Parasitism of secondary-stage juvenile of Heterodera glycines and four larva stages of Caenorhabditis elegans by Hirsutella spp

Parasitism of Hirsutella rhossiliensis on Different Nematodes and Its Endophytism Promoting Plant Growth and Resistance against Root-Knot Nematodes

The endoparasitic fungus Hirsutella rhossiliensis is an important biocontrol agent of cyst nematodes in nature. To determine the potential parasitism of the fungus on a non-natural host, the pinewood nematode (Bursaphelenchus xylophilus) living in pine trees and the endophytic ability of the fungus on plants, in this paper, we first constructed and utilized a green fluorescent protein (GFP)-tagged H. rhossiliensis HR02 transformant to observe the fungal infection process on B. xylophilus and its colonization on Arabidopsis roots. Then, we compared the fungal parasitism on three species of nematodes with different lifestyles, and we found that the fungal parasitism is correlated with nematode species and stages. The parasitic effect of H. rhossiliensis on adults of B. xylophilus is similar to that on second-stage juveniles (J2) of the root-knot nematode Meloidogyne incognita after 24 h of inoculation, although the virulence of the fungus to second-stage juveniles of M. incognita is stronger than that to those of B. xylophilus and Caenorhabditis elegans. Moreover, the endophytism of H. rhossiliensis was confirmed. By applying an appropriate concentration of H. rhossiliensis conidial suspension (5 × 106 spores/mL) in rhizosphere soil, it was found that the endophytic fungus can promote A. thaliana growth and reproduction, as well as improve host resistance against M. incognita. Our results provide a deeper understanding of the fungus H. rhossiliensis as a promising biocontrol agent against plant-parasitic nematodes.

Population dynamics and biocontrol efficacy of the nematophagous fungus Hirsutella minnesotensis in pot assay

Monitoring the population dynamics of a biocontrol agent and its target pest in soil is fundamental to the understanding of the mechanisms of biocontrol. In this study, the population dynamics and the control efficacy ofHirsutella minnesotensisagainst soybean cyst nematode (SCN)Heterodera glycineswas studied using quantitative real-time polymerase chain reaction (qPCR), confocal laser scanning microscopy and bioassay in sterilised soil. The qPCR test revealed that the amount of DNA was 1.12 × 107fg (g soil)−1after inoculation, subsequently decreased rapidly until 28 days (1.34 × 104-1.51 × 104fg (g soil)−1), then remained constant in SCN-free soil. While it decreased rapidly up to 14 days (around 1.40 × 104fg (g soil)−1), it then varied with the population changes of SCN over time. The amount ofH. minnesotensisDNA was highest in the rhizosphere soil with bothH. minnesotensisand SCN incubation. The percentage of nematodes parasitised was positively correlated with the amount ofH. minnesotensisDNA in soil with soybean. Microscopic observations indicated thatH. minnesotensiscould colonise the surface of soybean roots. In addition,H. minnesotensisinoculation significantly improved the biomass of soybean. Therefore, the rhizosphere colonisation, relationship between SCN parasitism and fungal density, and growth promotion of soybean supportH. minnesotensisas a potential biocontrol agent.

Development of a transformation system for Hirsutella spp. and visualization of the mode of nematode infection by GFP-labeled H. minnesotensis

Hirsutella rhossiliensis and H. minnesotensis are endoparasitic fungi of the second-stage juvenile (J2) of the soybean cyst nematode (Heterodera glycines) in nature. They also parasitize both H. glycines J2 and Caenorhabditis elegans on agar plates. Agrobacterium tumefaciens-mediated transformation conditions were established for these Hirsutella spp. The resulting transformants were similar to the corresponding wild-type strains. The infection processes of H. glycines J2 and C. elegans second larval stage (L2) by H. minnesotensis expressing ZsGreen were microscopically analyzed. Conidia of H. minnesotensis adhered to passing nematodes within 8 h post-inoculation (hpi), formed an infection peg between 8 and 12 hpi, and penetrated the nematode cuticle between 12 and 24 hpi for C. elegans L2 and between 12 and 32 hpi for H. glycines J2. Hyphal proliferation inside of the nematode coelom was observed at approximately 32 hpi for C. elegans L2 and at approximately 40 hpi for H. glycines J2. The fungus consumed the whole body and grew out to produce conidia at approximately 156 and 204 hpi for C. elegans L2 and H. glycines J2, respectively. The efficient transformation protocol and a better understanding of infection process provide a solid foundation for studying the molecular and cellular mechanisms underlying fungal parasitism of nematodes.