Polyamines produced by Sinorhizobium meliloti Rm8530 contribute to symbiotically relevant phenotypes ex planta and to nodulation efficiency on alfalfa

Abstract

In nitrogen-fixing rhizobia, emerging evidence shows significant roles for polyamines in growth and abiotic stress resistance. In this work we show that a polyamine-deficient ornithine decarboxylase null mutant (odc2) derived from Sinorhizobium meliloti Rm8530 had significant phenotypic differences from the wild-type, including greatly reduced production of exopolysaccharides (EPS; ostensibly both succinoglycan and galactoglucan), increased sensitivity to oxidative stress and decreased swimming motility. The introduction of the odc2 gene borne on a plasmid into the odc2 mutant restored wild-type phenotypes for EPS production, growth under oxidative stress and swimming. The production of calcofluor-binding EPS (succinoglycan) by the odc2 mutant was also completely or mostly restored in the presence of exogenous spermidine (Spd), norspermidine (NSpd) or spermine (Spm). The odc2 mutant formed about 25 % more biofilm than the wild-type, and its ability to form biofilm was significantly inhibited by exogenous Spd, NSpd or Spm. The odc2 mutant formed a less efficient symbiosis with alfalfa, resulting in plants with significantly less biomass and height, more nodules but less nodule biomass, and 25 % less nitrogen-fixing activity. Exogenously supplied Put was not able to revert these phenotypes and caused a similar increase in plant height and dry weight in uninoculated plants and in those inoculated with the wild-type or odc2 mutant. We discuss ways in which polyamines might affect the phenotypes of the odc2 mutant.

The <em>Sinorhizobium Meliloti</em> Nsps-Mbaa System affects Biofilm Formation, Exopolysaccharide Production and Motility in Response to Specific Polyamines

“…We previously reported that exogenous polyamines affect biofilm formation, EPS production and motility in S. meliloti Rm8530 [36]. We hypothesized that these effects were at least partly due to the S. meliloti NspS-MbaA homologs acting as a polyamine sensing-transducing system to modulate biofilm formation in a manner analogous to the V. cholerae system [23,24].…”

“…Individual colonies of the S. meliloti strains were stabbed into the swarming media with a toothpick and incubated at 30°C for 72 h. Swarm zones were quantitated by taking the average of two sides of a rectangle that framed the zone [35]. Swimming assays were performed in Bromfield medium containing 0.3 % Noble agar as described previously [36].…”

Microbial Musings – May 2020

“…Another strong paper from Michael Dunn's group at the Universidad Nacional Autónoma de México, Cuernavaca, Mexico, features in this issue, which is in fact the second from this group in 2020 (I might have to think of some kind of a prize) [10,11]. Continuing the theme of studying nitrogen metabolism in nitrogen-fixing bacteria, in this paper his group identify the arginase enzyme from Sinorhizobium meliloti, which is required for use of l-arginine as the sole nitrogen source by first breaking it down to l-ornithine and urea.…”

Microbial Musings – March 2020

“…The number of nodules was not reduced, but their function was poor, as reflected by decreases in plant weight and height. The authors think this nodule phenotype most likely results from the defects in EPS production, which likely underlie many of the other phenotypes, but the mechanism of this control is not known at present [4].…”

“…In our first paper we focus on the α-proteobacterium Sinorhizobium meliloti , which forms root nodules in leguminous plants that mature to become little cell factories for fixing nitrogen gas, ultimately to organic nitrogen, whilst at the same time being fed by the plant. A new paper from Michael F. Dunn's group in UNAM, Mexico, examined the function of polyamines in the biology of this bacterium [ 4 ]. These are small polycations that are accumulated by bacterial cells for a variety of cellular functions, and which his group, working with my colleague Jane Thomas-Oates at York ( @ChemistryatYork ), had shown in 2018 were required for normal cell growth [ 5 ].…”