Symbiotic Plant–Microbe Interactions: Stress Protection, Plant Growth Promotion, and Biocontrol by <i>Stenotrophomonas</i> Symbioses and Stress

The genus Stenotrophomonas is[COMP16] phylogenetically placed[COMP17] in the γ-subclass of Proteobacteria (Moore et al., 1997 ). The genus was first described with the type species Stenotrophomonas maltophilia (Palleroni and Bradbury, 1993 ), previously called Pseudomonas maltophilia (Hugh and Ryschenko, 1961 ) and later changed to Xanthomonas maltophilia (Swings et al., 1983 ). Actually, the genus comprises eight validly described species: S. maltophilia , S. nitritireducens (Finkmann et al., 2000 ), S. rhizophila (Wolf et al., 2002), S. acidaminophila (Assih et al., 2002 ), S. koreensis (Yang et al., 2006 ), S. terrae , S. humi (Heylen et al., 2007 ), and S. chelatiphaga (Kaparullina et al., 2009 ). However, pheno- and genotypic studies revealed much more differentiation at species level (Ryan et al., 2009 ). Only two species, S. maltophilia and S. rhizophila (Wolf et al., 2002 ), show a strong association with plant hosts. In comparison with S. maltophilia , the defining phenotypic characteristics of S. rhizophila are: growth at 4°C and no growth at 40°C; the utilization of xylose as a carbon source; higher osmotic tolerance (<5% NaCl [w/v]); and the absence of lipase and β-glucosidase production (Wolf et al., 2002 ). Both species produce osmoprotective substances (Roder et al., 2005 ). These are compounds compatible at very high internal concentrations with cellular functions, e.g., DNA replication, DNA–protein interactions, and cellular metabolism; they regulate the osmotic balance and are effective ­stabilizers of enzymes (Welsh, 2000 ). A molecular protocol was developed for the differentiation of the two Stenotrophomonas species. It targets specifically the ggpS gene responsible for glucosylglycerol (GG) synthesis because this marker occurs only in S. rhizophila strains and was absent from all S. maltophilia isolates ­(Ribbeck-Busch et al., 2005 ). As a further genetic marker the smeD gene was used, which is part of the operon coding for the multi-drug efflux pump SmeDEF only occurring in S. maltophilia (Alonso and Martinez, 2000 ).