Drought alters root microbiota due to iron and immunity issues

Drought favors the accumulation of Streptomyces in plant roots, but this increase does not, in itself, mean a benefit to the host. Studies point to another mechanism. Water deficiency reduces responses related to plant immunity and iron absorption. This suppression creates space for the proliferation of these bacteria inside the roots.

Scientists tested Arabidopsis thaliana on soils from 18 locations across the United States. The soils exhibited physical, chemical, and climatic differences. After two weeks in stress-free conditions, the plants spent six weeks under drought or irrigation regimes. Drought reduced biomass by an average of 4,3 times, with variations ranging from two to 10,5 times between soil types.

Drought and change

The team analyzed the microbiota of the soil, leaves, and different root fractions. Drought did not promote a uniform change throughout the bacterial community. The effect appeared more consistently in regions associated with the roots. The phylum Actinomycetota increased in the rhizoplane and in the root endophytic compartment. Within this group, Streptomyces showed the strongest and most frequent enrichment.

The result reinforces previous observations about an increase in Streptomyces in roots under drought. But the study questions the simple interpretation of the plant as a "cry for help." According to the data, the plant does not directly select these bacteria to alleviate stress. Drought reduces physiological and immune barriers. With less defense activity and lower iron absorption, the roots become a more permissive niche.

The expression of genes linked to salicylic acid and iron homeostasis decreased during drought. Salicylic acid participates in defense responses. Genes linked to iron, on the other hand, are involved in the acquisition and transport of this nutrient. The suppression of these two sets of genes accompanied the increase in Streptomyces in the roots.

Route manipulation

The researchers also manipulated these pathways. Foliar application of benzothiadiazole, an analog of salicylic acid, reduced the abundance of Streptomyces in the roots of irrigated and drought-treated plants. Increasing the soil pH through the addition of lime increased the enrichment of Streptomyces in the roots. The same occurred with the application of ferrozine, an iron chelator. These results indicate a link between iron availability, plant response, and bacterial colonization.

The suppression of iron absorption during drought did not appear only inArabidopsis thalianaScientists observed a similar effect in Solanum lycopersicum and Oryza sativa in addition to a reduction in iron content in the aerial parts ofTriticum aestivum, zea mays, Brassica and Pisum sativum cultivated in soil under drought conditions. The study points to the conservation of this mechanism over approximately 160 million years of divergence between monocotyledons and eudicotyledons.

Plant performance

Some lineages of Streptomyces They improved plant performance under conditions similar to drought and low iron availability. The CHAS_16 lineage increased biomass under iron-limited conditions and maintained chlorophyll production under iron limitation. It also maintained the iron content in the aerial part of Arabidopsis thaliana and restored chlorophyll production in Solanum lycopersicum in most of the iron concentrations tested.

This benefit depended on a live bacterium and a functional system of reductive iron import in the plant, involving IRT1 and FRO2. Activation of immunity reduced or eliminated the benefit. Therefore, the same condition of reduced plant defense associated with bacterial enrichment also influenced the ability of some strains to promote positive responses.

The central point of the study involves the dissociation between abundance and function. Lines enriched during drought did not necessarily show the capacity to benefit the plant. Consortia and isolates of Streptomyces They exhibited wide variation in biomass and chlorophyll recovery. Enrichment status, soil origin, and sequence variant identity did not allow for prediction of the effect on the plant.

Competition in the genre

Competition within the genus itself helped explain these results. In assays with synthetic communities, the BOIS_53 strain eliminated the ability of other strains to recover chlorophyll. The effect occurred due to secreted substances with inhibitory action against others. StreptomycesThus, interactions between lineages defined part of the final composition of the community and its functional effects.

The researchers propose a two-step model. First, drought reduces plant pathways linked to immunity and iron, which favors the enrichment of StreptomycesThen, competition between lineages of the same genus adjusts the composition of the community and determines effects on growth and nutrition.

The conclusion has implications for the agricultural use of microorganisms associated with drought tolerance. The presence or increase of Streptomyces The presence of microbial cells in roots alone is not sufficient as an indicator of benefit. The selection of inoculants or microbial communities must consider the behavior of specific strains, their competitive interactions, and the plant's physiological response.

Further information at doi.org/10.1016/j.cell.2026.04.027