Stenotrophomonas rhizophila – a novel biocontrol agent for saline soils

Abstract: Stenotrophomonas rhizophila DSM 14405T promoted plant growth of a widevariety 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 responsiblefor the plant growth promoting effect and to identify environmental parameters (soilsalinity, accompanying rhizosphere flora, crop species) influencing the efficacy andsurvival of this promising rhizosphere inoculant. In plant agar S. rhizophila builds uphigh 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 FISHby confocal laser scanning microscopy. Crops differed in response to the inoculant. Atlow 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 sweetpepper whereas high doses of 108 cells per seedling had negative effects in vitro. On thecontrary, oilseed rape did not show any significant response towards S. rhizophila at lowand high doses. S. rhizophila controlled seed borne diseases (Alternaria spec.) present inthe seed batches of cotton and sweet pepper. In non-sterile soil sweet pepper and tomatoshowed the strongest positive response, whereas no effects were visible in cotton andcucumber. The positive effects in non-sterile soil were more pronounced than in than ingnotobiotic systems (plant agar and autoclaved soil). Neither in gnotobiotic systems norin 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 orpromotes the growth of plants indirectly via the inhibition of pathogens or deleteriousmicroorganisms in the rhizosphere.