Relationship between Endopolyploidy and Cell Size in Epidermal Tissue of Arabidopsis

Relative quantities of DNA in individual nuclei of stem and leaf epidermal cells of Arabidopsis were measured microspectrofluorometrically using epidermal peels. The relative ploidy level in each nucleus was assessed by comparison to root tip mitotic nuclei. A clear pattern of regular endopolyploidy is evident in epidermal cells. Guard cell nuclei contain levels of DNA comparable to dividing root cells, the 2C level (i.e., one unreplicated copy of the nuclear DNA). Leaf trichome nuclei had elevated ploidy levels of 4C, 8C, 16C, 32C, and 64C, and their cytology suggested that the polyploidy represents a form of polyteny. The nuclei of epidermal pavement cells were 2C, 4C, and 8C in stem epidermis, and 2C, 4C, 8C, and 16C in leaf epidermis. Morphometry of epidermal pavement cells revealed a direct proportionality between nuclear DNA level and cell size. A consideration of the development process suggests that the cells of highest ploidy level are developmentally oldest; consequently, the developmental pattern of epidermal tissues can be read from the ploidy pattern of the cells. This observation is relevant to theories of stomate spacing and offers opportunities for genetic analysis of the endopolyploidy/polyteny phenomenon.