Abstract
In many root tropic behaviors, auxin is the essential phytohormone to regulate a cell growth directing root development. It was reported that light promotes the translocation of auxin carrier proteins such as PINs (PIN-FORMED) providing a polarity for roots to complete negative phototropism. These PIN proteins are known to be translocated via endocytic vesicle recycling in root cells. However, an direct influence of light conditions on endocytic vesicle recycling mechanism controlling tropic behaviors in Arabidopsis root cells are not well assessed. In this study, we compared the activity of endocytic vesicle recycling and PIN2 localization in root cells at root transition zone grown under (1) light regime (16 h light / 8 h dark) for 5 d, (2) light regime for initial 4 d followed by 24-h of dark, and (3) continuous dark for 5 d. In the result, dark-grown seedlings showed lower rate of endocytotic activities in root transition zones, compared to the light-grown roots. Interestingly, light-promoted endocytic recycling activity was attenuated to the level equivalent to dark-grown roots after 24-h of dark treatment. PIN2-GFP was shown to accumulate in vacuoles both in dark-grown and 24-h dark treatment seedlings. Moreover, the PIN2-GFP signal found in 24-h dark-treated roots was stronger than in the dark-grown sample. Here we propose a model for dynamic regulation of PIN2 localization regulated by endocytic vesicle recycling in the transition of light circumstances, which might be important for roots to prepare for upcoming unfavorable light.
