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

Confocal laser scanning microscopy (CLSM) has been used to image phloe m transport and unloading in the root tip of Arabidopsis. The fluoresc ent 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 pos ition of the prominent unloading 'zone' corresponded precisely with th at of the two protophloem files of sieve elements, demonstrating a fun ctional role of these cells in symplastic sieve-element unloading. Sym plastic transport following unloading was confined to the elongating z one of the root with little basipetal transport to more mature cells. Following photobleaching of the unloading zone, phloem transport was r estored immediately into the protophloem sieve elements, followed rapi dly by lateral, symplastic sieve-element unloading. The results demons trate that phloem transport processes can now be imaged in real time, and non-invasively, within an intact plant system.