애기장대의 어린 발달시기 큐티클 형성에 기여하는 ABC 수송체와 종자 지방 함량 증대에 기여하는 수송체의 기능 연구

Abstract

Lipids and lipophilic compounds are the building blocks of biological membranes, and constitute important energy reserves that are indispensable in the early phases of plant development. Moreover, they are major constituents of cuticle, as a surface coating material, and also play roles as signaling molecules. Very long chain fatty acids (VLCFAs) and VLCFA-derived chemicals act as non-autonomous mobile signals from epidermis to internal tissues and thereby fine-tune cell division activity and determine the plant growth. However, underlying transport mechanism of lipid and its precursors through membranes has remained limited up to date. To date, many ABC transporters have been shown to have important roles in plant development and growth. AtABCG11, 12, 13 and 32 transporters are reported to transport surface coating materials across the plasma membrane out of the cells in various tissues. Although single or double mutants of the ABC transporters exhibited strong reductions in total loads of cutin and wax, they were not completely devoid of them, suggesting that there are additional transporters involved in cutin and wax secretion. The aim of this study is to identify the transporters that is necessary for the cuticle formation during early seedling development. To address the question, I hypothesized that i) the transporter would be expressed with the genes involved in cuticle biosynthesis and transportation, ii) the transporter would belong to ABC transporter subfamily G member close in phylogeny to cuticle components transporter, and iii) the transporter mutant would exhibit increased cuticle permeability. From screening of mutant plants that are defective in expression of half-size ABCG members, I selected abcg5 knockout mutants as a candidate of putative cuticle transporter, because they exhibited increased cuticle permeability. Subsequent characterization on ABCG5 supported that ABCG5 is a transporter for cuticle components. First, ABCG5 is localized at plasma membrane of shoot in young seedlings. Second, the amount of cutin was decreased in abcg5 mutant cotyledons. Third, the abcg5 mutant seedlings looked water-soaked and vitrified, and were high in alcohol dehydrogenase activity, a representative hypoxia-responsive gene, which became more distinct under high humidity conditions. Such phenotypes can be explained by if water permeated through the defective cuticle of abcg5 cotyledons. In addition, abcg5 exhibited growth arrest, and disrupted structure and activity of shoot apical meristem when the medium lacked sucrose and/or under high humidity. Consequently, photosynthetic activity of abcg5 mutants was drastically reduced. These developmental defects of abcg5 may have been caused by the hypoxic stress under high humidity and/or by a defect in transport of a lipophilic signaling molecule which is necessary for shoot apical meristem activity. Second, to increase seed oil production, I used a transporter BILE ACID:SODIUM SYMPORTER FAMILY PROTEIN 2 (BASS2) known as a pyruvate transporter at plastid envelope. In this study, I hypothesized that increased pyruvate uptake into the plastids of developing seeds by overexpressing the pyruvate transporter BASS2 would increase the supply of carbon precursors, thus facilitating de novo fatty acid biosynthesis and eventually enhancing seed oil production. To test this hypothesis, I generated transgenic Arabidopsis plants that overexpressed BASS2 under the control of a seed-specific promoter from soybean (Glycine max). The resultant overexpressing plants produced seeds that were larger and heavier, and contained 8-27% more oil than those of the wild-type. The total seed number of plant did not differ significantly between the wild-type and overexpressing plant lines. Taken together, this study demonstrated that seed-specific overexpression of the pyruvate transporter BASS2 promotes oil production in Arabidopsis seeds. These studies revealed that ABCG5 transporter is necessary for cuticle formation and mediate many aspects of plant growth and development, and pyruvate transporter BASS2 can be useful to increase the seed oil content. Thus, studying the transporters is a feasible approach to manipulate and re-direct metabolism in plant for people’s interest.