Biochemical characterization of a recombinant plant class III chitinase from the pitcher of the carnivorous plant Nepenthes alata

Complex transcription regulation of acidic chitinase suggests fine-tuning of digestive processes in Drosera binata

MAIN CONCLUSION

DbChitI-3, Drosera binata's acidic chitinase, peaks at pH 2.5 from 15 °C to 30 °C. Gene expression is stimulated by polysaccharides and suppressed by monosaccharide digestion, implying a feedback loop in its transcriptional regulation. Here, we characterised a novel chitinase gene (DbChitI-3) isolated from the carnivorous plant species Drosera binata with strong homology to other Drosera species' extracellular class I chitinases with a role in digestive processes. The capability to cleave different forms of chitin was tested using recombinantly produced chitinase in Escherichia coli (rDbChitI-3S-His) and subsequent purification. The recombinant protein did not cleave chitin powder, the mono-, di- and tri- N-acetyl-D-glucosamine substrates, but cleaved acetic acid-swollen chitin. Fluorometric assay with acetic acid-swollen FITC-chitin as a substrate revealed the maximum enzyme activity at pH 2.5, spanning from 15 °C to 30 °C. Comparing enzymatic parameters with commercial chitinase from Streptomyces griseus showed rDbChitI-3S-His efficiency reaching 64.3% of S. griseus chitinase under optimal conditions. The highest basal expression of DbChitI-3 was detected in leaf blades. In other organs, the expression was either fivefold lower (petioles) or almost nondetectable (stems, roots and flowers). Application of gelatin, chitin, and pachyman resulted in a 3.9-, 4.6- and 5.7-fold increase in the mRNA transcript abundance of DbChitI-3 in leaves. In contrast, monosaccharides and laminarin decreased transcription of the DbChitI-3 gene by at least 70%, 5 h after treatment. The simultaneous application of suppressor and inducer (glucose and pachyman) indicated the predominant effect of the suppressor, implying that sufficient monosaccharide nutrients prioritize absorption processes in D. binata leaves over further digestion of the potential substrate.

Enzyme activities in two sister-species of carnivorous pitcher plants (Nepenthes) with contrasting nutrient sequestration strategies

The carnivorous pitcher plants of the genus Nepenthes usually attract, capture and digest arthropod prey to obtain mineral nutrients. But few members of the genus have evolved specialized nutrient sequestration strategies to acquire nitrogen from the faeces and urine of mutualistic mammals, which they attract. Because the plants obtain significant amounts of nitrogen in a more available form, we hypothesized that they have relaxed the production of digestive enzymes. If so, species that digest mammal faeces should show fewer digestive enzymes than closely related species that rely on arthropods. We tested this hypothesis by comparing digestive enzymes in 1) Nepenthes hemsleyana, whose pitchers serve as roosts for the mutualistic woolly bat Kerivoula hardwickii, which also defecate inside the pitchers, and 2) the close relative Nepenthes rafflesiana, a typical arthropod capturing species. To investigate the dynamics of aspartic proteases (nepenthesin I and II) and type III and IV chitinases in both species, we conducted qPCR, western blotting, mass spectrometry, and enzyme activity measurements. We found that mRNA in pitcher tissue and enzyme abundance in the digestive fluid is upregulated in both species in response to faeces and insect feeding. Contrary to our initial hypothesis, the final nepenthesin proteolytic activity in the digestive fluid is higher in response to faeces addition than to insect prey irrespective of Nepenthes species. This indicates that faeces can mimic arthropod prey triggering the production of digestive enzymes and N. hemsleyana retained capacity for production of them.

Expression of Drosera rotundifolia Chitinase in Transgenic Tobacco Plants Enhanced Their Antifungal Potential

In this study, a chitinase gene (DrChit) that plays a role in the carnivorous processes of Drosera rotundifolia L. was isolated from genomic DNA, linked to a double CaMV35S promoter and nos terminator in a pBinPlus plant binary vector, and used for Agrobacterium-mediated transformation of tobacco. RT-qPCR revealed that within 14 transgenic lines analysed in detail, 57% had DrChit transcript abundance comparable to or lower than level of a reference actin gene transcript. In contrast, the transgenic lines 9 and 14 exhibited 72 and 152 times higher expression level than actin. The protein extracts of these two lines exhibited five and eight times higher chitinolytic activity than non-transgenic controls when measured in a fluorimetric assay with FITC-chitin. Finally, the growth of Trichoderma viride was obviously suppressed when the pathogen was exposed to 100 μg of crude protein extract isolated from line 9 and line 14, with the area of mycelium growth reaching only 56.4% and 45.2%, of non-transgenic control, respectively. This is the first time a chitinase from a carnivorous plant with substrate specificity for long chitin polymers was tested in a transgenic plant with the aim of exploring its antifungal potential.

