Antimicrobial Functions of the Plant Hydrolases, Chitinase and ß-1,3-Glucanase

Bdelloid rotifers deploy horizontally acquired biosynthetic genes against a fungal pathogen

Coevolutionary antagonism generates relentless selection that can favour genetic exchange, including transfer of antibiotic synthesis and resistance genes among bacteria, and sexual recombination of disease resistance alleles in eukaryotes. We report an unusual link between biological conflict and DNA transfer in bdelloid rotifers, microscopic animals whose genomes show elevated levels of horizontal gene transfer from non-metazoan taxa. When rotifers were challenged with a fungal pathogen, horizontally acquired genes were over twice as likely to be upregulated as other genes - a stronger enrichment than observed for abiotic stressors. Among hundreds of upregulated genes, the most markedly overrepresented were clusters resembling bacterial polyketide and nonribosomal peptide synthetases that produce antibiotics. Upregulation of these clusters in a pathogen-resistant rotifer species was nearly ten times stronger than in a susceptible species. By acquiring, domesticating, and expressing non-metazoan biosynthetic pathways, bdelloids may have evolved to resist natural enemies using antimicrobial mechanisms absent from other animals.

A loss-of-function mutant allele of a glycosyl hydrolase gene has been co-opted for seed weight control during soybean domestication

The resultant DNA from loss-of-function mutation can be recruited in biological evolution and development. Here, we present such a rare and potential case of "to gain by loss" as a neomorphic mutation during soybean domestication for increasing seed weight. Using a population derived from a chromosome segment substitution line of Glycine max (SN14) and Glycine soja (ZYD06), a quantitative trait locus (QTL) of 100-seed weight (qHSW) was mapped on chromosome 11, corresponding to a truncated β-1, 3-glucosidase (βGlu) gene. The novel gene hsw results from a 14-bp deletion, causing a frameshift mutation and a premature stop codon in the βGlu. In contrast to HSW, the hsw completely lost βGlu activity and function but acquired a novel function to promote cell expansion, thus increasing seed weight. Overexpressing hsw instead of HSW produced large soybean seeds, and surprisingly, truncating hsw via gene editing further increased the seed size. We further found that the core 21-aa peptide of hsw and its variants acted as a promoter of seed size. Transcriptomic variation in these transgenic soybean lines substantiated the integration hsw into cell and seed size control. Moreover, the hsw allele underwent selection and expansion during soybean domestication and improvement. Our work cloned a likely domesticated QTL controlling soybean seed weight, revealed a novel genetic variation and mechanism in soybean domestication, and provided new insight into crop domestication and breeding, and plant evolution.

Tomato Bio-Protection Induced by Pseudomonas fluorescens N21.4 Involves ROS Scavenging Enzymes and PRs, without Compromising Plant Growth

Aims: to discover the interrelationship between growth, protection and photosynthesis induced by Pseudomonas fluorescens N21.4 in tomato (Lycopersicum sculentum) challenged with the leaf pathogen Xanthomonas campestris, and to define its priming fingerprint. Methods: Photosynthesis was determined by fluorescence; plant protection was evaluated by relative disease incidence, enzyme activities by specific colorimetric assays and gene expression by qPCR. Changes in Reactive Oxygen Species (ROS) scavenging cycle enzymes and pathogenesis related protein activity and expression were determined as metabolic and genetic markers of induction of systemic resistance. Results: N21.4 significantly protected plants and increased dry weight. Growth increase is supported by significant increases in photochemical quenching together with significant decreases in energy dissipation (Non-Photochemical Quenching, NPQ). Protection was associated with changes in ROS scavenging cycle enzymes, which were significantly increased on N21.4 + pathogen challenged plants, supporting the priming effect. Superoxide Dismutase (SOD) was a good indicator of biotic stress, showing similar levels in pathogen- and N21.4-treated plants. Similarly, the activity of defense-related enzymes, ß-1,3-glucanase and chitinase significantly increased in post-pathogen challenge state; changes in gene expression were not coupled to activity. Conclusions: protection does not compromise plant growth; N21.4 priming fingerprint is defined by enhanced photochemical quenching and decreased energy dissipation, enhanced chlorophylls, primed ROS scavenging cycle enzyme activity, and glucanase and chitinase activity.

