The etiolated maize (Zea mays L.) shoot has served as a model system to study red light (R)-regulated growth. Previous studies have shown that R inhibition of maize mesocotyl elongation involves a change in the auxin economy. Shown here is that R causes an increased tension in the epidermis relative to the inner tissue indicating that the growth of the epidermis is preferentially inhibited by R irradiation. This observation, taken together with previous indirect estimates of auxin within the epidermis, has prompted the hypothesis that R mediates the inhibition of mesocotyl elongation by preferentially decreasing auxin in the epidermis, a tissue which constrains the growth of the organ. We tested this hypothesis using gas chromatography-selected ion monitoring-mass spectrometry analysis of free indole-3-acetic acid (IAA) levels in both the apical 1 cm of the mesocotyl and the corresponding epidermis 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 the apical mesocotyl. However, within the peeled mesocotyl epidermis, R irradiation caused at least a 1.9-fold reduction in free IAA. To determine if the nearly twofold decrease in epidermal auxin occurring after R is physiologically significant, IAA was differentially applied to opposite sides of shoots. A twofold difference in IAA application rate caused asymmetrical growth. Thus, the twofold R-induced decrease in free IAA level in the epidermis, a difference sufficient to affect growth, and the rapid R-induced change in growth rate in the epidermis are consistent with the hypothesis that R causes growth of the mesocotyl to decrease by preferentially regulating the free IAA level in the mesocotyl epidermis.
Bibliographical noteFunding Information:
We thank Dr. Jim Shinkle (Trinity University, San Antonio, Tex., USA) for the loan of the transducers, helpful discussions, and suggesting the split-mesocotyl assay, Drs. Rainer Hertel and Peter Schopfer (University of Freiburg, Germany) for comments on the manuscript, and Ms. Susan Whitfield (University of North Carolina at Chapel Hill) for preparing the illustrations. This work was supported by a U.S. Department of Agriculture Competitive Research Grant Oce grant awarded to A.M.J, by a National Science Foundation ROA grant to M.B-B, and a U.S. Department of Energy grant to J.D.C. Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture, and does not imply its approval to the exclusion of other products or vendors that may also be suitable.
- Growth Epidermis
- Indole-3-acetic acid
- Red light
- Zea (mesocotyl)