Light Quality Regulates Lateral Root Development in Tobacco Seedlings by Shifting Auxin Distributions

Light is an important environmental regulator of diverse growth and developmental processes in plants. However, the mechanisms by which light quality regulates root growth are poorly understood. We analyzed lateral root (LR) growth of tobacco seedlings in response to three kinds of light qualities (red, white, and blue). Primary (1°) LR number and secondary (2°) LR density were elevated under red light (on days 9 and 12 of treatment) in comparison with white and blue lights. Higher IAA concentrations measured in roots and lower in leaves of plants treated with red light suggest that red light accelerated auxin transport from the leaves to roots (in comparison with other light qualities). Corroborative evidence for this suggestion was provided by elevated DR5::GUS expression levels at the shoot/root junction and in the 2° LR region. Applications of N-1-naphthylphthalamic acid (NPA) to red light-treated seedlings reduced both 1° LR number and 2° LR density to levels similar to those measured under white light; DR5::GUS expression levels were also similar between these light qualities after NPA application. Results were similar following exogenous auxin (NAA) application to blue light-treated seedlings. Direct [3H]IAA transport measurement indicated that the polar auxin transport from shoot to root was increased by red light. Red light promoted PIN3 expression levels and blue light reduced PIN1, 3–4 expression levels in the shoot/root junction and in the root, indicating that these genes play key roles in auxin transport regulation by red and blue lights. Overall, our findings suggest that three kinds of light qualities regulate LR formation in tobacco seedlings through modification of auxin polar transport.