Mosquitoes are considered one of the most important threats worldwide due to their ability to vector pathogens. They are responsible for the transmission of major pathogens such as malaria, dengue, zika, or chikungunya. Due to the lack of treatments or prophylaxis against many of the transmitted pathogens and an increasing prevalence of mosquito resistance to insecticides and drugs available, alternative strategies are now being explored. Some of these involve the use of microorganisms as promising agent to limit the fitness of mosquitoes, attract or repel them, and decrease the replication and transmission of pathogenic agents. In recent years, the importance of microorganisms colonizing the habitat of mosquitoes has particularly been investigated since they appeared to play major roles in their development and diseases transmission. In this issue, we will synthesize researches investigating how microorganisms present within water habitats may influence breeding site selection and oviposition strategies of gravid mosquito females. We will also highlight the impact of such microbes on the fate of females’ progeny during their immature stages with a specific focus on egg hatching, development rate, and larvae or pupae survival.

Symbiosis established between actinorhizal plants and Frankia spp., which are nitrogen-fixing actinobacteria, promotes nodule organogenesis, the site of metabolic exchange. The present study aimed to identify amino acid markers involved in Frankia-Alnus interactions by comparing nodules and associated roots from field and greenhouse samples. Our results revealed a high level of citrulline in all samples, followed by arginine (Arg), aspartate (Asp), glutamate (Glu), γ-amino-n-butyric acid (GABA), and alanine (Ala). Interestingly, the field metabolome approach highlighted more contrasted amino acid patterns between nodules and roots compared with greenhouse samples. Indeed, 12 amino acids had a mean relative abundance significantly different between field nodule and root samples, against only four amino acids in greenhouse samples, underlining the importance of developing “ecometabolome” approaches. In order to monitor the effects on Frankia cells (respiration and nitrogen fixation activities) of amino acid with an abundance pattern evocative of a role in symbiosis, in-vitro assays were performed by supplementing them in nitrogen-free cultures. Amino acids had three types of effects: i) those used by Frankia as nitrogen source (Glu, Gln, Asp), ii) amino acids stimulating both nitrogen fixation and respiration (e.g., Cit, GABA, Ala, valine, Asn), and iii) amino acids triggering a toxic effect (Arg, histidine). In this paper, a N-metabolic model was proposed to discuss how the host plant and bacteria modulate amino acids contents in nodules, leading to a fine regulation sustaining high bacterial nitrogen fixation.

A unique case of microbial symbiont capable of dormancy within its living host cells has been reported in actinorhizal symbioses. Some Frankia strains, named Sp+, are able to sporulate inside plant cells, contrarily to Sp− strains. The presence of metabolically slowed-down bacterial structures in host cells alters our understanding of symbiosis based on reciprocal benefits between both partners, and its impact on the symbiotic processes remains unknown. The present work reports a metabolomic study of Sp+ and Sp− nodules (from Alnus glutinosa), in order to highlight variabilities associated with in-planta sporulation. A total of 21 amino acids, 44 sugars and organic acids, and 213 secondary metabolites were detected using UV and mass spectrometric–based profiling. Little change was observed in primary metabolites, suggesting that in-planta sporulation would not strongly affect the primary functionalities of the symbiosis. One secondary metabolite (M27) was detected only in Sp+ nodules. It was identified as gentisic acid 5-O-β-d-xylopyranoside, previously reported as involved in plant defenses against microbial pathogens. This metabolite significantly increased Frankia in-vitro sporulation, unlike another metabolite significantly more abundant in Sp− nodules [M168 = (5R)-1,7-bis-(3,4-dihydroxyphenyl)-heptane-5-O-β-d-glucopyranoside]. All these results suggest that the plant could play an important role in the Frankia ability to sporulate in planta and allow us to discuss a possible sanction emitted by the host against less cooperative Sp+ symbionts.

Alnus glutinosa, able to establish symbiosis with mutualistic bacteria of the genus Frankia, is one of the main species in European riparian environments, where it performs numerous biological and socioeconomic functions. However, riparian ecosystems face a growing threat from Phytophthora alni, a highly aggressive waterborne pathogen causing severe dieback in A. glutinosa. To date, the tripartite interaction between the host plant, the symbiont Frankia, and the pathogen remains unexplored but is critical for understanding how pathogen-induced stress influences the nodule molecular machinery and thus the host-symbiont metabolism. In the present study, we aimed to explore for the first time how P. alni affects the overall molecular processes of Alnus glutinosa-Frankia nodules, with a special focus on unraveling the spatial expression of defense mechanisms within these tissues. We conducted a laboratory experiment based on P. alni infection of young A. glutinosa seedlings nodulated with Frankia alni ACN14a, noninfected or infected with the pathogen P. alni. Multi-omics analyses (i.e., transcriptomics, proteomics, and metabolomics) were carried out on nodules (N) and associated roots (AR) of the same plant to underline the impact on the nodule molecular processes (i.e., N/AR markers) when the host plant is infected compared with noninfected plants. Our results revealed that P. alni infection modified the molecular nodule processes and induced reprograming of defense-related markers by a shift in associated roots to the detriment of nodules. These findings suggest that A. glutinosa reinforces locally its immune responses in roots but moderates this activation in nodules to preserve its Frankia symbiont. [ ] Copyright © 2025 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .