To Live and Let Die: Genetic Analyses of Singlet Oxygen-Induced Chloroplast Quality Control and Programmed Cell Death Pathways

Plants have evolved complex signaling mechanisms to sense and respond to abiotic stresses. These stresses can lead to the excess production of reactive oxygen species (ROS) that damage cellular components and can lead to cellular dysfunction and, ultimately, cell death. Chloroplasts generate large amounts of ROS, including singlet oxygen (1O2), during photosynthesis. This 1O2 can act as a retrograde signal for the selective degradation of damaged chloroplasts and the activation of programmed cell death (PCD). While such processes allow plants to maintain efficient energy production and avoid toxic ROS accumulation, the signaling mechanisms and genes involved are largely unknown. This raises two intriguing questions: How do plants sense chloroplast dysfunction and selectively degrade damaged chloroplasts to sustain healthy chloroplast populations? -and- What mechanisms do plants use to activate PCD when their chloroplasts accumulate too much 1O2 to manage via selective degradation? To answer these questions, my dissertation research has utilized both forward and reverse genetic approaches to identify and characterize genes and pathways involved in 1O2-mediated chloroplast retrograde signaling (CRS), chloroplast quality control (CQC), and PCD. The Arabidopsis plastid ferrochelatase 2 (fc2) mutant has proven to be useful as a system in which to study 1O2-induced CRS, CQC, and PCD. These mutants conditionally accumulate 1O2 under diurnal (cycling) light conditions, leading to enhancements of chloroplast degradation and cell death in stressed fc2 plants. As these fc2 mutants have a clear conditional phenotype that correlates with chloroplast degradation, they are ideal for genetic suppressor screens to identify genes that play a role in 1O2-induced CRS, CQC, and PCD. In such screens, fc2 mutants are further mutagenized to screen for secondary mutations that lead to an fc2 suppressor (fts) phenotype. Such loss-of-function screens have already yielded many suppressor mutan