Modes of Action of Microbial Biocontrol in the Phyllosphere

A fast-growing field of research focuses on microbial biocontrol in the phyllosphere. Phyllosphere microorganisms possess a wide range of adaptation and biocontrol factors, which allow them to adapt to the phyllosphere environment and inhibit the growth of microbial pathogens, thus sustaining plant health. These biocontrol factors can be categorized in direct, microbe-microbe, and indirect, host-microbe, interactions. This review gives an overview of the modes of action of microbial adaptation and biocontrol in the phyllosphere, the genetic basis of the mechanisms, and examples of experiments that can detect these mechanisms in laboratory and field experiments. Detailed insights in such mechanisms are key for the rational design of novel microbial biocontrol strategies and increase crop protection and production. Such novel biocontrol strategies are much needed, as ensuring sufficient and consistent food production for a growing world population, while protecting our environment, is one of the biggest challenges of our time.

Role of plant defence in alfalfa during symbiosis

During effective symbiosis, rhizobia colonize their hosts, and avoid plant defence mechanisms. To determine whether the host defence responses can be elicited by the symbiotic bacteria, specific markers involved in incompatible pathogenic interactions are required. The available markers of alfalfa defence mechanisms are described and their use in the study of the symbiotic interaction discussed. As defence-related gene expression in roots is not always related to defence mechanisms, other model systems have been established allowing confirmation of an important role of bacterial surface components in alfalfa-Rhizobium meliloti interactions. Nod factors at high concentrations have been shown to elicit defence-like responses in Medicago cell suspensions and roots. Elicitation of defence mechanisms by high levels of Nod factors in Rhizobium-infected roots may be a part of the mechanism by which nodulation is feed-back regulated.

Glucan-rich diet is digested and taken up by the carnivorous sundew (Drosera rotundifolia L.): implication for a novel role of plant β-1,3-glucanases

Carnivory in plants evolved as an adaptation strategy to nutrient-poor environments. Thanks to specialized traps, carnivorous plants can gain nutrients from various heterotrophic sources such as small insects. Digestion in traps requires a coordinated action of several hydrolytic enzymes that break down complex substances into simple absorbable nutrients. Among these, several pathogenesis-related proteins including β-1,3-glucanases have previously been identified in digestive fluid of some carnivorous species. Here we show that a single acidic endo-β-1,3-glucanase of ~50 kDa is present in the digestive fluid of the flypaper-trapped sundew (Drosera rotundifolia L.). The enzyme is inducible with a complex plant β-glucan laminarin from which it releases simple saccharides when supplied to leaves as a substrate. Moreover, thin-layer chromatography of digestive exudates showed that the simplest degradation products (especially glucose) are taken up by the leaves. These results for the first time point on involvement of β-1,3-glucanases in digestion of carnivorous plants and demonstrate the uptake of saccharide-based compounds by traps. Such a strategy could enable the plant to utilize other types of nutritional sources e.g., pollen grains, fungal spores or detritus from environment. Possible multiple roles of β-1,3-glucanases in the digestive fluid of carnivorous sundew are also discussed.

Stress responses of tomato fruit tissue submitted to massive doses of ionising radiation

In plant tissue, massive doses of ionising radiation (0.5–3 kGy) induce an oxidative burst due to the overproduction of oxygen-centred free radicals. Changes in the protein metabolism of cherry tomato fruits were investigated in response to this peculiar stress. Although DNA damage definitively arrested cell division, the changes observed on a short-term basis were attributed to genetic regulation. Changes in protein metabolism were also maintained long term. Gamma-induced proteins (GIPs) were classified according to their induction kinetics. Group 1 proteins were induced immediately after the treatment and their synthesis was stopped within 24 h. During the same time period, global protein synthesis was restored and a new set of GIPs was induced. The function of these proteins is not yet known; but they may be involved in physiological disorders triggered by irradiation or in repair processes. Short-term typical changes involve the synthesis of ACC synthase – the ethylene pathway regulating enzyme - and most probably of some LMW-HSPs. A non-relevant response to irradiation has also been discovered, namely the long-term accumulation of chitinases. Irradiation induces both specific and non-specific responses which can be analysed by comparison with other types of oxidative stress and some GIPs seem to be specific to the treatment. The ability of irradiation to induce such different responses might be profitably applied for a better understanding of the oxidative mechanisms involved in signal transduction during environmental stress situations.

