UC Davis

Unlike most autotrophic plants, parasitic plants evolved to have a heterotrophic lifestyle and to steal water and nutrients from their host through specialized parasitic organs, haustoria. Some species of parasitic angiosperms parasitize major crop plants, which causes severe agricultural losses and threatens food security in many regions. Understanding how host plants sense and resist parasitic plants not only reveals the underlying mechanisms of various resistance systems, but also provides the foundation for agricultural improvements. Chapter One of this dissertation reviews the current knowledge on how host plants perceive stem and root parasitic plants and utilize different pre-attachment and post-attachment defense responses to deter these parasites.Cuscuta species (dodders) are stem holoparasitic angiosperms, which lack functional leaves and roots. Cuscuta campestris (C. campestris) is one of the most broadly distributed Cuscuta species and has a wide host range, which includes many important vegetable and fruit crops. Domesticated tomato (Solanum lycopersicum) is one of the crop plants that are vulnerable to C. campestris and reduces 70% yield upon Cuscuta infestations. Understanding the molecular mechanism of C. campestris haustorium formation will assist in parasitic weed management and the development of parasitic plant-resistant crops. Chapter Two of this dissertation investigated the transcriptome of six C. campestris tissues and identified LATERAL ORGAN BOUNDARIES DOMAIN 25 (CcLBD25) as a critical regulator in haustorium development. On the other hand, even though most cultivated tomatoes are usually susceptible to C. campestris, some specific Heinz hybrid tomato cultivars were discovered to be resistant to Cuscuta species. These tomato cultivars are used as a biocontrol method in the field, but the resistance mechanism remains unknown. Chapter Three of this dissertation dives into the underlying mechanism and genes involved in this lignin-based defense response. These resistant Heinz cultivars trigger post-attachment lignification in the stem cortex upon C. campestris infection. Although some studies have identified factors required for parasitic plant haustorium induction as well as genes involved in host defense responses, the signals involved in haustorium development at specific developmental stages and tissue-resolution communication between host and parasite during the haustorium penetration process are largely unknown. Chapter Four of this dissertation focused on the interface between the host and parasite. C. campestris haustorial tissue and tomato host tissue immediately surrounding haustoria were collected by laser-capture microdissection (LCM) to obtain tissue-resolution RNA-Seq profiles. These profiles were used to identify key genes regulating haustorial development and host responses, and describes my attempt to validate the function of these genes. Finally, Chapter Five summarizes the major discoveries of each research project and the potential of utilizing these findings in agriculture. This dissertation provides an overview of both haustorium development in C. campestris and defense response in tomato host plants. Therefore, this work will be of interest to academic researchers in plant biology and researchers interested in developing potential agricultural translational applications.