Stenotrophomonas rhizophila – a novel biocontrol agent for saline soils

Stenotrophomonas rhizophila DSM 14405T promoted plant growth of a wide variety of crops in saline soils of Uzbekistan. Contrary to its close relative S. maltophilia, it is non-pathogenic to humans. Aim of this study is to elucidate mechanisms responsible for the plant growth promoting effect and to identify environmental parameters (soil salinity, accompanying rhizosphere flora, crop species) influencing the efficacy and survival of this promising rhizosphere inoculant. In plant agar S. rhizophila builds up high populations (108-109 per g plant) on all tested crop species (cotton, sweet pepper, tomato, oilseed rape) irrespective of initial concentration (104 and 108 CFU/seedling). Ecto- and endophytic growth in roots could be shown using dsred-labelled cells or FISH by confocal laser scanning microscopy. Crops differed in response to the inoculant. At low doses (104 cells per seedling), positive effects (increase in root tip number or length, or increase in the number of secondary leaves) could be seen in cotton, tomato and sweet pepper whereas high doses of 108 cells per seedling had negative effects in vitro. On the contrary, oilseed rape did not show any significant response towards S. rhizophila at low and high doses. S. rhizophila controlled seed borne diseases (Alternaria spec.) present in the seed batches of cotton and sweet pepper. In non-sterile soil sweet pepper and tomato showed the strongest positive response, whereas no effects were visible in cotton and cucumber. The positive effects in non-sterile soil were more pronounced than in than in gnotobiotic systems (plant agar and autoclaved soil). Neither in gnotobiotic systems nor in non-sterile soil S. rhizophila alleviated salt stress directly. Our results indicate that S. rhizophila either needs a factor present in soil for optimal plant growth promotion or promotes the growth of plants indirectly via the inhibition of pathogens or deleterious microorganisms in the rhizosphere.