Characterization of a type D1A EUL-related lectin from rice expressed in Pichia pastoris

OsEUL Lectin Gene Expression in Rice: Stress Regulation, Subcellular Localization and Tissue Specificity

The Euonymus lectin (EUL) family is a unique group of carbohydrate-binding proteins that is omnipresent in plants. Sequences encoding EUL-related lectins have been retrieved from all completely sequenced plant genomes. The rice (Oryza sativa) genome contains 5 functional EUL genes referred to as OsEULS2, OsEULS3, OsEULD1a, OsEULD1b, and OsEULD2. In this study we focused on the tissue specific expression, stress inducibility and subcellular localization of the rice EULs. Even though the EUL domain sequence is highly conserved among the rice EULs (at least 80% sequence similarity) different biotic and abiotic stress treatments yielded unique responses for the different EULs. Transcript levels for OsEULs were differentially affected by drought and salt stress, ABA treatment, pathogen infection or insect infestation. Analysis of promoter activity revealed differential expression and tissue specificity for the 5 OsEUL genes, with most expression observed in the vascular system of roots and shoots, as well as in the root tips and seeds. At cell level, all OsEULs are located in the nucleus whereas OsEULD1b and OsEULD2 also locate to the cytoplasm. This paper contributes to the functional characterization of the EULs and provides insight in the biological importance of this family of proteins for rice.

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.

Signaling through plant lectins: modulation of plant immunity and beyond

Lectins constitute an abundant group of proteins that are present throughout the plant kingdom. Only recently, genome-wide screenings have unraveled the multitude of different lectin sequences within one plant species. It appears that plants employ a plurality of lectins, though relatively few lectins have already been studied and functionally characterized. Therefore, it is very likely that the full potential of lectin genes in plants is underrated. This review summarizes the knowledge of plasma membrane-bound lectins in different biological processes (such as recognition of pathogen-derived molecules and symbiosis) and illustrates the significance of soluble intracellular lectins and how they can contribute to plant signaling. Altogether, the family of plant lectins is highly complex with an enormous diversity in biochemical properties and activities.

Lectin domains at the frontiers of plant defense

Plants are under constant attack from pathogens and herbivorous insects. To protect and defend themselves, plants evolved a multi-layered surveillance system, known as the innate immune system. Plants sense their encounters upon perception of conserved microbial structures and damage-associated patterns using cell-surface and intracellular immune receptors. Plant lectins and proteins with one or more lectin domains represent a major part of these receptors. The whole group of plant lectins comprises an elaborate collection of proteins capable of recognizing and interacting with specific carbohydrate structures, either originating from the invading organisms or from damaged plant cell wall structures. Due to the vast diversity in protein structures, carbohydrate recognition domains and glycan binding specificities, plant lectins constitute a very diverse protein superfamily. In the last decade, new types of nucleocytoplasmic plant lectins have been identified and characterized, in particular lectins expressed inside the nucleus and the cytoplasm of plant cells often as part of a specific plant response upon exposure to different stress factors or changing environmental conditions. In this review, we provide an overview on plant lectin motifs used in the constant battle against pathogens and predators during plant defenses.

Transcriptional behavior of EUL-related rice lectins toward important abiotic and biotic stresses

The rice genome encodes several genes for putative carbohydrate-binding proteins belonging to the family of Euonymus related lectins (EULs). This lectin family was discovered recently and evidence shows that the expression of these proteins is subject to multiple environmental stresses. In this study, quantitative reverse transcription PCR (qRT-PCR) was conducted on rice seedlings exposed to various abiotic (150mM NaCl, 100mM mannitol, and 100μM abscisic acid (ABA)) and biotic (Xanthomonas oryzae pv. oryzae and Magnaporthe oryzae) stresses to compare the transcriptional behavior of the EULs and a known stress related lectin Orysata belonging to the family of jacalin-related lectins. All EUL transcripts were strongly up-regulated after ABA and NaCl treatments in the roots whereas the overall expression level was generally lower and more variable in the shoots. Moreover, all abiotic stresses induced Orysata in both tissues except for mannitol treatment which failed to show an effect in the roots. Orysata also strongly accumulated after X. oryzae pv. oryzae infection, as were various D-type EUL lectins. In contrast, some of the EUL proteins, including OrysaEULS3, OrysaEULD1A and OrysaEULD2, as well as Orysata were significantly down-regulated upon M. oryzae attack, suggesting fungal manipulation of these genes. Collectively, our results clearly show that rice expresses multiple carbohydrate-binding proteins in response to a wide variety of abiotic and biotic stress conditions. We hypothesize that the Euonymus related proteins fulfill a prominent role in sensing and responding to multiple environmental cues.