Effects of tomato inoculation with the entomopathogenic fungus Metarhizium brunneum on spider mite resistance and the rhizosphere microbial community

Abstract

Entomopathogenic fungi have been well exploited as biocontrol agents that can kill insects through direct contact. However, recent research has shown that they can also play an important role as plant endophytes, stimulating plant growth, and indirectly suppressing pest populations. In this study, we examined the indirect, plant-mediated, effects of a strain of entomopathogenic fungus, Metarhizium brunneum on plant growth and population growth of two-spotted spider mites (Tetranychus urticae) in tomato, using different inoculation methods (seed treatment, soil drenching and a combination of both). Furthermore, we investigated changes in tomato leaf metabolites (sugars and phenolics), and rhizosphere microbial communities in response to M. brunneum inoculation and spider mite feeding. A significant reduction in spider mite population growth was observed in response to M. brunneum inoculation. The reduction was strongest when the inoculum was supplied both as seed treatment and soil drench. This combination treatment also yielded the highest shoot and root biomass in both spider mite-infested and non-infested plants, while spider mite infestation increased shoot but reduced root biomass. Fungal treatments did not consistently affect leaf chlorogenic acid and rutin concentrations, but M. brunneum inoculation via a combination of seed treatment and soil drenching reinforced chlorogenic acid (CGA) induction in response to spider mites and under these conditions the strongest spider mite resistance was observed. However, it is unclear whether the M. brunneum-induced increase in CGA contributed to the observed spider mite resistance, as no general association between CGA levels and spider mite resistance was observed. Spider mite infestation resulted in up to two-fold increase in leaf sucrose concentrations and a three to five-fold increase in glucose and fructose concentrations, but these concentrations were not affected by fungal inoculation. Metarhizium, especially when applied as soil drench, impacted the fungal community composition but not the bacterial community composition which was only affected by the presence of spider mites. Our results suggest that in addition to directly killing spider mites, M. brunneum can indirectly suppress spider mite populations on tomato, although the underlying mechanism has not yet been resolved, and can also affect the composition of the soil microbial community.

Identification of an Entomopathogenic Fungus, Pseudozyma flocculosa (Traquair, Shaw & Jarvis), and Its Efficacy against Tetranychus urticae Koch

“…The tissue separation method was adopted to isolate and culture pathogens, with slight modifications [16,18]. Dead adult mites having been infected with fungi were collected from the laboratory, rinsed five times with sterile water, inoculated with a needle onto a potato dextrose agar (PDA, BD-Pharmingen, Becton, Dickinson and Company, New York, NY, USA) plate, and cultured in a 28 • C incubator (ZQZY-88BN, Zhichu Instrument Co., LTD, Shanghai, China) for 2-3 d. The grown mycelium was plated on a new PDA plate three times for repeated purification.…”

“…Some developed biocontrol products are being used effectively to control a wide range of insect pests worldwide, for example, in India, Korea, Japan, Thailand, Ukraine, and the USA [10,12]. Some typical entomopathogenic fungi, Beauveria bassiana (Balsamo) Vuillemin [8,13,14] and Metarhizium spp., have been developed as fungal agents for controlling different insect pests, including Cimex lectularius, Frankliniella occidentalis, and T. urticae, in the field [15,16]. Among these, some fungal strains have been reported to have significant pathogenicity on T. urticae, by inhibiting egg hatching and reducing the fecundity of females [15].…”

The Fungus Beauveria bassiana Alters Amounts of Sterols, Fatty Acids, and Hydroxycinnamic Acids in Potato Solanum tuberosum

“…Plants were grown at 22 • C and 40-60% relative humidity with a 16 h/8 h photoperiod (light/dark) using full-spectrum Flora phytolamps (Osram, Munich, Germany). We used soil drenching with the conidial suspension around seedlings ( [60][61][62] with modifications) because preliminary assays showed effective colonization of potatoes under the laboratory conditions described above. Soil around tubers was inoculated with 10 mL of the suspension (10 8 conidia per mL) in the 0.03% (v/v) Tween 80 solution 3 days after planting.…”