Numerous Arabidopsis genes have been cloned that correspond to putative pathogen defense-related genes identified in parsley (Petroselinum crispum). Treatment of Arabidopsis cells with fungal elicitor leads to rapid accumulation of the respective mRNAs with time courses comparable to those observed for their counterparts in parsley. Evolutionary sequence conservation of many of these genes in several plant species suggests they code for important plant functions.

In flowering plants, the developing embryo consists of growing populations of cells whose fates are determined in a position-dependent manner to form the adult organism. Mutations in the FACKEL(FK) gene affect body organization of theArabidopsis seedling. We report that FK is required for cell division and expansion and is involved in proper organization of the embryo. We isolated FK by positional cloning. Expression analysis in embryos revealed that FK mRNA becomes localized to meristematic zones. FK encodes a predicted integral membrane protein related to the vertebrate lamin B receptor and sterol reductases across species, including yeast sterol C-14 reductase ERG24. We provide functional evidence that FK encodes a sterol C-14 reductase by complementation of erg24. GC/MS analysis confirmed that fk mutations lead to accumulation of intermediates in the biosynthetic pathway preceding the C-14 reductase step. Although fk represents a sterol biosynthetic mutant, the phenotype was not rescued by feeding with brassinosteroids (BRs), the only plant sterol signaling molecules known so far. We propose that synthesis of sterol signals in addition to BRs is important in mediating regulated cell growth and organization during embryonic development. Our results indicate a novel role for sterols in the embryogenesis of plants.