Defence and signalling in Potato-Phytophthora infestans interactions

Sammanfattning: Popular Abstract in English Plants harvest sunlight to fix air carbon dioxide into a form which is the fundamental source of energy for other organisms such as animals. The domestication of crop plants and traditional breeding practices has resulted in increased yield and nutritional value of crops. This comes at a cost of loss in genetic variability that is required by living organisms to adapt to changing environment. Global warming is thought not only to contribute to unpredictable weather conditions but also to increase pests and disease causing microorganisms that can devastate crop production. One such example is the late blight disease on potato that caused Irish potato famine in the 1840’s leaving about a million Irish people starved to death and a similar number having to emigrate. With looming 9 billion of humans and added environmental variability, food security is one of the major concerns for the future well-being of humans. More food is required for human usage than ever before which has resulted in decreased forest area as increasing amount of forest is cut down to make land available for food production. Immediate solutions are needed to meet human food requirements for a sustainable future of different life forms on the planet earth. Potato has remarkable nutritional value with considerable amount of carbohydrates, proteins and vitamins, minerals and antioxidants (Brown 2005) that make potato a complete dietary source for survival if combined with milk and butter. Its yield-potential is more than any other major crop per hectare and potato requires a short growing period. With global climate change possibly leading to lower productivity and re-distribution of cereal products, potato plays a central role in global food security. In this thesis we have explored the wealth of genetic resources available in potato breeding material in order to find new mechanisms by which plants can cope with the late blight causing plant pathogen Phytophthora infestans (potatisbladmögel). Plants generally have strong defences such as cell walls that act as barrier against any foreign invaders. In exceptional cases plant pathogens have the ability to break these barriers and enter into inter- and intracellular space to access plant nutrients. This invasion is recognized by plants as plants have evolved receptor molecules that can detect conserved and usually essential molecules originating from the pathogen. This detection leads to an array of molecular cascades to switch on plant immunity. Successful pathogens have evolved specialized molecules (effectors) that sometimes are injected into the plant cell and block plant immunity that leads to disease. This is often refered to as effector triggered susceptibility (ETS). In some cases plants can recognize these effector molecules and switch on the immune system and avoid disease, such immunity is called effector triggered immunity (ETI) and usually involves small scale local cell death. In our field trials performed in Swedish conditions we found two promising potato genotypes that display such immunity. In our greenhouse experiments these two potato genotypes developed slightly different resistance phenotypes and one of the potato genotypes appeared to be ”paranoid” as it had its immunity activated without any pathogen stress. We studied these two potato genotypes in comparison with a classical susceptible potato genotype (ETS) to evaluate the differences at molecular level. We found that all potato genotypes displayed wide range of molecular level responses after the pathogen attack indicating that even in a susceptible interaction (ETS) pathogen is recognized and a wide range of responses is observed. Our comparison allowed us to find genes that were activated in resistance (ETI) but not in ETS. These genes maybe are involved in resistance and are good candidates for future studies in order to find more durable resistance mechanisms. In our unique proteomics study, we also found several proteins that are potentially targeted by the pathogen in order to suppress plant immunity for successful infection. Further studies are required to understand their role in the plant immune system. In a classical genetics approach to study resistance mechanism in one of our potato genotypes (paranoid potato) we found that resistance is most probably caused by a simple inheritance of one or more closely linked genes. Such resistance mechanisms are of great importance for classical breeding strategies for resistance. We also studied how plant pathogen P. infestans processes its proteins during its different growth stages that occur during infection process. We found sites on some of the pathogen’s proteins where phosphorus is added for signal processing. Some of these might be unique for the group in which P. infestans is classified. These studies will increase our understanding about how plants respond to pathogen stress and provide us with candidate genes to find new ways to improve resistance in plants.