Comparative genomics of Aeschynomene symbionts : insights into the ecological lifestyle of nod-independent photosynthetic Bradyrhizobia

Tropical aquatic species of the legume genus Aeschynomene are stem- and root-nodulated by bradyrhizobia strains that exhibit atypical features such as photosynthetic capacities or the use of a nod gene-dependent (ND) or a nod gene-independent (NI) pathway to enter into symbiosis with legumes. In this study we used a comparative genomics approach on nine Aeschynomene symbionts representative of their phylogenetic diversity. We produced draft genomes of bradyrhizobial strains representing different phenotypes: five NI photosynthetic strains (STM3809, ORS375, STM3847, STM4509 and STM4523) in addition to the previously sequenced ORS278 and BTAi1 genomes, one photosynthetic strain ORS285 hosting both ND and NI symbiotic systems, and one NI non-photosynthetic strain (STM3843). Comparative genomics allowed us to infer the core, pan and dispensable genomes of Aeschynomene bradyrhizobia, and to detect specific genes and their location in Genomic Islands (GI). Specific gene sets linked to photosynthetic and NI/ND abilities were identified, and are currently being studied in functional analyses

Infrared analysis and pressure measurements on a single loop pulsating heat pipe at different gravity levels

A Single Loop Pulsating Heat Pipe (SLPHP) with an inner diameter of 2 mm is tested in hyper/micro gravity conditions during the 68th ESA Parabolic Flight Campaign. The system is designed with two sapphire tubes that connect the heated and the cooled section, allowing simultaneous fluid flow high-speed visualization, and a direct to fluid IR analysis by using respectively a high-speed camera and a Medium-Wave Infrared Camera (MWIR). Three independent heaters are positioned at the evaporator in order to vary the power distribution and to promote different flow motions with specific heating configurations. Furthermore, two highly accurate pressure transducers measure the pressure drop between the condenser and the evaporator. Additionally, twelve thermocouples mounted on the external tube wall record local temperatures during parabolic flight tests. Such a complete thermo-fluid dynamic analysis at different gravity levels, coupled with the acquisition of high-speed and infrared images in the transparent section of the SLPHP, has the main objective of providing a better understanding on the relationship between the fluid flow motion and the thermal response of the device. Infrared Time-space temperature maps of the flow are correlated with pressure measurements, the external wall tube temperatures, the liquid slug velocity and the local void fraction; providing an exhaustive overview of such a PHP transparent tube both in microgravity and hyper-gravity conditions. Additionally, for the first time in microgravity, the effect of the condenser temperature on PHPs is explored. When the condenser temperature is set at a higher value than the environment, results highlight that the possibility to invert the flow motion direction by means of non-symmetrical heating configurations is hindered. These experimental data could assist the development of improved numerical models of Pulsating Heat Pipes at different gravity levels

Infrared analysis and pressure measurements on a single loop pulsating heat pipe at different gravity levels

A Single Loop Pulsating Heat Pipe (SLPHP) with an inner diameter of 2 mm is tested in hyper/micro gravity conditions during the 68th ESA Parabolic Flight Campaign. The system is designed with two sapphire tubes that connect the heated and the cooled section, allowing simultaneous fluid flow high-speed visualization, and a direct to fluid IR analysis by using respectively a high-speed camera and a Medium-Wave Infrared Camera (MWIR). Three independent heaters are positioned at the evaporator in order to vary the power distribution and to promote different flow motions with specific heating configurations. Furthermore, two highly accurate pressure transducers measure the pressure drop between the condenser and the evaporator. Additionally, twelve thermocouples mounted on the external tube wall record local temperatures during parabolic flight tests. Such a complete thermo-fluid dynamic analysis at different gravity levels, coupled with the acquisition of high-speed and infrared images in the transparent section of the SLPHP, has the main objective of providing a better understanding on the relationship between the fluid flow motion and the thermal response of the device. Infrared Time-space temperature maps of the flow are correlated with pressure measurements, the external wall tube temperatures, the liquid slug velocity and the local void fraction; providing an exhaustive overview of such a PHP transparent tube both in microgravity and hyper-gravity conditions. Additionally, for the first time in microgravity, the effect of the condenser temperature on PHPs is explored. When the condenser temperature is set at a higher value than the environment, results highlight that the possibility to invert the flow motion direction by means of non-symmetrical heating configurations is hindered. These experimental data could assist the development of improved numerical models of Pulsating Heat Pipes at different gravity levels

Rhizobium-legume symbiosis in the absence of Nod factors: two possible scenarios with or without the T3SS

International audienceThe occurrence of alternative Nod factor (NF)-independent symbiosis between legumes and rhizobia was first demonstrated in some Aeschynomene species that are nodulated by photosynthetic bradyrhizobia lacking the canonical nodABC genes. In this study, we revealed that a large diversity of non-photosynthetic bradyrhizobia, including B. elkanii, was also able to induce nodules on the NF-independent Aeschynomene species, A. indica. Using cytological analysis of the nodules and the nitrogenase enzyme activity as markers, a gradient in the symbiotic interaction between bradyrhizobial strains and A. indica could be distinguished. This ranged from strains that induced nodules that were only infected intercellularly to rhizobial strains that formed nodules in which the host cells were invaded intracellularly and that displayed a weak nitrogenase activity. In all non-photosynthetic bradyrhizobia, the type III secretion system (T3SS) appears required to trigger nodule organogenesis. In contrast, genome sequence analysis revealed that apart from a few exceptions, like the Bradyrhizobium ORS285 strain, photosynthetic bradyrhizobia strains lack a T3SS. Furthermore, analysis of the symbiotic properties of an ORS285 T3SS mutant revealed that the T3SS could have a positive or negative role for the interaction with NF-dependent Aeschynomene species, but that it is dispensable for the interaction with all NF-independent Aeschynomene species tested. Taken together, these data indicate that two NF-independent symbiotic processes are possible between legumes and rhizobia: one dependent on a T3SS and one using a so far unknown mechanism