Production of Destruxins from Metarhizium spp. Fungi in Artificial Medium and in Endophytically Colonized Cowpea Plants

Destruxins (DTXs) are cyclic depsipeptides produced by many Metarhizium isolates that have long been assumed to contribute to virulence of these entomopathogenic fungi. We evaluated the virulence of 20 Metarhizium isolates against insect larvae and measured the concentration of DTXs A, B, and E produced by these same isolates in submerged (shaken) cultures. Eight of the isolates (ARSEF 324, 724, 760, 1448, 1882, 1883, 3479, and 3918) did not produce DTXs A, B, or E during the five days of submerged culture. DTXs were first detected in culture medium at 2-3 days in submerged culture. Galleria mellonella and Tenebrio molitor showed considerable variation in their susceptibility to the Metarhizium isolates. The concentration of DTXs produced in vitro did not correlate with percent or speed of insect kill. We established endophytic associations of M. robertsii and M. acridum isolates in Vigna unguiculata (cowpeas) and Cucumis sativus (cucumber) plants. DTXs were detected in cowpeas colonized by M. robertsii ARSEF 2575 12 days after fungal inoculation, but DTXs were not detected in cucumber. This is the first instance of DTXs detected in plants endophytically colonized by M. robertsii. This finding has implications for new approaches to fungus-based biological control of pest arthropods.