The Arabidopsis BET bromodomain factor GTE4 regulates the mitotic cell cycle

Abstract

Proteins containing bromodomains are capable of binding to acetylated histone tails and have a role in recognizing and deciphering acetylated chromatin. Plant BET proteins contain one bromodomain. Twelve BET-encoding genes have been identified in the Arabidopsis genome. Two of these genes have been functionally characterized, one shows a role in seed germination, the other is involved in the establishment of leaf shape. Recently, we characterized a third AtBET gene, named GTE4. We demonstrated that GTE4 is involved in the activation and maintenance of cell division in the meristems and by this controls cell numbers in differentiated organs. Moreover, the quiescent center (QC) identity is partially lost in the apex of the primary root of gte4 mutant, and there is a premature switch from mitosis to endocycling. Genes involved in the retinoblastoma (RB)-E2F pathway, which is important for coupling cell division and cell differentiation in plants and animals, were either up- or down-regulated in the gte4 mutant. In this report we also show that the defect in germination observed in gte4 mutant seeds is not rescued by the action of GA3. Further the root pole of the mutant embryo shows irregular cytokinesis in the procambial stem cells, and the QC of the lateral root shows a partial, but not transient, loss of QC identity. These additional results reinforce the importance of GTE4 in the control of cell proliferation.

Figure 1

Effects of gte4 mutation on the histological structure of the leaf and the embryonic root pole, and the expression of a QC marker in the lateral root primordium. (A and B) Cross sections of the 6^th rosette leaf showing the cell types and the extension of the intercellular spaces in the mesophyll of wild-type (A) and gte4 mutant (B). (C and D) Expression of AGL42:GFP in the apex of a lateral root primordium. A strong GFP signal in the QC cells is present in the wild-type (C), whereas no signal is present in gte4 mutant containing the same construct (D). (E and F) Details of the longitudinal sections of the root pole of the embryo at the cotyledonary stage showing the regularly arranged cell files of the initials in the wild type (E), and the irregularly divided procambial initials (arrows) in the mutant (F). (A, B, E and F) toluidine blue-stained sections observed under light microscopy. (C and D) samples observed under an epifluorescence microscope equipped with a double wavelength filter set (BP 490/20 and BP 575/30) with dichroic filters RKPs 505 and 600 and emission filters BPs 525/20 and 635/40. Bars = 20 µm (A and B), 10 µm (C–F).

Figure 2

Percentage of intercellular spaces in the mesophyll of rosette leaves of 20- and 35-old gte4 and wild type plants. (5,000 µm² randomly chosen in the 6^th leaf of 10 specimens per genotype).

Figure 3

Seed germination in the presence and absence of GA₃, in gte4 in comparison with wild type. The percentages of germinated seeds on the total of sowed seeds are shown at different days in culture. ^aValues of gte4 statistically different at P < 0.01 level with respect to the corresponding values of wild type grown under the same conditions. ^bValue statistically different at P < 0.05 level with respect to wild type grown with GA₃ at the same day. Values of the same genotype and day followed by no symbol or the same symbol are not significantly different (N = 90).