Insights into the mechanisms of desiccation resistance of the Patagonian PAH-degrading strain Sphingobium sp. 22B.

Abstract

To analyse the physiological response of Sphingobium sp. 22B to water stress. The strain was grown under excess of carbon source and then subjected to low (60RH) and high (18RH) water stress conditions for 96h. Quantification of trehalose, glycogen, polyhydroxybutyrate (PHB) and transmission electron microscopy (TEM) was studied. Genes linked with desiccation were searched in Sphingobium sp. 22B and Sphingomonas 'sensu latu' genomes and their transcripts were quantified by real-time PCR. Results showed that, in the absence of water stress, strain 22B accumulated 4·76±1·41% of glycogen, 0·84±1·62% of trehalose and 44·9±6·4% of PHB per cellular dry weight. Glycogen and trehalose were mobilized under water stressed conditions, this mobilization was significantly higher in 60RH in comparison to 18RH. Gene treY was upregulated sixfold in 60RH relative to control condition. TEM and quantification of PHB revealed that PHB was mobilized under 60RH condition accompanied by the downregulation of the phbB gene. TEM images showed an extracellular amorphous matrix in 18RH and 60RH. Major differences were found in the presence of aqpZ and trehalose genes between strain 22B and Sphingomonas genomes. Strain 22B showed a carbon conservative metabolism capable of accumulation of three types of endogenous carbon sources. The strain responds to water stress by changing the expression pattern of genes related to desiccation, formation of an extracellular amorphous matrix and mobilization of the carbon sources according to the degree of water stress. Trehalose, glycogen and PHB may have multiple functions in different degrees of desiccation. The robust endowment of molecular responses to desiccation shown in Sphingobium sp. 22B could explain its survival in semi-arid soil. Understanding the physiology implicated in the toleration of the PAH-degrading strain Sphingobium sp 22B to environmental desiccation may improve the bioaugmentation technologies in semi-arid hydrocarbon-contaminated soils.