Introduction of Endophytic Pseudomonas rhodesiae and Acinetobacter sp. Effective on Seed Germination and Cucumber Growth Factors Improvement

Some bacteria are capable of entering the plant as endophytes that do not cause harm and could establish a mutualistic association with host plants. Endophytic bacteria are bacteria that live in plant tissues without doing substantive harm. They enter plant tissue primarily through different plant zones. Both Gram-positive and Gram-negative bacteria have been isolated from several tissue types in several plant species. In addition, several different bacterial species have been isolated from a single plant. Variation in endophytic bacteria populations referred to the time of sampling, type of plant tissue, age and environment conditions, as well. In general endophytic bacteria occur at lower population densities than rhizospheric bacteria or bacterial pathogens. Endophytic populations, like rhizospheric populations, are conditioned by biotic and abiotic factors, but endophytic bacteria could be better protected from biotic and abiotic stresses than rhizospheric bacteria. It is clear that the interaction between plants and some endophytic bacteria is associated with beneficial effects such as plant growth promotion and biocontrol potential against plant pathogens. These types of bacteria are often capable of eliciting significant physiological changes that modulate the growth and development of the plant. Most of the time, these beneficial effects of endophytes are greater than those of many rhizosphere-colonizing bacteria. Endophytic bacteria affect bacterial growth by numerous mechanisms directly or indirectly. Some genus of bacteria such as Azosprillium, Enterobacter, Azotobacter and Pseudomonas produces plant growth regulators which lead to plant growth improvement. Microorganism profit from plants due to the enhanced availability of nutrients, whereas plants can receive benefits from bacterial associates by growth enhancement or stress reduction. Therefore, mutualistic interactions between host plants and associated microorganisms could have emerged as a result of the clear positive selection exerted on these associations. The current study is conducted in order to evaluate the effectiveness of cucumber root endophytic bacteria on increasing plant growth indexes.
In this study, 45 strains were isolated from cucumber roots as mentioned in the literature. In the first step, all of them subjected to seed germination assay in lab conditions. Furthermore, they are evaluated for vigor index according to references. Seven strains (En 1 to En7) are selected for complementary research in greenhouse trials using randomized block design with 4 repetitions. Statistical analysis is done using SPSS v.22 by Duncan methods under 5% possibility levels. To detect preferred bacterial species, 16S rDNA-PCR product is sent for sequencing to Macrogen, Korea. The sequences are checked in BLAST program in NCBI database.
Based on medians comparison, cucumber seed cultivar Negin, are treated by 45 endophytic bacteria separately. Among them, seven strains showed a significant difference at 1% possibility level in comparison to control. These strains presented high vigor index from 1954.6 to 572 compared to control. They also showed better seed germination percentage rather than control in the range of 95 to 45 %, and selected for greenhouse trials. The results of medians comparison in pot experiments demonstrated that En1 strains leads to 60% root and shoot length enhancement comparing to control. Furthermore, highest shoot (84.6%) and root (55.8%) wet weight are recorded for En6 in comparison to control. Molecular data revealed that strains En1 and En6 belonging to Acinetobacter sp. and Pseudomonas rhodesiae, respectively. It is known that variation in endophytic bacteria may reflect differences in agriculture management methods. The P. rhodesiae is categorized in Pseudomonas fluorescent group and is isolated from mineral water for the first time. The positive effect of P. rhodesiae on different agricultural crops such as tomato, paper and wheat is reported. Genus Acinetobacter has an ability to produce plant hormones which changes plant chemical function towards growth improvement. One strain of this genus causes enhancement of sugar beet dry and wet weight to 19 and 69 % respectively, more than control treatment. More studies show that auxin production, phosphate solubilization, and nitrogen fixation are related to plants growth increasing. Since these entophytic bacteria have the effective role in inorganic materials increasing, application of this genus as biological fertilizer is suggested.
Based on our data, En1 and En6 isolates speed up seed germination and increase cucumber growth parameters. Complementary surveys concerning the application of these isolates as separate or combined samples under greenhouse and field conditions are recommended. This is the first report of isolation of Pseudomonas rhodesiae and Acinetobacter sp. as endophytic bacteria from cucumber in Iran.