Abstract
The yellow variegated2 (var2) is one of the best-characterized Arabidopsis (Arabidopsis thaliana) mutants showing leaf variegation. Leaf variegation of var2 results from the loss of an ATP-dependent metalloprotease, FtsH2, which is a major component of the FtsH heterocomplex in thylakoid membranes. While the functional role of FtsH2 in protein quality control has been extensively studied, the physiological state of plastids in white tissues of the var2 is not well characterized. Here we show that the white tissue in var2 is neither the result of photobleaching nor enhanced senescence. Visualization of plastids by plastid-targeted green fluorescent protein revealed that plastids in the white sector are distinct and have undifferentiated characteristics. The plastids are also distinct in that they contain large nucleoids, a complex structure of plastid DNA and proteins, that are typically found in undifferentiated plastids. Comparative analyses of protein profiles from green and white tissues suggested that the difference was observed in the proteins related to photosynthesis but not due to proteins of other organelles. Thus, cells in the white tissue are viable and their defect is limited to plastid function. The plastid accumulates normal levels of chloroplast transcripts, whereas a substantial repression of nuclear-encoded photosynthetic genes was evident in the white sector. Based upon these results, we inferred that the white sectors in var2 are made by viable cells that have plastids arrested in thylakoid formation. A proposed model to form the variegated sector in var2 is provided.
Highlights
The yellow variegated2 is one of the best-characterized Arabidopsis (Arabidopsis thaliana) mutants showing leaf variegation
We showed that plastid DNAs in the white tissues were organized as nucleoids that were typically detected in undifferentiated plastids
Leaf variegation in var1 and var2 mutants, which is caused by the loss of FtsH2 or FtsH5, respectively, has been extensively studied by several laboratories
Summary
The yellow variegated (var2) is one of the best-characterized Arabidopsis (Arabidopsis thaliana) mutants showing leaf variegation. The plastid accumulates normal levels of chloroplast transcripts, whereas a substantial repression of nuclear-encoded photosynthetic genes was evident in the white sector. Based upon these results, we inferred that the white sectors in var are made by viable cells that have plastids arrested in thylakoid formation. Leaf variegation is occasionally observed in higher plants It raises a fundamental question of why and how two sectors containing different cell types, a green cell with normal-appearing chloroplasts and a white cell with abnormal plastids, can be formed in leaf tissue. In Escherichia coli, ftsH occurs as a single copy essential gene (for review, see Ito and Akiyama, 2005)
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