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Abstract

Seed is an evolutionary invention that enables land plants to reproduce, and pause their life cycle to avoid adverse environments. In Arabidopsis thaliana, seeds acquire traits such as dormancy and desiccation tolerance during seed maturation, processes established by distinct molecular programs. The LAFL (LEC1, ABI3, FUS3, and LEC2) transcription factors (TFs) govern transcriptional networks that shape the maturation traits. Among LAFL, ABI3 collaborates with other TFs, including ABI4 and ABI5, to mediate ABA-regulated gene expression during late seed maturation and germination. To initiate seed germination, the LAFL expression must be turned off, mediated by Polycomb repressive complex 2 (PRC2), which deposits the repressive histone mark H3K27me3. VIVIPAROUS1/ABI3-LIKE1 and 2 (VAL1 and VAL2) recruit PRC complexes to repress the transcription of LAFL and their downstream genes. In this dissertation, I aimed to investigate how transcriptional regulation, mediated by both known and novel factors, governs the seed-to-seedling transition in Arabidopsis thaliana. I characterized a new transcriptional repressor of embryonic programming, Arabidopsis thaliana SEED DORMANCY 4-LIKE (AtSDR4L). Atsdr4l mutants exhibit increased seed dormancy, stunted seedling growth, elevated expression of ABI3, and over-accumulation of the seed storage lipids. I found that the penetrance and expressivity of embryonic phenotype in Atsdr4l seedlings are condition- and stage-dependent. Genetic analysis demonstrated that ABI3 partially contributes to the Atsdr4l mutant phenotype for embryonic traits and a feedback regulation occurs between ABI3 and AtSDR4L. I showed that AtSDR4L acts as a negative regulator of LAFL transcription and its own gene and contributes to H3K27me3 accumulation by interaction with VAL2. I investigated how ABI5 affects ABI3 regulome during ABA responses in germinating seeds. The absence of functional ABI5 led to decreased ABI3 binding in hundreds of regions, especially near ABA biosynthetic and catabolic genes such as NCED2, ABA1, and CYP707A4. In addition, ABI5 enhances ABI3 binding near genes that function in light responses. Our results have deepened our insights into the transcriptional regulation of the seed-to-seedling transition mediated by both known and novel transcription factors. These findings provided valuable resources for new discoveries and a better understanding of the underlying regulatory networks.