Understanding wheat stripe rust through studies on host and pathogen metabolism

Abstract

Wheat stripe rust, caused by the biotrophic Basidiomycete fungus Puccinia striiformis f. sp. tritici (Pst), is one of the most important crop diseases worldwide. Pst has a complex lifecycle with an asexual cycle and a sexual cycle, consisting of five possible spore types. The asexual cycle, which is the economically damaging phase on wheat, occurs repeatedly throughout the growing season through the germination of urediniospores, vegetative growth of the fungus and ultimate sporulation. During colonization, urediniospores germinate on the upper leaf surface and penetrate it via stomata. Vegetative growth starts when infectious hyphae branch off from a substomatal vesicle and ramify throughout the intercellular spaces. When the tip of a growing hypha touches a mesophyll cell, the fungus can develop an invasive structure called the haustorium. This specialized structure penetrates the cell wall but not the plasma membrane, expresses nutrient transporters and is the main site for secretion of virulence effector proteins. At this point the biotrophic growth starts. This stage If sufficient nutrients are available the fungus is able to complete the asexual cycle by developing spore-forming pustules that erupt from the leaf epidermis releasing millions of urediniospores (sporulation). Here I used physiological, molecular biology, metabolomics, and transcriptomics approaches to provide a broad view of metabolic processes during the infection. I measured the chitin content of infected wheat tissue as a proxy for fungal biomass, as chitin content is the major component of the fungal wall. Using this technique, I found that the Pst asexual lifecycle can be divided into two main phases. In the early part of Pst cycle from two to eight days after infection, it has limited requirements for nutrients, whereas the second phase from 9 to 15 dai leading to sporulation has high nutrient requirements. As all fungal nutrients are derived from the host, I measured aspects of carbon and nitrogen metabolic pathways. In the first phase, most photosynthesis parameters were unaffected. During the second phase, the high nutrient demand caused by the infection shifted the metabolic status of the infected tissue from source to strong sink. This conclusion was derived from the fact that all photosynthesis parameters related to carbon fixation and chlorophyll content decreased by at least half compared to healthy leaves. Moreover, genes related to asparagine and sucrose metabolism were up regulated in non-inoculated leaves while head weights were reduced. These data are consistent with a model in which the Pst infection induces nutrients reallocation from healthy to infected tissue in the second phase of infection to meet the sporulation phase demands. The second aspect that I studied was the relationship between plant development and Pst growth. One approach was to investigate the phenomenon of Adult Plant Resistance (APR), in this case conferred by the hexose transporter gene, Yr46. The Yr46 resistance is manifested as reduced production of Pst spores on the infected flag leaves of adult plants. I found that Yr46 seems to prevent the flag leaf from acting as sink tissue, possibly by accumulating sugars in the apoplast. This mechanism may reduce the cytoplasmic nutrients available for uptake by the haustoria. I suggest that immunity and plant development should be studied together, and observe that systemic responses provide additional information to understand the infection. I propose that in addition to the recognition of pathogen molecules, plant immunity may also concern the detection of pathogen manipulation of host metabolic pathways.