TyrR is involved in the transcriptional regulation of biofilm formation and D-alanine catabolism in Azospirillum brasilense Sp7

Reviewing and renewing the use of beneficial root and soil bacteria for plant growth and sustainability in nutrient-poor, arid soils

A rapidly increasing human population coupled with climate change and several decades of over-reliance on synthetic fertilizers has led to two pressing global challenges: food insecurity and land degradation. Therefore, it is crucial that practices enabling both soil and plant health as well as sustainability be even more actively pursued. Sustainability and soil fertility encompass practices such as improving plant productivity in poor and arid soils, maintaining soil health, and minimizing harmful impacts on ecosystems brought about by poor soil management, including run-off of agricultural chemicals and other contaminants into waterways. Plant growth promoting bacteria (PGPB) can improve food production in numerous ways: by facilitating resource acquisition of macro- and micronutrients (especially N and P), modulating phytohormone levels, antagonizing pathogenic agents and maintaining soil fertility. The PGPB comprise different functional and taxonomic groups of bacteria belonging to multiple phyla, including Proteobacteria, Firmicutes, Bacteroidetes, and Actinobacteria, among others. This review summarizes many of the mechanisms and methods these beneficial soil bacteria use to promote plant health and asks whether they can be further developed into effective, potentially commercially available plant stimulants that substantially reduce or replace various harmful practices involved in food production and ecosystem stability. Our goal is to describe the various mechanisms involved in beneficial plant-microbe interactions and how they can help us attain sustainability.

The GGDEF-EAL protein CdgB from Azospirillum baldaniorum Sp245, is a dual function enzyme with potential polar localization

Azospirillum baldaniorum Sp245, a plant growth-promoting rhizobacterium, can form biofilms through a process controlled by the second messenger cyclic diguanylate monophosphate (c-di-GMP). A. baldaniorum has a variety of proteins potentially involved in controlling the turnover of c-di-GMP many of which are coupled to sensory domains that could be involved in establishing a mutualistic relationship with the host. Here, we present in silico analysis and experimental characterization of the function of CdgB (AZOBR_p410089), a predicted MHYT-PAS-GGDEF-EAL multidomain protein from A. baldaniorum Sp245. When overproduced, CdgB behaves predominantly as a c-di-GMP phosphodiesterase (PDE) in A. baldaniorum Sp245. It inhibits biofilm formation and extracellular polymeric substances production and promotes swimming motility. However, a CdgB variant with a degenerate PDE domain behaves as diguanylate cyclase (DGC). This strongly suggest that CdgB is capable of dual activity. Variants with alterations in the DGC domain and the MHYT domain negatively affects extracellular polymeric substances production and induction of swimming motility. Surprisingly, we observed that overproduction of CdgB results in increased c-di-GMP accumulation in the heterologous host Escherichia coli, suggesting under certain conditions, the WT CdgB variant can behave predominantly as a DGC. Furthermore, we also demonstrated that CdgB is anchored to the cell membrane and localizes potentially to the cell poles. This localization is dependent on the presence of the MHYT domain. In summary, our results suggest that CdgB can provide versatility to signaling modules that control motile and sessile lifestyles in response to key environmental signals in A. baldaniorum.

Activation by TyrR in Escherichia coli K-12 by Interaction between TyrR and the α-Subunit of RNA Polymerase

