Host-Pathogen Interactions between Metarhizium spp. and Locusts

Abstract

The progress in research on the interactions between Metarhizium spp. and locusts has improved our understanding of the interactions between fungal infection and host immunity. A general network of immune responses has been constructed, and the pathways regulating fungal pathogenicity have also been explored in depth. However, there have been no systematic surveys of interaction between Metarhizium spp. and locusts. The pathogenesis of Metarhizium comprises conidial attachment, germination, appressorial formation, and colonization in the body cavity of the host locusts. Meanwhile, the locust resists fungal infection through humoral and cellular immunity. Here, we summarize the crucial pathways that regulate the pathogenesis of Metarhizium and host immune defense. Conidial hydrophobicity is mainly affected by the contents of hydrophobins and chitin. Appressorial formation is regulated by the pathways of MAPKs, cAMP/PKA, and Ca²⁺/calmodulin. Lipid droplets degradation and secreted enzymes contributed to fungal penetration. The humoral response of locust is coordinated by the Toll pathway and the ecdysone. The regulatory mechanism of hemocyte differentiation and migration is elusive. In addition, behavioral fever and density-dependent population immunity have an impact on the resistance of hosts against fungal infection. This review depicts a prospect to help us understand host-pathogen interactions and provides a foundation for the engineering of entomopathogenic fungi and the discovery of insecticidal targets to control insect pests.

Keywords: Metarhizium; immune response; locust; pathogenicity.

Figure 1

Schematic overview of the interactions between Metarhizium spp. and locusts. The dotted T represents indirect opposition. The solid T represents direct interactions.

Figure 2

A schematic model of the early phase interaction between Metarhizium spp. and locusts. The regulatory mechanisms of appressorial formation and the early phase immune response are depicted. The regulatory pathway of appressorial formation only represents the main signaling pathway, including Ca²⁺/CaM, cAMP/PKA, and MAPKs. The processes of LD synthesis and degradation are also shown in the model. The dotted arrows represent indirect actions to promote another component. The solid arrows or T indicate direct promotion or restraint to another component. DAG, diacylglycerol; TAG, triacylglycerol; LD, lipid droplet; GAG, galactosaminogalactan and the biosynthesis genes cluster; WB, woronin body; SP, serine protease; PPO, prophenoloxidase.

Figure 3

The late-phase interactions between Metarhizium spp. and locusts. The growth of the yeast-like hyphal bodies (HB) in the hemolymph of locusts relies on the ingestion of the locusts’ nutrients. The secreted protein and metabolites from Metarhizium to restrain humoral and cellular immunity. The pathways of ecdysone and Toll are opposed to the growth of HB. The dotted arrows or T represent indirect actions to promote or inhibit another component. The solid arrows or T indicate direct promotion or restraint to another component. 20E, 20-Hydroxyecdysone; EcR, ecdysone receptor; TLR, Toll-like receptor; IML-1, immunlectin-1; GNBP3, Gram-negative bacteria binding protein 3; AhR, aryl hydrocarbon receptor; SP, serine protease; Serpin, serine protease inhibitor; ROS, reactive oxygen species; PPO, prophenoloxidase; PO, phenoloxidase; AMP, antimicrobial peptides; PTPase, protein tyrosine phosphatase.