Exploring host-parasitic plant interactions by examining mechanisms of resistance and susceptibility

Abstract

Parasitic plants extract water and nutrients from host plants by using specialized structures called haustoria. These interactions can cause 100% crop losses, yet growers lack control options that are effective and affordable. Host crops with resistance to parasitism would be an ideal solution to the problem, but well-characterized examples of host resistance to parasitic plants are limited. This dissertation examines plant responses to parasitic plant infection. It includes studies on resistance mechanisms in carrot (Daucus spp.) against two major parasites, the root parasite Phelipanche aegyptiaca (Egyptian broomrape) and the shoot parasite Cuscuta gronovii (swamp dodder), as well as investigations into Arabidopsis and tomato molecular responses to parasitism. These comparative studies reveal parasite-induced changes in hosts across diverse plant systems. The chapter II of the dissertation examines interactions between P. aegyptiaca and wild carrot accessions (D. glaber, D. littoralis). Phenotypic and biochemical analyses revealed pre-attachment resistance due to reduced exudation of parasite germination stimulants called strigolactones from roots of the wild carrots. Additional post-attachment resistance was also documented. The chapter III evaluates responses of wild and cultivated carrot (D. glaber, D. carota cv. 0493B) to C. gronovii. While D. glaber was susceptible, carrot cultivar 0493B displayed complete resistance. Histochemical analysis showed that haustoria failed to establish vascular connections in resistant plants and triggered localized pigmentation. Metabolomic profiling indicated parasite-induced shifts in host metabolism. A resistance-associated QTL was mapped to Chromosome 1 (30–50 Mb), and transcriptomic analysis identified four candidate genes, including one potentially involved in lignin biosynthesis. Chapter IV broadens the investigation by characterizing host responses to P. aegyptiaca in a well-studied model species, Arabidopsis thaliana, and an economically important P. aegyptiaca host, Solanum lycopersicum. Rather than focusing on resistance, this chapter investigates how susceptible hosts respond to parasitic infection, with a focus on transcriptomic changes in the host during infection. Analyses revealed that both hosts shifted their gene expression related to defense and cell wall metabolism, while species-specific differences were found as well, with tomato inducing hormone signaling, particularly ethylene, more than Arabidopsis in early stages of parasitism. Chapter V again focuses on wild carrots and the biosynthesis of phenylpropenes, a group of secondary metabolites, which serve as natural defense compounds and have pharmaceutical and industrial applications. Tissue specific analyses of phenylpropenes and a corresponding differential gene expression analysis were conducted to determine key genes in phenylpropene formation. Gene candidates for cytochromes P450 and O-methyltransferases catalyzing the proposed enzymatic steps in phenylpropene metabolism were identified. These analyses provide a foundation for future investigations of phenylpropene formation in carrots and other plant species. Together, these studies enhance our understanding of resistance in host–parasite interactions and offer genetic and biochemical insights into protecting carrots and other crops to improve crop resilience, agricultural productivity and food security.