Saline-alkaline mediated chemotaxis in the plant growth-promoting rhizobacterium <i>Enterobacter cloacae</i>

The biomass, pH changes, and chemotaxis of Enterobacter cloacae (E. cloacae) DJ strain were assessed under various conditions using liquid culture and semi-solid agar plate. Concurrently, GABA concentration, GAD activity, and chemotactic gene expression were measured. The results demonstrated that DJ strain exhibited adaptability to saline-alkaline environments. After 4 h of culture, the pH value decreased, with more pronounced pH changes observed in the saline-alkaline groups. Semi-solid agar plate assays revealed that the DJ strain exhibited the strongest chemotaxis toward the saline-alkaline environment. The average migration radius of the DJ strain reached 1.64 ± 0.09 cm in the saline-alkaline environment after a 24-h cultivation, significantly exceeding the control group's value of 0.88 ± 0.097 cm. The DJ strain exhibited strong positive taxis toward the saline-alkaline environment. Na+ concentration was identified as the primary factor influencing the chemotactic behavior of DJ strain. The GABA content in the saline-alkali group and salt group was 13±0.38 µmol/l and 10.5±1.12 µmol/l, respectively. GAD enzyme activity peaked after 4 h of cultivation, then decreased progressively. qPCR results indicated that the expression of tsr and che-Y genes was up-regulated under saline-alkaline conditions. We propose a model whereby environmental Na+ activates GAD enzyme activity in the DJ strain, leading to increased GABA production that alters the bacterial microenvironment. In response, the DJ strain up-regulates chemotaxis-related gene expression, thereby modifying its behavior to adapt to the saline-alkaline environment.