Effects of auxin-related gene function on β-cedrene- and α-humulene-induced Arabidopsis thaliana root development. Number of root branches in A. thaliana col1 wildtype and in the T-DNA insertion mutant lines yuc1, aux1, pin1, pin3, arf19, and tir1 (a) grown in the presence of β-cedrene (shown in red) or in the absence of β-cedrene (shown in black), and (b) grown in the presence of α-humulene (shown in red) or in the absence of α-humulene (shown in black) (n = 15). Error bars indicate ± the SE. “*, P < 0.05; **, P < 0.01; “ns, no significant difference between treatments. c Photographs showing the influence of β-cedrene and α-humulene on root architecture. Scale bar: 2 cm.