The most obvious symptom of Al toxicity is the inhibition of root growth. However, the mechanism of Al-inhibiting root growth remains to be elucidated. In this study, auxin transport and vesicle movement of an auxin-efflux carrier (PIN2) were investigated in Arabidopsis roots in response to Al stress. Results indicated that Al inhibited the apical transport of auxin in root tips of Arabidopsis significantly. The severe inhibition was localized in the cells of transition zone, where the concentration of auxin was only 34% that of the control. Brefeldin A (BFA), an inhibitor of vesicle transport, induced the dot-like structure of PIN2 vesicle significantly. Al decreased the size of dot-like structure of PIN2 vesicles. Re- sults of real-time RT-PCR and Western-blotting analysis showed that Al increased the transcript level of PIN2 and the accumulation of PIN2 protein in horizontal direction of plasma membrane, but decreased its distribution in endosomes, suggesting that Al inhibited the transport of PIN2 vesicles from plasma membrane to endosomes. Results of cytoskeleton-depolymering drugs indicated that it was via the pathway of disruption of actin microfilaments that Al inhibited the transport of PIN2 vesicles. Exposed to Al stress, the cells of elongation zone had less Al uptake and less transport frequency of vesicles than cells of transition zone. Taken together, our results suggested that Al inhibited root growth mainly by modulating the transport of PIN2 vesicles between plasma membrane and endosomes, thus block- ing auxin transport and root growth.
