AHL-Priming Protein 1 mediates N-3-oxo-tetradecanoyl-homoserine lactone priming in Arabidopsis.

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.

NIN-Like Proteins: Interesting Players in Rhizobia-Induced Nitrate Signaling Response During Interaction with Non-Legume Host Arabidopsis thaliana.

Nitrogen is an essential macronutrient and a key cellular messenger. Plants have evolved refined molecular systems to sense the cellular nitrogen status. This is exemplified by the root nodule symbiosis between legumes and symbiotic rhizobia, where nitrate availability inhibits this mutualistic interaction. Additionally, nitrate also functions as a metabolic messenger, resulting in nitrate signaling cascades which intensively crosstalk with other physiological pathways. Nodule inception-like proteins (NLPs) are key players in nitrate signaling and regulate nitrate-dependent transcription during legume-rhizobia interactions. Nevertheless, the coordinated interplay between nitrate signaling pathways and rhizobacteria-induced responses remains to be elucidated. In our study, we investigated rhizobia-induced changes in the root system architecture of the non-legume host arabidopsis under different nitrate conditions. We demonstrate that rhizobium-induced lateral root growth and increased root hair length and density are regulated by a nitrate-related signaling pathway. Key players in this process are AtNLP4 and AtNLP5, because the corresponding mutants failed to respond to rhizobia. At the cellular level, AtNLP4 and AtNLP5 control a rhizobia-induced decrease in cell elongation rates, while additional cell divisions occurred independently of AtNLP4. In summary, our data suggest that root morphological responses to rhizobia are coordinated by a newly considered nitrate-related NLP pathway that is evolutionarily linked to regulatory circuits described in legumes.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Wounding and Insect Feeding Trigger Two Independent MAPK Pathways with Distinct Regulation and Kinetics.

Abiotic and biotic factors cause plant wounding and trigger complex short- and long-term responses at the local and systemic levels. These responses are under the control of complex signaling pathways, which are still poorly understood. Here, we show that the rapid activation of clade-A mitogen-activated protein kinases (MAPKs) MPK3 and MPK6 by wounding depends on the upstream MAPK kinases MKK4 and MKK5 but is independent of jasmonic acid (JA) signaling. In addition, this fast module does not control wound-triggered JA accumulation in Arabidopsis (Arabidopsis thaliana), unlike its orthologs in tobacco. We also demonstrate that a second MAPK module, composed of MKK3 and the clade-C MAPKs MPK1/2/7, is activated by wounding in a MKK4/5-independent manner. We provide evidence that the activation of this MKK3-MPK1/2/7 module occurs mainly through wound-induced JA production via the transcriptional regulation of upstream clade-III MAP3Ks, particularly MAP3K14. We show that mkk3 mutant plants are more susceptible to herbivory from larvae of the generalist lepidopteran herbivore Spodoptera littoralis, indicating that the MKK3-MPK1/2/7 module is involved in counteracting insect feeding.

Review: Mitogen-Activated Protein Kinases in nutritional signaling in Arabidopsis.

Mitogen-Activated Protein Kinase (MAPK) cascades are functional modules widespread among eukaryotic organisms. In plants, these modules are encoded by large multigenic families and are involved in many biological processes ranging from stress responses to cellular differentiation and organ development. Furthermore, MAPK pathways are involved in the perception of environmental and physiological modifications. Interestingly, some MAPKs play a role in several signaling networks and could have an integrative function for the response of plants to their environment. In this review, we describe the classification of MAPKs and highlight some of their biochemical actions. We performed an in silico analysis of MAPK gene expression in response to nutrients supporting their involvement in nutritional signaling. While several MAPKs have been identified as players in sugar, nitrogen, phosphate, iron and potassium-related signaling pathways, their biochemical functions are yet mainly unknown. The integration of these regulatory cascades in the current understanding of nutrient signaling is discussed and potential new avenues for approaches toward plants with higher nutrient use efficiencies are evoked.

Beneficial effects of bacteria-plant communication based on quorum sensing molecules of the N-acyl homoserine lactone group.

Bacterial quorum sensing (QS) mechanisms play a crucial role in the proper performance and ecological fitness of bacterial populations. Many key physiological processes are regulated in a QS-dependent manner by auto-inducers, like the N-acyl homoserine lactones (AHLs) in numerous Gram-negative bacteria. In addition, also the interaction between bacteria and eukaryotic hosts can be regulated by AHLs. Those mechanisms gained much attention, because of the positive effects of different AHL molecules on plants. This positive impact ranges from growth promotion to induced resistance and is quite contrasting to the rather negative effects observed in the interactions between bacterial AHL molecules and animals. Only very recently, we began to understand the molecular mechanisms underpinning plant responses to AHL molecules. In this review, we gathered the latest information in this research field. The first part gives an overview of the bacterial aspects of quorum sensing. Later we focus on the impact of AHLs on plant growth and AHL-priming, as one of the most understood phenomena in respect to the inter-kingdom interactions based on AHL-quorum sensing molecules. Finally, we discuss the potential benefits of the understanding of bacteria-plant interaction for the future agricultural applications.

N-acyl-homoserine lactones-producing bacteria protect plants against plant and human pathogens.

