Actinomycetes impacts on drought stress in maize (Zea Mays L.)

Abstract

Drought is a major cause of the present decrease in crop yield and agricultural productivity, accounting for the recent worldwide shortage in food availability. Drought results from the current change in climate conditions. The disastrous effects of drought on plants calls for a renewed concern on improved and effective strategies to improve plant growth and yield under drought stress. This work is therefore designed to identify rhizospheric actinomycetes and evaluate their potential to improve maize growth under drought stress condition. In this study, seven actinomycetes strains were isolated from the rhizosphere of two maize fields. Biochemical and morphological characteristics, sequencing of l 6S rDNA genes and phylogenetic analysis of nucleotide sequences obtained from the I 6S rDNA genes of the bacterial isolates revealed that five of the isolates belong to the genus Streptomyces, one to Arthrobacter and the other Microbacterium. Isolates were screened for drought tolerance and it was observed that all isolates successfully grew in 5% polyethylene glycol (PEG) 8000 medium with outstanding growths observed in isolates A. arilaitensis and S. pseudovenezuelae. Bacterial growths at different pH values, temperatures, sodium chloride (NaCl) concentrations and drought tolerance potentials of isolates at different PEG concentration and time were examined and it was observed that isolates showed better growth between pH 5 and 9, temperatures 25 and 35 °C and 0 to 4% NaCl concentration. Maximum growths for all bacterial isolates were observed at a PEG concentration of 5% and 120 h indicating that PEG concentration and time affected bacterial growth. The primers that amplified specific genes encoding proteins involved in drought tolerance : Glutathione peroxidase (GPX), glycine-rich RNA binding protein (GRP) , desiccation protectant protein (DSP) and Gtp-binding protein (GTP) were designed and polymerase chain reaction (PCR) performed on them including the plant growth promoting (PGP) genes for ACC deaminase activity (Aced) and siderophore production (Sid) and amplifications were observed as follows: four isolates for GPX and GRP, two for DSP, one for GTP, seven for Aced and six for Sid genes. The amplification of these genes by some of the isolates is an indication of their drought tolerance and PGP potentials. Isolates were screened for the production and amount of PG P traits and it was observed that the 7 isolates produced indole-3-actetic acid (IAA), l -aminocycloprpane-1-carboxylate (ACC) deaminase, ammonia and siderophore, 5 solubilized phosphate and I produced hydrogen cyanide. Streptomyces werraensis produced the highest IAA of I 0.12 ± 0.02 μg/ml followed by isolate A. arilaitensis (9.44 ± 0.01 μg/ml), S. pseudovenezuelae produced the highest ACC deaminase activity of 0.903 ± 0.024 μmo I/min and A. arilaitensis produced the highest siderophore of 51.3%. Two (2) isolates (A . arilaitensis and S. pseudovenezuelae) were selected to evaluate the effect of bacterial inoculation on drought tolerance in maize plants at three soil moisture levels (field capacity, moderately watered and completely dry) and two inoculation methods (directly inoculated and vermiculite coated seeds). The results obtained showed that inoculated plants were not only protected from the deleterious effects of drought but also showed significant increase in the root and shoot lengths, chlorophyll contents, number of leaves, leaf area, and root and shoot dry weights. However, greatest growths were observed in the inoculated plants at field capacity. Significant growths were observed in plants whose seeds were coated with vermiculite compared to un-coated ones and also at field capacity. The results obtained on the trials also confirms the ability of the isolates to resist drought by growing on PEG 8000 medium and the amplification of protein encoding genes involved in drought tolerance. The overall findings of this study indicates that the drought tolerant actinomycetes are important tools that can be developed into bio-inoculants for effective improvement of drought stress tolerance in plants.