On the Inside

摘要

The phenotypic abnormality exhibited by the immutans (im) mutant of Arabidopsis (Arabidopsis thaliana) is green and white variegation. The green sectors contain morphologically normal chloroplasts, whereas the white sectors contain abnormal chloroplasts that lack colored carotenoids due to a defect in phytoene desaturase (PDS) activity. IM serves as a terminal (quinol) oxidase in thylakoid membranes, where it accepts electrons from plastoquinol and transfers them to molecular oxygen to generate water. It has been hypothesized that a lack of IM function would lead to overreduction of the plastoquinone pool and to an accumulation of phytoene, thereby preventing the formation of carotenoids. Since carotenoids protect chloroplasts against photooxidation, this suggests that the plastids in im white (imW) sectors are photooxidized. This notion is consistent with results from plants treated with norflurazon (NF), which exhibit photobleached plastids and an albino phenotype due to the inhibition of PDS activity. Perhaps the most puzzling aspect of the im mutation is the variegation itself: Green and white cells of im have the same genotype, so how do the green sectors form? As a first step toward gaining insight into this question, Aluru et al. (pp. 904–923) compared the transcriptomes of imW sectors and im green (imG) sectors, and the transcriptomes of imW and NF-treated white Arabidopsis leaf tissues. Because the two white tissue types have many morphological, biochemical, and molecular similarities, their initial hypothesis was that they would have similar, if not identical, molecular phenotypes. Surprisingly, they found that there are distinct as well as shared gene expression patterns. Conceivably, these findings may be due to the fact that imW sectors are randomly interspersed with the photosynthetically active green leaf sectors while NF-treated tissues are not. This could perhaps lead to differences in the source-sink interactions between the two tissue types. Alternatively, another reason could be the differences in the mechanism of PDS inhibition; whereas NF affects PDS activity directly resulting only in phytoene accumulation, IM affects all reactions that transfer electrons into plastoquinone; thus, the redox status of the plastoquinone pool might be different in these two "white" tissues.