Plants are exposed to many stresses, including drought and pathogens. The phytohormone abscisic acid (ABA) is known to regulate major abiotic stress responses. The hormones salicyclic acid (SA), jasmonic acid (JA), and ethylene (ET) are known to regulate biotic stresses. Previous research has identified a molecule, [5-(3,4- dichlorophenyl) furan-2-yl]-piperidine-1-ylmethanethione (DFPM) that negatively regulates ABA signaling by activating a plant immune signaling pathway (Kim et al., 2011). In Chapter 1, the mutants reduced sensitivity to DFPM-inhibition of ABA (rda1 and rda2) are characterized by developing a method to genetically map the rda2 and rda1 loci to characterize their roles in mediating cross talk of an effector-triggered biotic stress response with abscisic acid (ABA) signal transduction. Through these mutants, the connection between abiotic and biotic stress signaling could uncover new insights. In addition, plants are also stressed if exposed to toxic metal(loid)s and one strategy plants use is to activate the sulfate assimilation pathway in order to enhance synthesis of thiols for detoxification of metal(loid)s. The mechanisms mediating metal(loid)-induced gene expression remain poorly understood. In Chapter 2, Slim1, a key regulator of the sulfate assimilation pathway during sulfur starvation, is investigated for its role in metal(loid) detoxification. To further identify activators and repressors of the pathway, a screen of an amiRNA library targeting transcription factors is screened on cadmium. Understanding the regulation of this pathway is thus an integral part of engineering plants that either prevent metal accumulation in edible plant tissues or alternatively accumulate high levels of toxic compounds for phytoremediation

Arsenic stress causes rapid transcriptional responses in plants. However, transcriptional regulators of arsenic-induced gene expression in plants remain less well known. To date, forward genetic screens have proven limited for dissecting arsenic response mechanisms. We hypothesized that this may be due to the extensive genetic redundancy present in plant genomes. To overcome this limitation, we pursued a forward genetic screen for arsenite tolerance using a randomized library of plants expressing >2,000 artificial microRNAs (amiRNAs). This library was designed to knock-down diverse combinations of homologous gene family members within sub-clades of transcription factor and transporter gene families. We identified six transformant lines showing an altered response to arsenite in root growth assays. Further characterization of an amiRNA line targeting closely homologous CBF and ERF transcription factors show that the CBF1,2 and 3 transcription factors negatively regulate arsenite sensitivity. Furthermore, the ERF34 and ERF35 transcription factors are required for cadmium resistance. Generation of CRISPR lines, higher-order T-DNA mutants and gene expression analyses, further support our findings. These ERF transcription factors differentially regulate arsenite sensitivity and cadmium tolerance.