Biological Control of the Bacterial Wilt of the Tomato ‘Micro-Tom' by Phenotypic Conversion Mutants of Ralstonia solanacearum

Abstract

Bacterial wilt caused by Ralstonia solanacearum Rsol is a devastating disease that has been observed in over 200 plant species such as tomatoes, eggplants, potatoes and other economically important crops around the world (Hayward, 1991). This pathogen invades the host plants through wounds in roots or sites where secondary roots emerge, then colonizes the cortex, infects the vascular parenchyma, and invades the xylem vessels (Vasse et al., 1995). After Rsol penetrates into the vessels, the bacterium rapidly spreads and increases throughout the vascular system, resulting in wilting and the death of the plants. Rsol spontaneously undergoes a phenotypic conversion (PC) after prolonged culture in plants, water extract of plants, soil or broth culture (Kelman, 1954; Buddenhagen and Kelman, 1964; Kelman and Hruschka, 1973; Shekhawat and Perombelon, 1991; Mori et al., 2011; 2012). PC mutants are either weak pathogenic or non-pathogenic, but these mutants are still able to colonize host tissue without causing symptoms (Kelman, 1954; Denny et al., 1988; Mori et al., 2011). The bacterial wilt diseases of tomato and tobacco were suppressed by pre-inoculation with PC mutants (Tanaka, 1983; Hara and Ono, 1991; Arwiyanto et al., 1994). The suppression of bacterial wilt by PC mutants in eggplant has been investigated (Ogawa et al., 2011; 2012a; 2012b). Those studies revealed that PC mutants suppressed bacterial wilt in eggplant by colonizing the root and stem and preventing subsequent colonization by the Rsol wildtype strain. It was speculated that PC mutants have the potential to induce resistance against the wild-type strain in eggplant. However, the molecular mechanisms underlying the induction of resistance are still unknown. In studies of the suppression of plant diseases by beneficial microorganisms, the plant disease severity was reduced because of the expression and accumulation of the pathogenesis-related (PR) proteins that inhibit infection of pathogen and have an antibacterial activity (Hase et al., 2006; Kawamura et al., 2009). PC mutant-infected plants seem to express and accumulate the PR proteins. In the present study, we investigate the expression of PR protein genes in ‘Micro-Tom’, a model cultivar of tomato suitable for genetic analysis (Meissner et al., 1997), after inoculation with PC mutants. The tomato ‘Micro-Tom’ that is miniature-dwarf cultivar, has a short life cycle in 2 3 months from sowing to fruit ripening in pots, and grows at high density (Meissner et al., 1997). Therefore, using this cultivar is suitable for the experiments that require many samples such as researches the plant growth or screening of PC mutants, and it is able to save the space and time for the tests in greenhouses or growth chambers. Among studies of disease control using microorganisms, the microorganisms showed either positive or negative effects on the growth of plants. As a positive example, Thanh et al. (2009) reported that plants pre-inoculated with plant growth-promoting rhizobacteria (PGPR) were sup-