Regulation of Gene Expression in Soybean Seed Development

Rie UzawaMarch 2021 Plant BiologyRegulation of Gene Expression in Soybean Seed DevelopmentAbstract Soybean (Glycine max) has been one of the most important crops to feed humans and animals for over 3000 years. Soybean seeds are a major source of proteins and oils. The goal of my dissertation is to understand how genes are regulated during soybean seed development. My dissertation is focused on two gene regulatory mechanisms for soybean seed development: gene regulation by micro RNAs (miRNAs) and by transcription factors. I first conducted studies on soybean seed development by profiling the miRNA populations in each seed subregion at different developmental stages. miRNAs have been shown to play important roles in plant development by regulating target mRNAs. Plant miRNAs have been profiled in different tissues of various plant species including soybean seeds. However, no studies have been comprehensively conducted to profile miRNAs at the single subregion level in soybean seeds. In order to get insight into the roles of miRNAs for soybean seed development, I have demonstrated that many miRNAs predominately accumulate in one subregion or stage. In particular, the endosperm subregion is enriched with miRNAs that have been identified only in soybean, whose functions have not yet been studied. Some of these non-conserved miRNAs specifically accumulate in the endosperm at a high level. I also explored the function of the target mRNAs of miRNAs. Based on the analysis of the target mRNAs, the endosperm-specific miRNAs may play important roles in biotic and abiotic stress responses. These results shed light on the role of miRNAs in soybean seed development. The second focus of my dissertation is to elucidate the gene regulatory network for soybean seed maturation. The seed maturation program is controlled by four transcription factors: LEAFY COTYLEDON1 (LEC1), ABA-RESPONSIVE ELEMENT BINDING PROTEIN3 (AREB3), BASIC LEUCINE ZIPPER67 (bZIP67) and ABA INSENSITIVE3 (ABI3). Two transcription factors, GROWTH-REGULATING FACTOR5 (GRF5) and HOMEOBOX22 (HB22) were identified as direct targets of LEC1, AREB3, bZIP67 and ABI3. I have investigated the genes regulated by GRF5 and HB22 to expand the maturation gene regulatory network. I have demonstrated that GRF5 and HB22 shared many of the same target genes that are jointly regulated by LEC1, AREB3, bZIP67 and ABI3. Furthermore, GRF5 binding sites are located close to those of LEC1, AREB3, bZIP67 and ABI3 on the common targets. By contrast, HB22 binding sites are more distantly located from those of LEC1, AREB3, bZIP67 and ABI3. My results suggest that GRF5 may regulate target genes involved in the soybean seed maturation program in combination with LEC1, AREB3, bZIP67 and ABI3 whereas HB22 may regulate the same genes independently. These results elucidate the roles of GRF5 and HB22 in soybean seed maturation program