A novel selection was developed for mutants of the C-terminal domain of RpoA (α-CTD) altered in activation by the TyrR regulatory protein of Escherichia coli K-12. This allowed the identification of an aspartate to asparagine substitution at residue 250 (DN250) as an activation-defective (Act^-) mutation. Amino acid residues known to be close to D250 were altered by in vitro mutagenesis, and the substitutions DR250, RE310, and RD310 were all shown to be defective in activation. None of these mutations caused defects in regulation of the upstream promoter (UP) element. The rpoA mutation DN250 was transferred onto the chromosome to facilitate the isolation of suppressor mutations. The TyrR mutations EK139 and RG119 caused partial suppression of rpoA DN250, and TyrR RC119, RL119, RP119, RA77, and SG100 caused partial suppression of rpoA RE310. Additional activation-defective rpoA mutants (DT250, RS310, and EG288) were also isolated, using the chromosomal rpoA DN250 strain. Several new Act^- tyrR mutants were isolated in an rpoA⁺ strain, adding positions R77, D97, K101, D118, R119, R121, and E141 to known residues S95 and D103 and defining the activation patch on the amino-terminal domain (NTD) of TyrR. These results support a model for activation of TyrR-regulated genes where the activation patch on the TyrR NTD interacts with the TyrR-specific patch on the α-CTD of RNA polymerase. Given known structures, both these sites appear to be surface exposed and suggest a model for activation by TyrR. They also help resolve confusing results in the literature that implicated residues within the 261 and 265 determinants as activator contact sites. IMPORTANCE Regulation of transcription by RNA polymerases is fundamental for adaptation to a changing environment and for cellular differentiation, across all kingdoms of life. The gene tyrR in Escherichia coli is a particularly useful model because it is involved in both activation and repression of a large number of operons by a range of mechanisms, and it interacts with all three aromatic amino acids and probably other effectors. Furthermore, TyrR has homologues in many other genera, regulating many different genes, utilizing different effector molecules, and in some cases affecting virulence and important plant interactions.

A regulatory network involving Rpo, Gac and Rsm for nitrogen-fixing biofilm formation by Pseudomonas stutzeri

Biofilm and nitrogen fixation are two competitive strategies used by many plant-associated bacteria; however, the mechanisms underlying the formation of nitrogen-fixing biofilms remain largely unknown. Here, we examined the roles of multiple signalling systems in the regulation of biofilm formation by root-associated diazotrophic P. stutzeri A1501. Physiological analysis, construction of mutant strains and microscale thermophoresis experiments showed that RpoN is a regulatory hub coupling nitrogen fixation and biofilm formation by directly activating the transcription of pslA, a major gene involved in the synthesis of the Psl exopolysaccharide component of the biofilm matrix and nifA, the transcriptional activator of nif gene expression. Genetic complementation studies and determination of the copy number of transcripts by droplet digital PCR confirmed that the regulatory ncRNA RsmZ serves as a signal amplifier to trigger biofilm formation by sequestering the translational repressor protein RsmA away from pslA and sadC mRNAs, the latter of which encodes a diguanylate cyclase that synthesises c-di-GMP. Moreover, RpoS exerts a braking effect on biofilm formation by transcriptionally downregulating RsmZ expression, while RpoS expression is repressed posttranscriptionally by RsmA. These findings provide mechanistic insights into how the Rpo/Gac/Rsm regulatory networks fine-tune nitrogen-fixing biofilm formation in response to the availability of nutrients.

Expression and function of the cdgD gene, encoding a CHASE-PAS-DGC-EAL domain protein, in Azospirillum brasilense

The plant growth-promoting bacterium Azospirillum brasilense contains several genes encoding proteins involved in the biosynthesis and degradation of the second messenger cyclic-di-GMP, which may control key bacterial functions, such as biofilm formation and motility. Here, we analysed the function and expression of the cdgD gene, encoding a multidomain protein that includes GGDEF-EAL domains and CHASE and PAS domains. An insertional cdgD gene mutant was constructed, and analysis of biofilm and extracellular polymeric substance production, as well as the motility phenotype indicated that cdgD encoded a functional diguanylate protein. These results were correlated with a reduced overall cellular concentration of cyclic-di-GMP in the mutant over 48 h compared with that observed in the wild-type strain, which was recovered in the complemented strain. In addition, cdgD gene expression was measured in cells growing under planktonic or biofilm conditions, and differential expression was observed when KNO₃ or NH₄Cl was added to the minimal medium as a nitrogen source. The transcriptional fusion of the cdgD promoter with the gene encoding the autofluorescent mCherry protein indicated that the cdgD gene was expressed both under abiotic conditions and in association with wheat roots. Reduced colonization of wheat roots was observed for the mutant compared with the wild-type strain grown in the same soil conditions. The Azospirillum-plant association begins with the motility of the bacterium towards the plant rhizosphere followed by the adsorption and adherence of these bacteria to plant roots. Therefore, it is important to study the genes that contribute to this initial interaction of the bacterium with its host plant.