Abstract
Chloroplast formation is associated with embryo development and seedling growth. However, the relationship between chloroplast differentiation and embryo development remains unclear. Five FtsHi genes that encode proteins with high similarity to FtsH proteins, but lack Zn2+-binding motifs, are present in the Arabidopsis genome. In this study, we showed that T-DNA insertion mutations in the Arabidopsis FtsHi4 gene resulted in embryo arrest at the globular-to-heart–shaped transition stage. Transmission electron microscopic analyses revealed abnormal plastid differentiation with a severe defect in thylakoid formation in the mutant embryos. Immunocytological studies demonstrated that FtsHi4 localized in chloroplasts as a thylakoid membrane-associated protein, supporting its essential role in thylakoid membrane formation. We further showed that FtsHi4 forms protein complexes, and that there was a significant reduction in the accumulation of D2 and PsbO (two photosystem II proteins) in mutant ovules. The role of FtsHi4 in chloroplast development was confirmed using an RNA-interfering approach. Additionally, mutations in other FtsHi genes including FtsHi1, FtsHi2, and FtsHi5 caused phenotypic abnormalities similar to ftshi4 with respect to plastid differentiation during embryogenesis. Taken together, our data suggest that FtsHi4, together with FtsHi1, FtsHi2, and FtsHi5 are essential for chloroplast development in Arabidopsis.
Highlights
Angiosperm embryo and endosperm initiate from double fertilization, where one of the two sperm cells fuse with an egg cell and the other fertilizes with the central cell, respectively [1]
Isolation and characterization of the ftshi4 mutant To investigate chloroplast biosynthesis during embryogenesis in Arabidopsis, we generated an Arabidopsis T-DNA insertion population [35] and screened for mutant plants whose siliques contained roughly 25% albino embryos arrested at early embryogenesis; other seeds appeared green due to chlorophyll accumulation
When the mature seeds harvested from this plant were plated on 1/2 Murashige and Skoog (MS) medium containing 50 mg/L kanamycin (Km), the ratio of kanamycin-sensitive versus kanamycin-insensitive seeds was 1:2 (231:476, p.0.05), which is consistent with a single insertion line of embryo-lethal defects
Summary
Angiosperm embryo and endosperm initiate from double fertilization, where one of the two sperm cells fuse with an egg cell and the other fertilizes with the central cell, respectively [1]. The elongated zygote undergoes an asymmetric cell division to produce a smaller apical and larger basal cell [2]. The apical cell undergoes one-cell, two-cell, four-cell, eight-cell, globular, heart-shaped, torpedo, and bent cotyledon-shaped embryo stages to refine embryonic patterns [3]. The chloroplast, which is an organelle derived from cyanobacteria through endosymbiosis, is a specific type of plastid. Chloroplasts are propagated from a pre-existing plastid through divisions, and formation is initiated from an undifferentiated plastid type; the proplastids. Proplastids develop grana, which are stacks of thylakoid membranes for light harvesting, electron transfer, and ATP synthesis. The chloroplast is responsible for photosynthesis, but for synthesis and storage of metabolic products (e.g., fatty acids, amino acids, starch) [2,4]
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