The Flavonoids Promote Polar Auxin Transport In Arabidopsis

Nitrate regulates LR development. On one hand, nitrate is necessarily for LR development, on the other hand, high level of nitrate inhibites LR growth. When the concentration is low (<50 mM), nitrate only influences LR development, and has no effect on primary root. But at higher concentration, nitrate obviously inhibits primary root elongation. Also, high concentration nitrate reduces the auxin sensitivity.Lateral root development and gravitropism are regulated by polar auxin transport (PAT), and auxin efflux inhibitors (AEI) inhibit LR growth and tropism. Because of>the structural similarity to PAT inhibitor NPA, flavonoid has long been regarded as endogenous polar auxin transport inhibitor. In our experiments, however, the inhibition of flavonoid on root system development is related to high nitrate, and quercetin, one kind of flavonoid, does not inhibit but promotes the gravitropism instead.A series of transfer experiments are carried out to study the effects of flavonoid on LR development at the most suitable nitrate concentration (0.2 mM). In our transfer system, quercetin inhibits both primary root and lateral root growth. The result of counting LR primordium indicates that quercetin does not influence primordium formation, but inhibits its elongation. This is essentially different to NPA, which inhibits both primordium formation and elongation. Also, quercetin increases hypocotyls length, which is linked toward elevated auxin transport level. GUS staining result of DR5::GUS report strain indicates that quercetin does not influence polar auxin transport. [ C]IAA is used to measure PAT ability in Arabidopsis stem under the treatment of quercetin, which promotes auxin transport at the concentration of 25 u M as a result. All of these phenotypes indicate that PAT is promoted but not inhibited under the treatment of quercetin.At the same time, we find quercetin enhances tropism: inducing hypergravitropism. On the one hand, quercetin promotes hypocqtyls phototropic growth, on the other hand, it promotes gravitropism of root tips. When the seedlingsare placed tilted at 80, quercetin induces root tips getting into medium, while on control case, it grows along the surface. And the root tips treated by quercetin arrives at right-angel ahead of schedule when rotate 90?for further culture. Pharmacological evidences show that quercetin-inducing hypergravitropism is structural specific activity, and the second massager such as Ca2+ and cAMP take parts in this process, and polar auxin transport is indispensable. A conclusion can be made from all these results that quercetin, one kind of flavonoid, does not act as PAT inhibitor but regulates auxin transport at the level of signal transduction in Arabidopsis.