Microbial allies against drought stress: an optimized screening method to improve seedling survival for forest restorations.

Improving drought tolerance of tree seedlings by plant growth-promoting microorganisms (PGPMs) is a promising approach for nature-based forest restoration. Identifying suitable microorganisms requires a robust selection, including efficient in planta screenings. We sampled at two forest sites in southern Germany with drought legacies and within a dry period to enhance the probability of isolating drought-tolerant microbes. Metabarcoding of the resident soil community revealed a broad on-site diversity with the potential for diverse plant growth-promoting and stress-resistance traits. We isolated 1,292 bacteria and 59 fungi from fine roots of Norway spruce and European beech. 429 isolates were identified to the Genus level. The most abundant genera were Paraburkholderia (121) and Bacillus (43) in bacteria and Penicillium (8) and Umbelopsis (8) in fungi. Isolates were scored in vitro for abiotic stress tolerance and plant growth-promoting traits, revealing diverse plant growth-promoting abilities for 31 bacteria and a particularly high stress tolerance for 8 fungi. Importantly, an axenic 24-well plate system was developed to investigate the influence of bacteria on spruce seedlings under drought conditions. The system allowed direct comparison of inoculation effects on seedling growth and survival with or without drought application. Six bacterial strains significantly promoted plant growth under well-watered conditions, while two bacterial strains improved survival and root length under drought. This study represents one of the first larger scale screenings for PGPMs isolated from forest soils on tree seedlings under drought and may contribute to finding nature-based drought mitigation strategies.