Introduction

One of the features that make plant growth distinct from that of animals and yeasts is the variety of the final size that individual cells reach in plant tissues. Most yeast and animal cells only double their size during their development but many plant cells often expand at least ten times their original size and some cells even expand more than 1000 times. Such massive increase in cell size is supported by a unique growth mechanism in plants that utilizes both macromolecular production in cytoplasm and water uptake into vacuoles. Given that cell size is fairly constant among different plant species, the determination of final cell size must be under genetic control. Elucidating these genetic mechanisms underlying plant cell growth has been challenging but thanks to the recent rapid accumulation of genetic, genomic and bioinformat-ics resources available in model plants, we have begun to uncover the complex pathways that determine cell size in plants. One of the key factors that often correlates with plant cell size is the nuclear DNA content or ploidy, and this correlation can be found either at a single cell level as the endoredu-plicated cell (Fig. 1) or at the whole plant level as tetraploid or polyploidy plants (Fig. 2). Herein we will first describe our current knowledge on how endoreduplication is controlled in higher plants. We will then examine how

Fig. 1 Various cell types support several cycles of endoreduplication in Arabidopsis and resulting increase in ploidy contributes to an increase in cell size. A Arabidopsis hypocotyls endoreduplicate up to 32C in the dark and up to 16C in the light. The increased ploidy positively correlates with the size of epidermal cells (insets). White triangles indicate the top and bottom of hypocotyls. Scale 500 |m (seedlings), 50 |m (insets). B The size of Arabidopsis leaf epidermal cells varies more than 500-fold and the variation in cell size is linked to ploidy. For example, the ploidy level of nuclei in a pair of guard cells remains 2C whereas ploidy in a fully developed trichome goes up to 32C. Scale 100 |im. C Arabidopsis root cells enter the endocycle after they exit from the mitotic cell cycle and the progression of the endocycle coincides with the post-mitotic cell expansion. Increased ploidy also contributes to the formation of large cell types including root hairs and vascular metaxylem cells. DAPI-stained nuclei from root cap cells (2C), root cortex cells (4C and 8C), metaxylem cells (16C) and root hair cells (16C) (Insets). Scale 10 |im (DAPI-stained nuclei), 500 |im (root tip)

Fig. 1 Various cell types support several cycles of endoreduplication in Arabidopsis and resulting increase in ploidy contributes to an increase in cell size. A Arabidopsis hypocotyls endoreduplicate up to 32C in the dark and up to 16C in the light. The increased ploidy positively correlates with the size of epidermal cells (insets). White triangles indicate the top and bottom of hypocotyls. Scale 500 |m (seedlings), 50 |m (insets). B The size of Arabidopsis leaf epidermal cells varies more than 500-fold and the variation in cell size is linked to ploidy. For example, the ploidy level of nuclei in a pair of guard cells remains 2C whereas ploidy in a fully developed trichome goes up to 32C. Scale 100 |im. C Arabidopsis root cells enter the endocycle after they exit from the mitotic cell cycle and the progression of the endocycle coincides with the post-mitotic cell expansion. Increased ploidy also contributes to the formation of large cell types including root hairs and vascular metaxylem cells. DAPI-stained nuclei from root cap cells (2C), root cortex cells (4C and 8C), metaxylem cells (16C) and root hair cells (16C) (Insets). Scale 10 |im (DAPI-stained nuclei), 500 |im (root tip)

endoreduplication and resulting increase in ploidy influence post-mitotic cell expansion in plants. Finally, we will also discuss recent findings that address how the control of cell size may be governed by overall developmental programmes.

Fig. 2 Cell and organ size correlate with the nuclear DNA content in Arabidopsis tetraploid plants. The nuclear DNA content in tetraploid plants is doubled compared to diploid plants, and this leads to the formation of larger cells in both flowers and leaves. An increase in cell size also contributes to an increase in flower size but does not appear to affect the overall leaf size. Scale 4 mm (flower), 10 |im (flower epidermis), 5 mm (seedlings), 100 |im (leaf epidermis). Light micrographs of flower epidermis are courtesy of Hirokazu Tsukaya (University of Tokyo)

Fig. 2 Cell and organ size correlate with the nuclear DNA content in Arabidopsis tetraploid plants. The nuclear DNA content in tetraploid plants is doubled compared to diploid plants, and this leads to the formation of larger cells in both flowers and leaves. An increase in cell size also contributes to an increase in flower size but does not appear to affect the overall leaf size. Scale 4 mm (flower), 10 |im (flower epidermis), 5 mm (seedlings), 100 |im (leaf epidermis). Light micrographs of flower epidermis are courtesy of Hirokazu Tsukaya (University of Tokyo)

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