A Phytopathogenic Cysteine Peptidase from Latex of Wild Rubber Vine Cryptostegia grandiflora

Review: Laticifer as a plant defense mechanism

Laticifers have been utilized as paradigms to enhance comprehension of specific facets of plant ecology and evolution. From the beginning of seedling growth, autonomous laticifer networks are formed throughout the plant structure, extending across all tissues and organs. The vast majority of identified products resulting from laticifer chemistry and metabolism are linked to plant defense. The latex, which is the fluid contained within laticifers, is maintained under pressure and has evolved to serve as a defense mechanism against both aggressors and invaders, irrespective of their capabilities or tactics. Remarkably, the latex composition varies among different species. The current goal is to understand the specific functions of various latex components in combating plant enemies. Therefore, the study of latex's chemical composition and proteome plays a critical role in advancing our understanding about plant defense mechanisms. Here, we will discuss some of these aspects.

Combined Effect of High Hydrostatic Pressure and Proteolytic Fraction P1G10 from Vasconcellea cundinamarcensis Latex against Botrytis cinerea in Grape Juice

The effect of high hydrostatic pressure (HHP) and the proteolytic fraction P1G10 from papaya latex was studied to find out whether a synergy exists in the growth inhibition of Botrytis cinerea in grape juice, contributing to the improvement of conservation techniques and extending the shelf life and quality of food products. Grape juice (GJ) diluted to 16 °Brix with a water activity (aw) of 0.980 was prepared from a concentrated GJ and used in this study. Results indicated a 92% growth inhibition of B. cinerea when exposed to 1 mg/mL of P1G10 and 250 MPa/4 min of pressure treatment. The proximate composition and antioxidant compounds present in the GJ were not significantly affected after the treatments. Eight phenolic compounds and two flavonoids in GJ were identified and quantified, with values fluctuating between 12.77 ± 0.51 and 240.40 ± 20.9 mg/L in the control sample (0.1 MPa). The phenolic compounds showed a significant decrease after the applied treatments, with the HHP sample having a content of 65.4 ± 6.9 mg GAE/100 mL GJ. In conclusion, a synergistic effect at moderate HHP of 250 MPa/4 min with the addition of P1G10 was observed, and the successful development of a stable and acceptable GJ product was possible.

Identification, characterization, and antifungal activity of cysteine peptidases from Calotropis procera latex

Cysteine peptidases (EC 3.4.22) are the most abundant enzymes in latex fluids. However, their physiological functions are still poorly understood, mainly related to defense against phytopathogens. The present study reports the cDNA cloning and sequencing of five undescribed cysteine peptidases from Calotropis procera (Aiton) Dryand (Apocynaceae) as well as some in silico analyses. Of these, three cysteine peptidases (CpCP1, CpCP2, and CpCP3) were purified. Their enzymatic kinetics were determined and they were assayed for their efficacy in inhibiting the hyphal growth of phytopathogenic fungi. The mechanism of action was investigated by fluorescence and atomic force microscopy as well as by induction of reactive oxygen species (ROS). The deduced amino acid sequences showed similar biochemical characteristics and high sequence homology with several other papain-like cysteine peptidases. Three-dimensional models showed two typical cysteine peptidase domains (L and R domains), forming a "V-shaped" active site containing the catalytic triad (Cys, His, and Asn). Proteolysis of CpCP1 was higher at pH 7.0, whereas for CpCP2 and CpCP3 it was higher at 7.5. All peptidases exhibited optimum activity at 35 °C and followed Michaelis-Menten kinetics. However, the major difference among them was that CpCP1 exhibited highest V_max, K_m, K_cat and catalytic efficiency. All peptidases were deleterious to the two fungi tested, with IC₅₀ of around 50 μg/mL. The peptidases promoted membrane permeabilization, morphological changes with leakage of cellular content, and induction of ROS in F. oxysporum spores. These results corroborate the hypothesis that latex cysteine peptidases play a role in defense against fungi.

Antifungal Proteins from Plant Latex

Latex, a milky fluid found in several plants, is widely used for many purposes, and its proteins have been investigated by researchers. Many studies have shown that latex produced by some plant species is a natural source of biologically active compounds, and many of the hydrolytic enzymes are related to health benefits. Research on the characterization and industrial and pharmaceutical utility of latex has progressed in recent years. Latex proteins are associated with plants' defense mechanisms, against attacks by fungi. In this respect, there are several biotechnological applications of antifungal proteins. Some findings reveal that antifungal proteins inhibit fungi by interrupting the synthesis of fungal cell walls or rupturing the membrane. Moreover, both phytopathogenic and clinical fungal strains are susceptible to latex proteins. The present review describes some important features of proteins isolated from plant latex which presented in vitro antifungal activities: protein classification, function, molecular weight, isoelectric point, as well as the fungal species that are inhibited by them. We also discuss their mechanisms of action.

