UC Riverside

Oomycetes, a class in the kingdom Stramenopila, are filamentous eukaryotic microorganisms that are known informally as water molds, but have adapted to many environmental niches. Phytophthora, an oomycete genus of approximately 100 plant-pathogenic species, has specifically adapted to several environmental niches. Environmental conditions play an influential role in the development of Phytophthora, particularly in sporangium formation. Sporangia, the main driver of Phytophthora epidemics, are especially susceptible to damage by environmental conditions, such as light-induced DNA lesions or desiccation by drying. Variable by species, light can stimulate (e.g. P. infestans) or repress (e.g. P. capsici) the production of sporangia. Light may have two developmental roles: an early pre-sporangiophore induction signal, and a later post-sporangiophore repression signal. In this work, I found that although many types of photoreceptors are described in eukaryotes, Phytophthora genomes encode only cryptochrome photoreceptors. The foliar-adapted species P. infestans encodes three cryptochromes, while the soilborne species P. capsici encodes two cryptochromes. Silencing of the P. infestans gene encoding cryptochrome PITG_01718, which lacks an ortholog in P. capsici, inhibits sporulation. While cryptochromes have traditionally been described in other organisms as blue light receptors, each red and blue light represses P. infestans sporulation. In contrast, P. capsici sporulation responds to blue light but not red light. This work also describes the use of RNA-seq to study the genetic control of sporulation by light and humidity. Constant light was found to repress the expression of genes induced in the post-sporangiophore stage by P. infestans, while constant darkness repressed the expression of genes in the pre-sporangiophore stage in P. capsici. Humidity also had a strong effect on repressing gene expression in the pre-sporangiophore stage of sporulation in P. infestans. This work also describes that specific types of nitrogen starvation induce sporulation. In addition to advancing our knowledge of sporulation control in oomycetes, this work also provides a better understanding of the homology-based gene silencing method. While this technique is an effective method for functional genomics in Phytophthora, work in this dissertation shows that the technique must be used with prudence due to the frequency of off-target effects.