Promotion of carob (Ceratonia siliqua L.) seedling growth by Streptomyces spp. and Pseudomonas fluorescens in an alkaline and infertile soil
Description of the subject. The sustainable rehabilitation of degraded soils is a major challenge. This study aims to evaluate the potential of indigenous plant-bacteria symbiotic associations for the phytoremediation of a degraded site. Objectives. To isolate and select Plant Growth-Promoting (PGP) bacterial strains adapted to local conditions to enhance the establishment of carob trees (Ceratonia siliqua), a species of significant socio-economic interest. Method. Sixty-four isolates consisting of actinobacteria or Pseudomonas fluorescens were obtained from the rhizosphere of pioneer legumes (Lotus creticus, Retama monosperma) from the site. A phenotypic screening assessed their PGP traits (indole-3-acetic acid [IAA] production, phosphate solubilization, fungal antagonism). The most promising isolates were used to inoculate carob seedlings grown for two months in a nursery on a sterilized substrate composed of the site's native alkaline and infertile soil. Growth parameters were measured and analyzed by Principal Component Analysis (PCA). Results. The screening identified isolates with complementary profiles: RP5 (high IAA production: 36.09 µg·mL-1), RP1 (phosphate solubilization: halo of 11 mm), and CAARL30 (antagonistic activity against Alternaria alternata, Aspergillus sp., Cladosporium sp., and Fusarium sp.). Isolate CAARL35 (Streptomyces sp.) showed a balanced profile of both IAA production and phosphate solubilization. Nursery trials confirmed the effectiveness of the inoculations. PCA identified the most effective treatments: isolate CAARL35, isolates RP1, RP2, RP5 (P. fluorescens), and the synergistic combination GAL4+RP1. Inoculation with CAARL35 induced the most significant improvements, increasing dry biomass by up to 41%, root length by 35%, and the number of leaflets by 77% compared to the non-inoculated control. Conclusions. This study demonstrates the potential of indigenous rhizobacterial strains, particularly Streptomyces sp. CAARL35, as an effective bioinoculant for enhancing carob tree growth under stressful soil conditions. The selection of high-performing plant-bacteria partnerships, adapted to local edaphic constraints, provides a promising and eco-friendly strategy for the rehabilitation of degraded ecosystems.