Molecular mechanisms for plant response to drought, insect, and pathogens

Global climate changes cause severe abiotic stresses to plants and make them more susceptible to biotic stresses. In this dissertation, I focus on elucidating the molecular mechanisms underlying plant responses to drought, pathogens, and insects.;Plants lose over 90% of the water through stomatal pores on the leaf surface. Under drought stress, plants synthesize abscisic acid (ABA), which triggers increases in cytosolic Ca2+ ([Ca]i) and induces stomatal closure. Extracellular Ca2+ (Ca2+ o) also induces [Ca]i increases and stomatal closure. However, the interaction between Ca2+o and ABA signaling pathways and the physiological relevance of Ca2+o signaling remain elusive. Recently, a receptor for Ca2+ o (CAS) in the Ca2+o signaling pathway was identified. In my study, I demonstrated the molecular mechanisms by which the Ca2+o-CAS signaling interacts with the ABA signaling in response to drought in Arabidopsis. I also found that CAS regulates plant defense against the insect Trichoplusia ni.;Plants respond to pathogens using multiple layers of defenses. This is especially the case against the obligate pathogen Hyaloperonospora parasitica (Hp) whose infection cycle involves several steps that causes downy mildew in Arabidopsis. Using transcriptional profiling and phenotypic screening against Hp infection, I identified 21 genes that affect 6 different defense responses mediated by the resistance protein RPP4 against Hp Emwa1. These genes are classified into two groups based on their mutant phenotypes: Group 1 mutants are defective in RPP4-mediated HR cell death and highly susceptible to Hp Emwa1; while Group 2 mutants are intact in HR cell death showing moderate levels of pathogen growth. Characterization of mutants in response to the virulent Hp Noco2 and to NPP1, a pathogen-associated molecule pattern, allowed for a more precise placement of the 21 genes in specific defense responses. Taken together, I have provided new insights into the interplay of different layers of defense mechanisms against Hp. In conclusion, through studying how Arabidopsis responds to drought, pathogens and insects, I have identified several molecular targets that could be used to improve crop resistance to abiotic and biotic stresses.