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Summary The implementation of beneficial microorganisms for plant protection has a long history. Many rhizobia bacteria are able to influence the immune system of host plants by inducing resistance towards pathogenic microorganisms. In this report, we present a translational approach in which we demonstrate the resistance‐inducing effect of Ensifer meliloti (Sinorhizobium meliloti) on crop plants that have a significant impact on the worldwide economy and on human nutrition. Ensifer meliloti is usually associated with root nodulation in legumes and nitrogen fixation. Here, we suggest that the ability of S. meliloti to induce resistance depends on the production of the quorum‐sensing molecule, oxo‐C14‐HSL. The capacity to enhanced resistance provides a possibility to the use these beneficial bacteria in agriculture. Using the Arabidopsis‐Salmonella model, we also demonstrate that the application of N‐acyl‐homoserine lactones‐producing bacteria could be a successful strategy to prevent plant‐originated infections with human pathogens. In this report, we present a translational approach in which we demonstrate that the resistance‐inducing effect of Sinorhizobium meliloti on crop plants depends on the production of the quorum sensing molecule, oxo‐C14‐HSL. In addition, using the Arabidopsis‐Salmonella model, we also demonstrate that the application of AHL‐producing bacteria could be a successful strategy to prevent plant‐originated infections with human pathogens.

The ability of plants to monitor their surroundings, for instance the perception of bacteria, is of crucial importance. The perception of microorganism-derived molecules and their effector proteins is the best understood of these monitoring processes. In addition, plants perceive bacterial quorum sensing (QS) molecules used for cell-to-cell communication between bacteria. Here, we propose a mechanism for how N-acyl-homoserine lactones (AHLs), a group of QS molecules, influence host defense and fortify resistance in Arabidopsis thaliana against bacterial pathogens. N-3-oxo-tetradecanoyl-L-homoserine lactone (oxo-C14-HSL) primed plants for enhanced callose deposition, accumulation of phenolic compounds, and lignification of cell walls. Moreover, increased levels of oxylipins and salicylic acid favored closure of stornata in response to Pseudomonas syringae infection. The AHL-induced resistance seems to differ from the systemic acquired and the induced systemic resistances, providing new insight into inter-kingdom communication. Consistent with the observation that shortchain AHLs, unlike oxo-C14-HSL, promote plant growth, treatments with C6-HSL, oxo-CIO-HSL, or oxo-C14-HSL resulted in different transcriptional profiles in Arabidopsis. Understanding the priming induced by bacterial QS molecules augments our knowledge of plant reactions to bacteria and suggests strategies for using beneficial bacteria in plant protection.

Background N -3-oxo-tetradecanoyl- L -homoserine lactone (oxo-C14-HSL) is one of the N -acyl homoserine lactones (AHL) that mediate quorum sensing in Gram-negative bacteria. In addition to bacterial communication, AHL are involved in interactions with eukaryotes. Short-chain AHL are easily taken up by plants and transported over long distances. They promote root elongation and growth. Plants typically do not uptake hydrophobic long sidechain AHL such as oxo-C14-HSL, although they prime plants for enhanced resistance to biotic and abiotic stress. Many studies have focused on priming effects of oxo-C14-HSL for enhanced plant resistance to stress. However, specific plant factors mediating oxo-C14-HSL responses in plants remain unexplored. Here, we identify the Arabidopsis protein ALI1 as a mediator of oxo-C14-HSL-induced priming in plants. Results We compared oxo-C14-HSL-induced priming between wild-type Arabidopsis Col-0 and an oxo-C14-HSL insensitive mutant ali1 . The function of the candidate protein ALI1 was assessed through biochemical, genetic, and physiological approaches to investigate if the loss of the ALI1 gene resulted in subsequent loss of AHL priming. Through different assays, including MAP kinase activity assay, gene expression and transcriptome analysis, and pathogenicity assays, we revealed a loss of AHL priming in ali1 . This phenomenon was reverted by the reintroduction of ALI1 into ali1 . We also investigated the interaction between ALI1 protein and oxo-C14-HSL using biochemical and biophysical assays. Although biophysical assays did not reveal an interaction between oxo-C14-HSL and ALI1, a pull-down assay and an indirect method employing biosensor E. coli LuxCDABE support such interaction. We expressed fluorescently tagged ALI1 in tobacco leaves to assess the localization of ALI1 and demonstrate that ALI1 colocalizes with the plasma membrane, tonoplast, and endoplasmic reticulum. Conclusions These results suggest that the candidate protein ALI1 is indispensable for oxo-C14-HSL-dependent priming for enhanced resistance in Arabidopsis and that the ALI1 protein may interact with oxo-C14-HSL. Furthermore, ALI1 protein is localized in the cell periphery. Our findings advance the understanding of interactions between plants and bacteria and provide an avenue to explore desired outcomes such as enhanced stress resistance, which is useful for sustainable crop protection.

