The effect of water stress on peanut soil microbiome and plant physiology

Abstract

Arachis hypogea is an economically important crop in the Southeastern region of the United States. The kernel has a high nutritional value containing oil, proteins, and carbohydrates. However, 90% of farmers in Alabama do not irrigate their peanuts due to expensive irrigation and/or water availability. Peanuts in Alabama and nearby regions tend to grow in sandy soil under hot and dry periods, possibly leading to drought. Drought alone can take over $50 million out of the U.S. yearly and is one of the most severe abiotic stresses that affect plants. As the temperature rises and water becomes restricted, soil solute concentration increases, and moisture availability and soil nutrient content decrease. Peanuts under water stress symptoms include wilting, folding, and drooping of the shoot; closure of stomata leads to an increase in photorespiration and a reduction of photosynthesis, the plant weakens, facilitating the incidence of infection, and a decrease of yield can be observed. Aspergillus flavus is a fungus that infects peanuts under hot and dry conditions. This fungus produces secondary metabolites (aflatoxins) that are carcinogenic and hepatotoxic in animals and humans. Studies have found a higher concentration of aflatoxins in the kernel, possibly due to its high oil and protein concentration. Conversely, some microbes can help by improving growth by regulating and producing phytohormones, enhancing water and nutrient availability, and conferring biotic and abiotic tolerance to plants. The family Mortierellaceae and the species Penicillium citrinum have shown plant growth-promoting properties on various plant hosts. Hence, detecting microbes that can antagonize A. flavus or confer drought tolerance to peanut plants is highly desired. Chapter one will provide an overview of peanuts and their importance, the effects of drought on the peanut plant and the soil microbiome, and soil fungal species known for plant growth-promoting properties in drought-stress environments. Chapter two focused on testing fungi that can alleviate drought stress in peanuts using two different fungal collections. For this study, the fungal collections were screened for salt and high temperatures, and five fungal cultures were selected and used for inoculating peanut seedlings under drought and no drought conditions. Fungal treatments were P. citrinum CCH_F37_B, Mortierella alpina OEO-305, M. calciphila OEO-304, Linnemannia elongata OEO-198, L. elongata OEO-196. Under no drought conditions, L. elongata OEO-196 significantly increased shoot biomass, while M. calciphila OEO-304 trended lower root and shoot biomass but was not significant. However, interestingly, M. calciphila OEO-304 had a positive trend in the dry biomass and was no different from control without drought. The treatment with P. citrinum CCH_F37_B altered photosynthetic efficiency in both droughted and no-drought experiments, but the plant was smaller overall. More studies are needed to understand the mechanisms of alleviating water stress from these treatments in peanut plants, but some fungi show promising results. In the third chapter, we designed experiments to understand how water regimes impact the microbial communities from two peanut soils. We hypothesized that applying different water regimes to the peanut soils would alter the microbial composition over time. Soils from two different fields at Wiregrass Research and Extension Center Headland, Alabama, were collected and transferred to a polyvinylchloride tube inside the growth chamber, where five different water treatments were applied each week for a total of nine weeks at 29oC. Each week soil was collected, DNA extracted, and sequenced. Results show that the water regime applied created a gradient and had a significant impact on the microbial communities in both soils. Actinobacteriota was an indicator taxon for drier soils, while Proteobacteria and Planctomycetota were more associated with moist ones. This study aims to show the necessity of alleviating peanut stress under water restriction that leads to yield loss, and understanding the microbiome under drought conditions can also guide the detection of this microbe or set of microbes. The conclusions and impacts of this study are presented in chapter four.