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Abstract
Several ascomycetous insect-pathogenic fungi, including species in the genera Beauveria
and Metarhizium, are plant root symbionts/endophytes and are termed as endophytic
insect-pathogenic fungi (EIPF). The endophytic capability and insect pathogenicity
of Metarhizium are coupled to provide an active method of insect-derived nitrogen
transfer to plant hosts via fungal mycelia. In exchange for the insect-derived nitrogen,
the plant provides photosynthate to the fungus. This symbiotic interaction offers
other benefits to the plant-EIPF can improve plant growth, they are antagonistic to
plant pathogens and herbivores and can enhance the plant tolerance to abiotic stresses.
The mechanisms and underlying biochemical and genetic features of insect pathogenesis
are generally well-established. However, there is a paucity of information regarding
the underlying mechanisms in this plant-symbiotic association. Here we review five
aspects of EIPF interactions with host plant roots: (i) rhizosphere colonization,
(ii) signalling factors from the plant and EIPF, (iii) modulation of plant defence
responses, (iv) nutrient exchange and (v) tripartite interactions with insects and
other micro-organisms. The elucidation of these interactions is fundamental to understanding
this symbiotic association for effective application of EIPF in an agricultural setting.