RED-LIGHT-REGULATED GROWTH - CHANGES IN THE ABUNDANCE OF INDOLEACETIC-ACID IN THE MAIZE (ZEA-MAYS L.) MESOCOTYL

The etiolated maize (Zea mays L.) shoot has served as a model system t o study red light (R)-regulated growth. Previous studies have shown th at R inhibition of maize mesocotyl elongation involves a change in the auxin economy. Shown here is that R causes an increased tension in th e epidermis relative to the inner tissue indicating that the growth of the epidermis is preferentially inhibited by R irradiation. This obse rvation, taken together with previous indirect estimates of auxin with in the epidermis, has prompted the hypothesis that R mediates the inhi bition of mesocotyl elongation by preferentially decreasing auxin in t he epidermis, a tissue which constrains the growth of the organ. We te sted this hypothesis using gas chromatography-selected ion monitoring- mass spectrometry analysis of free indole-3-acetic acid (IAA) levels i n both the apical 1 cm of the mesocotyl and the corresponding epidermi s of etiolated and 4-h, R-irradiated seedlings. Red light irradiation caused a 1.4-fold reduction in free IAA within the whole section of th e apical mesocotyl. However, within the peeled mesocotyl epidermis, R irradiation caused at least a 1.9-fold reduction in free IAA. To deter mine if the nearly twofold decrease in epidermal auxin occurring after R is physiologically significant, IAA was differentially applied to o pposite sides of shoots. A twofold difference in IAA application rate caused asymmetrical growth. Thus, the twofold R-induced decrease in fr ee IAA level in the epidermis, a difference sufficient to affect growt h, and the rapid R-induced change in growth rate in the epidermis are consistent with the hypothesis that R causes growth of the mesocotyl t o decrease by preferentially regulating the free IAA level in the meso cotyl epidermis.