Interactions between engineered tomato plants expressing antifungal enzymes and nontarget fungi in the rhizosphere and phyllosphere

The introduction of genetically modified (GM) plants in agroecosystems raises concern about possible effects on nontarget species. The impact of a tomato line transformed for constitutive expression of tobacco beta-1,3-glucanase and chitinase on indigenous nonpathogenic fungi was investigated. In greenhouse experiments, no significant differences were found in the colonization by arbuscular mycorrhizal fungi. Diversity indices computed from over 20 500 colonies of culturable rhizosphere and phyllosphere saprotrophic microfungi, assigned to 165 species (plus > 80 sterile morphotypes), showed no significant differences between GM and wild-type plants. Differences were found by discriminant analysis in both the rhizosphere and the phyllosphere, but such effects were minor compared with those linked to different plant growth stages.

In vitro antifungal activity and mechanism of action of chitinase against four plant pathogenic fungi

To determine why chitinase has different antifungal activity on different pathogenic fungi in vitro, we purified recombinant rice chitinase from Pichia pastoris and investigated its antifungal activity against four fungi - Rhizopus stolonifer (Ehrenb. et Fr.) Vuill, Botrytis squamosa Walker, Pythium aphanidermatum (eds.) Fitzp, and Aspergillus niger van Tiegh. Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) were used to analyze the surface microstructure and proportion of chitin in the cell wall of the four fungi, respectively. The results showed that the chitinase exhibited different antifungal activities against the four fungi, which was directly correlated to the surface microstructure and the proportion of chitin in the fungal cell wall. It will help understanding the antifungal mechanism of the recombinant chitinase and further determining its application scope on crop protection and post-harvest storage of fruits and vegetables.

Phytophthora infestans-triggered response of growth- and defense-related genes in potato cultivars with different levels of resistance under the influence of nitrogen availability

The effects of high and low N concentrations on the Solanum tuberosum-Phytophthora infestans interaction were studied in the potato cultivars Bettina, New York 121, Indira and Arkula, which exhibited different levels of resistance. Aboveground biomass and Chl and N content were significantly higher in all cultivars grown in higher N environments, while C:N ratios were lower, confirming successful application of N. High availability of N significantly increased susceptibility of three of the four potato cultivars, and amounts of pathogen within the infected leaflets determined in a quantitative real-time reverse transcriptase-polymerase chain reaction reflected this. Differential gene expression of P. infestans-induced and -repressed genes derived from three subtracted cDNA libraries at 0, 24, 48 and 72 h post-inoculation was studied in parallel. P. infestans attack led to an induction of defense-related and at the same time repression of growth-related potato genes mainly encoding photosynthetic genes. High N supply led to higher transcript abundance of photosynthetic genes such as Chl a/b-binding protein and ribulose bisphosphate carboxylase. N-dependent suppression of defense-related compounds in absence of the pathogen was not observed. Better N nutrition appeared to allow the plants to invest more resources in defense reactions.

Cell wall and membrane-associated exo-beta-D-glucanases from developing maize seedlings

A beta-D-glucan exohydrolase was purified from the cell walls of developing maize (Zea mays L.) shoots. The cell wall enzyme preferentially hydrolyzes the non-reducing terminal glucosyl residue from (1-->3)-beta-D-glucans, but also hydrolyzes (1-->2)-, (1-->6)-, and (1-->4)-beta-D-glucosyl units in decreasing order of activity. Polyclonal antisera raised against the purified exo-beta-D-glucanase (ExGase) were used to select partial-length cDNA clones, and the complete sequence of 622 amino acid residues was deduced from the nucleotide sequences of the cDNA and a full-length genomic clone. Northern gel-blot analysis revealed what appeared to be a single transcript, but three distinct polypeptides were detected in immunogel-blot analyses of the ExGases extracted from growing coleoptiles. Two polypeptides appear in the cell wall, where one polypeptide is constitutive, and the second appears at the time of the maximum rate of elongation and reaches peak activity after elongation has ceased. The appearance of the second polypeptide coincides with the disappearance of the mixed-linkage (1-->3), (1-->4)-beta-D-glucan, whose accumulation is associated with cell elongation in grasses. The third polypeptide of the ExGase is an extrinsic protein associated with the exterior surface of the plasma membrane. Although the activity of the membrane-associated ExGase is highest against (1-->3)-beta-D-glucans, the activity against (1-->4)-beta-D-glucan linkages is severely attenuated and, therefore, the enzyme is unlikely to be involved with turnover of the (1-->3), (1-->4)-beta-D-glucan. We propose three potential functions for this novel ExGase at the membrane-wall interface.

