Day and blue light modify growth, cell physiology and indole-3-acetic acid production of Azospirillum brasilense Az39 under planktonic growth conditions.

AIM: In this work, we evaluated the effects of light on growth, cell physiology and stress response of Azospirillum brasilense Az39, a non-photosynthetic rhizobacteria, under planktonic growth conditions. METHODS AND RESULTS: Exponential cultures of Az39 were exposed to blue (BL), red (RL) and daylight (DL) or maintained in darkness for 24, 48 and 72 h. The biomass production and indole 3-acetic acid (IAA) biosynthesis increased by exposition to DL. Conversely, BL decreased IAA concentration through a direct effect on the molecule. The DL increased superoxide dismutase activity, hydrogen peroxide and thiobarbituric acid reactive substances levels, but the last one was also increased by BL. Both DL and BL increased cell aggregation but only BL increased biofilm formation. CONCLUSIONS: We demonstrated that both BL and DL are stress effectors for A. brasilense Az39 under planktonic growth conditions. The DL increased biomass production, IAA biosynthesis and bacterial response to stress, whereas BL induced cell aggregation and biofilms formation, but decreased the IAA concentration by photooxidation. SIGNIFICANCE AND IMPACT OF THE STUDY: Blue light and DL changes growth capacity, cell physiology and plant growth promotion ability of A. brasilense Az39 and these changes could be considered to improve the production and functionality of biofertilizers.

Evaluation of growth and motility in non-photosynthetic Azospirillum brasilense exposed to red, blue, and white light.

Azospirillum brasilense is a non-photosynthetic rhizobacterium that promotes the growth of plants. In this work, we evaluated the effects of different light qualities on the growth, viability, and motility in combination to other culture conditions such as temperature or composition of the culture medium. Exponential cultures of A. brasilense Az39 were inoculated by drop-plate method on nutritionally rich (LB) or chemically defined (MMAB) media in the presence or absence of Congo Red indicator (CR) and exposed continuously to white light (WL), blue light (BL), and red light (RL), or maintained in dark conditions (control). The exposure to BL or WL inhibited growth, mostly in LB medium at 36 °C. By contrast, the exposure to RL showed a similar behavior to the control. Swimming motility was inhibited by exposure to WL and BL, while exposure to RL caused only a slight reduction. The effects of WL and BL on plant growth-promoting rhizobacteria should be considered in the future as deleterious factors that could be manipulated to improve the functionality of foliar inoculants, as well as the bacterial effects on the leaf after inoculation.

Comparison studies of dinitrogenase reductase ADP-ribosyl transferase/dinitrogenase reductase activating glycohydrolase regulatory systems in Rhodospirillum rubrum and Azospirillum brasilense.

Reversible ADP ribosylation of dinitrogenase reductase, catalyzed by the dinitrogenase reductase ADP-ribosyl transferase (DRAT)/dinitrogenase reductase activating glycohydrolase (DRAG) regulatory system, has been characterized in both Rhodospirillum rubrum and Azospirillum brasilense. Although the general functions of DRAT and DRAG are very similar in these two organisms, there are a number of interesting differences, e.g., in the timing and extent of the regulatory response to different stimuli. In this work, the basis of these differences has been studied by the heterologous expression of either draTG or nifH from A. brasilense in R. rubrum mutants that lack these genes, as well as the expression of draTG from R. rubrum in an A. brasilense draTG mutant. In general, these hybrid strains respond to stimuli in a manner similar to that of the wild-type parent of the recipient strain rather than the wild-type source of the introduced genes. These results suggest that the differences seen in the regulatory response in these organisms are not primarily a result of different properties of DRAT, DRAG, or dinitrogenase reductase. Instead, the differences are likely the result of different signal pathways that regulate DRAG and DRAT activities in these two organisms. Our results also suggest that draT and draG are cotranscribed in A. brasilense.

Construction of an Azospirillum brasilense Sp7 recA mutant.

Cosmid clones encoding the recA gene of Azospirillum brasilense were isolated by intergeneric complementation of an Escherichia coli recA mutant. Site-directed Tn5 mutagenesis and subcloning of one complementing cosmid clone allowed us to localize the A. brasilense recA gene on a 1.2 kb DNA fragment. One Tn5 insertion that inactivates the cloned recA gene was crossed into the chromosome of A. brasilense by marker exchange. The resulting A. brasilense recA mutant showed increased sensitivity to the DNA methylating agent methyl methanesulfonate and to ultraviolet light and had at least one hundredfold reduced recombinational activity compared to the parent strain.