Abstract
Consistent with their valve-like function in shoot–atmosphere gas exchange, guard cells are smaller than other epidermal cells and usually harbour 2C DNA levels in diploid plants. The paralogous Arabidopsis R2R3 MYB transcription factors, FOUR LIPS and MYB88, ensure that stomata contain just two guard cells by restricting mitosis. The loss of both FLP and MYB88 function in flp myb88 double mutants induces repeated mitotic divisions that lead to the formation of clusters of stomata in direct contact. By contrast, CYCLIN DEPENDENT KINASE B1 function is required for the symmetric division that precedes stomatal maturation. It was found that blocking mitosis by chemically disrupting microtubules or by the combined loss of FLP/MYB88 and CDKB1 function, causes single (undivided) guard cells (sGCs) to enlarge and attain mean DNA levels of up to 10C. The loss of both FLP and CDKB1 function also dramatically increased plastid number, led to the formation of multiple nuclei in GCs, altered GC and stomatal shape, and disrupted the fate of lineage-specific stem cells. Thus, in addition to respectively restricting and promoting symmetric divisions, FLP and CDKB1 together also conditionally restrict the G1/S transition and chloroplast and nuclear number, and normally maintain fate and developmental progression throughout the stomatal cell lineage.
Highlights
Cell differentiation is often associated with defined DNA levels that contribute to the function of specialized cell types (Nagl, 1976; Lee et al, 2009)
It was found that blocking mitosis by chemically disrupting microtubules or by the combined loss of FOUR LIPS (FLP)/MYB88 and CDKB1 function, causes single guard cells to enlarge and attain mean DNA levels of up to 10C
Chemical disruption of microtubules induces guard cells (GCs) endoreplication in a myb double mutant To probe the role of microtubule (MT) function in stomatal development, MT depolymerizing drugs were applied to developing Arabidopsis cotyledons and leaves
Summary
Cell differentiation is often associated with defined DNA levels that contribute to the function of specialized cell types (Nagl, 1976; Lee et al, 2009). While many plant cells contain 2C DNA levels throughout their lifespan, others undergo varying degrees of endoreplication meaning DNA replication without mitosis (Lee et al, 2009). Endoreplicaton occurs either through endocycling or endomitosis. G and S phases occur but not mitotic entry. The latter does take place in endomitosis, and chromosomes start to condense. The chromosomes fail to separate and, instead, the cells revert to a phase resembling G1 (Lee et al, 2009). Endomitosis can lead to nuclear division without cytokinesis, resulting in the formation of several nuclei in one cell (Meserve and Duronio, 2012; Pandit et al, 2012)
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