Identification of progestogens and androgens in plants and their role in plant defence against fungal infection

Fungal infections damage crops worldwide. Necrotrophic fungi of the genus Alternaria can destroy up to 86% of the yield of Brassicaceae plants such as broccoli, cauliflower, and cabbage. This forces farmers to use fungicides, which are often ecologically harmful and to which fungi can develop resistance. The goal of this thesis is to discover alternative methods of crop protection by first investigating whether steroid hormones such as progestogens and androgens are synthesized by plants and what role they play in plant defense against fungal infections. To move closer to this goal, the presence of progestogens and androgens and their metabolism in viridiplantae were examined (Manuscript I). In manuscript II the role of the steroid DHEA during the infection of Arabidopsis thaliana by Alternaria brassicicola is investigated. Molecular analysis using UHPLC-ESI-MS/MS of algae, mosses, gymnosperms, monocots, and dicots has revealed the widespread presence of progestogens and androgens in the plant kingdom. Androgens are found in all monophyletic groups examined, whereas progestogens only appear in land plants. This implies that androgen biosynthesis evolved first and that progestogen biosynthesis evolved after the transition to land to protect plants from oxidative stress. In this thesis, it was shown that the biosynthesis of progestogens and androgens is evolutionarily conserved in plants and that they play a role in plant defense against fungi. However, further investigation is needed to determine the role dehydroepiandrosterone might play in the plant defense of monocots against biotrophic fungi and whether other steroids also play important roles.