Active auxin import is required for correct protoxylem formation.

(A) Auxin concentration was plotted for the cells of the xylem axis, marked with a white line. (B) Plot of the auxin concentration along the xylem axis in simulations of wild type (black), pin7 (green), and the importer mutant (blue). px = protoxylem; mx = metaxylem. (C, D) Frequency of protoxylem defects in auxin importer mutants (C) without and (D) with a 48 h treatment of 10nm 6-benzylaminopurine. ‘Defective’ protoxylem has breaks in one or both strands; ‘normal’ protoxylem is continuous. Asterisks indicate a significant difference from wild type (Col-0). (E) Chloral hydrate cleared aux1 roots with long gaps in one (top) or both (bottom) protoxylem strands. Yellow arrows indicate protoxylem; white arrows indicate missing protoxylem. (F–H) Expression of DR5::GFP in the (F, G) aux1lax1lax2 mutant and (H) wild type. Scale bar: 10μm.