Laticifers, Latex, and Their Role in Plant Defense

Latex, a sap produced by cells called laticifers, occurs in plants of wide taxonomic diversity. Plants exude latex sap in response to physical damage. Questions about the function of latex or the underlying mechanisms persist, but a role in defense is likely. The presence of constitutive peptidases in latex sap in addition to inducible and de novo synthesized pathogenesis-related proteins (PR-proteins), raises the question about the role that each sap component plays to protect plants and how synergism occurs among sap proteins in the course of herbivory or infection. Here we discuss a variety of functions for laticifer and latex in plant defense. We propose that latex peptidases build the front line of defense against herbivores or pathogens.

Antimicrobial peptides from different plant sources: Isolation, characterisation, and purification

Antimicrobial peptides (AMPs), the self-defence products of organisms, are extensively distributed in plants. They can be classified into several groups, including thionins, defensins, snakins, lipid transfer proteins, glycine-rich proteins, cyclotides and hevein-type proteins. AMPs can be extracted and isolated from different plants and plant organs such as stems, roots, seeds, flowers and leaves. They perform various physiological defensive mechanisms to eliminate viruses, bacteria, fungi and parasites, and so could be used as therapeutic and preservative agents. Research on AMPs has sought to obtain more detailed and reliable information regarding the selection of suitable plant sources and the use of appropriate isolation and purification techniques, as well as examining the mode of action of these peptides. Well-established AMP purification techniques currently used include salt precipitation methods, absorption-desorption, a combination of ion-exchange and reversed-phase C18 solid phase extraction, reversed-phase high-performance liquid chromatography (RP-HPLC), and the sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) method. Beyond these traditional methods, this review aims to highlight new and different approaches to the selection, characterisation, isolation, purification, mode of action and bioactivity assessment of a range of AMPs collected from plant sources. The information gathered will be helpful in the search for novel AMPs distributed in the plant kingdom, as well as providing future directions for the further investigation of AMPs for possible use on humans.

The Latex Protein MLX56 from Mulberry (Morus multicaulis) Protects Plants against Insect Pests and Pathogens

Biotic stresses are major constraints limiting the leaf quality and productivity of mulberry. MLX56 is a unique chitin-binding protein isolated from Shin-Ichinose (Morus alba) latex that displays toxicity against lepidopteran caterpillars. In this study, the full-length cDNA encoding MLX56 was isolated from Husang 32 (M. multicaulis) and designated HMLX56. Amino acid sequence analysis and protein modeling of three MLX56 proteins showed that they were highly conserved among Morus species. Tissue expression pattern analysis showed that the HMLX56 gene was strongly expressed in mulberry bark and leaves but only slightly expressed in fruits. In addition, analysis of GUS expression indicated that the promoter of HMLX56 showed higher transcriptional activity along the vascular strands, and its activity can be regulated by various environmental factors. Like the MLX56 protein from M. alba, the HMLX56 protein showed toxicity to Plutella xylostella. Moreover, when the HMLX56 gene was ectopically expressed in Arabidopsis, the transgenic plants showed enhanced resistance to aphids, the fungal pathogen Botrytis cinerea and the bacterial pathogen Pseudomonas syringae pv. tomato DC3000. Our data suggest that the HMLX56 protein has a lectin-like molecular structure consisting of two hevein-like chitin-binding domains which provide not only chitin-binding activities but also other mechanisms of defense. The information provided here improves our understanding of the potential functions and defense mechanisms of MLX56 proteins, enabling in-depth functional analysis of latex exudates and perhaps facilitating mulberry genetic improvement in the future.

First insights into the diversity and functional properties of chitinases of the latex of Calotropis procera

Chitinases (EC 3.2.1.14) found in the latex of Calotropis procera (Ait) R. Br. were studied. The proteins were homogeneously obtained after two ion exchange chromatography steps. Most proteins were identified individually in 15 spots on 2-D gel electrophoresis with isoelectric points ranging from 4.6 to 6.0 and molecular masses extending from 27 to 30 kDa. Additionally, 66 kDa proteins were identified as chitinases in SDS-PAGE. Their identities were further confirmed by mass spectrometry (MS) analysis of the tryptic digests of each spot and MS analysis of the non-digested proteins. Positive reaction for Schiff's reagent suggested the proteins are glycosylated. The chitinases exhibited high catalytic activity toward to colloidal chitin at pH 5.0, and this activity underwent decay in the presence of increasing amounts of reducing agent dithiothreitol. Spore germination and hyphae growth of two phytopathogenic fungi were inhibited only marginally by the chitinases but were affected differently. This suggested a complex relationship might exist between the specificity of the proteins toward the fungal species. The chitinases showed potent insecticidal activity against the Bruchidae Callosobruchus maculatus, drastically reducing survival, larval weight and adult emergence. It is concluded that closely related chitinases are present in the latex of C. procera, and the first experimental evidence suggests these proteins are involved more efficiently in defence strategies against insects rather than fungi.

