The involvement of Medicago truncatula non-specific lipid transfer protein N5 in the control of rhizobial infection

Legume nodulation and nitrogen fixation require interaction of DnaJ-like protein and lipid transfer protein

The lipid transport protein (LTP) product of the AsE246 gene of Chinese milk vetch (Astragalus sinicus) contributes to the transport of plant-synthesized lipids to the symbiosome membranes (SMs) that are required for nodule organogenesis in this legume. However, the mechanisms used by nodule-specific LTPs remain unknown. In this study, a functional protein in the DnaJ-like family, designated AsDJL1, was identified and shown to interact with AsE246. Immunofluorescence showed that AsDJL1 was expressed in infection threads (ITs) and in nodule cells and that it co-localized with rhizobium, and an immunoelectron microscopy assay localized the protein to SMs. Via co-transformation into Nicotiana benthamiana cells, AsDJL1 and AsE246 displayed subcellular co-localization in the cells of this heterologous host. Co-immunoprecipitation assays confirmed that AsDJL1 interacted with AsE246 in nodules. The essential interacting region of AsDJL1 was determined to be the zinc finger domain at its C-terminus. Chinese milk vetch plants transfected with AsDJL1-RNAi had significantly decreased numbers of ITs, nodule primordia and nodules as well as reduced (by 83%) nodule nitrogenase activity compared with the controls. By contrast, AsDJL1 overexpression led to increased nodule fresh weight and nitrogenase activity. RNAi-AsDJL1 also significantly affected the abundance of lipids, especially digalactosyldiacylglycerol, in early-infected roots and transgenic nodules. Taken together, the results of this study provide insights into the symbiotic functions of AsDJL1, which may participate in lipid transport to SMs and play an essential role in rhizobial infection and nodule organogenesis.

Identification and evolution of nsLTPs in the root nodule nitrogen fixation clade and molecular response of Frankia to AgLTP24

Non-specific lipid transfer proteins (nsLTPs) are antimicrobial peptides, involved in several plant biological processes including root nodule nitrogen fixation (RNF). Nodulating plants belonging to the RNF clade establish symbiosis with the nitrogen-fixing bacteria rhizobia (legumes symbiosis model) and Frankia (actinorhizal symbiosis model) leading to root nodule formation. nsLTPs are involved in processes active in early step of symbiosis and functional nodule in both models. In legumes, nsLTPs have been shown to regulate symbiont entry, promote root cortex infection, membrane biosynthesis, and improve symbiosis efficiency. More recently, a nsLTP, AgLTP24 has been described in the context of actinorhizal symbiosis between Alnus glutinosa and Frankia alni ACN14a. AgLTP24 is secreted at an early step of symbiosis on the deformed root hairs and targets the symbiont in the nitrogen-fixing vesicles in functional nodules. nsLTPs are involved in RNF, but their functions and evolutionary history are still largely unknown. Numerous putative nsLTPs were found up-regulated in functional nodules compared to non-infected roots in different lineages within the RNF clade. Here, results highlight that nodulating plants that are co-evolving with their nitrogen-fixing symbionts appear to have independently specialized nsLTPs for this interaction, suggesting a possible convergence of function, which opens perspectives to investigate nsLTPs functions in RNF.

A Nonspecific Lipid Transfer Protein with Potential Functions in Infection and Nodulation

The response of Alnus glutinosa to Frankia alni ACN14a is driven by several sequential physiological events from calcium spiking and root-hair deformation to the development of the nodule. Early stages of actinorhizal symbiosis were monitored at the transcriptional level to observe plant host responses to Frankia alni. Forty-two genes were significantly upregulated in inoculated compared with noninoculated roots. Most of these genes encode proteins involved in biological processes induced during microbial infection, such as oxidative stress or response to stimuli, but a large number of them are not differentially modulated or downregulated later in the process of nodulation. In contrast, several of them remained upregulated in mature nodules, and this included the gene most upregulated, which encodes a nonspecific lipid transfer protein (nsLTP). Classified as an antimicrobial peptide, this nsLTP was immunolocalized on the deformed root-hair surfaces that are points of contact for Frankia spp. during infection. Later in nodules, it binds to the surface of F. alni ACN14a vesicles, which are the specialized cells for nitrogen fixation. This nsLTP, named AgLTP24, was biologically produced in a heterologous host and purified for assay on F. alni ACN14a to identify physiological effects. Thus, the activation of the plant immunity response occurs upon first contact, while the recognition of F. alni ACN14a genes switches off part of the defense system during nodulation. AgLTP24 constitutes a part of the defense system that is maintained all along the symbiosis, with potential functions such as the formation of infection threads or nodule primordia to the control of F. alni proliferation. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Structure and Development of the Legume-Rhizobial Symbiotic Interface in Infection Threads

