Rhizosphere Microbes From <i>Populus euphratica</i> Conferred Salt Stress Resistance to <i>Populus alba</i>  ×  <i>Populus glandulosa</i>

The rhizosphere microbiomes of halophytes are crucial for plant adaptation to high-salinity soil conditions, but how to harness rhizosphere microbes to confer salt stress resistance to plants remains obscure. This study aimed to establish a framework (isolate-select-construct) for tailoring simplified salt-tolerant synthetic microbial communities (SynComs) and explore how they confer salt stress resistance to the plant. First, a total of 512 strains were isolated from the high-salt rhizosphere soil of Populus euphratica through high-throughput cultivation. Among these, nine strains were further selected for their salt-tolerant and growth-promoting abilities, with three isolates identified as key microbes, including hub microbes, keystone taxa and biomarkers. Guided by a function-driven strategy, we constructed five distinct SynComs, with SynCom5, SynCom7 and SynCom9 showing the most significant improvement in the growth of hybrid Poplar 84K (Populus alba × Populus glandulosa). Mechanistic investigations revealed that these SynComs can increase resistance to salt stress by directly reducing oxidative stress, adjusting osmolytes and balancing ions. Additionally, these SynComs were observed to recruit specific root-associated bacterial consortia that enhance the adaptability of poplar to salt stress. Overall, this study lays the groundwork for designing SynComs that promote plant growth and offers insights into harnessing specific microbial communities to boost plants' salt resistance.