Effect of auxin on mesocotyl elongation of dark-grown maize under different seeding depths

In order to elucidate the physiological mechanism of maize mesocotyl elongation induced by auxin under different seeding depths, seeds of five maize inbred lines, including 3681-4 line tolerant to deep seeding, were treated with IAA and triiodobenzoic acid (TIBA) under seeding depths of 20 or 2 cm. Under deep seeding conditions, maize mesocotyls grew by 1.5-2.0 times faster than under shallow seeding conditions. IAA (10(-6) to 10(-4) M) applied to roots stimulated mesocotyl elongation only of 3681-4 line and only under deep seeding conditions. TIBA (10(-5) and 10(-4) M) applied to roots inhibited mesocotyl elongation in all lines, but only 3681-4 was sensitive to 10(-6) M TIBA. IAA promoted only cell elongation, and TIBA inhibited both cell elongation and cell division. After IAA and TIBA treatments, endogenous IAA content changed in parallel with the mesocotyl growth rate under different seeding depths. Furthermore, ABP1 gene expression changed in parallel with the mesocotyl growth rate under deep seeding conditions. Therefore, deep seeding tolerance of 3681-4 line was achieved due to auxin-regulated rapid mesocotyl elongation.