Comparison of bacterial community structures in the rhizoplane of tomato plants grown in soils suppressive and conducive towards bacterial wilt

Abstract

It has been reported that the growth of Ralstonia solanacearum is suppressed at the rhizoplane of tomato plants and that tomato bacterial wilt is suppressed in plants grown in a soil (Mutsumi) in Japan. To evaluate the biological factors contributing to the suppressiveness of the soil in three treated Mutsumi soils (chloroform fumigated soil; autoclaved soil mixed with intact Mutsumi soil; and autoclaved soil mixed with intact, wilt-conducive Yamadai soil) infested with R. solanacearum, we bioassayed soil samples for tomato bacterial wilt. Chloroform fumigation increased the extent of wilt disease. More of the tomato plant samples wilted when mixed with Yamadai soil than when mixed with Mutsumi soil. Consequently, the results indicate that the naturally existing population of microorganisms in Mutsumi soil was significantly able to reduce the severity of bacterial wilt of tomato plants. To characterize the types of bacteria present at the rhizoplane, we isolated rhizoplane bacteria and classified them into 22 groups by comparing their 16S restriction fragment length polymorphism patterns. In Yamadai soil a single group of bacteria was extremely predominant (73.1%), whereas in Mutsumi soil the distribution of the bacterial groups was much more even. The 16S rDNA sequence analysis of strains of dominant groups suggested that gram-negative bacteria close to the beta-proteobacteria were most common at the rhizoplane of the tomato plants. During in vitro assays, rhizoplane bacteria in Mutsumi soil grew more vigorously on pectin, one of the main root exudates of tomato, compared with those in Yamadai soil. Our results imply that it is difficult for the pathogen to dominate in a diversified rhizobacterial community that thrives on pectin.

FIG. 1

Occurrence of tomato bacterial wilt grown in chloroform-fumigated (○) and nonfumigated (●) Mutsumi soil infested with R. solanacearum SL8 at an initial density of 10³ CFU/g of soil. Standard errors are indicated by bars or are within each symbol.

FIG. 2

Occurrence of tomato bacterial wilt grown in nine parts autoclaved Mutsumi soil mixed with either one part intact Mutsumi soil (●) or 1 part intact Yamadai soil (○) and infested with R. solanacearum SL8 at an initial density of 10³ CFU/g of dry soil. Standard errors are indicated by bars or are within each symbol.

FIG. 3

Relationships among 22 groups composed of 316 isolates of culturable bacteria from the rhizoplanes of tomato plants. The square distance (genetic similarity) was determined from 16S rDNA-RFLP patterns obtained by digestion with HinfI and MboI by the Ward method. Isolates within 130 of the square distance were combined into a group.

FIG. 4

Percentage of 16S rDNA-RFLP pattern groups in the rhizoplanes of tomato plants in Yamadai (gray bar) or Mutsumi (black bar) soils.