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The Arabidopsis thaliana F-box protein FBL17 is essential for progression through the second mitosis during pollen development.

Gusti A, Baumberger N, Nowack M, Pusch S, Eisler H, Potuschak T, De Veylder L, Schnittger A, Genschik P - PLoS ONE (2009)

Bottom Line: FBL17 expression is strongly enhanced in plants co-expressing E2Fa and DPa, transcription factors that promote S-phase entry.Seed abortion can, however, be overcome by mutations in FIE, a component of the Polycomb group complex, overall resembling loss-of-function mutations in the A. thaliana cyclin-dependent kinase CDKA;1.Finally we identified ASK11, as an SKP1-like partner protein of FBL17 and discuss a possible mechanism how SCF(FBL17) may regulate cell division during male gametogenesis.

View Article: PubMed Central - PubMed

Affiliation: Institut de Biologie Moléculaire des Plantes du CNRS, Université de Strasbourg, Strasbourg, France.

ABSTRACT
In fungi and metazoans, the SCF-type Ubiquitin protein ligases (E3s) play a critical role in cell cycle regulation by degrading negative regulators, such as cell cycle-dependent kinase inhibitors (CKIs) at the G1-to-S-phase checkpoint. Here we report that FBL17, an Arabidopsis thaliana F-box protein, is involved in cell cycle regulation during male gametogenesis. FBL17 expression is strongly enhanced in plants co-expressing E2Fa and DPa, transcription factors that promote S-phase entry. FBL17 loss-of-function mutants fail to undergo pollen mitosis II, which generates the two sperm cells in mature A. thaliana pollen. Nonetheless, the single sperm cell-like cell in fbl17 mutants is functional but will exclusively fertilize the egg cell of the female gametophyte, giving rise to an embryo that will later abort, most likely due to the lack of functional endosperm. Seed abortion can, however, be overcome by mutations in FIE, a component of the Polycomb group complex, overall resembling loss-of-function mutations in the A. thaliana cyclin-dependent kinase CDKA;1. Finally we identified ASK11, as an SKP1-like partner protein of FBL17 and discuss a possible mechanism how SCF(FBL17) may regulate cell division during male gametogenesis.

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fbl17 T-DNA insertion mutants.(A) FBL17 transcript accumulation in plants over-expressing the E2Fa transcription factor and its dimerization partner, DPa. Quantitative RT-PCR on RNA extracted from E2Fa-DPa overexpressing (OE) seedlings show a 15-fold increase in the relative abundance of FBL17 transcript compared to control RNA (Col-0). The experiment was three times repeated. Data are means±SE. (B) Diagram of the genomic locus of FBL17. The two T-DNA insertions disrupt the 7th exon and the 6th intron in the fbl17-1 and fbl17-2 allele respectively. Light grey filling indicate non-translated region of the transcript whereas dark grey filling indicates coding sequence. (C) Wild type silique opened to reveal the seed content. (D) Heterozygous fbl17-1+/− silique displaying a reduced fertility and aborted seeds (marked by white arrows). (E) Homozygous fbl17-1 mutant complemented with the FBL17 genomic clone show wild type siliques and normal seed development. (C, D, E, bar = 500 µm).
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pone-0004780-g001: fbl17 T-DNA insertion mutants.(A) FBL17 transcript accumulation in plants over-expressing the E2Fa transcription factor and its dimerization partner, DPa. Quantitative RT-PCR on RNA extracted from E2Fa-DPa overexpressing (OE) seedlings show a 15-fold increase in the relative abundance of FBL17 transcript compared to control RNA (Col-0). The experiment was three times repeated. Data are means±SE. (B) Diagram of the genomic locus of FBL17. The two T-DNA insertions disrupt the 7th exon and the 6th intron in the fbl17-1 and fbl17-2 allele respectively. Light grey filling indicate non-translated region of the transcript whereas dark grey filling indicates coding sequence. (C) Wild type silique opened to reveal the seed content. (D) Heterozygous fbl17-1+/− silique displaying a reduced fertility and aborted seeds (marked by white arrows). (E) Homozygous fbl17-1 mutant complemented with the FBL17 genomic clone show wild type siliques and normal seed development. (C, D, E, bar = 500 µm).

Mentions: To better characterize the role of the SCF in plant cell cycle function, we searched for F-box protein encoding genes that are expressed in a cell cycle dependent manner. Among the ∼700 A. thaliana F-box proteins [26], one of them, called FBL17 (At3g54650) was of particular interest because its expression was found to be 2.5 fold increased in S-phase based on available microarray data [27]. Moreover, this gene was in silico identified as a putative target of E2F transcription factors [28]. A. thaliana plants co-expressing ectopically E2Fa with its dimerization partner, DPa, display strongly induced cell proliferation rates [29] and indeed in these plants, we found a 15-fold increase in FBL17 transcript accumulation (Figure 1A).


