LACs are necessary and sufficient to direct lignin polymerization in Arabidopsis cell wall domains. A, Maximum projection image of hypocotyl epidermal cells induced to transdifferentiate into protoxylem TEs in VND7-VP16-GR seedlings, showing NBD-CA fluorescence in the secondary cell walls (arrowheads). B, VND7-induced protoxylem TE showing the following: i, cellulose deposition (Pontamine S4B stain); ii, whole mount immunolocalization of xylan (LM10 antibody); and iii, merged images. C, Maximum projection image of epidermal cells induced to transdifferentiate into protoxylem TE, in VND7-VP16-GR seedlings in lac4 lac17 double-mutant backgrounds showing that NBD-CA was not incorporated into the secondary cell walls (arrowheads). D, VND7-VP16-GR induced protoxylem TE in lac4 lac17 double mutants showing identical deposition patterns for the following: i, cellulose (Pontamine S4B stain); ii, xylan (LM10 immunolocalization); and iii, merged images, as observed in VND7-VP16-GR alone shown in B. E and F, Constitutive expression of LAC4 (E) or LAC17 (F) catalyzes lignin polymerization of NBD-CA in primary cell walls of cotyledon epidermal cells. G, Control wild-type cotyledon epidermal cells do not incorporate NBD-CA into their cell walls. H, Corresponding bright-field image of control wild-type cotyledons with one cell outlined. Bars = 25 μm.