Regulation of enzyme activities in carnivorous pitcher plants of the genus Nepenthes

Nepenthes regulates enzyme activities by sensing stimuli from the insect prey. Protein is the best inductor mimicking the presence of an insect prey. Carnivorous plants of the genus Nepenthes have evolved passive pitcher traps for prey capture. In this study, we investigated the ability of chemical signals from a prey (chitin, protein, and ammonium) to induce transcription and synthesis of digestive enzymes in Nepenthes × Mixta. We used real-time PCR and specific antibodies generated against the aspartic proteases nepenthesins, and type III and type IV chitinases to investigate the induction of digestive enzyme synthesis in response to different chemical stimuli from the prey. Transcription of nepenthesins was strongly induced by ammonium, protein and live prey; chitin induced transcription only very slightly. This is in accordance with the amount of released enzyme and proteolytic activity in the digestive fluid. Although transcription of type III chitinase was induced by all investigated stimuli, a significant accumulation of the enzyme in the digestive fluid was found mainly after protein and live prey addition. Protein and live prey were also the best inducers for accumulation of type IV chitinase in the digestive fluid. Although ammonium strongly induced transcription of all investigated genes probably through membrane depolarization, strong acidification of the digestive fluid affected stability and abundance of both chitinases in the digestive fluid. The study showed that the proteins are universal inductors of enzyme activities in carnivorous pitcher plants best mimicking the presence of insect prey. This is not surprising, because proteins are a much valuable source of nitrogen, superior to chitin. Extensive vesicular activity was observed in prey-activated glands.

Discovery of digestive enzymes in carnivorous plants with focus on proteases

BACKGROUND

Carnivorous plants have been fascinating researchers with their unique characters and bioinspired applications. These include medicinal trait of some carnivorous plants with potentials for pharmaceutical industry.

METHODS

This review will cover recent progress based on current studies on digestive enzymes secreted by different genera of carnivorous plants: Drosera (sundews), Dionaea (Venus flytrap), Nepenthes (tropical pitcher plants), Sarracenia (North American pitcher plants), Cephalotus (Australian pitcher plants), Genlisea (corkscrew plants), and Utricularia (bladderworts).

RESULTS

Since the discovery of secreted protease nepenthesin in Nepenthes pitcher, digestive enzymes from carnivorous plants have been the focus of many studies. Recent genomics approaches have accelerated digestive enzyme discovery. Furthermore, the advancement in recombinant technology and protein purification helped in the identification and characterisation of enzymes in carnivorous plants.

DISCUSSION

These different aspects will be described and discussed in this review with focus on the role of secreted plant proteases and their potential industrial applications.

Nepenthes: State of the art of an inspiring plant for biotechnologists

Plant carnivory results from the adaptation of plants to their environment. The capture and digestion of preys, followed by their assimilation by the plant is a source of additional nutrients to overcome scarce nutrient in poor soils. Nepenthes are highly studied carnivorous plants and have developed a number of ecological traits which have attracted the attention of plant biologists. Multiple adaptive strategies developed by these plants make them a source of inspiration for many applications ranging from therapeutic treatments to biocontrol solution in agriculture. The outstanding tissue organization of the digestive pitcher can help to create new and original materials usable in everyday life. In this review article, we propose a state of the art of the latest studies carried out on these particular plants and we establish a list of potential tracks for their exploitation.

Structural and functional characterisation of a class I endochitinase of the carnivorous sundew (Drosera rotundifolia L.)

Chitinase gene from the carnivorous plant, Drosera rotundifolia , was cloned and functionally characterised. Plant chitinases are believed to play an important role in the developmental and physiological processes and in responses to biotic and abiotic stress. In addition, there is growing evidence that carnivorous plants can use them to digest insect prey. In this study, a full-length genomic clone consisting of the 1665-bp chitinase gene (gDrChit) and adjacent promoter region of the 698 bp in length were isolated from Drosera rotundifolia L. using degenerate PCR and a genome-walking approach. The corresponding coding sequence of chitinase gene (DrChit) was obtained following RNA isolation from the leaves of aseptically grown in vitro plants, cDNA synthesis with a gene-specific primer and PCR amplification. The open reading frame of cDNA clone consisted of 978 nucleotides and encoded 325 amino acid residues. Sequence analysis indicated that DrChit belongs to the class I group of plant chitinases. Phylogenetic analysis within the Caryophyllales class I chitinases demonstrated a significant evolutionary relatedness of DrChit with clade Ib, which contains the extracellular orthologues that play a role in carnivory. Comparative expression analysis revealed that the DrChit is expressed predominantly in tentacles and is up-regulated by treatment with inducers that mimick insect prey. Enzymatic activity of rDrChit protein expressed in Escherichia coli was confirmed and purified protein exhibited a long oligomer-specific endochitinase activity on glycol-chitin and FITC-chitin. The isolation and expression profile of a chitinase gene from D. rotundifolia has not been reported so far. The obtained results support the role of specific chitinases in digestive processes in carnivorous plant species.

