Flavonols Accumulate Asymmetrically and Affect Auxin Transport in Arabidopsis      

Abstract

Flavonoids represent a class of secondary metabolites with diverse functions in plants including ultraviolet protection, pathogen defense, and interspecies communication. They are also known as modulators of signaling processes in plant and animal systems and therefore are considered to have beneficial effects as nutraceuticals. The rol1-2 (for repressor of lrx1) mutation of Arabidopsis (Arabidopsis thaliana) induces aberrant accumulation of flavonols and a cell-growth phenotype in the shoot. The hyponastic cotyledons, aberrant shape of pavement cells, and deformed trichomes in rol1-2 mutants are suppressed by blocking flavonoid biosynthesis, suggesting that the altered flavonol accumulation in these plants induces the shoot phenotype. Indeed, the identification of several transparent testa, myb, and fls1 (for flavonol synthase1) alleles in a rol1-2 suppressor screen provides genetic evidence that flavonols interfere with shoot development in rol1-2 seedlings. The increased accumulation of auxin in rol1-2 seedlings appears to be caused by a flavonol-induced modification of auxin transport. Quantification of auxin export from mesophyll protoplasts revealed that naphthalene-1-acetic acid but not indole-3-acetic acid transport is affected by the rol1-2 mutation. Inhibition of flavonol biosynthesis in rol1-2 fls1-3 restores naphthalene-1-acetic acid transport to wild-type levels, indicating a very specific mode of action of flavonols on the auxin transport machinery.

7-Rhamnosylated Flavonols Modulate Homeostasis of the Plant Hormone Auxin and Affect Plant Development

“…The rol1-2 mutant was identified in a screen for alterations in cell wall structures and shows modifications in the Rha-rich cell wall component pectin (43). In addition, rol1-2 also shows changes in the flavonol glycosylation profile, mainly a reduction in the degree of rhamnosylation, whereas the overall quantity of flavonols is not affected (30,44). This confirms the importance of RHM1, which is coexpressed with genes important for flavonol biosynthesis (26) and for flavonol glycosylation.…”

“…The rol1-2 allele and fls1-3 allele used in this study are described elsewhere (43,44). For all analyses described, the ugt89c1-3 nonsense allele was used.…”

“…For propagation and crossings, 10-day-old plants were transferred to soil, grown in a 16-h light, 8-h dark cycle at 22°C, and irradiated with white light (Biolux, Osram). The EMS screen was performed as described (44), and 75,000 M2 seedlings were screened for a suppressed rol1-2 mutant phenotype. All ugt89c1 alleles were backcrossed at least three times to Col-0 and rol1-2 prior to analysis.…”

“…Both rol1-2 and ugt78d2 show changes in auxin concentration or transport activity. For rol1-2, it was shown that auxin transport is reverted to wildtype levels when blocking flavonol accumulation (44), thus indicating that flavonols do interfere with auxin distribution.…”

“…Mutations affecting steps in flavonoid biosynthesis ( Fig. 1) upstream of flavonols such as tt4 or tt6, mutations in the flavonol synthase (FLS1), or mutations in a positive regulator of flavonol biosynthesis encoded by MYB111 all suppress rol1-2, resulting in wild type-like shoot development, whereas root development is comparable with the rol1-2 single mutant (30,44). A mutation in TT7, blocking quercetin accumulation, or any step farther downstream in the pathway has no effect on the rol1-2 shoot phenotype (30,44).…”