Author: Miaoying Tian

Publisher:

Published: 2005

Total Pages:

ISBN-13:

Abstract: The oomycetes form one of several lineages within the eukaryotes that independently evolved a parasitic lifestyle and are thought to have developed unique mechanisms of pathogenicity. The devastating oomycete plant pathogen Phytophthora infestans causes late blight, a ravaging disease of potato and tomato. Little is known about processes associated with P. infestans pathogenesis, particularly the suppression of host defense responses. We used data mining of P. infestans sequence databases to identify 18 extracellular protease inhibitors belonging to two major structural classes: (i) Kazal-like serine protease inhibitors (EPI1 to EPI14) and (ii) cystatin-like cysteine protease inhibitors (EPIC1 to EPIC4). A variety of molecular, biochemical and bioinformatic approaches were employed to functionally characterize these genes and investigate their roles in pathogen virulence. The 14 EPI proteins form a diverse family and appear to have evolved by domain shuffling, gene duplication, and diversifying selection to target a diverse array of serine proteases. Recombinant EPI1 and EPI10 proteins inhibited subtilisin A among major serine proteases, and inhibited and interacted with tomato P69B subtilase, a pathogenesis-related protein belonging to PR7 class. The recombinant cystatin-like cysteine protease inhibitor EPIC2B interacted with a novel tomato papain-like extracellular cysteine protease PIP1 with an implicated role in plant defense. PIP1 is closely related to Rcr3, an apoplastic cysteine protease required for tomato Cf-2 and Cladosporium fulvm Avr2-dependent defense response. Both EPIC1 and EPIC2B interacted with Rcr3. Interactions with plant defense-related proteases suggest a counterdefense role of these extracellular protease inhibitors. Interestingly, EPIC1 and EPIC2B were degraded by tomato pathogenesis-related P69B subtilase and EPI1 protected both proteins from degradation, indicating that EPI1 contributes to virulence by protecting pathogen proteins from degradation by defense-related proteases. In addition, our overall results suggest that complex cascades of inhibition of host proteases by diverse extracellular protease inhibitors of P. infestans might occur in the plant apoplast during infection, thus leading to multifaceted suppression of plant defense responses. Both Kazal-like and cystatin-like inhibitors are widespread in the oomycetes, but have not been reported in other microbial plant pathogens. Inhibition of host proteases by P. infestans protease inhibitors is proposed to be a novel mechanism of pathogen suppression of plant defense.