Endocytosis of Xanthomonas campestris pathovar campestris lipopolysaccharides in non-host plant cells of Nicotiana tabacum

The specific recognition of phytopathogenic bacteria by plant cells is generally mediated by a number of signal molecules. The elicitor-active lipopolysaccharides (LPS) of the phytopathogenic bacterium Xanthomonas campestris pv. campestris (X.c.c) are recognized by its non-host plant Nicotiana tabacum (N.t.). This LPS was purified and labelled with fluorescein isothiocyanate (FITC) for monitoring the fate of these signal molecules in intact plant cells of tobacco. In this study we were able to show that the so-labelled LPS rapidly bound to the cell wall and was then internalized into the cells in a temperature- and energy-dependent way. This uptake of LPS could be outcompeted by the addition of an excess of unlabelled LPS. Furthermore, it was blocked by amantadine, an inhibitor of receptor-mediated endocytosis of mammalian cells. Immunolocalization experiments showed for the first time a significant co-localization of the LPS-elicitor with endosomal structures using an anti-Ara6 antibody. These observations suggest specific endocytosis of LPSX.c.c. into tobacco cells. The possibility for a receptor-mediated endocytosis comparable to the mammalian system will be discussed.