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.

N-Acyl-Homoserine Lactone Primes Plants for Cell Wall Reinforcement and Induces Resistance to Bacterial Pathogens via the Salicylic Acid/Oxylipin Pathway.

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 stomata 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 short-chain AHLs, unlike oxo-C14-HSL, promote plant growth, treatments with C6-HSL, oxo-C10-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.

AHL-priming functions via oxylipin and salicylic acid.

Collaborative action between the host plant and associated bacteria is crucial for the establishment of an efficient interaction. In bacteria, the synchronized behavior of a population is often achieved by a density-dependent communication called quorum sensing. This behavior is based on signaling molecules, which influence bacterial gene expression. N-acyl homoserine lactones (AHLs) are such molecules in many Gram-negative bacteria. Moreover, some AHLs are responsible for the beneficial effect of bacteria on plants, for example the long chain N-3-oxo-tetradecanoyl-L-homoserine lactone (oxo-C14-HSL) can prime Arabidopsis and barley plants for an enhanced defense. This AHL-induced resistance phenomenon, named AHL-priming, was observed in several independent laboratories during the last two decades. Very recently, the mechanism of priming with oxo-C14-HSL was shown to depend on an oxylipin and salicylic acid (SA). SA is a key element in plant defense, it accumulates during different plant resistance responses and is the base of systemic acquired resistance. In addition, SA itself can prime plants for an enhanced resistance against pathogen attack. On the other side, oxylipins, including jasmonic acid (JA) and related metabolites, are lipid-derived signaling compounds. Especially the oxidized fatty acid derivative cis-OPDA, which is the precursor of JA, is a newly described player in plant defense. Unlike the antagonistic effect of SA and JA in plant-microbe interactions, the recently described pathway functions through a synergistic effect of oxylipins and SA, and is independent of the JA signaling cascade. Interestingly, the oxo-C14-HSL-induced oxylipin/SA signaling pathway induces stomata defense responses and cell wall strengthening thus prevents pathogen invasion. In this review, we summarize the findings on AHL-priming and the related signaling cascade. In addition, we discuss the potential of AHL-induced resistance in new strategies of plant protection.

Modified N-acyl-homoserine lactones as chemical probes for the elucidation of plant-microbe interactions.

Gram-negative bacteria often use N-acyl-homoserine lactones (AHLs) as signal molecules to monitor their local population densities and to regulate gene-expression in a process called "Quorum Sensing" (QS). This cell-to-cell communication allows bacteria to adapt to environmental changes and to behave as multicellular communities. QS plays a key role in both bacterial virulence towards the host and symbiotic interactions with other organisms. Plants also perceive AHLs and respond to them with changes in gene expression or modifications in development. Herein, we report the synthesis of new AHL-derivatives for the investigation and identification of AHL-interacting proteins. We show that our new compounds are still recognised by different bacteria and that a novel biotin-tagged-AHL derivative interacts with a bacterial AHL receptor.

Arabidopsis growth and defense are modulated by bacterial quorum sensing molecules.

N-acyl-homoserine lactones (AHLs) play an important role in the communication within the rhizosphere; they serve as a chemical base for interactions within and between different species of Gram-negative bacteria. Not only bacteria, also plants perceive and react to AHLs with diverse responses. Here we describe a negative correlation between the length of AHLs' lipid chains and the observed growth promotion in Arabidopsis thaliana. Moreover, we speculate on a positive correlation between the reinforcement of defense mechanisms and the length of the lipid moieties. Observation presented here may be of great importance for understanding of the complex interplay between plants and their environment, as well as for agronomic applications.

N-acyl-homoserine lactone confers resistance toward biotrophic and hemibiotrophic pathogens via altered activation of AtMPK6.

Pathogenic and symbiotic bacteria rely on quorum sensing to coordinate the collective behavior during the interactions with their eukaryotic hosts. Many Gram-negative bacteria use N-acyl-homoserine lactones (AHLs) as signals in such communication. Here we show that plants have evolved means to perceive AHLs and that the length of acyl moiety and the functional group at the γ position specify the plant's response. Root treatment with the N-3-oxo-tetradecanoyl-L-homoserine lactone (oxo-C14-HSL) reinforced the systemic resistance to the obligate biotrophic fungi Golovinomyces orontii in Arabidopsis (Arabidopsis thaliana) and Blumeria graminis f. sp. hordei in barley (Hordeum vulgare) plants. In addition, oxo-C14-HSL-treated Arabidopsis plants were more resistant toward the hemibiotrophic bacterial pathogen Pseudomonas syringae pv tomato DC3000. Oxo-C14-HSL promoted a stronger activation of mitogen-activated protein kinases AtMPK3 and AtMPK6 when challenged with flg22, followed by a higher expression of the defense-related transcription factors WRKY22 and WRKY29, as well as the PATHOGENESIS-RELATED1 gene. In contrast to wild-type Arabidopsis and mpk3 mutant, the mpk6 mutant is compromised in the AHL effect, suggesting that AtMPK6 is required for AHL-induced resistance. Results of this study show that AHLs commonly produced in the rhizosphere are crucial factors in plant pathology and could be an agronomic issue whose full impact has to be elucidated in future analyses.