Root Colonization and Plant Growth Promotion of the Coastal Dune Legume Retama monosperma (L.) Boiss. by Pseudomonas sp. PK31

Abstract

Non-rhizobial endophytes constitute a diverse and significant component of the microbial communities associated with legume nodules. However, their colonization patterns within native host tissues and their contribution to plant growth promotion remain poorly understood. This study investigated the plant growth-promoting potential and root colonization behaviour of Pseudomonas sp. PK31, a non-rhizobial endophyte isolated from surface-sterilized root nodules of Retama monosperma collected from coastal dunes in northwestern Algeria. Strain PK31 was identified through 16S rRNA gene sequencing and phylogenetic analysis. R. monosperma seedlings were grown under nitrogen-free hydroponic conditions with or without PK31 inoculation. Root colonization was examined 10 days post-inoculation using bright-field microscopy and histological analysis. Plant growth parameters were assessed 30 days after inoculation. PK31 significantly increased shoot length (+36.9%, adjusted p = 0.022) and shoot fresh mass (+46.7%, adjusted p = 0.014). Histological observations revealed preferential bacterial colonization of the elongation and root-hair zones, with bacterial cells present in the epidermis and outer cortex, forming discrete intra- and intercellular pockets accompanied by localized cortical cell divisions. This study provides the first description of the epi–endophytic colonization behaviour of Pseudomonas sp. PK31 in its native host, demonstrating that root colonization is associated with enhanced seedling growth under nitrogen-free conditions. These findings suggest that PK31 is a promising candidate for further evaluation as a bioinoculant for low-input agriculture and ecosystem restoration in semi-arid environments.