Early stages of germination, plant colonization and production of indole-3-acetic acid (IAA) by species of Metarhizium

Abstract

Fungi in the genus Metarhizium are known for their ability to successfully colonize both insect and plant hosts. Some species, called generalists, can infect a broader range of insects, while infection by specialists is restricted to specific insects. Generalists and specialists require different environmental cues to initiate conidial germination, an essential step in the process of host colonization. In this study, the early morphological development of several strains of Metarhizium was evaluated under different nutritional conditions and substrates. Overall, generalist strains have less restricted nutritional requirements to initiate germination, while specialists germinated poorly in the tested conditions. Several Metarhizium germinated in corn (Zea mays) and bean (Phaseolus vulgaris) root exudate, indicating an ability to utilize plant derived compounds for their development. Change in surface hydrophobicity had a significant effect in rates of differentiation of germ tubes into appressoria, infective structures required for insect colonization. However, some strains can still produce some appressoria in hydrophilic surfaces, particularly M. anisopliae. Microscopic analysis showed that the generalist M. robertsii germinates in both insect and plant tissues, but with morphological variations. Appressorial formation was restricted to insect tissues while hyphal growth was observed in plant root tissue. In addition to early morphological development, generalists and specialists were evaluated for their ability to produce the plant hormone indole-3-acetic acid (IAA), which is used by plant-associated microorganisms to promote plant root growth and is also known to be involved in insect pathogenicity by Metarhizium. Regardless of lifestyle (differences in environmental condition preferences and life cycle), all strains were able to produce IAA in vitro in the presence of the exogenous precursor L-tryptophan. M. anisopliae strains showed high intraspecific variation in level of IAA production and were selected for plant inoculation assays. No correlation was observed between IAA production in vitro and effects on corn plant growth or rates of root colonization, as the most competent endophytic colonizer was not the highest IAA producer. Our results indicate that other mechanisms might be involved in successful root colonization by Metarhizium, and IAA production could play other important roles in their ecology.