[Show abstract][Hide abstract] ABSTRACT: Very-long-chain fatty acids (VLCFAs) are important functional components of various lipid classes, including cuticular lipids in the higher plant epidermis and lipid-derived second messengers. Here, we report the characterization of transgenic Arabidopsis thaliana plants that epidermally express FATTY ACID ELONGATION1 (FAE1), the seed-specific beta-ketoacyl-CoA synthase (KCS) catalyzing the first rate-limiting step in VLCFA biosynthesis. Misexpression of FAE1 changes the VLCFAs in different classes of lipids but surprisingly does not complement the KCS fiddlehead mutant. FAE1 misexpression plants are similar to the wild type but display an essentially glabrous phenotype, owing to the selective death of trichome cells. This cell death is accompanied by membrane damage, generation of reactive oxygen species, and callose deposition. We found that nuclei of arrested trichome cells in FAE1 misexpression plants cell-autonomously accumulate high levels of DNA damage, including double-strand breaks characteristic of lipoapoptosis. A chemical genetic screen revealed that inhibitors of KCS and phospholipase A2 (PLA2), but not inhibitors of de novo ceramide biosynthesis, rescue trichome cells from death. These results support the functional role of acyl chain length of fatty acids and PLA2 as determinants for programmed cell death, likely involving the exchange of VLCFAs between phospholipids and the acyl-CoA pool.
[Show abstract][Hide abstract] ABSTRACT: The eukaryotic N-end rule pathway mediates ubiquitin- and proteasome-dependent turnover of proteins with a bulky amino-terminal residue. Arabidopsis locus At5g02310 shows significant similarity to the yeast N-end rule ligase Ubr1. We demonstrate that At5g02310 is a ubiquitin ligase and mediates degradation of proteins with amino-terminal Arg residue. Unlike Ubr1, the Arabidopsis protein does not participate in degradation of proteins with amino-terminal Phe or Leu. This modified target specificity coincides with characteristic differences in domain structure. In contrast to previous publications, our data indicate that At5g02310 is not identical to CER3, a gene involved in establishment of a protective surface wax layer. At5g02310 has therefore been re-designated PROTEOLYSIS 6 (PRT6), in accordance with its ubiquitin ligase function.
[Show abstract][Hide abstract] ABSTRACT: In vegetative leaf tissues, cuticles including cuticular waxes are important for protection against nonstomatal water loss and pathogen infection as well as for adaptations to environmental stress. However, their roles in the anther wall are rarely studied. The innermost layer of the anther wall (the tapetum) is essential for generating male gametes. Here, we report the characterization of a T-DNA insertional mutant in the Wax-deficient anther1 (Wda1) gene of rice (Oryza sativa), which shows significant defects in the biosynthesis of very-long-chain fatty acids in both layers. This gene is strongly expressed in the epidermal cells of anthers. Scanning electron microscopy analyses showed that epicuticular wax crystals were absent in the outer layer of the anther and that microspore development was severely retarded and finally disrupted as a result of defective pollen exine formation in the mutant anthers. These biochemical and developmental defects in tapetum found in wda1 mutants are earlier events than those in other male-sterile mutants, which showed defects of lipidic molecules in exine. Our findings provide new insights into the biochemical and developmental aspects of the role of waxes in microspore exine development in the tapetum as well as the role of epicuticular waxes in anther expansion.
