Shauna Somerville

Mechanisms of Disease Resistance

Powdery mildew diseases occur on more than 9,000 plant species, including agriculturally important field and horticultural crops. The powdery mildew pathogens are obligate, biotrophic fungal pathogens that grow only on living host tissues. To successfully colonize a host, biotrophic pathogens must strike a balance between drawing sufficient nutrients from the host to thrive but not so much that the host dies. The powdery mildews extract nutrients and water from their hosts via a haustorium -- a fungal structure encapsulated by an invagination of the host plasma membrane. This specialized host membrane is the site of nutrient transfer to the powdery mildew pathogen. In addition, the successful powdery mildew isolates must be able to evade or must be tolerant of normal host defenses.

In one of Shauna Somerville's lab projects, postdoctoral fellow John Vogel screened more than 60,000 mutagenized powdery-mildew-susceptible plants for resistant mutants. From this screen, two classes of mutants were expected. In one class, the researchers predicted that the defense responses would be successfully activated when normally they are not. This class includes both mutants in which defense responses are induced following powdery mildew infection, and those in which defense responses are constitutively activated -- or turned on all the time. The second class consists of mutants with defects in host compatibility factors -- components required by the pathogen for the successful colonization of the host. To aid in classifying the mutants into one of these two classes, Vogel assayed the mutants for three features: the expression of known defense responses, the stage of powdery mildew arrest, and changes in susceptibility to other pathogens. Detailed analysis of four mutant groups was completed.

The researchers found that the mutants in the largest complementation group are characterized by reduced pollen transmission, enhanced accumulation of pathogenesis-related protein 1 (PR1) mRNA, and susceptibility to a closely related powdery mildew, Erysiphe orontii. Thus, this group may fall into the class with enhanced defense responses. However, the defenses are not broad spectrum since they are effective only against E. cichoracearum.

Mutants in a second group did not exhibit morphological aberration, had enhanced accumulation of PR1 mRNA, and were resistant to two additional biotrophic pathogens, E. orontii and Peronospora parasitica -- the downy mildew pathogen. Surprisingly, these mutants did not produce callose either at sites of fungal penetration, a common host response, or at wound sites. (Callose accumulation in pollen tubes is normal, suggesting that these mutants are not defective in callose synthesis per se.) This mutant group may also fall into the class of mutants with enhanced defense responses, with the caveat that whatever defense responses are induced they do not include the callose response. This mutant class is potentially interesting for commercial applications since it confers resistance on multiple pathogens.

A third mutant group developed microlesions of dead cells in mesophyll tissue. (The powdery mildew fungus never penetrates beyond the epidermal layer.) These microlesions appear to arise regardless of the presence of the pathogen, and the lesions do not increase in either number or size following inoculation with the powdery mildew pathogen. Thus, although host-cell death of attacked cells -- the hypersensitive necrosis response -- is often associated with the expression of resistance, the researchers believe that the spontaneous microlesions observed in this mutant group do not play a direct role in disease resistance. PR1 mRNA accumulation is reduced relative to the susceptible wild-type plants, and the disease resistance phenotype is expressed only under high light conditions where the mutants are dwarfed. Under low light conditions, the mutants resemble the wild-type parent in stature and disease susceptibility. Given this spectrum of properties, it is difficult to place the mutants of this group in either of the two predicted classes.

The final mutant group consisted of plants with no significant increase in any of the known defense responses. This mutant is the best candidate for the class in which host compatibility factors have been disrupted or a totally novel defense pathway has been activated.

Relatively little is known about which host components contribute to disease development in susceptible plants. However, with a better understanding of such factors, it seems possible that disease control strategies based on disrupting the successful colonization by the pathogen may offer a useful alternative to current strategies based on known host resistance genes. For example, a typical barley powdery-mildew-resistance gene has a useful life span in the field of only five years. Disease control strategies based on disrupting host compatibility factors may be more difficult for the powdery mildew pathogen to overcome in the field and thus be more stable.

In future studies, John Vogel will clone and characterize the genes from each mutant group. These genes will provide necessary tools both to evaluate any potentially novel defense pathways or pathway regulators uncovered by the mutant screen and to evaluate the hypothesis that genes recovered from this project encode host compatibility factors. The latter genes are potential sources of stable powdery mildew resistance.