Question

Give an account of the biological mechanisms and molecules used by biotrophic fungi to survive plant defence responses, after their initial colonisation of plants.

Answer 1

Answer:

The biotrophic fungi and their plant host have highly specialized relationship structurally and also biochemically. Biotrophc fungi penetrate the host cell wall and colonizing the intercellular space using feeding structures like haustoria to absorb nutrients and suppress host defenses without disrupting the plasma membrane. A constant balance between virulence and evading host detection show a very sophisticated form of pathogenesis of biotrophic fungi. By contrast, necrotrophs overpowering the host by utilizing a variety of secreted pathogenicity and virulence factors throughout infection instead of producing specialized infection structures.

By their feeding acitivities, biotrophic fungi create a nutrient sink to the infection site, so that the host is disadvantaged and shows serious yield lost. In many ways, this type of parasitism is very sophisticated - keeping the host alive as a long-term source of food. In this review the most important groups of biotrophic fungi plant pathogens like powdery mildew fungi (Ascomycota), the rust fungi (Basidiomycota) and plant defense mechanism have been considered.

Plant pathogens have to pass the complex multilayered defense system for compatible interaction. Fungus protection may include fungal chitin shield, scavenger, which protect the fungal cell wall and the chitin fragments from chitinases. For example effector of Cladosporium fulvum holds a functional chitin-binding domain. Plants secrete beta-1,3-glucanases to damage fungal cell walls but some pathogen produces glucanase inhibitor protein. Other effectors (proteinase inhibitors and phytoalexin detoxifying enzymes) may aid the pathogen success too.

For the success of pathogenesis including attachment, host recognition, penetration and proliferation biotrophic fungi form infection structure. The structure formation is restricted by regulated gene expression and complex regulatory pathways. For valuable virulence activity biotrophic fungi have: highly developed infection structures; limited secretory activity, especially of lytic enzymes; carbohydrate rich and protein-containing interfacial layers, which separate fungal and plant plasma membranes; long-term suppression of host defense; haustoria that used for nutrient absorption and metabolism.

Biotrophic fungi also have several mechanisms to defend their effectors from plant receptor molecules. Once the fungal effector passes plant defense mechanism the plant will not resist. Subsequently the plant reduces production of defense signaling molecule like salicylic acid.