Abstract
Cell cycle entry and quiescence are regulated by the E2F transcription factors in association with RETINOBLASTOMA-RELATED (RBR). E2FB is considered to be a transcriptional activator of cell cycle genes, but its function during development remains poorly understood. Here, by studying E2FB-RBR interaction, E2F target gene expression, and epidermal cell number and shape in e2fb mutant and overexpression lines during leaf development in Arabidopsis (Arabidopsis thaliana), we show that E2FB in association with RBR plays a role in the inhibition of cell proliferation to establish quiescence. In young leaves, both RBR and E2FB are abundant and form a repressor complex that is reinforced by an autoregulatory loop. Increased E2FB levels, either by expression driven by its own promoter or ectopically together with DIMERIZATION PARTNER A, further elevate the amount of this repressor complex, leading to reduced leaf cell number. Cell overproliferation in e2fb mutants and in plants overexpressing a truncated form of E2FB lacking the RBR binding domain strongly suggested that RBR repression specifically acts through E2FB. The increased number of small cells below the guard cells and of fully developed stomata indicated that meristemoids preferentially hyperproliferate. As leaf development progresses and cells differentiate, the amount of RBR and E2FB gradually declined. At this stage, elevation of E2FB level can overcome RBR repression, leading to reactivation of cell division in pavement cells. In summary, E2FB in association with RBR is central to regulating cell proliferation during organ development to determine final leaf cell number.
Highlights
Cell cycle entry and quiescence are regulated by the E2F transcription factors in association with RETINOBLASTOMARELATED (RBR)
According to the textbook model established in animal systems, cell cycle entry is guarded by cyclin-dependent kinases (CDKs), which, upon activation by mitogenic signals, phosphorylate and thereby inactivate Rb and other related pocket proteins
To follow E2FB protein level in its native context during leaf development, we generated Arabidopsis plants carrying the genomic region of E2FB under the control of its own promoter and tagged its C terminus with 33 Venus yellow fluorescent protein (YFP), a modified YFP
Summary
Cell cycle entry and quiescence are regulated by the E2F transcription factors in association with RETINOBLASTOMARELATED (RBR). By studying E2FB-RBR interaction, E2F target gene expression, and epidermal cell number and shape in e2fb mutant and overexpression lines during leaf development in Arabidopsis (Arabidopsis thaliana), we show that E2FB in association with RBR plays a role in the inhibition of cell proliferation to establish quiescence. In young leaves, both RBR and E2FB are abundant and form a repressor complex that is reinforced by an autoregulatory loop. E2FA was shown to play roles in maintaining genome integrity and viability in meristematic cells (Horvath et al, 2017)
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