REAL-TIME IMAGING OF PHLOEM UNLOADING IN THE ROOT-TIP OF ARABIDOPSIS

Confocal laser scanning microscopy (CLSM) has been used to image phloem transport and unloading in the root tip of Arabidopsis. The fluorescent probe 5(6) carboxyfluorescein (CF) was ester loaded into a single cotyledon and the entire seedling placed within an observation chamber under the microscope. Translocation of CF to the root tip was rapid, followed by unloading into discrete concentric files of cells. The position of the prominent unloading 'zone' corresponded precisely with that of the two protophloem files of sieve elements, demonstrating a functional role of these cells in symplastic sieve-element unloading. Symplastic transport following unloading was confined to the elongating zone of the root with little basipetal transport to more mature cells. Following photobleaching of the unloading zone, phloem transport was restored immediately into the protophloem sieve elements, followed rapidly by lateral, symplastic sieve-element unloading. The results demonstrate that phloem transport processes can now be imaged in real time, and non-invasively, within an intact plant system.