Uptake of endocytic markers by rice cells: variations related to the growth phase

Endocytosis is now considered a basic cellular process common to plant tell s. Although both non-specific and receptor-mediated endocytosis appear to t ake place in plant cells, the physiological role of the latter remains uncl ear. We have investigated the endocytic process in rice cell suspensions us ing two biotinylated proteins, peroxidase and bovine serum albumin (bHRP an d bBSA), as markers. First, we show that markers are internalized by rice c ells and appear in intracellular membranes. The uptake of the two markers i s temperature dependent, saturable with time and markers dose and it is com peted by free biotin. Thus, it shows the properties of a receptor-mediated process. We also show that uptake of markers is strongly influenced by grow th phase as optimal uptake occurs during the lag phase, but the initiation of the exponential growth phase decreases uptake drastically. Arrest of the cell cycle by starvation of either a nutrient (phosphate) or a growth regu lator (2,4-dichlorophenoxyacetic acid), both components of the culture medi um, does not modify the rate of bBSA uptake. Subsequent readdition of these components results in growth recovery and a dramatic decrease in bBSA upta ke. On the other hand, nocodazole treatment, a method to arrest the cell cy cle by microtubule depolymerization, inhibited bBSA uptake. The possible ca uses for this arrest of endocytosis are discussed.