Evaluation of the Plant Growth Promoting Effect of Root Contact, Diffusible and Volatile Compounds Produced by Rhizobacteria and Microalgae on Arabidopsis Thaliana

Bacteria and microalgae have beneficial impact on plant growth and survival through host functional and adaptive traits via complex mechanisms. Volatile and non-volatile metabolites produced by microorganisms have a continuous effect on plants by providing nutrients and regulating various plant metabolic and signaling pathways. The aim of this study was to assess the plant promoting effect of two Chlorella spp. microalgae under mixotrophic conditions, as well as the effects of plant growth promoting rhizobacteria (PGPR) Bacillus sp. WCC-B36, Azospirillum sp. WCC-ASP12 and Azotobacter sp. WCC-IZA56 on the model plant Arabidopsis thaliana. Growth and quality parameters were followed in three different co-cultivation systems as (i) direct root contact supplemented with density effect, (ii) contact with diffusible compounds and (iii) effects of volatile compounds. Direct effect mediated by rhizobacteria promoted significant shoot and root length growth with well-developed root architecture at low bacterial densities (<105 CFU – colony forming unit mL−1), which became more pronounced over time. At a higher microbial density (>107 CFU mL−1), plant growth was retarded regardless of the bacteria present. This suggests that the microenvironment surrounding the colonies was altered and there was competition for nutrients. Our results indicate that the metabolites, diffusible and volatile organic substances produced by the microalgae enhanced lateral root growth and root hair formation, while inhibited primary root elongation. Volatile and diffusible substances of Chlorella sp. CHL13 and Bacillus sp. WCC-B36 have the most significant effect on seedlings and primary root growth.