INTRODUCTION

Many bacterial pathogens of plants and animals use a conserved type III secretion system (TTSS) to infect their eukaryotic hosts during pathogenesis. For plant pathogenic bacteria in the genera Pseudomonas, Xanthomonas, Ralstonia, Erwinia, and Pantoea, the TTSS is encoded by the hypersensitive response and pathogenicity (hrp) gene cluster. These pathogens use the TTSS within plant tissue at early stages of infection to establish cell-to-cell contact. Once contact is made, the bacterial TTSS facilitates the direct secretion and translocation of a diverse group of proteins (referred to as TTSS effectors) into host plant cells. Mutant bacteria lacking a functional TTSS cannot multiply within plant tissue. Thus, the TTSS is an essential virulence determinant required for bacterial colonization and persistence in plant hosts.

The most characterized TTSS effectors in plant pathogenic bacteria are avirulence (Avr) proteins. Avr effectors translocated into plant cells have two general fates. If the host is resistant, it recognizes the Avr effector and immediately activates a resistance (R) protein-dependent defense response that limits the growth and spread of the invading pathogen. Conversely, if the host is susceptible, the Avr effector will go unnoticed, allowing the pathogen to multiply outside of the plant cell. Thus, the outcome of complex bacterial-plant interactions is defined at the molecular level by the presence or absence of functional alleles of bacterial Avr effectors and plant R proteins.

Owing to selective pressures, one might predict that pathogens would lose Avr effectors to avoid plant surveillance systems. On the contrary, Avr effectors are maintained in strains and can mutate to avoid recognition by host R proteins, implying that they may play an important role in establishing bacterial-plant interactions. In fact, recent molecular investigations show that TTSS effectors can independently and collectively function in planta to suppress basal- and R protein-mediated immune responses. For example, TTSS effectors can interfere with R protein activation, repress salicylic acid (SA)-mediated defense pathways, inhibit programmed cell death (PCD), and suppress plant cell wall remodeling. These seminal studies clearly demonstrate that TTSS effectors target distinct plant signaling pathways. However, at present, the biochemical basis for these effector-induced plant phenotypes remains a mystery.


RESEARCH OBJECTIVES

The central question my laboratory investigates is how bacterial pathogens employ TTSS effectors to manipulate plant signal transduction to promote disease. We study the plant pathogen Xanthomonas campestris pathovar vesicatoria (Xcv), the causal agent of bacterial spot disease of pepper and tomato. Our primary focus is to elucidate the biochemical role of two Xcv effector families predicted to encode cysteine proteases that target the plant SUMO pathway. We are investigating protease activity and regulation, identifying plant substrates and target pathways, and dissecting the mechanism for effector recognition in planta. In addition, we are isolating and characterizing the Xcv TTSS effector proteome. To do so, we apply modern biochemical, cell biological, and genetic approaches using the natural host tomato.  When appropriate, we also use two model pathosystems, Xanthomonas-Arabidopsis and Xanthomonas-Nicotiana benthamiana, to exploit the available genomic resources and biochemical techniques, respectively.

Currently, the lab is divided into three groups, each working on the mechanistic action of a unique Xcv effector protein.  (1) The XopD group has discovered that the XopD is a DNA-binding protein with SUMO protease activity that operates in the plant nucleus as a repressor to alter transcription.  Tomato leaves infected with Xcv mutants lacking XopD die faster than leaves infected with wild type Xcv, indicating that XopD may actively repress hormone signal transduction associated with the onset of plant senescence.  The identification of XopD-targeted promoters and the underlying mechanism for altering gene transcription is in progress. (2) The AvrBsT group has discovered that SOBER1, a carboxylesterase, inhibits AvrBsT-triggered responses in Arabidopsis.  In sober1 mutant plants, AvrBsT alters host physiology resulting in the activation of salicylic acid-dependent and independent defense signaling which limits bacterial growth.  The immediate goal is to identify AvrBsT target(s) and SOBER1 substrates to elucidate how SOBER1 is able to suppress AvrBsT action in planta.  (3) The XopN group has discovered that XopN physically interacts with an atypical receptor-like kinase in tomato referred to as TARK1.  XopN is an unusual protein consisting of alpha-helical repeats, predicted to share structural similarity with the regulatory subunit of protein phosphatase 2A, a HEAT-repeat containing protein.  XopN is essential for Xcv pathogenesis on tomato.  Thus, we are actively dissecting the biochemical nature of XopN-TARK1 interactions at the plasma membrane and its potential impact on downstream signaling processes involved in defense and plant development.