Date Published: January 20, 2012
Author(s): Jan AC Vriezen, Frans J de Bruijn, Klaus R Nüsslein.
Sinorhizobium meliloti is a microorganism commercially used in the production of e.g. Medicago sativa seed inocula. Many inocula are powder-based and production includes a drying step. Although S. meliloti survives drying well, the quality of the inocula is reduced during this process. In this study we determined survival during desiccation of the commercial strains 102F84 and 102F85 as well as the model strain USDA1021.
Rhizobia form root nodules in symbiosis with legumes in which they fix atmospheric nitrogen and supply the fixed nitrogen to the plants (Jones et al. 2007). This system can be used to replenish soils with biologically-fixed nitrogen, and reduces the need for chemical fertilizers and pollution. However, this process is affected by salinity, drought and desiccation stress (Zahran 1999; Vriezen et al. 2007). To make optimal use of this system, inoculants are employed which allow for close contact between the microorganisms and the germinating seed (Smith 1992; Kosanke et al. 1999; Deaker et al. 2004). Many inoculants are powder-based, and a drying step during production reduces their quality (Kosanke et al. 1992). According to Catroux et al. (2001) many inoculants remain unreliable because of the inability of bacterial cells to persist under adverse conditions, including desiccation.
The symbiotic system of rhizobia with legumes can be used to replenish soils with biologically-fixed nitrogen and prevents the need for chemical fertilizers and pollution. To make optimal use of this system, inoculants are employed allowing close contact between the microorganisms and the germinating seed (Kosanke et al. 1999; Smith 1992; Deaker et al. 2004). Powder-based seed inocula require a drying step to enable long-term storage. This drying step can result in a reduction in inoculum quality (Smith 1992), i.e. reduced culturability and infectivity. Although culturability is reduced, a substantial part of the cells are in a viable but non-culturable state (VBNC). The induction of this physiological state by desiccation is a novel and very relevant observation since rhizobial VBNC cells do not infect plants, while those able to form colonies do as reported for strain S. meliloti 41 (Basaglia et al. 2007).
The authors declare that they have no competing interests.