To gain insight into the processes controlling leaf development, we characterized an Arabidopsis mutant, varicose(vcs), with leaf and shoot apical meristem defects. The vcsphenotype is temperature dependent; low temperature growth largely suppressed defects, whereas high growth temperatures resulted in severe leaf and meristem defects. VCS encodes a putative WD-domain containing protein,suggesting a function involving protein-protein interactions. Temperature shift experiments indicated that VCS is required throughout leaf development,but normal secondary vein patterning required low temperature early in leaf development. The low-temperature vcs phenotype is enhanced in axr1-3 vcs double mutants and in vcs mutants grown in the presence of polar auxin transport inhibitors, however, vcs has apparently normal auxin responses. Taken together, these observations suggest a role for VCS in leaf blade formation.