Membrane Homeoviscous Adaptation in Sinorhizobium Submitted to a Stressful Thermal Cycle Contributes to the Maintenance of the Symbiotic Plant-Bacteria Interaction.

Natalia Soledad Paulucci,Adriana Belén Cesari,Marta Susana Dardanelli,María Angélica Perillo,María Alicia Biasutti

doi:10.3389/fmicb.2021.652477

Abstract

Here, we estimate fast changes in the fluidity of Sinorhizobium meliloti membranes submitted to cyclic temperature changes (10°C–40°C–10°C) by monitoring the fluorescence polarization (P) of DPH and TMA-DPH of the whole cell (WC) as well as in its outer (OM) and inner (IM) membranes. Additionally, the long-term response to thermal changes is demonstrated through the dynamics of the phospholipid and fatty acid composition in each membrane. This allowed membrane homeoviscous adaptation by the return to optimal fluidity levels as measured by the PDPH/TMA-DPH in WC, OM, IM, and multilamellar vesicles of lipids extracted from OM and IM. Due to probe-partitioning preferences and membranes’ compositional characteristics, DPH and TMA-DPH exhibit different behaviors in IM and OM. The rapid effect of cyclic temperature changes on the P was the opposite in both membranes with the IM being the one that exhibited the thermal behavior expected for lipid bilayers. Interestingly, only after the incubation at 40°C, cells were unable to recover the membrane preheating P levels when cooled up to 10°C. Solely in this condition, the formation of threads and nodular structures in Medicago sativa infected with S. meliloti were delayed, indicating that the symbiotic interaction was partially altered but not halted.

Highlights

Sinorhizobium meliloti are Gram-negative bacteria usually applied in the formulation of commercial inoculants because of their ability to promote the growth and yield of legume crops (e.g., Medicago sativa, alfalfa) through a symbiotic atmospheric nitrogen (N)-fixing relationship.N is fundamental for the development of plants because it is needed for the synthesis of proteins and nucleic acids
We measured the value of PDPH in isolated outer membrane (OM) and spheroplasts (IM) as well as in multilamellar vesicles (MLV) prepared with the lipids extracted from OM and inner membrane (IM) of cells previously subjected to successive 24 h incubation periods at 10, 40, and 10◦C (Figure 5)
Because the impairment would not be absolute in whole cell (WC) and is absent in IM, the DPH molecules that reach the IM would find an environment capable of exhibiting PDPH changes that accompany the T-induced anisotropy dynamics (Figures 2A,B)

Summary

INTRODUCTION

Sinorhizobium meliloti are Gram-negative bacteria usually applied in the formulation of commercial inoculants because of their ability to promote the growth and yield of legume crops (e.g., Medicago sativa, alfalfa) through a symbiotic atmospheric nitrogen (N)-fixing relationship. When DPH is used as a probe in Gram-negative bacteria, the complex cell envelope may produce misleading results from fluorescence polarization experiments obtained in WC and OM, leading to erroneous interpretations This is because the OM is a lipid bilayer containing phospholipids (PL), which are confined to the inner monolayer of the lipid bilayer, whereas the outer monolayer is mainly composed of lipopolysaccharides (LPS) (Bos et al, 2007). The process of symbiotic interaction between S. meliloti and alfalfa is deeply studied (Sieberer et al, 2005; Peck et al, 2006; Jones et al, 2007), to our knowledge, there are no studies that analyze the effects of environmental conditions exerted on biochemical changes in the cell envelope and the subsequent events of early colonization of alfalfa root simultaneously in the same samples. Studying the thermal-induced changes in lipid composition of the S. meliloti envelope sheds light on its biochemical mechanisms of adaptation, but might contribute to explain the thermal-mediated Sinorhizobium and Medicago crosstalk at the molecular level

MATERIALS AND METHODS

RESULTS

DISCUSSION

DATA AVAILABILITY STATEMENT