Cold atmospheric pressure (CP) plasma irradiation of seeds has been shown to promote plant growth, but the molecular basis of this phenomenon is poorly understood. In our study, optimum irradiation of common sunflower seeds using a dielectric barrier discharge CP device stimulated growth of sunflower lateral organs and roots by 9-14% compared to the control. Metagenomic analysis revealed that the structure of plant-associated bacterial assembly was greatly modified upon CP treatment and could be attributed to the antimicrobial effect of CP-generated reactive species. The treatment resulted in the domination of spore forming Mycobacterium sp. in the above-ground tissues of the seedlings. While the overall bacterial diversity in the roots was barely affected, the CP-induced shift in microbial composition is the likely basis for the observed seedling root growth stimulation and the long-term effect on lateral organ growth and could be mediated by increase in water uptake and/or direct root signaling. Low amplitude protein abundance differences were detected in the roots of the emerging seedlings that are characteristic to low intensity stress stimuli response and could be linked to the changes in plant-associated microbiome upon CP treatment.

Bacterial endophytes are common inhabitants of plant tissues that have been shown to play an important role in regulation of plant growth and to have the potential as biological agent for plant disease protection. Only fragmented knowledge is present about endophytes that reside in the phyllosphere of cultivated tree plants such as domestic apple (Malus × domestica Borkh.). Therefore the goal of this study was to identify culturable endophytic bacteria characteristic of an apple phyllosphere and to establish biochemical traits involved in plant growth promoting activity as well as to study microbial growth suppressing activity of the endophytes. Thirty-eight putative endophytic bacteria were isolated from apple buds of cultivars 'Gala', 'Golden Delicious' and 'Orlovim' grown under field conditions and 13 of the isolates were assigned to Curtobacterium, Pantoea and Pseudomonas species. Biochemical tests revealed traits important for plant growth stimulation and microbial growth suppression characteristics of the isolates, including nitrogen fixation, production of indole-3-acetic acid (IAA), phosphate solubilization, production of siderophores and hydrogen cyanide. Several isolates displayed antagonistic activity against selected non-pathogenic and pathogenic bacterial strains: 17 isolates were able to inhibit growth of Micrococcus luteus, 4 -Pseudomonas aeruginosa, 2 -Escherichia coli and Bacillus subtilis. In addition, it was determined that two isolates of Pantoea sp. (D_8 and D_10) and Pseudomonas fluorescens group isolate D_7 were able to inhibit growth of the apple scab pathogen (Venturia inaequalis (Cke) Wint.), suggesting a role of the endophytes in disease resistance and a potential use for biocontrol applications.

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The objective of this study was to establish changes in the composition of plant-associated microbiome of Arabidopsis thaliana (L.) induced by cold plasma (CP) treatment of seeds. Metagenomic analysis revealed that growth-stimulating CP treatment largely reduced the abundance of actinobacteria of Mycobacteriaceae family, resulting in the domination of Bacillaceae in germinated seedlings. Changes in the composition of mature plant microbiota were mainly manifested by the enhanced relative abundance of Sinobacteraceae and Nocardiaceae as well as Pseudomonadaceae, Paenibacillaceae and Alcaligenaceae families which include the common rhizosphere and growth-promoting bacteria.

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