Submitted to: Planta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/1/1997
Publication Date: N/A
Interpretive Summary: Plant growth is controlled, in part, by the interaction of the plant hormone auxin and light quality (plants are most sensitive, for example, to the amount of red light). This interaction is important in cropping systems since high planting density increases the amount of red light due to transmission through and reflection from leaves. Changes in red light can effect plant growth by decreasing nutrients going to storage organs such as edible roots or fruit and increasing the growth of shoots and stems. It is likely that these changes are controlled by the effect of red light on auxin levels, but more information is required. Dark grown corn seedlings were used as a model system to study the effect of red light on auxin levels. Red light was shown to decrease auxin levels and the effect was primarily seen in the epidermal tissue that are most involved in plant growth. These findings suggest strategies where it may be possible to decrease the negative effects of plant density and thus increase the size and quality of harvested roots or fruit. These results will be useful to plant biochemists, plant breeders and molecular biologist interested in improving plant performance under high density planting conditions and to consumers interested in the potential for obtaining variety improvements.
Technical Abstract: The etiolated maize 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 suggesting 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 IAA levels in both the apical 1 cm of the mesocotyl and the corresponding epidermis of etiolated and 4 hour, R-irradiated seedlings. R irradiation caused a 1.4-fold reduction in free IAA within the whole section of the apical mesocotyl. However, within the peeled mesocotyl epidermis, red light irradiation caused at least a 1.9-fold reduction in free IAA. To determine if the nearly two-fold decrease in epidermal auxin occurring after R is physiologically significant, IAA was differentially applied to opposite sides of shoots. A 2-fold difference in IAA application rate caused asymmetrical growth. Thus, the two-fold 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 levels in the mesocotyl epidermis.