Role of an Atypical E2F Transcription Factor in the Control of Arabidopsis Cell Growth and Differentiation

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Abstract
The balance between cell proliferation and differentiation is crucial in multicellular organisms, where it is regulated by complex gene expression networks. This is particularly relevant in plants because organogenesis is a continuous postembryonic process. Here, we investigate the function of Arabidopsis thaliana E2Ff, an atypical member of the E2F family of transcription factors, which acts independently of a dimerization partner. We have focused our analysis on roots and hypocotyls, organs where (1) cell proliferation and differentiation are spatially and/or temporally separated, (2) growth depends on cell expansion in the longitudinal axis, and (3) the AtE2Ff promoter is active. AtE2Ff overexpression produced a reduction in the size of differentiated cells of these organs. Cells of mutant e2ff-1 plants with reduced levels of AtE2Ff mRNA were larger, especially in the hypocotyl, suggesting a role as a growth regulator. These effects of AtE2Ff are not associated with changes in nuclear ploidy levels or in the expression of cell cycle marker genes. However, expression of a subset of cell wall biogenesis genes is misregulated in an AtE2Ff-dependent manner, and based on chromatin immunoprecipitation experiments, they seem to be direct E2F targets. Our results highlight the complex regulatory function exerted by E2F and suggest a possible role of AtE2Ff in repressing cell wall biosynthesis genes during cell elongation in differentiated cells.