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3,3'-Thiodipropionic acid (TDP) is used as an additive in food and cosmetic industry and as precursor substrate for biotechnical polythioester production. Its catabolism was investigated in Variovorax paradoxus TBEA6 previous to this study. It was reported that the insertion of the transposon Tn5::mob into a gene showing high homology to flavin-dependent oxidoreductases (fox) resulted in impaired growth with TDP. Therefore, it was assumed that the initial cleavage of TDP is catalyzed by an FAD-dependent oxidoreductase (Fox, VPARA_05580). Accordingly, fox was heterologously expressed as a thioredoxin fusion protein. Analytical size exclusion chromatography revealed a homodimeric structure and the presence of the cofactor FAD. In vitro experiments showed that FoxTBEA6 is a D-2-hydroxy acid specific dehydrogenase and that its activity is enhanced in presence of either Ni2+, Co2+ or Zn2+. Cleavage of TDP by FoxTBEA6 was not observed. The findings are contrary to restricted growth with TDP of the transposon mutants and the previously published deletion mutant V. paradoxus TBEA6 Δfox. In this study, this contradiction was investigated by generation of additional deletion mutants, in which partial or complete deletion of fox did not affect utilization of TDP, and the mapping of single nucleotide polymorphisms (SNPs) in V. paradoxus TBEA6 Δfox.

Utilization of rhizobacteria that have associated with plant roots in harsh environments could be a feasible strategy to deal with limits to agricultural production caused by soil salinity. Halophytes occur naturally in high-salt environments, and their roots may be associated with promising microbial candidates for promoting growth and salt tolerance in crops. This study aimed to isolate efficient halotolerant plant-growth-promoting rhizobacterial strains from halophytes and evaluate their activity and effects on sugar beet (Beta vulgaris L.) growth under salinity stress. A total of 23 isolates were initially screened for their ability to secrete 1-aminocyclopropane-1-carboxylate deaminase (ACD) as well as other plant-growth-promoting characteristics and subsequently identified by sequencing the 16S rRNA gene. Three isolates, identified as Micrococcus yunnanensis, Planococcus rifietoensis and Variovorax paradoxus, enhanced salt stress tolerance remarkably in sugar beet, resulting in greater seed germination and plant biomass, higher photosynthetic capacity and lower stress-induced ethylene production at different NaCl concentrations (50-125 mM). These results demonstrate that salinity-adapted, ACD-producing bacteria isolated from halophytes could promote sugar beet growth under saline stress conditions.

Soil-microorganism symbioses are of fundamental importance for plant adaptation to the environment. Research in microbial ecology has revealed that some soil bacteria are associated with arbuscular mycorrhizal fungi (AMF). However, these interactions may be much more complex than originally thought. To assess the type of bacteria associated with AMF, we initially isolated spores of Glomus irregulare from an Agrostis stolonifera rhizosphere. The spores were washed with sterile water and plated onto G. irregulare mycelium growing in vitro in a root-free compartment of bicompartmented Petri dishes. We hypothesized that this system should select for bacteria closely associated with the fungus because the only nutrients available to the bacteria were those derived from the hyphae. Twenty-nine bacterial colonies growing on the AMF hyphae were subcultured and identified using 16S rRNA gene sequences. All bacterial isolates showed high sequence identity to Bacillus cereus, Bacillus megaterium, Bacillus simplex, Kocuria rhizophila, Microbacterium ginsengisoli, Sphingomonas sp. and Variovorax paradoxus. We also assessed bacterial diversity on the surface of spores by PCR-denaturating gradient gel electrophoresis. Finally, we used live cellular imaging to show that the bacteria isolated can grow on the surface of hyphae with different growing patterns in contrast to Escherichia coli as a control.

A newly isolated strain of Variovorax paradoxus could grow on homovanillate and several monohydroxylated phenylacetic acids. During growth on homovanillate, the organism formed separate NAD(P)H-dependent hydroxylases with activity towards 4-hydroxyphenylacetic acid and homovanillate. Homovanillate hydroxylase catalysed a typical monooxygenase reaction and had little activity towards 4-hydroxyphenylacetic acid. GC-MS and TLC analysis suggested that homovanillate was 1-hydroxylated to yield a dihydroxymonomethoxyphenylacetic acid which served as a substrate for homogentisate 1,2-dioxygenase. Methanol, but not formaldehyde, was released either during ring-cleavage or subsequent metabolism of the ring-cleavage product.