The changing climate and plant pathogens present significant threats to global food production. There is a vast need for crop varieties that could endure more extreme weather conditions and ever-evolving pathogens. Plants have developed molecular mechanisms to battle pathogen

invasion, while pathogens have co-evolved strategies to overcome these mechanisms. Therefore, the interactions between plants and pathogens are very complex, and it is crucial to elucidate them in order to develop more resilient and durable crops. This work focuses on the biotrophic fungus Puccinia striiformis f. sp. tritici (Pst), which causes wheat yellow rust and is one of the most devastating plant pathogens.

Biotrophic fungi colonise living tissue, and to do so, they secrete effector proteins (effectors) that interact with the host's proteins to suppress plant defences and enable the spread of the fungus. Pst is estimated to secrete more than a thousand effector proteins, but most of their functions remain unknown. The main limitation is the lack of efficient methodology for screening large numbers of effector candidates. Yeast (Saccharomyces cerevisiae) has been used as a model to study the functions of bacterial type III secretion system (T3SS) effectors but, to our knowledge, it has not yet been utilised to study fungal effectors.

In this project, I used yeast as a model for identifying and characterising predicted Pst effectors.

Expression of effectors in yeast under various stress conditions can induce growth retardation phenotypes and provide information about potential effector targets. Yeast-based screening of Pst effector candidates revealed many different phenotypes. After sorting the effector candidates into groups based on the phenotypes, I focused on candidates that induced more severe growth retardation under osmotic and salt stress conditions. I investigated the subcellular localisation of selected effector candidates and their potential interactions with osmotic stress signalling components of yeast. I identified two effector candidates (KNE96582, KNE91991) that could facilitate infection of wheat with Pst, and I identified a putative target of one of the effector candidates.