Acyl lipids are essential constituents of all cells, but acyl chain requirements vary greatly and depend on the cell type considered. This implies a tight regulation of fatty acid production so that supply fits demand. Isolation of the Arabidopsis thaliana WRINKLED1 (WRI1) transcription factor established the importance of transcriptional regulation for modulating the rate of acyl chain production. Here, we report the isolation of two additional regulators of the fatty acid biosynthetic pathway, WRI3 and WRI4, which are closely related to WRI1 and belong to the APETALA2-ethylene-responsive element binding protein family of transcription factors. These three WRIs define a family of regulators capable of triggering sustained rates of acyl chain synthesis. However, expression patterns of the three WRIs differ markedly. Whereas only WRI1 activates fatty acid biosynthesis in seeds for triacylglycerol production, the three WRIs are required in floral tissues to provide acyl chains for cutin biosynthesis and prevent adherence of these developing organs and subsequent semisterility. The targets of these WRIs encode enzymes providing precursors (acyl chain and glycerol backbones) for various lipid biosynthetic pathways, but not the subsequent lipid-assembling enzymes. These results provide insights into the developmental regulation of fatty acid production in plants.

In yeast and animals, the anaphase-promoting complex or cyclosome (APC/C) is an essential ubiquitin protein ligase that regulates mitotic progression and exit by controlling the stability of cell cycle regulatory proteins, such as securin and the mitotic cyclins. In plants, the function, regulation, and substrates of the APC/C are poorly understood. To gain more insight into the roles of the plant APC/C, we characterized at the molecular level one of its subunits, APC2, which is encoded by a single-copy gene in Arabidopsis. We show that the Arabidopsis gene is able to partially complement a budding yeast apc2 ts mutant. By yeast two-hybrid assays, we demonstrate an interaction of APC2 with two other APC/C subunits: APC11 and APC8/CDC23. A reverse-genetic approach identified Arabidopsis plants carrying T-DNA insertions in the APC2 gene. apc2 null mutants are impaired in female megagametogenesis and accumulate a cyclin-␤ -glucuronidase reporter protein but do not display metaphase arrest, as observed in other systems. The APC2 gene is expressed in various plant organs and does not seem to be cell cycle regulated. Finally, we report intriguing differences in APC2 protein subcellular localization compared with that in other systems. Our observations support a conserved function of the APC/C in plants but a different mode of regulation.

We investigated the role of the ubiquitin proteasome system (UPS), which allows proteins to be selectively degraded, during gametophyte development in Arabidopsis thaliana. Three mutant alleles altering the UPS were isolated in the Wassilewskija (Ws) accession: they affect the Regulatory Particle 5a (RPT5a) gene, which (along with RPT5b) encodes one of the six AAAATPases of the proteasome regulatory particle. In the heterozygous state, all three mutant alleles displayed 50% pollen lethality, suggesting that RPT5a is essential for male gametophyte development. However, a fourth mutant in the Columbia (Col) accession did not display such a phenotype because the RPT5b Col allele complements the rpt5a defect in the male gametophyte, whereas the RPT5b Ws allele does not. Double rpt5a rpt5b mutants showed a complete male and female gametophyte lethal phenotype in a Col background, indicating that RPT5 subunits are essential for both gametophytic phases. Mitotic divisions were affected in double mutant gametophytes correlating with an absence of the proteasome-dependent cyclinA3 degradation. Finally, we show that RPT5b expression is highly increased when proteasome functioning is defective, allowing complementation of the rpt5a mutation. In conclusion, RPT5 subunits are not only essential for both male and female gametophyte development but also display accession-dependent redundancy and are crucial in cell cycle progression.

As part of an ongoing research program dedicated to the understanding of proanthocyanidin (PA) accumulation in Brassica napus seed coat, transgenic rapeseed plants carrying a 2.3-kb fragment of the Arabidopsis thaliana BAN promoter (ProAtBAN) fused to the uidA reporter gene (GUS) were generated. Analysis of these plants revealed that ProAtBAN was activated in B. napus seed coat, following a spatio-temporal pattern that was very similar to the PA deposition profile in rapeseed and also to the one previously described in Arabidopsis. ProAtBAN activity occurred as soon as the early stages of embryogenesis and was restricted to the cells where PAs were shown to accumulate. Therefore, the Arabidopsis BAN promoter can be used to trigger gene expression in B. napus seed coat for both genetic engineering and functional validation of candidate genes. In addition, these data strongly suggest that the transcriptional regulatory network of the BAN gene is conserved between Arabidopsis and rapeseed. This is consistent with the fact that similarity searches of the public rapeseed sequence databases allowed recovering the rapeseed homologs for several BAN regulators, namely TT1, TT2, TT8, TT16 and TTG1, which have been previously described in Arabidopsis.