The intracellular infection thread initiated in a root hair cell is a unique structure associated with Rhizobium-legume symbiosis. It is characterized by inverted tip growth of the plant cell wall, resulting in a tunnel that allows invasion of host cells by bacteria during the formation of the nitrogen-fixing root nodule. Regulation of the plant-microbial interface is essential for infection thread growth. This involves targeted deposition of the cell wall and extracellular matrix and tight control of cell wall remodeling. This review describes the potential role of different actors such as transcription factors, receptors, and enzymes in the rearrangement of the plant-microbial interface and control of polar infection thread growth. It also focuses on the composition of the main polymers of the infection thread wall and matrix and the participation of reactive oxygen species (ROS) in the development of the infection thread. Mutant analysis has helped to gain insight into the development of host defense reactions. The available data raise many new questions about the structure, function, and development of infection threads.

Comprehensive classification of the plant non-specific lipid transfer protein superfamily towards its sequence-structure-function analysis

Background

Non-specific Lipid Transfer Proteins (nsLTPs) are widely distributed in the plant kingdom and constitute a superfamily of related proteins. Several hundreds of different nsLTP sequences—and counting—have been characterized so far, but their biological functions remain unclear. It has been clear for years that they present a certain interest for agronomic and nutritional issues. Deciphering their functions means collecting and analyzing a variety of data from gene sequence to protein structure, from cellular localization to the physiological role. As a huge and growing number of new protein sequences are available nowadays, extracting meaningful knowledge from sequence–structure–function relationships calls for the development of new tools and approaches. As nsLTPs show high evolutionary divergence, but a conserved common right handed superhelix structural fold, and as they are involved in a large number of key roles in plant development and defense, they are a stimulating case study for validating such an approach.

Methods

In this study, we comprehensively investigated 797 nsLTP protein sequences, including a phylogenetic analysis on canonical protein sequences, three-dimensional structure modeling and functional annotation using several well-established bioinformatics programs. Additionally, two integrative methodologies using original tools were developed. The first was a new method for the detection of (i) conserved amino acid residues involved in structure stabilization and (ii) residues potentially involved in ligand interaction. The second was a structure–function classification based on the evolutionary trace display method using a new tree visualization interface. We also present a new tool for visualizing phylogenetic trees.

Results

Following this new protocol, an updated classification of the nsLTP superfamily was established and a new functional hypothesis for key residues is suggested. Lastly, this work allows a better representation of the diversity of plant nsLTPs in terms of sequence, structure and function.

Genome-Wide Transcriptional Changes and Lipid Profile Modifications Induced by Medicago truncatula N5 Overexpression at an Early Stage of the Symbiotic Interaction with Sinorhizobium meliloti

Plant lipid-transfer proteins (LTPs) are small basic secreted proteins, which are characterized by lipid-binding capacity and are putatively involved in lipid trafficking. LTPs play a role in several biological processes, including the root nodule symbiosis. In this regard, the Medicago truncatula nodulin 5 (MtN5) LTP has been proved to positively regulate the nodulation capacity, controlling rhizobial infection and nodule primordia invasion. To better define the lipid transfer protein MtN5 function during the symbiosis, we produced MtN5-downregulated and -overexpressing plants, and we analysed the transcriptomic changes occurring in the roots at an early stage of Sinorhizobium meliloti infection. We also carried out the lipid profile analysis of wild type (WT) and MtN5-overexpressing roots after rhizobia infection. The downregulation of MtN5 increased the root hair curling, an early event of rhizobia infection, and concomitantly induced changes in the expression of defence-related genes. On the other hand, MtN5 overexpression favoured the invasion of the nodules by rhizobia and determined in the roots the modulation of genes that are involved in lipid transport and metabolism as well as an increased content of lipids, especially galactolipids that characterize the symbiosome membranes. Our findings suggest the potential participation of LTPs in the synthesis and rearrangement of membranes occurring during the formation of the infection threads and the symbiosome membrane.

