The increasing world population, estimated to reach 9 billion by 2050 (ref. 1), and the exacerbating effect of climate change on plants pose substantial challenges to food security. Drought is one of the consequences of climate change that has a devastating effect on humans directly by affecting food security. Plants have developed various adaptation mechanisms to drought stress, including early maturity, modifications in root architecture and synthesis of root exudates, the ‘cry for help’ phenomenon2,3. Several plant exudates have been reported to mediate plant interaction with their microbiome when exposed to various stresses, and the potential role of these stress-induced exudates in helping plants has been described. However, a full understanding of the mechanisms involved in these interactions is yet to be fully elucidated. A recent study by Li et al. published in Cell Host & Microbe reveals that wheat under drought stress fosters the proliferation of certain bacterial species such as Streptomyces coeruleorubidus and Leifsonia shinshuensis via metabolite-driven recruitment, resulting in observable advantages for plant performance.
Two popular varieties of wheat in Australia, Scepter and Illabo, were used. Both varieties have traits that fit various farming strategies and environments. Illabo offers early seeding and extended grazing, whereas Scepter has a typical sowing window and great yield potential. In the study, they were grown in sandy and clay soils while subjected to controlled drought conditions (30% water holding capacity) and compared with well-watered controls (80% water holding capacity). The authors used 16S ribosomal RNA amplicon sequencing, metagenomics and metabolomics to investigate the alterations in microbial communities in bulk soil, the rhizosphere and the root endosphere. A significant increase was observed in S. coeruleorubidus (30-fold increase) and L. shinshuensis (3-fold increase), especially under drought conditions in sandy soils. In their metabolomic profiling results, 4-oxoproline emerged as the most significantly enriched metabolite in drought-stressed rhizospheres, showing a 4.51-fold increase. Network analysis revealed a noteworthy positive correlation between the concentration of 4-oxoproline and the abundance of S. coeruleorubidus. Inoculation experiments demonstrated that S. coeruleorubidus markedly enhanced wheat performance, especially under drought conditions. The presence of L. shinshuensis led to a significant improvement in root biomass. Physiological assessments revealed that the inoculation of S. coeruleorubidus resulted in a 36.57% increase in stomatal density and a 134.03% rise in leaf H2O2 content, as well as a 2.27-fold upregulation of the drought-responsive gene TaCHLD. H2O2 acts as a long-distance signal promoting stomatal closure during drought stress and improving drought tolerance by activating antioxidant defence systems and the ABA pathway, while TaCHLD gene upregulation during stress helps to maintain plants’ stay-green phenotype under drought, which prevents premature senescence and leaf yellowing. The genomic analysis of S. coeruleorubidus revealed genetic elements linked to stress resilience and identified 48 genes linked to the levels of 4-oxoproline. The analysis of metabolites confirmed the production of trehalose, glycine betaine and glutathione by S. coeruleorubidus under conditions of osmotic stress. Soil conditioned with drought-stressed wheat showed increased populations of beneficial bacteria, leading to enhanced performance in subsequent wheat crops. Plants grown in soil conditioned under drought stress showed notable increases in shoot weight (91.05%), root weight (113.24%) and grain weight (49.06%) compared with those cultivated in well-watered soil.
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