The Arabidopsis thaliana F-box protein FBL17 is essential for progression through the second mitosis during pollen development.

Gusti A, Baumberger N, Nowack M, Pusch S, Eisler H, Potuschak T, De Veylder L, Schnittger A, Genschik P - PLoS ONE (2009)

fbl17 T-DNA insertion mutants.(A) FBL17 transcript accumulation in plants over-expressing the E2Fa transcription factor and its dimerization partner, DPa. Quantitative RT-PCR on RNA extracted from E2Fa-DPa overexpressing (OE) seedlings show a 15-fold increase in the relative abundance of FBL17 transcript compared to control RNA (Col-0). The experiment was three times repeated. Data are means±SE. (B) Diagram of the genomic locus of FBL17. The two T-DNA insertions disrupt the 7th exon and the 6th intron in the fbl17-1 and fbl17-2 allele respectively. Light grey filling indicate non-translated region of the transcript whereas dark grey filling indicates coding sequence. (C) Wild type silique opened to reveal the seed content. (D) Heterozygous fbl17-1+/− silique displaying a reduced fertility and aborted seeds (marked by white arrows). (E) Homozygous fbl17-1 mutant complemented with the FBL17 genomic clone show wild type siliques and normal seed development. (C, D, E, bar = 500 µm).
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getmorefigures.php?uid=PMC2651519&req=5

pone-0004780-g001: fbl17 T-DNA insertion mutants.(A) FBL17 transcript accumulation in plants over-expressing the E2Fa transcription factor and its dimerization partner, DPa. Quantitative RT-PCR on RNA extracted from E2Fa-DPa overexpressing (OE) seedlings show a 15-fold increase in the relative abundance of FBL17 transcript compared to control RNA (Col-0). The experiment was three times repeated. Data are means±SE. (B) Diagram of the genomic locus of FBL17. The two T-DNA insertions disrupt the 7th exon and the 6th intron in the fbl17-1 and fbl17-2 allele respectively. Light grey filling indicate non-translated region of the transcript whereas dark grey filling indicates coding sequence. (C) Wild type silique opened to reveal the seed content. (D) Heterozygous fbl17-1+/− silique displaying a reduced fertility and aborted seeds (marked by white arrows). (E) Homozygous fbl17-1 mutant complemented with the FBL17 genomic clone show wild type siliques and normal seed development. (C, D, E, bar = 500 µm).
Mentions: To better characterize the role of the SCF in plant cell cycle function, we searched for F-box protein encoding genes that are expressed in a cell cycle dependent manner. Among the ∼700 A. thaliana F-box proteins [26], one of them, called FBL17 (At3g54650) was of particular interest because its expression was found to be 2.5 fold increased in S-phase based on available microarray data [27]. Moreover, this gene was in silico identified as a putative target of E2F transcription factors [28]. A. thaliana plants co-expressing ectopically E2Fa with its dimerization partner, DPa, display strongly induced cell proliferation rates [29] and indeed in these plants, we found a 15-fold increase in FBL17 transcript accumulation (Figure 1A).

Bottom Line: FBL17 expression is strongly enhanced in plants co-expressing E2Fa and DPa, transcription factors that promote S-phase entry.Seed abortion can, however, be overcome by mutations in FIE, a component of the Polycomb group complex, overall resembling loss-of-function mutations in the A. thaliana cyclin-dependent kinase CDKA;1.Finally we identified ASK11, as an SKP1-like partner protein of FBL17 and discuss a possible mechanism how SCF(FBL17) may regulate cell division during male gametogenesis.

View Article: PubMed Central - PubMed

Affiliation: Institut de Biologie Moléculaire des Plantes du CNRS, Université de Strasbourg, Strasbourg, France.

ABSTRACT
In fungi and metazoans, the SCF-type Ubiquitin protein ligases (E3s) play a critical role in cell cycle regulation by degrading negative regulators, such as cell cycle-dependent kinase inhibitors (CKIs) at the G1-to-S-phase checkpoint. Here we report that FBL17, an Arabidopsis thaliana F-box protein, is involved in cell cycle regulation during male gametogenesis. FBL17 expression is strongly enhanced in plants co-expressing E2Fa and DPa, transcription factors that promote S-phase entry. FBL17 loss-of-function mutants fail to undergo pollen mitosis II, which generates the two sperm cells in mature A. thaliana pollen. Nonetheless, the single sperm cell-like cell in fbl17 mutants is functional but will exclusively fertilize the egg cell of the female gametophyte, giving rise to an embryo that will later abort, most likely due to the lack of functional endosperm. Seed abortion can, however, be overcome by mutations in FIE, a component of the Polycomb group complex, overall resembling loss-of-function mutations in the A. thaliana cyclin-dependent kinase CDKA;1. Finally we identified ASK11, as an SKP1-like partner protein of FBL17 and discuss a possible mechanism how SCF(FBL17) may regulate cell division during male gametogenesis.

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