In plants, the topological organization of membranes has mainly been attributed to the cell wall and the cytoskeleton. Additionally, few proteins, such as plant-specific remorins have been shown to function as protein and lipid organizers. Root nodule symbiosis requires continuous membrane re-arrangements, with bacteria being finally released from infection threads into membrane-confined symbiosomes. We found that mutations in the symbiosis-specific SYMREM1 gene result in highly disorganized perimicrobial membranes. AlphaFold modelling and biochemical analyses reveal that SYMREM1 oligomerizes into antiparallel dimers and may form a higher-order membrane scaffolding structure. This was experimentally confirmed when expressing this and other remorins in wall-less protoplasts is sufficient where they significantly alter and stabilize de novo membrane topologies ranging from membrane blebs to long membrane tubes with a central actin filament. Reciprocally, mechanically induced membrane indentations were equally stabilized by SYMREM1. Taken together we describe a plant-specific mechanism that allows the stabilization of large-scale membrane conformations independent of the cell wall. In plants, plasma membrane topologies are predominantly driven by the cell wall. In this study, the authors demonstrate that remorin proteins can take over these functions at specialized, unwalled plasma membranes such as infection droplets associated with symbiotic infection threads.

The initiation of intracellular host cell colonization by symbiotic rhizobia in Medicago truncatula requires repolarization of root hairs, which includes the re-arrangement of cytoskeletal filaments. The molecular players governing microtubule (MT) re-organization during infection remain to be discovered. Here, we identified the M. truncatula DREPP protein and investigated its functions during rhizobial infections. We show that rhizobial colonization of drepp mutant roots as well as transgenic roots over-expressing DREPP is impaired. DREPP re-localizes into symbiosis-specific membrane nanodomains in a stimulus-dependent manner. This subcellular segregation coincides with DREPP-dependent MT fragmentation and a partial loss of the ability to re-organize the MT cytoskeleton in response to rhizobia, which might relay on an interaction between DREPP and MT organizing protein SPIRAL2 (SPR2). Taken together, our results reveal that establishment of symbiotic associations in M. truncatula require DREPP in order to regulate MT reorganization during initial root hair responses to rhizobia.

The implementation of beneficial microorganisms for plant protection has a long history. Many rhizobia bacteria are able to influence the immune system of host plants by inducing resistance towards pathogenic microorganisms. In this report, we present a translational approach in which we demonstrate the resistance‐inducing effect of Ensifer meliloti (Sinorhizobium meliloti) on crop plants that have a significant impact on the worldwide economy and on human nutrition. Ensifer meliloti is usually associated with root nodulation in legumes and nitrogen fixation. Here, we suggest that the ability of S. meliloti to induce resistance depends on the production of the quorum‐sensing molecule, oxo‐C14‐HSL. The capacity to enhanced resistance provides a possibility to the use these beneficial bacteria in agriculture. Using the Arabidopsis‐Salmonella model, we also demonstrate that the application of N‐acyl‐homoserine lactones‐producing bacteria could be a successful strategy to prevent plant‐originated infections with human pathogens.

Quorum sensing (QS) molecules mediate communication between bacterial cells. N-acyl homoserine lactones (AHL) are one of the best-studied groups of QS molecules. In addition to bacterial communication, AHL are involved in interactions with eukaryotes. Short side-chain AHL are readily taken up by plants. They induce root elongation and growth promotion. Hydrophobic long side-chain AHL are usually not transported over long distances although, they may prime plants for enhanced resistance. Unfortunately, studies elucidating the plant factors required for response to AHL are sparse. Here, we provide evidence of a plant protein, namely the AHL-priming protein 1 (ALI1), indispensable for enhanced resistance response induced by the N-3-oxotetradecanoyl-homoserine lactone (oxo-C14-HSL). Comparing Col-0 and the ali1 mutant, we revealed loss of AHL-priming in ali1. This phenomenon is reverted with the reintroduction of ALI1 into ali1. Additional transcriptome analysis revealed that ali1 is less sensitive to oxo-C14-HSL treatment compared to the wild-type. Our results suggest, therefore, that ALI1 is required for oxo-C14-HSL-dependent priming for enhanced resistance in Arabidopsis. Competing Interest Statement The authors have declared no competing interest. Footnotes * https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE156726

Organization of membrane topologies in plants has so far been mainly attributed to the cell wall and the cytoskeleton. Taking rhizobial infections of legume root cells, where plasma membranes undergo dynamic and large-scale topology changes, as an initial model, we challenged this paradigm and tested whether additional scaffolds such as plant-specific remorins that accumulate on highly curved and often wall-less plasma membrane domains, control local membrane dynamics. Indeed, loss-of-function mutants of the remorin protein SYMREM1 failed to develop stabilized membrane tubes as found in colonized cells in wild-type plants, but released empty membrane spheres instead. Expression of this and other remorins in wall-less protoplasts allowed engineering different membrane topologies ranging from membrane blebs to long membrane tubes. Reciprocally, mechanically induced membrane indentations were equally stabilized by SYMREM1. This function is likely supported by remorin oligomerization into antiparallel dimers and the formation of higher order membrane scaffolding structures. Taken together we describe an evolutionary confined mechanism that allows the stabilization of large-scale membrane conformations and curvatures in plants. The remorin SYMREM1 evolved as structural membrane scaffold that stabilizes membrane tubulation and curvature during symbiotic intracellular infections.