Strigolactones (SLs) are considered to be a novel class of phytohormone involved in plant defense responses. Currently, their relationships with other plant hormones, such as abscisic acid (ABA), during responses to salinity stress are largely unknown. In this study, the relationship between SL and ABA during the induction of H2O2 – mediated tolerance to salt stress were studied in arbuscular mycorrhizal (AM) Sesbania cannabina seedlings. The SL levels increased after ABA treatments and decreased when ABA biosynthesis was inhibited in AM plants. Additionally, the expression levels of SL-biosynthesis genes in AM plants increased following treatments with exogenous ABA and H2O2. Furthermore, ABA-induced SL production was blocked by a pre-treatment with dimethylthiourea, which scavenges H2O2. In contrast, ABA production was unaffected by dimethylthiourea. Abscisic acid induced only partial and transient increases in the salt tolerance of TIS108 (a SL synthesis inhibitor) treated AM plants, whereas SL induced considerable and prolonged increases in salt tolerance after a pre-treatment with tungstate. These results strongly suggest that ABA is regulating the induction of salt tolerance by SL in AM S. cannabina seedlings.

Salinity is an important but little-studied abiotic stressor affecting plant growth. Although several previous reports have examined salt-tolerant plant microbial communities, we still lack a comprehensive understanding about the functional characteristics and genomic information of this population.

Chemoreceptors provide sensory specificity and sensitivity that enable motile bacteria to seek optimal positions for growth and metabolism in gradients of various physicochemical cues. Despite the abundance of chemoreceptors, little is known regarding the sensory specificity and the exact contribution of individual chemoreceptors to the lifestyle of bacteria. Azospirillum brasilense are motile bacteria that can fix atmospheric nitrogen under microaerophilic conditions. Here, we characterized a chemoreceptor in this organism, named AerC, which functions as a redox sensor that enables the cells to seek microaerophilic conditions that support optimum nitrogen fixation. AerC is a representative of a widespread class of soluble chemoreceptors that monitor changes in the redox status of the electron transport system via the FAD cofactor associated with its PAS domains. In A. brasilense, AerC clusters at the cell poles. Its cellular localization and contribution to the behavioral response correlate with its expression pattern and with changes in the overall cellular FAD content under nitrogen-fixing conditions. AerC-mediated energy taxis in A. brasilense prevails under conditions of nitrogen fixation, illustrating a strategy by which cells optimize chemosensing to signaling cues that directly affect current metabolic activities and thus revealing a mechanism by which chemotaxis is coordinated with dynamic changes in cell physiology.

Sesbania cannabina is a plant that grows naturally along the seashores in Rudong County, China (RDC) and it has been introduced into the Yellow River Delta (YRD) as a pioneer plant to improve the saline-alkaline soils. In order to investigate the diversity of S. cannabina rhizobia in these soils, a total of 198 rhizobial isolates were characterized and phylogenetic trees were constructed based on data from multilocus sequence analysis (MLSA) of the housekeeping genes recA, atpD and glnII, as well as 16S rRNA. Symbiotic features were also studied by establishing the phylogeny of the symbiotic genes nodA and nifH, and by performing nodulation assays. The isolates had highly conserved symbiotic genes and were classified into nine genospecies belonging to the genera Ensifer, Agrobacterium, Neorhizobium and Rhizobium. A unique community structure was detected in the rhizobia associated with S. cannabina in the saline-alkaline soils that was characterized by five novel genospecies and four defined species. In addition, Ensifer sp. I was the predominant rhizobia in YRD, whereas Ensifer meliloti and Neorhizobium huautlense were the dominant species in RDC. Therefore, the study demonstrated for the first time that this plant strongly selected the symbiotic gene background but not the genomic background of its microsymbionts. In addition, biogeographic patterns existed in the rhizobial populations associated with S. cannabina, which were mainly correlated with pH and salinity, as well as the mineral nutrient contents. This study provided novel information concerning the interaction between soil conditions, host plant and rhizobia, in addition to revealing the diversity of S. cannabina rhizobia in saline-alkaline soils.