Fungal growth and proteinaceous toxins in net blotch disease of barley

Abstract

The fungus that causes net form net blotch disease (NFNB), Pyrenophora teres f. teres (Ptt), uses proteinaceous toxins to cause necrosis but little is known about the relationship between fungal growth, toxin production and virulence in NFNB. More virulent isolates had higher rates of conidial germination (both in vitro and in planta) and fungal development in planta, represented by longer hyphae and more appressoria, compared with less virulent isolates. However, the cell free filtrates of all Ptt isolates, regardless of their virulence, were able to induce symptoms in a susceptible cultivar suggesting that the timing of toxin production in relation to fungal growth may be important. A proteomics approach was used to identify four virulence-related candidate proteins: isochorismatase (PttCHFP1), an endo-1,4-β-xylanase A (PttXyn11A), a glycophosphatidylinositol (GPI)-anchored common in fungal extracellular membrane (CFEM) domain-containing protein (PttGPI-CFEM) and an unknown proteinaceous secreted (but conserved) hypothetical protein (PttSP1). The first three proteins have been previously proposed to have roles in plant defence; cell wall degradation and necrosis induction; and hyphal attachment and fungal networking, respectively. PttSP1 contains an effector motif but has an unknown function. Of these, PttXyn11A was expressed more abundantly by the more virulent isolates during the plant-pathogen interaction suggesting that endo-1,4--xylanase A plays a role in the virulence of Ptt on barley.