A Study on an ABCI Protein Involved in Early Seedling Development in Plant

DoctorThe genome of Arabidopsis thaliana, a model plant, encodes numerous ATP-binding cassette (ABC) proteins that exhibit diverse functions. Most eukaryotic ABC transporters are encoded either as half-size or full-size transporters, consisting of one or two nucleotide-binding domains (NBDs) and transmembrane domains (TMDs). However, some eukaryotic ABC proteins, including ABC group I, lack TMDs. ABC group I is a subfamily of ABC proteins with features similar to prokaryotic ABC proteins. Some ABC group I proteins (ABCIs), however, exhibit distinctive features from prokaryotic ones, i.e., plant specific manner.Early growth and development of plants are regulated by environmental and internal cues. Light is a representative environmental factor regulating diverse processes such as seed germination, photomorphogenesis, and accumulation of pigments. Plant hormones are internal factors that control responses to various environmental factors. Abscisic acid (ABA) is one of these hormones and an important regulator of seed germination and seedling development.Here, I provide multiple lines of evidence that Arabidopsis thaliana ABCI20 is involved in early plant growth and development regulated by light and hormone. AtABCI20 was expressed at early stage of seedling development mainly in the root. The gene was highly induced by light, and more or less by ABA. atabci20 knockout mutant seedlings exhibited retarded growth under normal conditions, and this phenotype was aggravated in the presence of ABA. However, there was no difference in ABA content in the wild type and atabci20. Expression of AtABCI20 in the atabci20 mutant background complemented this phenotype. In atabci20, the expression of genes involved in the ABA signal transduction pathway, such as ABI3, ABI4, and ABI5, was altered, which corresponded with the increased sensitivity of the atabci20 mutant to ABA, but no genes involved in ABA biosynthesis exhibited different expression in atabci20 compared to the wild type.A transcriptome analysis revealed that the expression of many genes encoding ABA response-related proteins and endomembrane proteins was different between atabci20 and the wild type under normal conditions, and this difference became even more pronounced when plants were treated with ABA. Interestingly, genes involved in secondary metabolic process such as flavonoid pathway were also over-represented in atabci20 mutants. Furthermore, these genes exhibited high co-expression values with AtABCI20 suggesting a role of AtABCI20 in the secondary metabolism. Supporting this possibility, atabci20 accumulated larger amount of anthocyanin compared to the wild type. Moreover, the expression of AtABCI20 was down-regulated in hy5 mutant in which HY5, a key regulator of light and hormone signaling, is knocked down, suggesting an involvement of AtABCI20 in light response. Localization studies using AtABCI20 fused to green fluorescent protein and biochemical assays indicated that AtABCI20 is targeted to and tightly associated with the endoplasmic reticulum (ER) membrane. Taken together, I conclude that AtABCI20 performs an important function in early seedling development by regulating ABA and light responses at the ER