rich hevein domain present in class I chitinases and in other chitin-binding proteins. The protein could be purified from seed coats by single step binding to a chitin bead matrix and displayed chitinase activity by an electrophoretic zymogram assay. The corresponding cDNA and genomic clones for the chitinase protein were isolated and characterized, and the expression pattern determined by RNA blot analysis. The deduced peptide sequence of 320 amino acids included an N-terminal signal peptide and conserved chitin-binding and catalytic domains interspaced by a proline hinge. An 11.3 kb EcoRI genomic fragment bearing the 2.4 kb chitinase gene was fully sequenced. The gene contained two introns and was flanked by A+T-rich tracts. Analysis by DNA blot hybridization showed that this is a single or low copy gene in the soybean genome. The chitinase is expressed late in seed development, with particularly high expression in the seed coat. Expression was also evident in the late stages of development of the pod, root, leaf, and embryo, and in tissues responding to pathogen infection. This study further illustrates the differences in protein composition of the various seed tissues and demonstrates that defence-related proteins are prevalent in the seed coat.

The structure and action of chitinases

Chitin is second only to cellulose in biomass and it is an important component of many cell wall structures. Several families of enzymes, of distinctly different structure, have evolved to hydrolyze this important polysaccaride. Glycohydrolase family 18 enzymes, chitinases, are characterized by an eight-fold alpha/beta barrel structure; it has representatives among bacteria, fungi, and higher plants. In general these chitinases act through a retaining mechanism in which beta linked polymer is cleaved to release a beta anomer product. Family 19 chitinases are found primarily in plants but some are found in bacteria. Members of this family are related to one another by amino acid sequence, but are unrelated to family 18 proteins. They have a bilobal structure with a high alpha-helical content. Despite any significant sequence homology with lysozymes, structural analysis reveals that family 19 chitinases, together with family 46 chitosanases, are similar to several lysozymes including those from T4-phage and from goose. The structures reveal that the different enzyme groups arose from a common ancestor glycohydrolase antecedent to the procaryotic/eucaryotic divergence. In general, the family 19 enzymes operate through an inverting mechanism.

Host-parasite interactions: elicitation of defense responses in plants with chitosan

The plant's defense response against pathogens can be elicited by numerous external signals. Plant pathogens known to be incompatible on a given plant species can elicit strong disease resistance responses, whereas an adapted compatible pathogen generates a weaker response and thus can more readily infect the plant tissue. The plant's response can be manipulated genetically by the transfer of "R" genes (single dominant genes for race-specific disease resistance) or by treatment with elicitors such as chitosan. Both of these manipulations can result in the rapid activation of a subset of genes called PR (pathogenesis-related) genes, generally regarded as the genes that functionally develop disease resistance. There appear to be multiple modes by which chitosan can increase PR gene function, including activating cell surface or membrane receptors and internal effects on the plant's DNA conformation that in turn influence gene transcription. A novel strategy for controlling PR gene expression proposes to transform plants with a chitosan-inducible gene promoter linked in line with a single signal gene capable of rapid, intense induction of an entire set of PR genes, thereby enabling the control of disease resistance by external chitosan applications.

Characterization of two class II chitinase genes from peanut and expression studies in transgenic tobacco plants

Two different genes encoding class II chitinases from peanut (Arachis hypogaea L. cv. NC4), A.h.Chi2;1 and A.h.Chi2;2, have been cloned. In peanut cell suspension cultures, mRNA levels of A.h.Chi2;2 increased after ethylene or salicylate treatment and in the presence of conidia from Botrytis cinerea. The second gene, A.h.Chi2;1, was only expressed after treatment with the fungal spores. Transgenic tobacco plants containing the complete peanut A.h.Chi2;1 gene exhibited essentially the same expression pattern in leaves as observed in peanut cell cultures. Expression characteristics of transgenic tobacco carrying a promoter-GUS fusion of A.h.Chi2;1 are described.