Mediterranean Tree Root Exudates Rise During Drought and Associate With Specific Bacteria

Root exudates from mature trees in a mixed Mediterranean forest increased by an average of 3.3-fold during the dry season compared to the wet season, reaching exudation rates of up to 41.7 micrograms of carbon per day per square centimeter. Seasonal drought also shifted chemical metabolite profiles across species while preserving the overall taxonomic stability of the root bacterial community.

The research addressed a key pathway influencing forest tree health: the interaction between root exudates and rhizosphere bacteria, which has remained poorly understood at the chemical level in forest soil. To examine how root dynamics and bacterial communities interact under seasonal drought, researchers conducted a two-year field experiment studying mature trees of Pinus halepensis, Quercus calliprinos, and Pistacia lentiscus.

Throughout the two-year investigation, the researchers tracked root exudate rates and metabolite composition across four seasons. In parallel, they conducted 16S rRNA sequencing down to the species level to characterize bacterial communities within both the soil and root environments.

Chemical analyses identified 89 metabolites responsive to seasonal drought, largely consisting of 24 amino acids, 21 phenolics, 11 carbohydrates, and 9 terpenoids. Although metabolite composition varied according to tree species and season, eight specific metabolites responded to drought consistently across all three tree species.

In contrast to the seasonal chemical shifts observed in root exudates, root bacterial communities changed with host tree species rather than with season. However, multiple bacterial taxa correlated either positively or negatively with the drought-responsive metabolites, demonstrating varied microbial responses to drought-induced chemical changes.

Among these correlations, 19 actinobacteria species were associated with specific drought-responsive compounds, including the terpenoid glaucocalyxin A, the carbohydrate deoxyribose, and a C5 sugar alcohol. The findings suggest that drought-induced shifts in exudate chemistry act as metabolic cues associated with rhizosphere bacterial shifts while maintaining broad taxonomic community stability.

The two-year field experiment was established in a mixed Mediterranean forest to investigate how belowground interactions between tree root exudates and rhizosphere bacteria operate at the chemical level under seasonal drought. In forest ecosystems, the exchange between root exudation and rhizosphere bacteria represents a key pathway influencing tree health, yet the chemical dynamics governing this interaction in forest soil have remained unclear. To capture these interactions under natural field conditions, the investigation focused on mature trees representing three co-occurring species: Pinus halepensis, Quercus calliprinos, and Pistacia lentiscus. Over the course of the two-year study period, the research team analyzed root exudate rates and metabolite composition simultaneously with 16S rRNA sequencing down to the species level across four seasons, examining bacterial communities in both the root systems and the surrounding soil.

Measurements revealed that seasonal drought induced substantial quantitative and qualitative shifts in tree root exudation across the study site. On average, root exudate rates increased 3.3-fold during the dry season compared to the wet season across Pinus halepensis, Quercus calliprinos, and Pistacia lentiscus, with exudation rates reaching up to 41.7 micrograms of carbon per day per square centimeter. In parallel with this quantitative increase, chemical analysis identified 89 metabolites that were responsive to seasonal drought. These drought-responsive compounds were largely distributed among four major chemical classes: 24 amino acids, 21 phenolics, 11 carbohydrates, and 9 terpenoids. Although the overall metabolite composition varied across the different tree species and across seasons, a specific group of eight metabolites was drought-responsive across all three tree species.

While root exudate rates and metabolite profiles varied markedly across seasons, the root bacterial communities exhibited a contrasting ecological pattern. The 16S rRNA sequencing revealed that root bacterial communities changed with the host tree species rather than with the season, maintaining overall taxonomic stability throughout the seasonal transitions. However, statistical analyses showed that multiple bacterial taxa correlated either positively or negatively with specific drought-responsive metabolites. In particular, 19 actinobacteria species were associated with individual exudate compounds, including the terpenoid glaucocalyxin A, the carbohydrate deoxyribose, and a C5 sugar alcohol, illustrating diverse bacterial responses to shifts in root exudation triggered by seasonal drought.

These findings indicate that in a Mediterranean forest subject to recurrent seasonal drought, drought-induced changes in root exudate composition can serve as metabolic cues associated with changes in the rhizosphere bacterial community while preserving the overarching taxonomic stability of the community. By conducting simultaneous chemical metabolite profiling and species-level 16S rRNA sequencing of both root and soil bacterial communities across four seasons, the study demonstrates that seasonal drought alters the chemical output of mature tree roots in species-specific and shared patterns, linking specific root exudate metabolites to distinct bacterial taxa in forest soil.

Subject of Research: Tree root exudate and rhizosphere bacterial community dynamics under seasonal drought

Article Title: Pine root exudate terpenoids associate with Mycobacterium sp. under summer drought in a mixed Mediterranean forest

Article References: Obersteiner, S., Oppenheimer-Shaanan, Y., Yalin, D., Sade, D., Zavaro, V., Reich, Z., & Klein, T. (2026). Pine root exudate terpenoids associate with Mycobacterium sp. under summer drought in a mixed Mediterranean forest. Environmental Microbiome. https://doi.org/10.1186/s40793-026-00921-8

Image Credits: AI Generated

DOI: 10.1186/s40793-026-00921-8

Keywords: root exudates, rhizosphere bacteria, seasonal drought, Mediterranean forest, Pinus halepensis, Quercus calliprinos, Pistacia lentiscus, 16S rRNA sequencing, terpenoids, glaucocalyxin A, deoxyribose, actinobacteria, amino acids, phenolics, carbohydrates, metabolite composition, sugar alcohol