[Show abstract][Hide abstract] ABSTRACT: In plants, extracellular matrix polymers built from polysaccharides and cuticular lipids have structural and protective functions. The cuticle is found to be ten times thinner in Arabidopsis thaliana (L.) Heynh than in many other plants, and there is evidence that it is unusual in having a high content of alpha-,omega-dicarboxylic fatty acids (FAs) in its polyesters. We designated the new organ fusion mutant hth-12 after it appeared to be allelic to adhesion of calyx edges (ace) and hothead (hth), upon molecular cloning of the gene by transposon tagging. This mutant is deficient in its ability to oxidize long-chain omega-hydroxy FAs to omega-oxo FAs, which results in leaf polyesters in decreased alpha-,omega-dicarboxylic FAs and increased omega-hydroxy FAs. These chemical phenotypes lead to disorder of the cuticle membrane structure in hth-12. ACE/HTH is a single-domain protein showing sequence similarity to long-chain FA omega-alcohol dehydrogenases from Candida species, and we hypothesize that it may catalyze the next step after cytochrome P450 FA omega-hydroxylases in the omega-oxidation pathway. We show that ACE/HTH is specifically expressed in epidermal cells. It appears very likely therefore that the changes in the amount of alpha-,omega-dicarboxylic FAs in hth-12 reflect the different composition of cuticular polyesters. The ACE/HTH gene is also expressed in root epidermal cells which do not form a polyester membrane on the exterior surface, thereby making it possible that the end products of the pathway, alpha-,omega-dicarboxylic FAs, are generally required for the cross-linking that ensures the integrity of the outer epidermal cell wall.
[Show abstract][Hide abstract] ABSTRACT: The outermost epidermal cell wall is specialized to withstand pathogens and natural stresses, and lipid-based cuticular polymers are the major barrier against incursions. The Arabidopsis thaliana mutant bodyguard (bdg), which exhibits defects characteristic of the loss of cuticle structure not attributable to a lack of typical cutin monomers, unexpectedly accumulates significantly more cell wall-bound lipids and epicuticular waxes than wild-type plants. Pleiotropic effects of the bdg mutation on growth, viability, and cell differentiation are also observed. BDG encodes a member of the alpha/beta-hydrolase fold protein superfamily and is expressed exclusively in epidermal cells. Using Strep-tag epitope-tagged BDG for mutant complementation and immunolocalization, we show that BDG is a polarly localized protein that accumulates in the outermost cell wall in the epidermis. With regard to the appearance and structure of the cuticle, the phenotype conferred by bdg is reminiscent of that of transgenic Arabidopsis plants that express an extracellular fungal cutinase, suggesting that bdg may be incapable of completing the polymerization of carboxylic esters in the cuticular layer of the cell wall or the cuticle proper. We propose that BDG codes for an extracellular synthase responsible for the formation of cuticle. The alternative hypothesis proposes that BDG controls the proliferation/differentiation status of the epidermis via an unknown mechanism.
[Show abstract][Hide abstract] ABSTRACT: Cutinized and suberized cell walls form physiological important plant-environment interfaces as they act as barriers limiting water and nutrient loss and protect from radiation and invasion by pathogens. Due to the lack of protocols for the isolation and analysis of cutin and suberin in Arabidopsis, the model plant for molecular biology, mutants and transgenic plants with a defined altered cutin or suberin composition are unavailable, causing that structure and function of these apoplastic barriers are still poorly understood. Transmission electron microscopy (TEM) revealed that Arabidopsis leaf cuticle thickness ranges from only 22 nm in leaf blades to 45 nm on petioles, causing the difficulty in cuticular membrane isolation. We report the use of polysaccharide hydrolases to isolate Arabidopsis cuticular membranes, suitable for depolymerization and subsequent compositional analysis. Although cutin characteristic omega-hydroxy acids (7%) and mid-chain hydroxylated fatty acids (8%) were detected, the discovery of alpha,omega-diacids (40%) and 2-hydroxy acids (14%) as major depolymerization products reveals a so far novel monomer composition in Arabidopsis cutin, but with chemical analogy to root suberin. Histochemical and TEM analysis revealed that suberin depositions were localized to the cell walls in the endodermis of primary roots and the periderm of mature roots of Arabidopsis. Enzyme digested and solvent extracted root cell walls when subjected to suberin depolymerization conditions released omega-hydroxy acids (43%) and alpha,omega-diacids (24%) as major components together with carboxylic acids (9%), alcohols (6%) and 2-hydroxyacids (0.1%). This similarity to suberin of other species indicates that Arabidopsis roots can serve as a model for suberized tissue in general.