Aims Low numbers of rhizobia in soil or inoculants delay nodulation and decrease symbiotic legume productivity. This study investigated the effect of coinoculation with a helper bacterium, Pseudomonas fluorescens WSM3457 on the Medicago truncatulaEnsifer (Sinorhizobium ) medicae WSM419 symbiosis challenged by a low inoculum dose. Methods In a glasshouse experiment the effect of coinoculation with WSM3457 on the kinetics of nodule initiation and development was assessed 5, 7, 10, 14, 17, 21, and 42 days after inoculation of M. truncatula cv. Caliph with 10 3 cells/plant of E. medicae WSM419. Results Co-inoculated plants had enhanced rate of nodule initiation and development, greater numbers of larger crown nodules, and by day 42 accumulated more N than plants inoculated with E. medicae WSM419 alone. Nodule development was altered by co-inoculation. Approximately 25% of nodule initials on co-inoculated plants formed in closely associated pairs, young nodules were larger with multiple meristems and developed into cluster-like multilobed nodules compared to those on WSM419 inoculated plants. Molecular typing showed WSM3457 occupied a significant proportion of root nodules on co-inoculated plants. Conclusion Co-inoculation with P. fluorescens WSM3457 enhanced symbiotic effectiveness of M. truncatula when inoculated with a low inoculum dose of E. medicae WSM419.

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