Fig. 6. Model for the regulation of the phototropic response of maize coleoptiles via indole-3-acetic acid (IAA) redistribution and differential expression of Zea mays K⁺ channel 1 (ZMK1). (a) Schematic representation of IAA-myo-inositol (IAA_m) and K⁺ fluxes from the kernel to the growing tissues of the maize seedling. (b) Model for the auxin redistribution followed by differential expression of ZMK1 in the phototropically stimulated coleoptile tip. The photoreceptor kinase phot1 (gray circles) is evenly distributed in the coleoptile tip, whereas the K⁺ uptake channel ZMK1 (black squares) accumulates in the shaded flank as a result of the IAA gradient. The arrow indicates the direction of light. (c) Potassium accumulation in the fast-growing shaded half of the coleoptile is a consequence of ZMK1 redistribution and is required for phototropic bending.