Mechanism of tomato bacterial wilt suppression in soil amended with lysine

The effect of four amino acids, unutilizable by Ralstonia solanacearum, on tomato bacterial wilt (TBW) was tested in three Japanese soils. Results confirmed our previous findings that the addition (2.5 mg g(-1) soil) of lysine and serine suppressed TBW, but that of tyrosine and valine did not. The number of the pathogen in non-rhizosphere soil, rhizosphere soil and the rhizoplane was markedly lower in the lysine and serine treatments than in the tyrosine and valine treatments, while the opposite result was obtained for the total bacterial population. Substrate-induced respiration analysis revealed that bacteria became more predominant in the amino acid treatments, especially in lysine. There were no apparent correlations between the microbial activities in soil and the disease index. PCR-DGGE targeting the 16S rRNA gene of the rhizoplane samples from lysine-added Nagoya, Fuchu and Iwate soils revealed 10, 1 and 2 unique bands, respectively, indicating shifts in bacterial community. DGGE patterns in the suppressive treatments belonged to the same cluster while the not suppressive treatments clustered differently. The lower disease incidence of TBW following lysine treatment is likely related to a specific bacterial community in the rhizoplane that developed on the addition of lysine.