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

Abstract Background Interactions between tree root exudates and rhizosphere bacteria are a key pathway influencing tree health in the forest. However, how this interaction occurs at the chemical level belowground in forest soil still remains unclear. We conducted a 2-year field experiment in a mixed Mediterranean forest with mature trees of Pinus halepensis , Quercus calliprinos and Pistacia lentiscus to examine the interplay between tree roots and bacterial community dynamics under seasonal drought . Root exudate rate and metabolite composition were analyzed simultaneously with 16S rRNA sequencing to the species level of soil and root bacterial communities across four seasons. Results Root exudates increased, on average, 3.3-fold in the dry vs. wet season across the three species, reaching up to 41.7 μg C day −1 cm −2 . Eighty-nine metabolites were responsive to seasonal drought, consisting largely of amino acids (24), phenolics (21), carbohydrates (11) and terpenoids (9). Across all tree species, eight metabolites were drought-responsive. Metabolite composition varied with tree species and season. However, root bacterial communities changed with host tree species but not with season. Multiple bacterial taxa correlated either positively or negatively with drought-responsive metabolites; for example, 19 actinobacteria species were associated with compounds such as the terpenoid glaucocalyxin A and the carbohydrates deoxyribose and a C5 sugar alcohol, illustrating diverse bacterial responses to seasonal drought-induced exudation shifts. Conclusion Our results suggest that in a Mediterranean forest experiencing recurrent seasonal drought, drought-induced shifts in root exudate composition may act as metabolic cues that are associated with changes in the rhizosphere bacterial community, while preserving its overall taxonomic stability.