Dynamic and distinct physiological responses by a soil bacterium promote survival along a desiccation continuum.

Abstract

Soil bacteria play a central role in global biogeochemical cycles and are critical for soil health and agricultural productivity. The dynamic nature of soil hydration status affects bacterial habitats by changing the energy state of soil water and disrupting aqueous connections critical for nutrient diffusion. To study how soil bacteria respond to desiccation, we used the rhizobacterium Pseudomonas synxantha 2-79 as a model organism and quantified its response to co-occurring water and nutrient limitations at the single-cell level. We hypothesized that the relative importance of osmolyte synthesis and starvation responses to desiccation tolerance is context dependent, with the optimal strategy determined by the trajectory of nutrient and water deprivation. We constructed a transcriptional reporter to track P. synxantha's expression of biosynthesis genes for the osmolyte N-acetylglutaminylglutamine amide (NAGGN) and collected extensive single-cell growth rate, cell size, and reporter expression data through experiments that mimicked different rates and extents of soil drying. Only actively growing cells responded to an osmotic shock by synthesizing NAGGN; this response was not observed for pre-starved bacteria. Despite the lack of osmolyte NAGGN synthesis, prior starvation enhanced P. synxantha's ability to recover from osmotic stress once water and nutrients were restored. In line with our observation that prior starvation prevented cell lysis upon rewetting, starved cells had more rigid membranes. Together, our results indicate that diverse cellular properties contribute to soil bacterial desiccation tolerance, whose relative response and fitness are tuned to different challenges imposed by soil drying dynamics.IMPORTANCESoil bacteria are critical to agriculture, but it is unclear how these organisms respond to desiccation, a common and worsening stress. Desiccation both dehydrates bacterial cells and eliminates the liquid water connections between soil pores that bacteria use to access nutrients. We studied how a model soil bacterium responds to (co)-occurring starvation and water stress at the single-cell level, focusing on osmolyte synthesis and physiological adjustments that take place under starvation. We describe the desiccation and regrowth trajectories in these conditions at single-cell resolution. We find that starvation restricts synthesis of a dipeptide osmolyte but rigidifies the membrane, enabling bacteria to withstand more severe water stress. Distinct cellular factors thus contribute differentially to desiccation tolerance along a drying trajectory.