Abstract
BackgroundLeaves are determinate organs; hence, precise control of cell proliferation and post-mitotic cell expansion is essential for their growth. A defect in cell proliferation often triggers enhanced post-mitotic cell expansion in leaves. This phenomenon is referred to as ‘compensation’. Several lines of evidence from studies on compensation have shown that cell proliferation and post-mitotic cell expansion are coordinately regulated during leaf development. Therefore, compensation has attracted much attention to the mechanisms for leaf growth. However, our understanding of compensation at the subcellular level remains limited because studies of compensation have focused mainly on cellular-level phenotypes. Proper leaf growth requires quantitative control of subcellular components in association with cellular-level changes. To gain insight into the subcellular aspect of compensation, we investigated the well-known relationship between cell area and chloroplast number per cell in compensation-exhibiting lines, and asked whether chloroplast proliferation is modulated in response to the induction of compensation.ResultsWe first established a convenient and reliable method for observation of chloroplasts in situ. Using this method, we analyzed Arabidopsis thaliana mutants fugu5 and angustifolia3 (an3), and a transgenic line KIP-RELATED PROTEIN2 overexpressor (KRP2 OE), which are known to exhibit typical features of compensation. We here showed that chloroplast number per cell increased in the subepidermal palisade tissue of these lines. We analyzed tetraploidized wild type, fugu5, an3 and KRP2 OE, and found that cell area itself, but not nuclear ploidy, is a key parameter that determines the activity of chloroplast proliferation. In particular, in the case of an3, we uncovered that promotion of chloroplast proliferation depends on the enhanced post-mitotic cell expansion. The expression levels of chloroplast proliferation-related genes are similar to or lower than that in the wild type during this process.ConclusionsThis study demonstrates that chloroplast proliferation is promoted in compensation-exhibiting lines. This promotion of chloroplast proliferation takes place in response to cell-area increase in post-mitotic phase in an3. The expression of chloroplast proliferation-related genes were not promoted in compensation-exhibiting lines including an3, arguing that an as-yet-unknown mechanism is responsible for modulation of chloroplast proliferation in these lines.
Highlights
Leaves are determinate organs; precise control of cell proliferation and post-mitotic cell expansion is essential for their growth
Kinetics of the cell-area changes indicated that increase in cell area takes place in three distinct manner in compensation-exhibiting lines represented by angustifolia3, fugu5 and a cyclin dependent kinase inhibitor gene KIP-RELATED PROTEIN2 overexpressor (KRP2 OE) [10,13]
Chloroplast number per cell increased in the leaves of compensation-exhibiting lines We investigated the number of chloroplasts per cell in the subepidermal palisade tissue of 21-day-old compensation-exhibiting lines including fugu5, an3 and KRP2 OE
Summary
Leaves are determinate organs; precise control of cell proliferation and post-mitotic cell expansion is essential for their growth. This fact suggests that enhanced post-mitotic cell expansion in an is not a simple result of a defect in cell proliferation, but a result of an active motion for leaf growth in response to a defect in cell proliferation This idea is supported by our recent analysis of chimeric leaves for AN3 expression: an mutant cells are considered to actively produce and transmit an inter-cellular signal for enhanced post-mitotic cell expansion [12]. These studies have deepened our understanding of the mechanism of compensation, and leaf growth, at the cellular level
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