New Inoculation Strategy for Legume Based on Rhizobium-Metabolite Co-encapsulation.

The new strategies that are trying to be developed to protect microorganisms for a successful application have generated various types of granulated, powdered, or liquid formulations. In this work, we have developed a rhizobial encapsulation system for legumes accompanied by metabolites to enhance microorganism-plant communication. This novel way of producing a biofertilizer for legumes was developed based on alginate, a degradable compound that allows environmentally friendly use. This way of generating an inoculant allows it designing by making different molecular combinations for different purposes, being a double inoculant, biological and molecular.

Immobilization of Bradyrhizobium and Azospirillum in alginate matrix for long time of storage maintains cell viability and interaction with peanut.

Immobilizarion of PGPR for agricultural applications aims to provide temporary physical protection from stressful environmental conditions and the gradual release of cells for successful root colonization, release the cells gradually. In this work, we immobilized Bradyrhizobium sp. SEMIA6144 or Azospirillum brasilense Az39 cells in 2% alginate beads prepared by ionic gelation process, and then stored up to 12 months at 4 °C. Alginate matrix showed interaction with the immobilized bacteria (FTIR), allowed a constant release of cells, and improved their viability and capability to interact with Arachis hypogaea. Cell number into beads reached 107 CFU.bead-1; however, viability decreased from 4 months of storage for Az39, while it was maintained up to 12 months for SEMIA6144, showing a low metabolic activity measured by the MTT assay. Adhesion of SEMIA6144 and Az39 from new beads to peanut root was 11.5% and 16%, respectively, higher than non-immobilized bacteria. Peanut inoculation with 12 months storage SEMIA6144 beads significantly increased root length and biomass at 30 days of growth, and under restrictive water condition (RWC), nodulation and total plant N content increased compared with liquid inoculation. Our results demonstrate that immobilization of SEMIA6144 and Az39 in alginate matrix is a potential alternative to enhance peanut growth even under RWC. KEY POINTS: • Alginate encapsulation enhances viability of SEMIA6144 or Az39 under storage at 4 °C for 1 year. • Alginate beads 2% ensure the gradual release of the microorganisms. • Cells from beads stored for long periods present chemotaxis and adhesion to peanut root. • Peanut inoculation with 1-year-old SEMIA6144 beads improves nodulation and growth in RWC.

Changes in the lipid composition of Bradyrhizobium cell envelope reveal a rapid response to water deficit involving lysophosphatidylethanolamine synthesis from phosphatidylethanolamine in outer membrane.

We evaluate the behavior of the membrane of Bradyrhizobium sp. SEMIA6144 during adaptation to polyethylene glycol (PEG). A dehydrating effect on the morphology of the cell surface, as well as a fluidizing effect on the membrane was observed 10 min after PEG shock; however, the bacteria were able to restore optimal membrane fluidity. Shock for 1 h caused an increase of lysophosphatidylethanolamine in the outer membrane at the expense of phosphatidylcholine and phosphatidylethanolamine (PE), through an increase in phospholipase activity. The amount of lysophosphatidylethanolamine did not remain constant during PEG shock, but after 24 h the outer membrane was composed of large amounts of phosphatidylcholine and less amount of lysophosphatidylethanolamine similar to the control. The inner membrane composition was also modified after 1 h of shock, observing an increase of phosphatidylcholine at the expense of PE, the proportions of these phospholipids were then modified to reach 24 h of shock values similar to the control. Vesicles prepared with the lipids of cells exposed to 1 h shock presented higher rigidity compared to the control, indicating that changes in the composition of phospholipids after 1 h of shock restoring fluidity after the PEG effect and would allow cells to maintain surface morphology.

Arachis hypogaea PGPR isolated from Argentine soil modifies its lipids components in response to temperature and salinity.

The aim of this work was to clarify the mechanism related to plant growth promoting of a bacterial strain (L115) isolated from Arachis hypogaea rhizospheres and the effects of high growth temperature and salinity on phospholipids and fatty acids composition. L115 was isolated from peanut rhizospheres and identified according to the sequence analysis of the 16S rRNA gene. Phenotypic, metabolic and plant growth promoting rhizobacteria (PGPR) characteristics of L115 were tested. Inoculation test in plant growth chamber was performed. In addition, L115 was exposed to a 37 °C and 300 mM NaCl and phospholipids and fatty acid composition were evaluated. L115 strain was identified as Ochrobactrum intermedium and was able to increase the peanut shoot and root length as well as dry weight, indicating a PGPR role by being able to produce indole acetic acid and siderophores and present ACC deaminase activity. In addition, L115 showed tolerance to both high growth temperature and 300 mM NaCl. The most striking change was a decreased percentage of 18:1 fatty acid and an increase in 16:0 and 18:0 fatty acids, under high growth temperature or a combination of increased temperature and salinity. The most important change in phospholipid levels was an increase in phosphatidylcholine biosynthesis in all growth conditions. L115 can promote the growth of peanut and can tolerate high growth temperature and salinity modifying the fatty acid unsaturation degree and increasing phosphatidylcholine levels. This work is the first to report the importance of the genus Ochrobactrum as PGPR on peanut growth as well as on the metabolic behaviour against abiotic stresses that occur in soil. This knowledge will be useful for developing strategies to improve the growth of this bacterium under stress and to enhance its bioprocess for the production of inoculants.