Arabidopsis and natural pathogens

Arabidopsis thaliana

The small crucifer A.thaliana is a small annual plant, common along agricultural fields, roads, and other disturbed habitats. It is native to Eurasia and naturalized almost worldwide. Due to its rapid life cycle, large number of progeny, and small genome size, it is a widely used model for molecular, genetic and evolutionary studies.
The efforts of many scientists since the first part of the last century have resulted in a global collection of many thousands of inbred strains, originating from diverse habitats, and with considerable genetic and phenotypic variation. In conjunction with whole-genome sequences of over 1,000 of these strains, A. thaliana stands as an ideal organism for rapid mapping of adaptive traits using genome-wide association studies (GWAS).

Arabidopsis pathogens

With the advance of A. thaliana as a model for molecular genetics during the 1980s and 1990s, two types of pathosystems were set up to identify genes involved in plant defense: readily available crop pathogens that turned out to infect A.thaliana (such as Pseudomonas syringae pv. tomato DC3000), and natural pathogens isolated from the wild (especially Hyaloperonospora arabidopsidis, originally known as Peronospora parasitica). These pathosystems enabled the cloning of many resistance (R) genes, and paved the way for subsequent biochemical studies that have greatly advanced our mechanistic understanding of plant-pathogen interaction. However, with few exceptions, we know much less about how either specialist or generalist pathogens interact with A. thaliana in the wild, and how these interactions are shaped by abiotic and biotic components of the natural environment. This is where Pathodopsis comes in. The goal of this project is to take these previous efforts to a new level, by mapping genetic diversity of wild pathogens onto the genetic diversity of wild A. thaliana hosts.

Pseudomonas image

The Pseudomonas genus of gram-negative Proteobacteria is among the most abundant colonizers of plants. Several species within this genus are pathogenic, while others serve as biocontrol agents protecting plants from disease. Much is known about the mechanisms of pathogenicity, that is, why certain strains cause disease. The responsible genes evolve quickly between host species and even within host populations. As a genetically tractable pathogen, Pseudomonas syringae was adopted early on as a model pathosystem for A. thaliana, and important discoveries have been made with this system. On the plant, Pseudomonas infections cause a range of symptoms, including yellow or black leaf spots and leaf necrosis.

Hyaloperonospora arabidopsidis
The causal agent of downy mildew in A.thaliana, Hyaloperonospora arabidopsidis (HpA), is an oomycete (water mold) highly specialized to the species. Upon successful infection of a host plant, HpA hyphae invade its extracellular space and derive nutrition through specialized feeding structures. Asexual spores form on sporangiophores, visible as little ‘trees’ growing out of stomata, primarily on the underside of leaves. Young plants, when infected, can be severely debilitated and sometimes be killed. Suited immunity components, however, can restrict the growth and propagation of HpA. Notably, most A.thaliana disease resistance genes that have been characterized molecularly were originally identified based on their role in resistance to specific strains of HpA.