Monoxenic Root Organ Culture Enables High-Yield Production of Viable Indigenous Rhizophagus irregularis Inoculum for Arid Oasis Agroecosystems

Arbuscular mycorrhizal fungi (AMF) play a pivotal role in plant adaptation to arid ecosystems, yet their widespread agricultural use is constrained by the scarcity of high-quality, locally adapted inoculum. This study established a reliable monoxenic culture system for mass-producing an indigenous AMF isolate from the date palm (Phoenix dactylifera L.) rhizosphere in the Figuig oasis, southeastern Morocco. The isolate was identified as Rhizophagus irregularis based on spore morphology and Large Subunit ribosomal DNA (LSU rDNA) phylogeny. Two propagule types, surface-sterilized spores and mycorrhizal root fragments of Plantago lanceolata L., were compared for initiation of in vitro cultures on Ri T-DNA-transformed carrot (Daucus carota L.) hairy roots. By week 16, cultures initiated from mycorrhizal root fragments produced 1414 ± 65 spores per plate and showed significantly higher performance than spore-derived cultures in terms of propagule viability, root colonization, and hairy root growth. Propagule viability reached 84% and 68%, root colonization frequencies were 95% and 72%, and hairy root lengths averaged 81 and 63 cm in root fragment- and spore-derived cultures, respectively (p < 0.01). In a subsequent whole-plant assay using P. lanceolata, in vitro-produced spores induced markedly higher mycorrhizal colonization frequency (91.0 ± 1.6% compared with 74.8 ± 1.9%) and intensity (70.0 ± 1.6% compared with 55.0 ± 1.6%) than spores obtained from conventional trap cultures (p < 0.001). These results demonstrate that monoxenic root-organ culture using root fragments is a robust, reproducible method for generating abundant, contaminant-free, and functionally superior inoculum of native R. irregularis. This advance provides a solid platform for developing tailored bio-inoculants to enhance crop resilience and sustainability in arid and semi-arid agroecosystems.