This dissertation aims to elucidate the behaviors of interfacially active molecules that self-assemble at fluid-fluid interfaces and stabilize them. Examples of such molecules include amphiphilic surfactant molecules that comprise hydrophobic (oil-loving) and hydrophilic (water-loving) moieties and therefore have a natural affinity for oil-water as well as water-air interfaces. Surfactant molecules are ever present in typical household soaps and detergents, and when dissolved in water, self-assemble at the air water interface as well as into multi-molecule aggregates known as micelles. These assemblies, in turn, facilitate foaming and form the basis of the cleaning actions of the soaps and detergents. The first part of this dissertation examines the role of micellar and interfacially self-assembled surfactants on the stability of thin foam films. Specifically, the correspondence between inter-micellar interactions in bulk solutions and in thin films, and the influence of these interactions on the average lifetime of foam films are investigated. I envision that my studies will inspire colloidal scientists to explore foam film studies as a simple but effective method for characterizing nanoscopic colloidal interactions and forces. In the second part of this dissertation, development of interfacially active polymers that stabilize water-water interfaces, designed specifically for stabilization of polyelectrolyte complex coacervates, is demonstrated. Complex coacervates form upon electrostatic complexation of oppositely charged macromolecules and their subsequent condensation into aqueous macromolecules-rich phase. This aqueous two-phase system has been demonstrated to exhibit unique capabilities to achieve dynamic spatial compartmentalization as well as spontaneous sequestering of biological molecules. Despite these exciting prospects, their use has been limited owing to our inability to stabilize coacervate droplets and prevent their macro-phase separation. In my research, I have developed block polymers and comb polymers that have successfully stabilized micron-sized coacervate droplets, leading to first demonstrations of stable coacervate emulsions. I have also demonstrated sequestration of proteins (enzymes) into the stabilized coacervate droplets, paving way for these emulsions to be employed as protein-based bioreactors with selective small molecule transport.

Small RNAs, including microRNAs (miRNAs) and small interfering RNAs (siRNAs), are geared to respond to the complexity of developmental and physiological processes and negatively regulated gene expression by guiding chromatin modification mRNA degradation, and translational repression of their targets.

Increasing evidence indicates that host small RNAs play a crucial role during the plant-pathogen interactions. In the first chapter of this thesis, we reviewed pathogen-responsive endogenous miRNAs and siRNAs in plants. In addition, we discussed the important components involved in the canonical RNA silencing pathways, including dsRNA-specific endoribonuclease (RNase) Dicer-like proteins (DCLs), RNA-dependent RNA polymerases (RDRs) and Argonaute (AGO) Proteins.

MiRNAs are short noncoding RNA molecules with 21-24 nucleotides and are found in most eukaryotic cells. In the second chapter of this thesis, we globally profiled plant miRNAs in response to infection of bacterial pathogen Pseudomonas syringae pv. tomato (Pst) in Arabidopsis and found 19 miRNA precursors that can yield multiple distinct miRNA-like RNAs in addition to miRNAs and miRNA*s. These miRNA precursor-derived miRNA-like RNAs are often arranged in phase and form duplexes with approximately 2 nucleotides 3'-end overhang. These miRNA-like RNAs are potentially functional at least under certain conditions.

In the third chapter of this thesis, we identified 15, 27 and 20 miRNA families being differentially regulated after Pst DC3000 hrcC, Pst DC3000 EV and Pst DC3000 avrRpt2 infections, respectively, through the deep sequencing results from 13 small-RNA libraries. Some bacteria-regulated miRNAs targets have been annotated and believed to be involved in plant hormone biosynthesis and signaling pathways, including auxin, abscisic acid, and jasmonic acid pathways, which suggests the regulation of plant innate immunity through fine-tuning of multiple plant hormone pathways. Moreover, we discussed the procedural differences between sequencing-based profiling and small RNA Northern blot and stated the possibilities that cause the inconsistency.

AtlsiRNA-1, as the first example of the long siRNAs identified in Arabidopsis, contributes to resistance against Pseudomonas syringae infection by degrading the negative regulator of disease resistance, AtRAP. How does AtRAP regulate defense response became the major focus of my study to elucidate the mechanisms of plant innate immunity. In the fourth chapter of this thesis, I further investigated the molecular mechanism of AtRAP by characterizing the subcellular localization of AtRAP and determining the function of C-terminal RAP domain in disease resistance.