Studies On The Responses Of The Parasitic Plant Cuscuta Australis To Phytohormones And C.Australis-mediated Transfer Of Systemic Herbivory-Induced Signal Between Different Soybean Plants

Parasites are defined as the organisms that obtain nutrients from hosts and cause harm but not immediate death. Parasitic plants can affect the growth and development of their hosts, and even could lead to host death when hosts are severely parasitized. The plants in the genus Cuscuta(dodder) are shoot obligate parasites. This genus consists of about 200 species that occur on all continents. Cuscuta seedlings have no roots and cotyledons only have thread-shaped hypocotyls. After contacting hosts, Cuscuta seedlings induce haustorium formation and then penetrate into host tissues, and the vascular systems fuse to get water and nutrients to survive. Due to their abnormal lifestyles, compared with autotrophic plants, parasitic plants are likely very different in development and stress adaptation. Except for water and nutrients, Cuscuta not only extracts small molecules from hosts, such as sugars and amino acids, but also secondary metabolites. Thus far, little is known about the development and how the interactions between hosts and parasites on a molecular level. In this Ph.D thesis, I studied 1) how parasitic plant C. australis responds to phytohormones and 2) discovered that C. australis transmits systemic defense signaling between different hosts. The results are shown below:Using germination and seedling growth assays, we examined the responses of C. australis to ABA. Remarkably, germination and seedling assays indicated that C. australis was almost completely insensitive to ABA. Tomato seedlings were sensitive to ABA treatment and their hypocotyl lengths were significantly reduced at a concentration of 1 μM ABA. In contrast, C. australis seedlings did not show growth inhibition at 1 μM of ABA and even up to 200 μM of ABA(a concentration at which tomato seedling had died already); only when 700 μM of ABA were applied, the lengths of C. australis seedlings were moderately reduced(25%). Thus, C. australis seedlings were extremely hyposensitive to ABA-induced suppression of hypocotyl elongation. Tomato seeds had very poor germination rates on ABA-containing plates: even after 20 days, the germination rates in all treatment groups, including 1 μM, were below 10%. In contrast, all C. australis seeds germinated within 14 days, even after being treated with 10 μM of ABA, indicating that C. australis seeds are very insensitive to ABA-induced inhibition of seeds germination. Furthermore, transcriptome data-mining suggested that C. australis likely had only 4 functional ABA receptors and this is much less than those in Arabidopsis, tomato, and rice. These data suggest that C. australis has a distinct ABA hormonal physiology, and we propose that the highly adapted and specialized lifestyle may have relaxed the selection pressure from drought stress in C. australis and have led to loss of responses to ABA.We also tested how C. australis responses to other phytohormones. The seedlings of C. australis responded to cytokines and jasmonic acid similarly to tomato hypocotyls, but they were less insensitive to SA, indole-3-acetic acid and gibberellic acid 3. These data also support the hypothesis that C. australis has distinct physiology in development and defense.When plants are attacked by insects, plants respond to insect feeding not only in the attacked tissues(local) but also in the systemic undamaged tissues. It is not uncommon that individual dodder parasitizes several hosts linking their vascular systems together. We hypothesized that these networks can serve as a conduit for transferring herbivore-induced defense signals. We established C. australis connections between two soybean(Glycine max) plants, and challenged a “local” plant with the caterpillar Spodoptera litura. We investigated the defense responses and insect resistance in the neighboring C. australis-connected “systemic” plants. We show that caterpillar infestation on “local” plants led to increased insect resistance, induction of defense-related genes and activation of jasmonate acid(JA) pathway in the “systemic” plant. These results indicate that Cuscuta can transmit herbivore-induced defense signals and contribute to interplant defense communication.In total, our data suggest that C. australis has a unique ABA hormonal physiology, and we speculate that ABA signaling in C. australis might not be as important as in other normal plants, given that these parasites live on host plants which can reserve water under drought stress, and C. australis may have partially lost its ABA signaling. C. australis also show less sensitive to other phytohormones, suggesting that it has a special hormone physiology. This study illustrates that the parasitic plant C. australis responds specifically to phytohormones, providing important data for understanding the unique physiology of parasitic plants. Furthermore, for the first time, we discover that C. australis can transfer the systemic signaling in different hosts to induce defenses against herbivores, providing important insight into higher plant-mediated inter-plant signaling.