INVOLVEMENT OF MEMBRANE SULFOLIPIDS IN THE ACID TOLERANCE RESPONSE (ATR) OF Sinorhizobium meliloti

MORENO-OCAMPO, A. ; LÓPEZ-LARA, I.M.; DRAGHI, W.O.; LAGARES, A.; MARTÍNEZ-AGUILAR, L.; GEIGER, O.

Congreso; LVI SAIB Meeting XV SAMIGE Meeting; 2020

SAIB SAMIGE

Acid stress is an abiotic condition that severely limits the growth and symbiotic performance of Sinorhizobium meliloti (Sme). In order to cope with low pH, bacteria display a multigenic response to surpass the deleterious effects of environmental acidity. A common mechanism triggered by acidity is the remodeling of bacterial cell membranes to resist the high proton concentration, modifying membrane permeability to maintain cellular homeostasis. In a previous microarray experiment, we showed that the sqdB gene, which is involved in the sulfoquinovosyldiacylglycerol biosynthesis, was differentially expressed in acid-grown Sme cultures. This result suggests the involvement of sulfolipids (SL) in the remodeling membranes in acidgrown Sme. In order to determine a different membrane composition, we performed a quantitative analysis of SL in neutraland acid-grown cultures. [35S]sulfate-labeled cultures of S. meliloti 1021 were harvested at O.D.600nm=0.2±0.1, and membrane lipids were extracted and separated by thin-layer chromatography. Results showed a higher presence of SL in cell membranes of S. meliloti 1021 when grown at low pH, which is in agreement with a higher expression of the sqdB gene, as previously observed. At early log-phase, S. meliloti triggers an Acid Tolerance Response (ATR(+)), a cell phenotype that involves a differential death rate as well as an increased competition for nodule occupancy in neutral and acid conditions, turning it into a desirable phenotype for improving N fixation rates in acid soils. In order to establish the involvement of SL in the ATR(+) phenotypes, we determined the decimal reduction time (D10) of wild type and mutant strains altered in SL synthesis. Results of Sme strain unable to synthesize SL (Sme SLD12) showed a higher death rate, and the wild type phenotype was restored in complemented Sme strain (Sme W38). Thus, the presence of SL in membranes of acid-grown bacteria seems to be essential for displaying an ATR(+) response of Sme. The ATR(+) symbiotic performance was also evaluated in Sme strains altered in SL biosynthesis. The infection kinetic in alfalfa seedlings in acid conditions, as well as the nodule number at 21 days postinfection showed no differences in wild type, null mutant and complemented strains. A Sme strain with overexpression of SL synthesis (Sme C22), however, showed significant differences in their symbiosis behaviour, with delayed nodulation kineticas well as minor nodule numbers. Thus, results showed that in Sme the presence and the regulation of the synthesis of membrane sulfolipids are critical for surpassing acid stressful conditions. Additional experimental approaches are carried on in order to determine if a higher presence of SL alter the proton permeability of cellular membranes.