It has long been observed that rare earth elements (REEs) regulate multiple facets of plant growth and development. However, the underlying mechanisms remain largely unclear. Here, using electron microscopic autoradiography, we show the life cycle of a light REE (lanthanum) and a heavy REE (terbium) in horseradish leaf cells. Our data indicate that REEs were first anchored on the plasma membrane in the form of nanoscale particles, and then entered the cells by endocytosis. Consistently, REEs activated endocytosis in plant cells, which may be the cellular basis of REE actions in plants. Moreover, we discovered that a portion of REEs was successively released into the cytoplasm, self-assembled to form nanoscale clusters, and finally deposited in horseradish leaf cells. Taken together, our data reveal the life cycle of REEs and their cellular behaviors in plant cells, which shed light on the cellular mechanisms of REE actions in living organisms.activation | endocytic vesicle S ome trace elements, such as rare earth elements (REEs), have been observed for a long time to be beneficial to plant growth (1, 2). REEs are f-block elements in the periodic table, including 15 lanthanides, plus scandium and yttrium (2). The beneficial effect of REEs on plants was first reported in 1917, when it was found that cerium, an REE, improved physiological activities of water-floss (Spirogyra) (3). Since the 1970s, REEs have been widely used in agriculture as plant growth stimulants (4). Currently, more than 100 of crop species have been applied with REEs, which has increased crop yields by 5-15% (4). In addition, REEs also serve to protect plants against certain plant diseases and some stresses, such as drought, cold, acid rain, or heavy metals (2). The wide use of REEs has caused an overaccumulation of REEs in the ecosystem, including soil (5), atmosphere (6), and water (7). Moreover, it was shown that REEs have dual effects on plant growth: low concentrations of REEs exert positive effects on growth and yield of crops, whereas high concentrations of REEs seem to be harmful for plants, indicating that REEs are not absolutely good for plants (2). Therefore, it is imperative to elucidate how REEs act on plant cells to ensure food safety.Despite of many advances achieved in recent years, the action mechanisms of REEs on plant cells still remain poorly understood (2). Several explanations, which are mainly focused on the physiological roles of REEs in plant cells, have been proposed (2). These mechanisms include the possible changes of photosynthesis, mineral nutrient uptake and metabolism, catalytic activities of some enzymes, hormonal balance, and so forth (2). However, these hypotheses are not connected to any cellular or molecular principles and are therefore far from explaining the action mechanisms of REEs in plants (2,8). In addition, little consensus has yet been reached regarding the life cycle of REEs in plant cells. Some studies report that REEs cannot enter the plant cells but only be localized in apoplast (8-14); other st...