Report of Crown Gall, Caused by Agrobacterium

Crown gall is caused by bacteria belonging to the genera Agrobacierium, Allorhizobium and Rhizobium of the family Rhizobiaceae. They are aerobic, motile, non-spore form-ing, flagellated, Gram-negative, rod-shaped bacteria. 

The taxonomy of bacteria causing crown gall is debatable and still not fully resolved. Initially, their taxonomy was based on pathogenic features and all bacteria causing crown gall were classified as Agrobacterium tumefaciens, at this stage, A. vitis are therefore classified as A. tumefaciens biovar 3. However, the pathogenicity of tumorigenic strains is determined mainly by the presence of a conjugative tumour-inducing (Ti) plasmid in their genome. Since Ti plasmid acquisition or loss may lead to a change in taxonomic status, classification based on pathogenic properties was not considered stable.

Taking into account new biochemical and genomic criteria, especially the percentage of DNA-DNA hybridization between the different strains, it appears that the strains of biovar 3 are different from other biovars to represent a separate species; the name Agrobacterium vitis is then proposed (Ophel and Kerr, 1990). As early as 1993, early studies based on the gene encoding 16S rRNA showed that these strains were phylogenetically closer to certain Rhizobium strains than A. tumefaciens biovar 1 (Willems and Collins, 1993, Yanagi and Yamasato, 1993). In 2001, a study based on the analysis of the sequences of the 16S rRNA gene integrates all the bacteria called Agrobacterium, Allorhizobium, Rhizobium and Ensifer in the same bacterial genus called Rhizobium and thus modifies the name of Agrobacterium vitis to Rhizobium vitis and Agrobacterium tumefacians to Rhizobium radiobacter (Young et al., 2001). In 2003, a new study called into question the grouping of all these bacteria in the same genus (Farrand, 2003). Via biochemical and genomic criteria, it is shown that biovars 1 and 3 are part of a different genus from the other Rhizobium which is again named the genus Agrobacterium. The name Agrobacterium vitis and Agrobacterium tumefacians are therefore used again. Knowing that the gene coding for 16S rRNA is sometimes highly conserved between strains belonging to different species, it has been shown that the use of MLSA (multilocus sequence analysis) is better suited to the study of phylogenetically related strains or species. (Farrand 2003, Martens et al., 2007). A recent study led to a phylogeny of the Rhizobiaceae family by an MLSA approach targeting 6 housekeeping genes (atpD, glnA, recA, ginli, rpoB and thrC) in 114 strains mainly belonging

to the genera Agrobacterium and Rhizobium. This study led to the emergence of a new genus, the genus Neorhizobium, for the reclassification of Rhizobium rhizogenes into Agrobacterium rhizogenes and finally for the reclassification of Agrobacterium vitis to Allorhizobium vitis (Mousavi et al., 2014). Another argument supporting the separation A. tumefacians and A. vitis is the presence of a linear chromid in bacteria of the genus Agrobacterium via the acquisition of the telA gene in the family Rhizobiaceae (Figure 7). The telA gene encodes a protelomerase involved in the double-strand linearization of the chromid DNA characteristic of this family (Slater et al., 2009). The presence of this gene and therefore of a linear chromid in Agrobacterium thus makes it possible to differentiate them from other genera which possess a circular chromid (and thus do not have the telA gene), such as Allorhizobium vitis (Ramirez¬Bahena et al. 2014).