Keywords
Fungal endophytes · Insect-pathogenic fungi · Plant defense · Plant growth
promotion · Stress · Sustainable agriculture
13.1 Introduction
The soil microbiome comprises diverse and complex microbial communities that
provide many beneficial services to agroecosystems (Spence and Bais 2013; Zolla
et al. 2013). Several biotic and abiotic factors affect the recruitment and maintenance
of microbial communities in the rhizosphere, the thin layer of soil in close proximity
to the plant roots, and in plants (Rosier et al. 2016). For example, plant species, root
morphology, and soil characteristics give rise to distinct microbial communities in
the rhizosphere (Berg and Smalla 2009). Rhizodeposition, the organic compounds
released from plant roots into the surrounding environment, contains a large amount
of photosynthetically-fixed carbon and is a major resource that supports the soil
microbial community (Rosier et al. 2016).
The relationship between plants and soil microbes may be beneficial, harmful, or
neutral for plants. Hence, understanding the diversity in community structure and
key functions of the microbiome can provide valuable information for improving
agricultural sustainability (Ahmad and Zaib 2020). Endophytes are plant symbionts,
often bacteria or fungi, that can live within a plant for at least part of their life cycle
without causing apparent disease symptoms and can inhabit the tissues of most
terrestrial plants (reviewed in Vega 2018). Endophytes can affect plant physiology
and growth by conferring abiotic and biotic stress tolerance, increasing growth and
biomass, reducing water consumption, improving drought tolerance, and altering
resource allocation (Dara 2019; Hu and Bidochka 2019; Vega 2018; Yan et al.
2019). Other benefits of endophytes include enhanced mineral uptake (Ren et al.
2011), improved N use efficiency (Behie et al. 2012, 2017), protection against plant
diseases (Dara 2019), and defense against herbivory (Ahmad et al. 2020b; Akutse
et al. 2013; Lopez and Sword 2015; Sullivan et al. 2007; Zhang et al. 2009).
However, in some cases, endophytic colonization can improve the performance of
plant-feeding insects, e.g. aphids (Clifton et al. 2018).
Endophytic insect pathogens are a ubiquitous but often-overlooked group of
natural enemies, and fungi are among the most common insect pathogens. There
are almost 700 species of fungal insect pathogens belonging to more than 100 genera,
the majority of them belonging to the Hypocreales. Endophytic insect-pathogenic
fungi (EIPF) have multifunctional lifestyles. As direct pathogens of insects, they can
infect and kill susceptible insect by the germination of spores on the host insect’s
cuticle, and formation of specialized infection structures that penetrate the insect
cuticle that result in fatal infections in the insect host. Metarhizium (Clavicipitaceae)
and Beauveria (Cordycipitaceae) genera are hypocrealean EIPF that are ubiquitously
found in agricultural soils and can endophytically colonize a broad range of plant
hosts (Vega 2018). As endophytes, they can promote plant growth, translocate
380
I. Ahmad et al.
Fungal endophytes · Insect-pathogenic fungi · Plant defense · Plant growth
promotion · Stress · Sustainable agriculture
13.1 Introduction
The soil microbiome comprises diverse and complex microbial communities that
provide many beneficial services to agroecosystems (Spence and Bais 2013; Zolla
et al. 2013). Several biotic and abiotic factors affect the recruitment and maintenance
of microbial communities in the rhizosphere, the thin layer of soil in close proximity
to the plant roots, and in plants (Rosier et al. 2016). For example, plant species, root
morphology, and soil characteristics give rise to distinct microbial communities in
the rhizosphere (Berg and Smalla 2009). Rhizodeposition, the organic compounds
released from plant roots into the surrounding environment, contains a large amount
of photosynthetically-fixed carbon and is a major resource that supports the soil
microbial community (Rosier et al. 2016).
The relationship between plants and soil microbes may be beneficial, harmful, or
neutral for plants. Hence, understanding the diversity in community structure and
key functions of the microbiome can provide valuable information for improving
agricultural sustainability (Ahmad and Zaib 2020). Endophytes are plant symbionts,
often bacteria or fungi, that can live within a plant for at least part of their life cycle
without causing apparent disease symptoms and can inhabit the tissues of most
terrestrial plants (reviewed in Vega 2018). Endophytes can affect plant physiology
and growth by conferring abiotic and biotic stress tolerance, increasing growth and
biomass, reducing water consumption, improving drought tolerance, and altering
resource allocation (Dara 2019; Hu and Bidochka 2019; Vega 2018; Yan et al.
2019). Other benefits of endophytes include enhanced mineral uptake (Ren et al.
2011), improved N use efficiency (Behie et al. 2012, 2017), protection against plant
diseases (Dara 2019), and defense against herbivory (Ahmad et al. 2020b; Akutse
et al. 2013; Lopez and Sword 2015; Sullivan et al. 2007; Zhang et al. 2009).
However, in some cases, endophytic colonization can improve the performance of
plant-feeding insects, e.g. aphids (Clifton et al. 2018).
Endophytic insect pathogens are a ubiquitous but often-overlooked group of
natural enemies, and fungi are among the most common insect pathogens. There
are almost 700 species of fungal insect pathogens belonging to more than 100 genera,
the majority of them belonging to the Hypocreales. Endophytic insect-pathogenic
fungi (EIPF) have multifunctional lifestyles. As direct pathogens of insects, they can
infect and kill susceptible insect by the germination of spores on the host insect’s
cuticle, and formation of specialized infection structures that penetrate the insect
cuticle that result in fatal infections in the insect host. Metarhizium (Clavicipitaceae)
and Beauveria (Cordycipitaceae) genera are hypocrealean EIPF that are ubiquitously
found in agricultural soils and can endophytically colonize a broad range of plant
hosts (Vega 2018). As endophytes, they can promote plant growth, translocate
380
I. Ahmad et al.