Impact of organic and conventional cropping systems on plant growth-promoting rhizobacteria (PGPR)

Maintaining soil health and productivity requires an understanding of soil biodiversity and the interactions within the soil microbiome and rhizosphere. This study focuses on Plant Growth-Promoting Rhizobacteria (PGPR) associated with horticultural crops (Solanaceae and Cucurbitaceae), grown under both conventional and organic no-till systems (within the MOnsampolo VEgetables Long Term Experiment - MOVE-LTE - established in 2001 by CREA-OF). We examined how these farming systems affect the PGPR population. Moreover, we investigated the ability of PGPR to induce indole-3-acetic acid (IAA) and siderophores production, phosphate solubilization, and antagonistic activity against Fusarium oxysporum and Alternaria alternata. A total of 147 bacterial isolates were tested; 28 identified strains with plant growth-promoting (PGP) traits were identified by sequencing the 16S rRNA gene, with their genomes matched to those in the NCBI database. Bacillus megaterium showed high siderophore units (90.55), P-solubilization Index (3.58), and antagonism activities. B. altitudinis and B. halotolerans were notable for high IAA production (46.23 and 40.7 ppm, respectively) while Paenarthrobacter nitroguajacolicus and Variovorax paradoxus exhibited high P-solubilization Index (3.97 and 4.39, respectively). Moreover, the distribution of strains with high relative abundance (>10%) among treatments reveal key characteristics of the PGPR community of the cropping systems: microbial community is significantly influenced by crop (tomato vs zucchini and pumpkin), with organic no-till management enhancing strain diversity more than abundance.

These results advance PGPR knowledge and indicate specific strengths and weaknesses around which consortia could be strategically developed to test performance under real conditions and ultimately improve crop yield and health across all agricultural systems.