Consistent with a role of auxin in vascular differentiation, a number of mutations in auxin signal transduction genes are associated with reduced vascular system. The figure shows two mutants, in which the vascular system is reduced in embryos and postembryonic stages.
A,B: Impaired vascular strand formation in Arabidopsis mutants. The MP gene has been implicated in auxin signal transduction and encodes an Auxin Response Factor type protein. In triangular stage wild-type Arabidopsis embryos (A), narrow cells in the center of the embryo mark the vascular cylinder of the hypocotyl/root axis (vp). In mp mutant embryos (B), cells appear not to be oriented in the apical-basal direction and mutant embryos develop into seedlings that lack the hypocotyl-root axis, indicating a role of auxin as an orienting signal in embryonic cell differentiation. At all stages, vascular continuity is impaired in mp mutant organs: Wild-type rosette leaf venation pattern (C), mp mutant leaf venation (D). E: Wild type seedling vascularture. F, G: Vascular differentiation is confined to central and basal areas in cotyledons of seedlings of the mutant mp and, to a lesser extent, of the insertion tagged monopteros-like (mpl) mutant (G). Note that both mutants lack hypocotyl and root.