The Effect of the Agrobacterium tumefaciens attR Mutation on Attachment and Root Colonization Differs between Legumes and Other Dicots

Abstract

Infections of wound sites on dicot plants by Agrobacterium tumefaciens result in the formation of crown gall tumors. An early step in tumor formation is bacterial attachment to the plant cells. AttR mutants failed to attach to wound sites of both legumes and nonlegumes and were avirulent on both groups of plants. AttR mutants also failed to attach to the root epidermis and root hairs of nonlegumes and had a markedly reduced ability to colonize the roots of these plants. However, AttR mutants were able to attach to the root epidermis and root hairs of alfalfa, garden bean, and pea. The mutant showed little reduction in its ability to colonize these roots. Thus, A. tumefaciens appears to possess two systems for binding to plant cells. One system is AttR dependent and is required for virulence on all of the plants tested and for colonization of the roots of all of the plants tested except legumes. Attachment to root hairs through this system can be blocked by the acetylated capsular polysaccharide. The second system is AttR independent, is not inhibited by the acetylated capsular polysaccharide, and allows the bacteria to bind to the roots of legumes.Infections of wound sites on dicotyledonous plants by the soil bacterium Agrobacterium tumefaciens result in the formation of crown gall tumors. The ability of the bacteria to attach to plant cells is required for virulence. All known nonattaching bacterial mutants are avirulent (1,2,7,15). In previous research we have identified a region of the bacterial chromosome (att) containing a number of genes required for attachment to host cells and for virulence (10,11). One of these genes (attR) encodes a putative transacetylase. AttR mutants lack an acetylated capsular polysaccharide found in the wild-type parent strain (14). Preparations of this polysaccharide were able to block the binding of wild-type bacteria to carrot suspension culture cells (14). This polysaccharide is believed to play a role in the virulence-associated binding of the bacteria to host cells.Agrobacteria are capable of living in the rhizosphere as well as in plant wounds. We have previously examined the colonization by A. tumefaciens of the roots of two plants (tomato and Arabidopsis thaliana) which are susceptible to the induction of crown gall tumors (9). When nonattaching mutants of A. tumefaciens (including AttR mutants) were compared to wildtype bacteria, the mutants were found to have more than 10,000-fold-reduced ability to colonize the roots of tomato, suggesting that att genes are involved in the colonization of intact roots as well as in bacterial attachment at wound sites and in virulence.Previous studies of nonattaching mutants of agrobacteria carried out in our laboratory and by others have used a variety of cells and organs from plants, including tobacco, tomato, carrot, zinnia, A. thaliana, and Bryophyllum daigremontiana (1, 2, 6-8, 12, 14). When we extended the study of the interaction of attachment of wild-type A. tumefaciens and att mutants to include legumes, we discovered that the...

Detection of and Response to Signals Involved in Host-Microbe Interactions by Plant-Associated Bacteria

“…A 30-kb cluster of A. tumefaciens att genes has also been described as being required for attachment and tumorigenesis (316,317,319). However, the recently published A. tumefaciens genome sequence revealed that the att genes are located on the cryptic plasmid pAtC58, which is not essential for virulence (233,423).…”

A model for the development of the rhizobial and arbuscular mycorrhizal symbioses in legumes and its use to understand the roles of ethylene in the establishment of these two symbioses

“…Mort and Grover (1988), studying the sugar composition of RH cell walls, demonstrated that among legumes, the RH carbohydrate composition did not differ much; on the contrary, there was a large variation when legumes were compared with nonlegumes. This difference among families was further emphasized by Matthysse and McMahan (2001) who studied the attachment of Agrobacterium tumefaciens to RH. AttR mutants of Agrobacterium tumefaciens are able to differentiate RH of nonlegumes from those of legumes; the bacterium is unable to attach to the former whereas it binds to the latter (Matthysse and McMahan 2001).…”

“…This difference among families was further emphasized by Matthysse and McMahan (2001) who studied the attachment of Agrobacterium tumefaciens to RH. AttR mutants of Agrobacterium tumefaciens are able to differentiate RH of nonlegumes from those of legumes; the bacterium is unable to attach to the former whereas it binds to the latter (Matthysse and McMahan 2001). Since Agrobacterium is closely related to Rhizobium (Young and Johnston 1989), especially Rhizobium leguminosarum and Rhizobium meliloti, it is plausible to assign a similar behaviour to Rhizobium.…”

The Low-Molecular-Weight Fraction of Exopolysaccharide II from Sinorhizobium meliloti Is a Crucial Determinant of Biofilm Formation

“…Bacteria associated with plants frequently form biofilms on leaves, on root surfaces, and within the intercellular spaces of plant tissues (1,3,5,13,34,38,43). Microscopic studies show that the rhizobial cells migrate down the infection threads as biofilm-like filaments toward the root interior (15,43).…”