An Update on Jacalin-Like Lectins and Their Role in Plant Defense

Lectin Sequence Distribution in QTLs from Rice (Oryza sativa) Suggest A Role in Morphological Traits and Stress Responses

Rice (Oryza sativa) is one of the main staple crops worldwide but suffers from important yield losses due to different abiotic and biotic stresses. Analysis of quantitative trait loci (QTL) is a classical genetic method which enables the creation of more resistant cultivars but does not yield information on the genes directly involved or responsible for the desired traits. Lectins are known as proteins with diverse functions in plants. Some of them are abundant proteins in seeds and are considered as storage/defense proteins while other lectins are known as stress-inducible proteins, implicated in stress perception and signal transduction as part of plant innate immunity. We investigated the distribution of lectin sequences in different QTL related to stress tolerance/resistance, morphology, and physiology through mapping of the lectin sequences and QTL regions on the chromosomes and subsequent statistical analysis. Furthermore, the domain structure and evolutionary relationships of the lectins in O. sativa spp. indica and japonica were investigated. Our results revealed that lectin sequences are statistically overrepresented in QTLs for (a)biotic resistance/tolerance as well as in QTLs related to economically important traits such as eating quality and sterility. These findings contribute to the characterization of the QTL sequences and can provide valuable information to the breeders.

Overview of the Structure⁻Function Relationships of Mannose-Specific Lectins from Plants, Algae and Fungi

To date, a number of mannose-binding lectins have been isolated and characterized from plants and fungi. These proteins are composed of different structural scaffold structures which harbor a single or multiple carbohydrate-binding sites involved in the specific recognition of mannose-containing glycans. Generally, the mannose-binding site consists of a small, central, carbohydrate-binding pocket responsible for the "broad sugar-binding specificity" toward a single mannose molecule, surrounded by a more extended binding area responsible for the specific recognition of larger mannose-containing N-glycan chains. Accordingly, the mannose-binding specificity of the so-called mannose-binding lectins towards complex mannose-containing N-glycans depends largely on the topography of their mannose-binding site(s). This structure⁻function relationship introduces a high degree of specificity in the apparently homogeneous group of mannose-binding lectins, with respect to the specific recognition of high-mannose and complex N-glycans. Because of the high specificity towards mannose these lectins are valuable tools for deciphering and characterizing the complex mannose-containing glycans that decorate both normal and transformed cells, e.g., the altered high-mannose N-glycans that often occur at the surface of various cancer cells.

The Complement System of Agnathans

Agnathans (lamprey and hagfish) are a group of primitive jawless fish. Jawed vertebrates possess adaptive immunity including immunoglobulins, while agnathans lack immunoglobulins but have alternative adaptive immunity in which variable lymphocyte receptors (VLRs) function as antibodies. The complement system consists of many proteins involved in the elimination of pathogens. In mammals, it is activated via the three different pathways, resulting in the generation of C3b followed by the lytic pathway. Complement components including C3, mannose-binding lectin (MBL), and MBL-associated serine proteases (MASP) of the lectin pathway and factor B of the alternative pathway have been identified from lamprey and/or hagfish, while lytic pathway components have not been identified. In mammals, C1q binds to IgM/IgG-antigen complexes and activates the classical pathway in association with C1r and C1s. Lamprey also has C1q (LC1q), but its function differs from that of mammalian C1q. LC1q acts as a lectin and activates C3 in association with MASP via the lectin pathway. Furthermore, LC1q may interact with a secreted type of VLR (VLRB) in complex with antigens and mediate activation of C3, potentially via MASP, leading to cytolysis. Cytolysis is mediated by a newly identified serum protein named lamprey pore-forming protein (LPFP). In conclusion, lamprey has a complement activation pathway, which could be regarded as the classical pathway and also has a cytolytic system that is distinct from the mammalian lytic pathway. Thus, it appears that the complement system of agnathans is very unique and may have developed independently from jawed vertebrates.

Internalization of a sunflower mannose-binding lectin into phytopathogenic fungal cells induces cytotoxicity

Lectins are carbohydrate-affinity proteins with the ability to recognize and reversibly bind specific glycoconjugates. We have previously isolated a bioactive sunflower mannose-binding lectin belonging to the jacalin-related family called Helja. Despite of the significant number of plant lectins described in the literature, only a small group exhibits antifungal activity and the mechanism by which they kill fungi is still not understood. The aim of this work was to explore Helja activity on plant pathogenic fungi, and provide insights into its mechanism of action. Through cellular and biochemical experimental approaches, here we show that Helja exerts an antifungal effect on Sclerotinia sclerotiorum, a sunflower pathogen. The lectin interacts with the fungal spore surface, permeabilizes its plasma membrane, can be internalized into the cell and induces oxidative stress, finally leading to the cell death. On the other hand, Helja is inactive towards Fusarium solani, a non-pathogen of sunflower, showing the selective action of the lectin. The mechanistic basis for the antifungal activity of an extracellular jacalin lectin is presented, suggesting its initial interaction with fungal cell wall carbohydrates and further internalization. The implication of our findings for plant defense is discussed.

Targeting the Immune System with Plant Lectins to Combat Microbial Infections

The arsenal of drugs available to treat infections caused by eukaryotic and prokaryotic microbes has been declining exponentially due to antimicrobial resistance phenomenon, leading to an urgent need to develop new therapeutic strategies. Host-directed immunotherapy has been reported as an attractive option to treat microbial infections. It consists in the improvement of host defenses by increasing the expression of inflammatory mediators and/or controlling of inflammation-induced tissue injury. Although the in vitro antimicrobial and immunomodulatory activities of lectins have been extensively demonstrated, few studies have evaluated their in vivo effects on experimental models of infections. This review aims to highlight the experimental use of immunomodulatory plant lectins to improve the host immune response against microbial infections. Lectins have been used in vivo both prophylactically and therapeutically resulting in the increased survival of mice under microbial challenge. Other studies successfully demonstrated that lectins could be used in combination with parasite antigens in order to induce a more efficient immunization. Therefore, these plant lectins represent new candidates for management of microbial infections. Furthermore, immunotherapeutic studies have improved our knowledge about the mechanisms involved in host–pathogen interactions, and may also help in the discovery of new drug targets.