EndophytIic Bacterial Species for Plant

Indirect mechanism of plant growth promoting endophytes includes production of secondary metabolite which protects host plants from infection causing organisms by hydrocyanic acid production which is synthesized by the decarboxylation of glycine. HCN serves as an effective biological control agent against plant pathogens. HCN mainly stops electron transport chain and prevents energy supply to the cell, leading to death of the pathogen. In our study, 6.2 % endophytic bacterial isolates showed positive results for HCN production. These native endophytes could be used as biocontrol agents to suppress pathogenic microorganisms and improve tef crops growth, yield as well as grain quality.

HCN secreted by Pseudomonas fluorescent strain CHAO has been demonstrated to stimulate root hair formation and suppress back root rot caused by Thielaviopsis basicola in tobacco plant and improve yield (38).
Lytic enzymes act as agents for prevention of disease causing organisms by secreting cell wall lytic enzymes. In the present study, 14.9 %, 12.3% and 6.2% of the endophytic bacterial isolates showed effective result on synthesis of amylase, protease and cellulase, respectively and could be used as bacterial biocontrol agent to improve tef crops yield and reduce pesticides application.

PGP endophytes that produce different types of lytic enzymes are effectively control plant pathogenic fungi and bacteria. Bull et al [39] reported that Lysobacter and Myxobacteria produces lytic enzymes which have shown efficacy against some plant pathogenic fungi.
Abiotic stress factors such as temperature, pH, salinity and heavy metal contamination are the major factors that limit sustainable crops productivity and cause for more than 30 % of worldwide crops damages [40]. Even though many plant growth-promoting bacteria endophytes show good results during laboratory evaluation, they fail in the field when applied as bioinoculants.

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One main reason for their failure is the stress imposed on them by the sudden change of soil physical, chemical and biological properties in the environment [41].
Salinity is one of the most important factors that unfortunately affect plant growth and yield. It affects about 20 % of cultivated lands and 50 % of irrigated areas are affected by salinity. Plants cultivated in saline soil commonly increase their ethylene production in the body of the plants in order to recruit programmed cell death. Soil salinity has been reported to decrease plants productivity by affecting plants metabolism, and total nitrogen contents. Our present study 36.9 % of the bacterial isolates were grown well at 5% NaCl w/v. Around 12.8%, 11.3% and 6.2 % of bacterial isolates were grown well on 10 % NaCl, 15 % NaCl and 20 % NaCl w/v, respectively. These identified PGP endophytes showed ability to survive in a wide range of saline environment and thus, can improve tef crop productivity.
pH is another factors that affects life of the endophytic PGP bacteria species in the rhizosphere. In the present study, all of the tef endophytic bacterial isolates were grown well at pH-7 and none of them were grown at pH-13. Majorities (46.2 %) of bacterial isolates survived at pH-5 and few (4.6%) of them survived at pH-11. Temperature is also another plant growth limiting factors that affect PGP bacterial properties. In the present study, all of the identified tef endophytic bacterial isolates having PGP properties survived at 20 and 30oC, 36.4 % of the isolates survived at 40oC, and only 1 % survived at 50oC. No any endophytic bacterial isolates survived at 60oc. This indicates that tef crops colonizing endophytic bacteria can tolerate a wide range of pH and temperature that confirmed as potential plant growth promoting bacteria to sustain crop productivity.
For seed germination and seedling growth evaluation, a seeds of tef was inoculated with three tef endophytic PGP bacterial species (Pseudomonas fluorescens biotype G, Pseudomonas fluorescens and Pantoea agglomerans). Seed coating was made using single and consortium endophytic bacterial inoculation system. All of the tef seeds inoculated with potential endophytic bacterial species or strains were germinate tef seeds up to hundred percent on the third and fourth days after inoculation. Increase means shoot length (MSL) and roots length (RL) of the inoculated seeds up to 3 and 2.6 cm respectively and vigor index of seedling was measured from 460 to 520 on the last day of the experiment. Pradhan [42], reported that seeds inoculated with Bacillus sp. were significantly increased the germination, root and shoot length of the crops as compared to none inoculated one. Besides, Pieterse and Van Loon [43] reported that thirty percent growth improvement of Arabidopsis accession was achieved due to inoculation with Pseudomonas fluorescens. According to Woyessa and Assefa [44], inoculation of tef crops with Pseudomonas fluorescent increased mean root dry weight up to 39 % percent, root shoot ratio up to 42 %, and grain yield up to 28 % and also tef crops inoculated with Bacillus subtilis increased mean root dry weight of tef up to twenty eight, root shoot ratio up to nineteen nine percent and grain yield up to forty four percent.

CONCLUSIONS

These promising identified tef endophytic bacterial species or strains having PGP, biocontrol and abiotic stress tolerance properties are strong candidates for the development of bioinoculants as single strains or as consortia. They can be used to improve soil fertility, and conserve biodiversity. They have the potential to establish themselves as endophytes in tef roots and seeds, and can contribute to greater yield and productivity of crops in an eco-friendly manner. This strategy would work towards solving the problem of over-utilizing chemical fertilizers, reducing environmental pollution while increasing tef growth and yield.