Pathogenesis-related proteins in barley

Plants in nature are generally resistant to most of the pathogens they encounter. However, many flingal pathogens can cause severe diseases and significant yield losses in crops. Plants defend themselves against pathogens through a combination of constitutive and inducible defenses. The induced plant defense is particularly characterized by an increased accumulation of pathogenesis-related proteins (PRs). Since the discovery of PRs in 1970, several PR families have been identified. The specific function of many PRs is still unknown, although several are postulated to play a role in preventing pathogen invasion. The aim of my research was to study spatial and temporal localization of PRs and accumulation of their corresponding mRNAs to better understand the role and possible function of PRs in plant defense. As a model system I studied the interaction between barley and the hemibiotrophic fungus Bipolaris sorokiniana, which causes severe yield losses worldwide. The studies are mainly focused on three PRs (PR-I, PR-3 and PR-5). These PRs accumulated in both leaves and roots of barley seedlings as a response to infection of the respective tissues. However, the accumulation of PRs in roots was dependent on whether young or old root segments were infected. A stronger response was found when young root segments were inoculated in comparison to old roots. All three PRs appear to be part of a prefoimed defense since both PR-I and PR-5 were constitutively present in both xylem and phloem tissues and the root epidermis, and PR-3 showed a high constitutive presence mainly in the epidermis of leaves and to some extent in the phloem. In response to infection, all three proteins were highly induced in the ground tissues and to some extent in the vascular tissues. PR-I, PR-3 and PR-5 mRNAs accumulated in a biphasic pattern in leaves from both leaf- and root-infected seedlings. This biphasic accumulation pattern of transcripts was not detectable in roots. These results suggest that different induction strategies may be active in barley seedlings depending on the primary site of infection by B. sorokiniana. PR-I, PR-3 and PR-S were all recovered over the cell walls of inter- and intracellular hyphae of B. sorokiniana indicating that they may influence fungal growth