Metarhizium anisopliae's persistence as a saprophyte, genetic basis of adaptation and role as a plant symbiont

Abstract

Metarhizium anisopliae is a model for elucidating the basis of entomopathogenicity. However, many aspects of the saprophytic life of M. anisopliae remain unclear, including: 1) how the rhizosphere maintains populations of M. anisopliae; 2) the potential for microevolution; 3) the genetic factors that allow M. anisopliae to adapt to a saprophytic life and 4) the extent to which plant ecology is impacted by the rhizospheric competence of M. anisopliae. To extend our knowledge of plant-insect-fungal interactions, a field trial was conducted with M. anisopliae applied to turf. The specific objectives were to: 1) investigate the genetic basis of adaptation and strain stability of M. anisopliae; 2) monitor long-term survival of M. anisopliae, 3) compare the winter survival of M. anisopliae ΔMcl1 (disrupted in an immune evasion gene and nonpathogenic) with M. anisopliae ΔMad2 (adhesin-deficient and unable to adhere to plant epidermis) and 4) investigate root colonization and its impact on plant growth of Triticum aestivum seeds coated with conidia. Results showed that M. anisopliae ΔMad2 had a linear decrease in population and did not colonize roots, while M. anisopliae and M. anisopliae ΔMcl1 cycled with seasons in the soil and colonized roots. Microarray analyses were used to assay for any mutations affecting gene expression during survival in the field. After 3.5 years, field recovered Metarhizium isolates differed in gene expression by an average of 0.26 % for the 1,749 expressed sequence tags. Mutations disproportionately affected cell wall and stress responses, while genes coding for pathogenicity determinants such as adhesins and toxins were highly conserved. Triticum seeds inoculated with M. anisopliae ΔMcl1 and M. anisopliae ΔMad2 prior to planting in the field produced a 14.92%, 4% and 0% increase in seed weight respectively. M. anisopliae increased growth rates of winter wheat in microcosms and may act as a biofertilizer by making insoluble phosphate available to plants. This study showed Metarhizium benefits plants by protecting them from insect pests and by potentially solubilizing otherwise unavailable nutrients. Metarhizium, therefore, may be implemented in an integrated pest management (IPM) approach to reduce the use of chemical insecticides and fertilizers.