Drought is a major cause of the present decrease in crop yield and agricultural productivity around the globe. The disastrous effects of drought on plants call for a renewed concern on effective strategies to improve plant growth and yield under drought stress. This study was designed to evaluate the effectiveness of Arthrobacter arilaitensis and Streptomyces pseudovenezuelae to improve maize growth under drought stress at three soil moisture levels using two inoculation methods. Seven rhizosphere actinomycetes isolates were screened for the production of plant growth promoting (PGP) traits and it was observed that all isolates produced one or more PGP properties. The inoculated plants were not only protected from the deleterious effects of drought, but also showed significant increases in the physiological parameters measured. The findings of this study suggest that these isolates are important tools capable of being developed into bio-inoculants to effectively improve drought tolerance in plants.

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Many studies have shown that the maize rhizosphere comprises several plant growth-promoting microbes, but there is little or no study on the effects of land-use and management histories on microbial functional gene diversity in the maize rhizosphere soils in Africa. Analyzing microbial genes in the rhizosphere of plants, especially those associated with plant growth promotion and carbon cycling, is important for improving soil fertility and crop productivity. Here, we provide a comparative analysis of microbial genes present in the rhizosphere samples of two maize fields with different agricultural histories using shotgun metagenomics. Genes involved in the nutrient mobilization, including nifA, fixJ, norB, pstA, kefA and B, and ktrB were significantly more abundant (α = 0.05) in former grassland (F1) rhizosphere soils. Among the carbon-cycling genes, the abundance of 12 genes, including all those involved in the degradation of methane were more significant (α = 0.05) in the F1 soils, whereas only five genes were significantly more abundant in the F2 soils. α-diversity indices were different across the samples and significant differences were observed in the β diversity of plant growth-promoting and carbon-cycling genes between the fields (ANOSIM, p = 0.01 and R = 0.52). Nitrate-nitrogen (N-NO3) was the most influential physicochemical parameter (p = 0.05 and contribution = 31.3%) that affected the distribution of the functional genes across the samples. The results indicate that land-use and management histories impact the composition and diversity of plant growth-promoting and carbon-cycling genes in the plant rhizosphere. The study widens our understanding of the effects of anthropogenic activities on plant health and major biogeochemical processes in soils.

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