Carnivorous Nutrition in Pitcher Plants (Nepenthes spp.) via an Unusual Complement of Endogenous Enzymes

Plants belonging to the genus Nepenthes are carnivorous, using specialized pitfall traps called "pitchers" that attract, capture, and digest insects as a primary source of nutrients. We have used RNA sequencing to generate a cDNA library from the Nepenthes pitchers and applied it to mass spectrometry-based identification of the enzymes secreted into the pitcher fluid using a nonspecific digestion strategy superior to trypsin in this application. This first complete catalog of the pitcher fluid subproteome includes enzymes across a variety of functional classes. The most abundant proteins present in the secreted fluid are proteases, nucleases, peroxidases, chitinases, a phosphatase, and a glucanase. Nitrogen recovery involves a particularly rich complement of proteases. In addition to the two expected aspartic proteases, we discovered three novel nepenthensins, two prolyl endopeptidases that we name neprosins, and a putative serine carboxypeptidase. Additional proteins identified are relevant to pathogen-defense and secretion mechanisms. The full complement of acid-stable enzymes discovered in this study suggests that carnivory in the genus Nepenthes can be sustained by plant-based mechanisms alone and does not absolutely require bacterial symbiosis.

Proteome analysis of digestive fluids in Nepenthes pitchers

BACKGROUND AND AIMS

Carnivorous plants have developed strategies to enable growth in nutrient-poor soils. For the genus Nepenthes, this strategy represents producing pitcher-modified leaves that can trap and digest various prey. These pitchers produce a digestive fluid composed of proteins, including hydrolytic enzymes. The focus of this study was on the identification of these proteins.

METHODS

In order to better characterize and have an overview of these proteins, digestive fluid was sampled from pitchers at different stages of maturity from five species of Nepenthes (N. mirabilis, N. alata, N. sanguinea, N. bicalcarata and N. albomarginata) that vary in their ecological niches and grew under different conditions. Three complementary approaches based on transcriptomic resources, mass spectrometry and in silico analysis were used.

KEY RESULTS

This study permitted the identification of 29 proteins excreted in the pitchers. Twenty of these proteins were never reported in Nepenthes previously and included serine carboxypeptidases, α- and β-galactosidases, lipid transfer proteins and esterases/lipases. These 20 proteins display sequence signals allowing their secretion into the pitcher fluid.

CONCLUSIONS

Nepenthes pitcher plants have evolved an arsenal of enzymes to digest prey caught in their traps. The panel of new proteins identified in this study provides new insights into the digestive process of these carnivorous plants.

Identification and characterization of a novel chitinase with antifungal activity from 'Baozhu' pear (Pyrus ussuriensis Maxim.)

A novel chitinase from the 'Baozhu' pear was found, purified, and characterized in this report. This chitinase was a monomer with a molecular mass of 28.9 kDa. Results of the internal peptide sequence analyses classify this chitinase as a class III chitinase. In the enzymatic hydrolytic assay, this chitinase could hydrolyze chitin derivatives into di-N-acetylchitobiose (GlcNAc2) as a major product in the initial phase, as well as hydrolyze GlcNAc2 into N-acetylglucosamine (GlcNAc), which represents both chitobiosidase and β-N-acetylglucosaminase activity. Biological analyses showed that this chitinase exhibits strong antifungal activity toward agricultural pathogenic fungi. In total, chitinase from 'Baozhu' pear is a novel bifunctional chitinase that could be a potential fungicide in the biological control of plant diseases.

Feeding on prey increases photosynthetic efficiency in the carnivorous sundew Drosera capensis

BACKROUND AND AIMS: It has been suggested that the rate of net photosynthesis (AN) of carnivorous plants increases in response to prey capture and nutrient uptake; however, data confirming the benefit from carnivory in terms of increased AN are scarce and unclear. The principal aim of our study was to investigate the photosynthetic benefit from prey capture in the carnivorous sundew Drosera capensis.

METHODS

Prey attraction experiments were performed, with measurements and visualization of enzyme activities, elemental analysis and pigment quantification together with simultaneous measurements of gas exchange and chlorophyll a fluorescence in D. capensis in response to feeding with fruit flies (Drosophila melanogaster).

KEY RESULTS

Red coloration of tentacles did not act as a signal to attract fruit flies onto the traps. Phosphatase, phophodiesterase and protease activities were induced 24 h after prey capture. These activities are consistent with the depletion of phosphorus and nitrogen from digested prey and a significant increase in their content in leaf tissue after 10 weeks. Mechanical stimulation of tentacle glands alone was not sufficient to induce proteolytic activity. Activities of β-D-glucosidases and N-acetyl-β-D-glucosaminidases in the tentacle mucilage were not detected. The uptake of phosphorus from prey was more efficient than that of nitrogen and caused the foliar N:P ratio to decrease; the contents of other elements (K, Ca, Mg) decreased slightly in fed plants. Increased foliar N and P contents resulted in a significant increase in the aboveground plant biomass, the number of leaves and chlorophyll content as well as AN, maximum quantum yield (Fv/Fm) and effective photochemical quantum yield of photosystem II (ΦPSII).

CONCLUSIONS

According to the stoichiometric relationships among different nutrients, the growth of unfed D. capensis plants was P-limited. This P-limitation was markedly alleviated by feeding on fruit flies and resulted in improved plant nutrient status and photosynthetic performance. This study supports the original cost/benefit model proposed by T. Givnish almost 30 years ago and underlines the importance of plant carnivory for increasing phosphorus, and thereby photosynthesis.

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.