Root environment is a key determinant of fungal entomopathogen endophytism following seed treatment in the common bean, Phaseolus vulgaris

Soroush Parsa,Viviana Ortiz,María I Gómez-Jiménez,Matthew Kramer,Fernando E Vega

doi:10.1016/j.biocontrol.2016.09.001

Abstract

The common bean is the most important food legume in the world. We examined the potential of the fungal entomopathogens Beauveria bassiana and Metarhizium anisopliae applied as seed treatments for their endophytic establishment in the common bean. Endophytic colonization in sterile sand:peat averaged ca. 40% higher for fungus treatments and ca. six times higher for volunteer fungi (other fungal endophytes naturally occurring in our samples), relative to sterile vermiculite. Colonization by B. bassiana and M. anisopliae was least variable in sterile vermiculite and most variable in sterile soil:sand:peat. The impact of soil sterilization on endophytic colonization was assessed in a separate experiment using six different field-collected soils. Soil sterilization was the variable with the largest impact on colonization (70.8% of its total variance), while the fungal isolate used to inoculate seeds explained 8.4% of the variance. Under natural microbial soil conditions experienced by common bean farmers, seed inoculations with B. bassiana and M. anisopliae are unlikely to yield predictable levels of endophytic colonization.

Highlights

Very little is known about the relative contributions of genetic factors and environmental factors, to the effectiveness of seed treatments with fungal entomopathogens. We addressed this imperative by screening multiple isolates/strains of Beauveria bassiana sensu lato (Balsamo-Crivelli) Vuillemin and Metarhizium anisopliae (Metchnikoff) Sorokin sensu lato (Ascomycota: Hypocreales) under various soil conditions in seed treatments intended to establish the fungal entomopathogens as endophytes in the common bean
Preliminary screening experiments in vermiculite with 22 fungal isolates suggested that B. bassiana is generally more successful at colonizing bean seedlings than M. anisopliae, with the top seven isolates belonging to the former species
At least 62 peer-reviewed papers have been published on the inoculation of 30 plant species in 15 families with various fungal entomopathogens, e.g., B. bassiana, Lecanicillium sp., L. lecanii, M. anisopliae, M. brunneum, M. robertsii, and Purpureocillium lilacinum (Vega and Parsa, 2017)

Summary

Introduction

Living inconspicuously within plant tissues, fungal endophytes play important roles in plant community ecology (Carroll, 1988; Hyde and Soytong, 2008; Omacini et al, 2001; Rodriguez et al, 2009; Saikkonen et al, 1998; Wani et al, 2015), and can have negative effects on insect pests and plant pathogens (Backman and Sikora, 2008; Giménez et al, 2007; Gond et al, 2010) as well as positive effects on plant growth (Behie et al, 2012; Kabaluk⇑ Corresponding author.and Ericsson, 2007; Sasan and Bidochka, 2012). There is widespread interest in learning how to use fungal endophytes with insect pathogenic capabilities (i.e., entomopathogenic fungal endophytes) to enhance plant tolerance to insect pests (Parsa et al, 2013; Vega et al, 2008, 2009). Notable among these fungi are species of Beauveria and Metarhizium, which are globally distributed and commercially available as biological control agents (Lacey et al, 2001, 2015).

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