Nadia Efremova

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.

Suberin is a hydrophobic polyester found in the cell walls of various plant-environment interfaces, including shoot and root peridermal tissue, and the root hypodermis and endodermis. Suberin deposits form apoplastic barriers that control water and nutrient transport, protect against pathogens and seal wounded tissue. Despite this physiological importance, and the detailed information on the suberin composition of many plants, there is a great gap in our knowledge of the molecular mechanism of suberin biosynthesis, caused in part by a lack of mutants in suberin formation. Here, we report the characterization of daisy, an Arabidopsis mutant that is defective in a fatty acid elongase condensing enzyme. The daisy mutant roots exhibit disturbed growth, and the suberin level is reduced in C(22) and C(24) very long chain fatty acid derivatives, whereas C(16), C(18) and C(20) derivatives accumulate, compared with wild-type suberin, indicating that DAISY functions as a docosanoic acid synthase. Consistent with a significantly increased level of suberin in the roots of NaCl-stressed plants, DAISY is transcriptionally activated by NaCl application, and also by polyethylene glycol-induced drought stress and wounding. Expression analysis using RT-PCR and promoter-GUS fusions demonstrated a distinct DAISY expression pattern in the root stele, senescing sepals, siliques abscission zones and the chalaza-micropyle region of seeds. Together, these results indicate that DAISY is involved in suberin biosynthesis and in the formation of protective layers in these tissues, and in the response to unfavourable environmental conditions.

Although the multilayered structure of the plant cuticle was discovered many years ago, the molecular basis of its formation and the functional relevance of the layers are not understood. Here, we present the permeable cuticle1 (pec1) mutant of Arabidopsis thaliana, which displays features associated with a highly permeable cuticle in several organs. In pec1 flowers, typical cutin monomers, such as ω-hydroxylated fatty acids and 10,16-dihydroxypalmitate, are reduced to 40% of wild-type levels and are accompanied by the appearance of lipidic inclusions within the epidermal cell. The cuticular layer of the cell wall, rather than the cuticle proper, is structurally altered in pec1 petals. Therefore, a significant role for the formation of the diffusion barrier in petals can be attributed to this layer. Thus, pec1 defines a new class of mutants. The phenotypes of the pec1 mutant are caused by the knockout of ATP BINDING CASSETTEG32 (ABCG32), an ABC transporter from the PLEIOTROPIC DRUG RESISTANCE family that is localized at the plasma membrane of epidermal cells in a polar manner toward the surface of the organs. Our results suggest that ABCG32 is involved in the formation of the cuticular layer of the cell wall, most likely by exporting particular cutin precursors from the epidermal cell.