Expression of Endochitinase from Trichoderma harzianum in Transgenic Apple Increases Resistance to Apple Scab and Reduces Vigor

ABSTRACT The goal of this research was to improve scab resistance of apple by transformation with genes encoding chitinolytic enzymes from the bio-control organism Trichoderma harzianum. The endochitinase gene, as cDNA and genomic clones, was transferred into apple cv. Marshall McIntosh by Agrobacterium-transformation. A total of 15 lines were identified as transgenic by NPTII enzyme-linked immunosorbent assay and polymerase chain reaction and confirmed by Southern analysis. Substantial differences in endochitinase activity were detected among different lines by enzymatic assay and western analysis. Eight lines propagated as grafted and own-rooted plants were inoculated with Venturia inaequalis. Six of these transgenic lines expressing endochitinase were more resistant than nontransformed cv. Marshall McIntosh. Disease severity compared with cv. Marshall McIntosh was reduced by 0 to 99.7% (number of lesions), 0 to 90% (percentage of leaf area infected), and 1 to 56% (conidia recovered) in the transgenic lines tested. Endochitinase also had negative effects on the growth of both inoculated and uninoculated plants. There was a significant negative correlation between the level of endochitinase production and both the amount of disease and plant growth.

Rationale and perspectives on the development of fungicides

Fungicides continue to be essential for the effective control of plant diseases. New classes of fungicides with novel modes of action are being developed in the 1990s. These include the strobilurins, phenylpyrroles, anilinopyrimidines, phenoxyquinolines, and compounds that trigger defense mechanisms in the plant. For the foreseeable future, new toxophores will be identified through a process of random screening, with natural products representing a rich source of fungicide leads. Progress is being made in the development of high-throughput screens comprised of target enzyme sites or cell-based assays; these techniques will improve the probability of discovery. Following the identification of suitable leads, biorational design is used to optimize specific properties. In vivo glasshouse screens and field trials are expected to remain the dominant methods for characterizing new compounds. Low toxicity to humans and wildlife, low environmental impact, low residues in food, and compatibility with integrated pest management (IPM) programs are increasingly important considerations in the selection of fungicides for development.

Expression of beta-1,3-glucanase and chitinase in healthy, stem-rust-affected and elicitor-treated near-isogenic wheat lines showing Sr5-or Sr24-specified race-specific rust resistance

Pathogenesis-related expression of the two antifungal hydrolases beta-1,3-glucanase (EC 3.2.1.39) and chitinase (EC 3.2.1.14) was studied in wheat (Triticum aestivum L.) as part of the defence response to stem rust (Puccinia graminis f.sp. tritici, Pgt), mediated by the semi-dominantly acting resistance genes Sr5 and Sr24. Complete resistance (infection type 0), mediated by the Sr5 gene in cultivar Pre-Sr5, closely correlates with the hypersensitive response of penetrated cells at early stage of the interaction, when the first haustorium is formed. In contrast, cultivar Pre-Sr24 shows intermediate resistance (infection type 2-3) which is not directly linked to cell death. In both cases, the plant response included a rapid increase in beta-1,3-glucanase activity between 24 and 48 h after inoculation. One main extracellular 30-kDa isform of beta-1,3-glucanase was present in both lines, as shown by polyacrylamide-gel electrophoresis. Two additional minor isoforms (32 and 23 kDa) were detected only in Pre-Sr24, and only at later time points. Increased enzme activity and the appearance of new isoforms in the resistance lines was preceded by accumulation of mRNAs encoding beta-1,3-glucanase and chitinases. However, there were no changes in chitinase activity or isoforms. A high constitutive level of chitinase activity was observed in all wheat genotypes. Serological studies indicated the presence of a class II chitinase of 26 kDa. Accumulation of beta-1,3-glucanase and chitinase transcripts was detected before the pathogen penetrated the leaves through stomata and approximately 16 h before the typical hypersensitive response was observed, indicating that signal(s) for defense gene activation were recognised by the host plant long before a tight contact between the pathogen and a host cell is established. A glycoprotein (Pgt elicitor) derived from hyphal walls, strongly induced beta-1,3-glucanase. We discuss the possible role of the elicitor in the early signalling mediating Sr5- and Sr24-specified resistance in wheat.