Research

Molecular Basis of Broad-Spectrum Disease Resistance

Plants have evolved different forms of resistance mechanisms to combat potential pathogens.

The most common is race-specific resistance triggered by a plant resistance (R) gene upon recognition of a cognate factor termed avirulence (Avr) gene from a pathogen. This type of resistance is highly specific---effective against one or a few strains of a particular pathogen carrying the corresponding Avr gene and may be readily overcome by pathogens. Most of such R genes encode proteins containing a nucleotide-binding site and leucine-rich-repeats (NBS-LRR) that show homology to animal proteins participating in innate immunity and apoptosis.

A much less characterized but more desirable form is broad-spectrum (or non race-specific) and durable resistance. We isolated a novel type of R gene RPW8 from Arabidopsis thaliana that confers broad-spectrum resistance in Arabidopsis (Xiao et al., 2001) and tobacco (Xiao et al., 2003a) to powdery mildew isolates from distinct Erysiphe species. Powdery mildew is a very important biotrophic fungal pathogen that causes

disease on numerous plant species (Figure 1). Our recent work suggested that RPW8 originated several millions of years ago (Xiao et al., 2004) and resistance conferred by RPW8 seems to be durable. Interestingly, like most race-specific resistance, RPW8-mediated broad-spectrum resistance is associated with hypersensitive response (HR) (Figure 2) and PR (pathogenesis-related) gene expression, and requires signaling components salicylic acid, EDS1, PAD4, EDS5, SGT1 and NPR1 that are also recruited by a subset of NBS-LRR R genes (Xiao et al., 2005). It seems likely that different forms of resistance may be super-imposed to a common, conserved innate immunity system. We are now trying to determine (i) how RPW8-mediated broad-spectrum resistance has functionally originated; (ii) whether RPW8-like genes have evolved in crops; and (iii) how RPW8-derived signals are integrated to the conserved signaling pathway leading to HR and resistance. In addition, we are interested in characterization of other powdery mildew R genes from Arabidopsis accessions that lack RPW8.

Regulation of Programmed Cell Death and Resistance

Plant disease resistance is often accompanied with hypersensitive response (HR) (Figure 2), which is manifested as a rapid, localized host cell death at the pathogen infection site. HR is a form of programmed cell death (PCD) that limits spread of the invading pathogen and may play a role in activation of plant systemic resistance. HR is thought to be under tight control of both positive and negative regulation for adequate activation of resistance while minimizing unnecessary host cell death. Many cell death-related (also referred to as "lesion mimic") mutants have been isolated and some of the corresponding genes have been cloned (reviewed by Lorrain et al., 2003), yet very little has been known about the precise control of HR cell death. We found that enhanced expression of RPW8 under control of the native promoter results in spontaneous HR-like cell death (SHL) and constitutive activation of defenses (Xiao et al., 2003b). High level expression of RPW8 leads to lethality at seedling stage due to massive cell death (Figure 3). One project in the lab is to identify both positive and negative regulators of the RPW8-mediated HR pathway. We mutagenized a Col-0 (the most frequently used accession but lacks RPW8) transgenic line with moderate level of RPW8 expression. This line exhibits intermediate level of SHL, which expedites visual screening for mutants with no cell death (suppressors) or exaggerated cell death (enhancers). We are currently characterizing over 10 "enhancer" mutants and screening for "suppressor" mutants from over 30,000 independent T-DNA tagged lines.

Research in our laboratory is supported by the USDA-NRI (Grant# 2005-35319-15656; 2006-35301-16883) and National Science Foundation (IOS#0842877).