PhysMlogical Plant Pathology (1982) 20,27-33 Binding of Agrobacterium fumefaciens to carrot protoplasts ANN G. MATTHYSSE~, KATHRYN V. HOLMESI and ROBIN H. G. GURLITzt 7Department of Botany, University of .North Carolina, Chapel Hill, North Carolina 27514, U.S.A. $%partment of Pathology, Unt$ormed Services University of the Health Sciences, Bethesda, Maryland 20014, iY.S.A. (Accepted&r publication July 1981) Agrobacterium tumefaciens attached both to intact carrot cells in vitro and to carrot protoplasts. After attachment to the protoplast membrane the bacteria elaborated cellulose fibrils, which anchored them to the surface of the plant cell. These fibrils entrapped other bacteria forming large bacterial clusters on the plant cell surface. The kinetics and bacterial strain specificity of attachment to protoplasts were similar to those observed with intact cells. Thus the plant cell membrane may contain a specific receptor for virulent Agrobacterium cells. It is likely that the transfer of tumor-inducing plasmid DNA from the bacterium to the plant cell would be facilitated by the tight binding of large numbers of bacteria to the plant cell membrane. INTRODUCTION Crown gall tumors result from the extracellular infection at a wound site by Agro- bacterium tumefaciens. The first step in tumor formation is the attachment of the the bacteria to the host cells (71. During the course of the infection a large tumor- inducing (pTi) plasmid is transferred from the virulent bacteria to the host plant cells. A portion of the pTi DNA is maintained and transcribed in the tumor cells II% 2, 3, 141. The mechanism of plasmid transfer from the bacterium to the plant cell is unknown. In order to enter the plant cell the plasmid DNA must be transferred across the bacterial plasma membrane and the plant cell plasma membrane. We have demonstrated that virulent strains of A. tumefaciens are capable of specific attachment to intact plant tissue culture cells [IO]. Avirulent strains of A. tumefaciew which lack the Ti plasmid show markedly less attachment than do virulent strains. This attachment is a complex process with several intermediate steps. During bind- ing, the bacterial cells are induced to synthesize cellulose fibrils, which hold the bacteria to the plant cell surface [8]. The bacteria also possessenzymes capable of digesting the pectin portion of the plant cell wall [S]. This complex process may facilitate the intimate contact of the bacterial membrane with the plasmalemma of the plant cell. In addition some areas of plant plasma membrane may become exposed during formation of a wound. Intimate contact of the bacterial plasma membrane with the plant plasma membrane may thus occur by either of two routes. This manuscript describes experiments with carrot protoplasts documenting that virulent A. tumefaciens cells can indeed bind specifically to the plasma membrane of plant cells. 0048~4059/82/010027+09 $02,00/O @ 1982 Academic Press Inc. (London) Limited