Agriculture Reference
In-Depth Information
Suppression Mechanisms of Used Pumice -Contribution of Biological
Factors
Dominant microbial groups were estimated by the SIR method (Figure 5). The addition
of cycloheximide into the used pumice did not decrease CO
2
emission rate significantly,
suggesting that fungal contribution to total microbial biomass may be small. In contrast, the
addition of rifampicin and kanamycin significantly decreased CO
2
emission rate roughly by
50%. It was concluded that pumice is originally colonized by little microbes and then
microbes, especially bacteria, were enriched with repeated cultivation, which provide
substrates to microbes in the pumice through root exudations and plant residues.
Then, dominant bacteria colonizing tomato roots grown in unused and used pumice were
isolated and screened for their disease suppressing potential against bacterial wilt. Phylogenic
analysis suggested that bacterial community of tomato roots was diverse both in unused and
used pumice, and that there seemed to be no specific group detected only in the used pumice.
When 50 strains isolated from the used pumice were tested for growth suppression of
R.
solanacearum
YU1Rif43 in a root exudates medium, five strains suppressed the growth of the
pathogen markedly. Then, five strains were inoculated into unused pumice infested with the
pathogen, only one strain W3, nearest to
Burkholderia
sp., suppressed DI significantly
(Figure 6). Interestingly, the other strains did not decreased DI, suggesting that most
rhizobacteria did not interfere with the pathogen. W3 completely recovered the disease
suppressing property of used pumice by inoculation into sterilized used pumice (Figure 7).
Microbial community structures were compared of tomato roots grown in different treatments
of pumice media using PCR-DGGE (Figure 8). The band corresponding to the strain W3 was
not detected in the tomato roots grown in unused pumice, but was present in those in used
pumice. When tomato was grown in unused pumice inoculated with W3, DI decreased
significantly (and the band for W3 was detected. These results may indicate that W3 is
involved in disease suppression of bacterial wilt in used pumice and unused pumice
inoculated with the strain. However, single inoculation of W3 was not so effective at reducing
DI compared to used pumice (Figure 9). Considering physical and chemical conditions of
unused and used pumices, it was considered that W3 seemed not to perform well in the
unused pumice because of low C content. Thus, W3 were inoculated into unused pumice
together with carbon sources, such as glucose and xylose. It was observed that disease
suppressive properties of W3 were further enhanced by the addition of glucose or xylose.
Glucose is utilizable to
R. solanacearum
YU1Rif43, but xylose is not. Therefore, we
imagined that xylose be more effective at reducing bacterial wilt, but the result showed that
glucose and xylose were similarly effective, suggesting that utility of carbon substrate by the
antagonist be more important in successful biological control.
