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.