Abstract
In October 2011, hairy root symptoms were observed on commercially cultivated tomato (Lycopersicon esculentum) grown on coco slabs (a plant cultivation medium made with finely flaked coconut fibers) in large hydroponic installations equipped with recirculating drip irrigation systems in Hiroshima Prefecture, Japan. Abundant proliferative adventitious hairy roots appeared and thickly covered the slab surfaces, leading to a “root mat” symptom. The dense growth of roots was also observed inside the slabs. The causal bacterium, which was demonstrated by inoculation and reisolation to be pathogenic on tomato, was a gram-negative, aerobic rod with 2 to 4 peritrichous flagella. It formed opaque, milky or pale yellowish-colored, circular colonies, and had biochemical and physiological characters similar to those of Rhizobium species. Phylogenetic analysis based on 16S rDNA sequences also confirmed that it was a member of the genus Rhizobium. MLSA analyses based on the concatenated sequences of genes atpD, glnA, and recA revealed that the isolates are in the R. radiobacter clade. Thus, the pathogen was identified as Rhizobium radiobacter (Beijerinck and van Delden 1902) Young, Kuykendall, Martinez-Romero, Kerr and Sawada 2001. PCR analyses clarified that the pathogen harbored a cucumopine Ri plasmid. This is the first report on the occurrence of root mat (hairy root) of tomato caused by R. radiobacter (Ri) in Japan. Phylogenetic analyses revealed the existence of chromosomal polymorphisms among the present isolates, which proved to belong to genomovars G7 and G9 of R. radiobacter species complex. On the other hand, the cucumopine Ri plasmids harbored by the present isolates and the root mat pathogens isolated in Europe were highly homogeneous, which suggests that plasmids derived from the same origin might have been transferred to the indigenous bacteria in Europe and Japan, respectively.