Proteomic analysis and purification of an unusual germin-like protein with proteolytic activity in the latex of Thevetia peruviana

The latex from Thevetia peruviana is rich in plant defense proteins, including a 120 kDa cysteine peptidase with structural characteristics similar to germin-like proteins. More than 20,000 plant species produce latex, including Apocynaceae, Sapotaceae, Papaveraceae and Euphorbiaceae. To better understand the physiological role played by latex fluids, a proteomic analysis of Thevetia peruviana (Pers.) Schum latex was performed using two-dimensional gel electrophoresis and mass spectrometry. A total of 33 proteins (86 %) were identified, including storage proteins, a peptidase inhibitor, cysteine peptidases, peroxidases and osmotins. An unusual cysteine peptidase, termed peruvianin-I, was purified from the latex by a single chromatographic step involving gel filtration. The enzyme (glycoprotein) was inhibited by E-64 and iodoacetamide and exhibited high specific activity towards azocasein (K m 17.6 µM), with an optimal pH and temperature of 5.0-6.0 and 25-37 °C, respectively. Gel filtration chromatography, two-dimensional gel electrophoresis, and mass spectrometry revealed that peruvianin-I possesses 120 kDa, pI 4.0, and six subunits (20 kDa). A unique N-terminal amino acid sequence was obtained to oligomer and monomers of peruvianin-I (1ADPGPLQDFCLADLNSPLFINGYPCRNPALAISDDF36). High-resolution images from atomic force microscopy showed the homohexameric structure of peruvianin-I may be organized as a trimer of dimers that form a central channel similar to germin-like proteins. Peruvianin-I exhibited no oxalate oxidase and superoxide dismutase activity or antifungal effects. Peruvianin-I represents the first germin-like protein (GLP) with cysteine peptidase activity, an activity unknown in the GLP family so far.

New constitutive latex osmotin-like proteins lacking antifungal activity

Proteins that share similar primary sequences to the protein originally described in salt-stressed tobacco cells have been named osmotins. So far, only two osmotin-like proteins were purified and characterized of latex fluids. Osmotin from Carica papaya latex is an inducible protein lacking antifungal activity, whereas the Calotropis procera latex osmotin is a constitutive antifungal protein. To get additional insights into this subject, we investigated osmotins in latex fluids of five species. Two potential osmotin-like proteins in Cryptostegia grandiflora and Plumeria rubra latex were detected by immunological cross-reactivity with polyclonal antibodies produced against the C. procera latex osmotin (CpOsm) by ELISA, Dot Blot and Western Blot assays. Osmotin-like proteins were not detected in the latex of Thevetia peruviana, Himatanthus drasticus and healthy Carica papaya fruits. Later, the two new osmotin-like proteins were purified through immunoaffinity chromatography with anti-CpOsm immobilized antibodies. Worth noting the chromatographic efficiency allowed for the purification of the osmotin-like protein belonging to H. drasticus latex, which was not detectable by immunoassays. The identification of the purified proteins was confirmed after MS/MS analyses of their tryptic digests. It is concluded that the constitutive osmotin-like proteins reported here share structural similarities to CpOsm. However, unlike CpOsm, they did not exhibit antifungal activity against Fusarium solani and Colletotrichum gloeosporioides. These results suggest that osmotins of different latex sources may be involved in distinct physiological or defensive events.

Plant antifungal proteins and their applications in agriculture

Fungi are far more complex organisms than viruses or bacteria and can develop numerous diseases in plants that cause loss of a substantial portion of the crop every year. Plants have developed various mechanisms to defend themselves against these fungi which include the production of low-molecular-weight secondary metabolites and proteins and peptides with antifungal activity. In this review, families of plant antifungal proteins (AFPs) including defensins, lectins, and several others will be summarized. Moreover, the application of AFPs in agriculture will also be analyzed.

Biologically active and antimicrobial peptides from plants

Bioactive peptides are part of an innate response elicited by most living forms. In plants, they are produced ubiquitously in roots, seeds, flowers, stems, and leaves, highlighting their physiological importance. While most of the bioactive peptides produced in plants possess microbicide properties, there is evidence that they are also involved in cellular signaling. Structurally, there is an overall similarity when comparing them with those derived from animal or insect sources. The biological action of bioactive peptides initiates with the binding to the target membrane followed in most cases by membrane permeabilization and rupture. Here we present an overview of what is currently known about bioactive peptides from plants, focusing on their antimicrobial activity and their role in the plant signaling network and offering perspectives on their potential application.