Abstract
Flavonoids are synthesized in response to developmental and environmental signals and perform many functions in plants. Arabidopsis (Arabidopsis thaliana) roots grown in complete darkness do not accumulate flavonoids since the expression of genes encoding enzymes of flavonoid biosynthesis is light dependent. Yet, flavonoids accumulate in root tips of plants with light-grown shoots and light-shielded roots, consistent with shoot-to-root flavonoid movement. Using fluorescence microscopy, a selective flavonoid stain, and localized aglycone application to transparent testa mutants, we showed that flavonoids accumulated in tissues distal to the application site, indicating uptake and movement systems. This was confirmed by time-course fluorescence experiments and high-performance liquid chromatography. Flavonoid applications to root tips resulted in basipetal movement in epidermal layers, with subsequent fluorescence detected 1 cm from application sites after 1 h. Flavonoid application to midroot or cotyledons showed movement of flavonoids toward the root tip mainly in vascular tissue. Naringenin, dihydrokaempferol, and dihydroquercetin were taken up at the root tip, midroot, or cotyledons and traveled long distances via cell-to-cell movement to distal tissues, followed by conversion to quercetin and kaempferol. In contrast, kaempferol and quercetin were only taken up at the root tip. Using ATP-binding cassette (ABC) transporter and H(+)-ATPase inhibitors suggested that a multidrug resistance-associated protein ABCC transporter facilitated flavonoid movement away from the application site.
Highlights
Genomic Interactions Group, Australian Research Council Centre of Excellence for Integrative Legume Research, Research School of Biological Sciences, The Australian National University, Canberra, Australian Capital Territory 0200, Australia (C.S.B., M.A.D.); and Biology Department, Wake Forest University, Winston-Salem, North Carolina 27109 (G.K.M.)
Light is required for flavonoid synthesis in Arabidopsis seedlings (Pelletier and Shirley, 1996; Kubasek et al, 1998; Jenkins et al, 2001), and, when seedlings are grown with roots exposed to light, the presence of flavonoids
To verify that flavonoids were capable of root-to-shoot and shootto-root movement, we used HPLC analysis of tissues distal to the application site to show that downstream products of the flavonoid pathway were present in these tissues. To determine how this movement was facilitated, we tested various inhibitors of ATPbinding cassette (ABC) transporters and H1-ATPase and described a putative mechanism for this movement. These results showed that flavonoids can move in Arabidopsis and suggest that this movement may be mediated by an ATP-binding cassette (ABC)-type transporter
Summary
Genomic Interactions Group, Australian Research Council Centre of Excellence for Integrative Legume Research, Research School of Biological Sciences, The Australian National University, Canberra, Australian Capital Territory 0200, Australia (C.S.B., M.A.D.); and Biology Department, Wake Forest University, Winston-Salem, North Carolina 27109 (G.K.M.). A selective flavonoid stain, and localized aglycone application to transparent testa mutants, we showed that flavonoids accumulated in tissues distal to the application site, indicating uptake and movement systems. This was confirmed by timecourse fluorescence experiments and high-performance liquid chromatography. Flavonoid Movement in Arabidopsis regulates root branching (Brown et al, 2001) and gravitropism (Buer and Muday, 2004) For these regulatory roles of flavonoids to be relevant when plants are grown in light with roots in the dark, it may be necessary for either light signals or flavonoids themselves be transported to the roots.
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