From early lineages to angiosperms, plants have evolved mechanisms to protect themselves against biotic and abiotic challenges. Plant hormones are the primary regulators of growth and defense responses, and elucidation of the hormone signaling networks has provided insight into the molecular mechanisms that protect the plant. The hormone jasmonate (JA) is critical for defenses against herbivores and necrotrophic pathogens and is involved in developmental regulation such as senescence and reproduction. In the core JA signaling pathway, JAZ repressors bind to and inhibit the MYC transcription factors (TFs). Upon a stressor such as insect attack, the F-box protein, COI1, forms a JA-isoleucine (JA-Ile) co-receptor with the JAZ repressors. This association results in ubiquitin-degradation of JAZ, thereby relieving repression on the MYCs. While constitutive JA signaling mutants such as the jaz decuple mutant, jazD, displays robust resistance to insects and necrotrophic pathogens, these elevated defenses come at the cost of growth and reproduction. Although the molecular mechanisms are well-characterized for JA responses, a remaining question in the field of plant hormone signaling is how response specificity is achieved by a limited set of components. In the case of JA signaling, this question has been difficult to address because of the large size of the JAZ gene family and lack of obvious phenotypes in single jaz mutants. Additionally, the molecular mechanisms underlying growth and defense tradeoffs are not fully elucidated. Lastly, JA has long been associated with regulation of senescence. However, a lack of senescence phenotypes in JA biosynthetic or signaling mutants suggests that JA is not critical for natural, age-dependent senescence. Most studies of JA-induced senescence have been performed under dark conditions with high concentrations of exogenous JA, which has hindered insight into the physiological relevance of this phenomenon and its connection to defense and metabolism. Further advances in these areas may inform biotechnological applications to tune the growth and defense balance as a means to increase crop yields. In this dissertation, I examined the subfunctionalization of JAZ paralogs in Arabidopsis thaliana using a novel screen combining forward and reverse genetics. Among ten JAZ paralogs tested, I identified four members that function to repress tryptophan biosynthesis and defense responses to necrotrophic pathogens. The JA-responsive transcription factor MYC3 was also identified as a major positive regulator of these responses. I also performed genetic suppressor screens to identify molecular components that contribute to JA-regulated growth-defense balance and senescence. These studies identified six mutations in COI1 and six mutations in MYC2. I also used soil-grown jazD plants as a novel system to understand the process of JA-mediated leaf senescence. Characterization of senescing jazD plants over a 72-hour time course shows that the JA signaling pathway controls chloroplast metabolism and photosynthesis during the growth-to-defense transition. This collective work provides new insights into mechanisms of response specificity in the JA signaling pathway. It additionally contributes to an understanding of regulatory factors required for senescence and the growth-defense balance.
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