Lipid Transfer Proteins As Components of the Plant Innate Immune System: Structure, Functions, and Applications

Among a variety of molecular factors of the plant innate immune system, small proteins that transfer lipids and exhibit a broad spectrum of biological activities are of particular interest. These are lipid transfer proteins (LTPs). LTPs are interesting to researchers for three main features. The first feature is the ability of plant LTPs to bind and transfer lipids, whereby these proteins got their name and were combined into one class. The second feature is that LTPs are defense proteins that are components of plant innate immunity. The third feature is that LTPs constitute one of the most clinically important classes of plant allergens. In this review, we summarize the available data on the plant LTP structure, biological properties, diversity of functions, mechanisms of action, and practical applications, emphasizing their role in plant physiology and their significance in human life.

Non-specific lipid transfer proteins in plants: presenting new advances and an integrated functional analysis

Plant non-specific lipid-transfer proteins (nsLTPs) are small, basic proteins present in abundance in higher plants. They are involved in key processes of plant cytology, such as the stablization of membranes, cell wall organization, and signal transduction. nsLTPs are also known to play important roles in resistance to biotic and abiotic stress, and in plant growth and development, such as sexual reproduction, seed development and germination. The structures of plant nsLTPs contain an eight-cysteine residue conserved motif, linked by four disulfide bonds, and an internal hydrophobic cavity, which comprises the lipid-binding site. This structure endows stability and increases the ability to bind and/or carry hydrophobic molecules. There is growing interest in nsLTPs, due to their critical roles, resulting in the need for a comprehensive review of their form and function. Relevant topics include: nsLTP structure and biochemical features, their classification, identification, and characterization across species, sub-cellular localization, lipid binding and transfer ability, expression profiling, functionality, and evolution. We present advances, as well as limitations and trends, relating to the different topics of the nsLTP gene family. This review collates a large body of research pertaining to the role of nsLTPs across the plant kingdom, which has been integrated as an in depth functional analysis of this group of proteins as a whole, and their activities across multiple biochemical pathways, based on a large number of reports. This review will enhance our understanding of nsLTP activity in planta, prompting further work and insights into the roles of this multifaceted protein family in plants.

Genome-wide survey and expression analysis of the putative non-specific lipid transfer proteins in Brassica rapa L

BACKGROUND

Plant non-specific lipid transfer proteins (nsLtps) are small, basic proteins encoded by multigene families and have reported functions in many physiological processes such as mediating phospholipid transfer, defense reactions against phytopathogens, the adaptation of plants to various environmental conditions, and sexual reproduction. To date, no genome-wide overview of the Brassica rapa nsLtp (BrnsLtp) gene family has been performed. Therefore, as the first step and as a helpful strategy to elucidate the functions of BrnsLtps, a genome-wide study for this gene family is necessary.

METHODOLOGY/PRINCIPAL FINDING

In this study, a total of 63 putative BrnsLtp genes were identified through a comprehensive in silico analysis of the whole genome of B. rapa. Based on the sequence similarities, these BrnsLtps was grouped into nine types (I, II, III, IV, V, VI, VIII, IX, and XI). There is no type VII nsLtps in B. rapa, and a new type, XI nsLtps, was identified in B. rapa. Furthermore, nine type II AtLtps have no homologous genes in B. rapa. Gene duplication analysis demonstrated that the conserved collinear block of each BrnsLtp is highly identical to those in Arabidopsis and that both segmental duplications and tandem duplications seem to play equal roles in the diversification of this gene family. Expression analysis indicated that 29 out of the 63 BrnsLtps showed specific expression patterns. After careful comparison and analysis, we hypothesize that some of the type I BrnsLtps may function like Arabidopsis pathogenesis-related-14 (PR-14) proteins to protect the plant from phytopathogen attack. Eleven BrnsLtps with inflorescence-specific expression may play important roles in sexual reproduction. Additionally, BrnsLtpI.3 may have functions similar to Arabidopsis LTP1.

CONCLUSIONS/SIGNIFICANCE

The genome-wide identification, bioinformatic analysis and expression analysis of BrnsLtp genes should facilitate research of this gene family and polyploidy evolution and provide new insight towards elucidating their biological functions in plants.