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The plant growth-promoting bacteria Herbaspirillum seropedicae is an endophytic diazotroph found in several economically important crops. Reliable, high-yield, and cost-effective methods are needed to determine bacterial viability in inoculant formulations or in plant. The present study aims to develop a PMA-qPCR assay to evaluate viable cells of H. seropedicae in pure culture and maize roots grown in vitro. H. seropedicae grown in a culture medium was submitted to heat treatment at 48°C for different periods of time. Maize roots were inoculated, grown in vitro and collected seven days after inoculation. The bacteria viable cells were quantified using qPCR, PMA-qPCR assays, and plate counting. Standard curves were prepared, and the efficiency obtained ranged from 85 to 99%. The limit of detection (LOD) was 101 genome copies, corresponding to 60.3 pg of DNA. Enumeration obtained in pure cultures by qPCR, PMA-qPCR and plate count were 8.85 ± 0.16, 6.51 ± 0.12 and 2.25 ± 0.30 log CFU.mL− 1 after heat treatment, respectively. These results showed that PMA-qPCR is a powerful approach for quantifying viable and viable but non-culturable cells in inoculants and plants. PMA-qPCR allowed reliable obtained results much faster than culture-dependent methods.

Herbaspirillum seropedicae is a plant growth-promoting bacteria isolated from diverse plant species. In this work, the main objective was to investigate the efficiency of H. seropedicae strain SmR1 in colonizing and increasing maize growth in the early stages of development under greenhouse conditions. Inoculation with H. seropedicae resulted in 10.51 and 19.43% in mean of increase of root biomass concerning non-inoculated controls, mainly in the initial stages of plant development, at 21 days after emergence (DAE). Quantification of H. seropedicae in roots and leaves was performed by quantitative PCR.. H. seropedicae was detected only in maize inoculated roots by qPCR, and a slight decrease in DNA copy number g−1 of fresh root weight was observed from 7 to 21 DAE, suggesting that there was initial effective colonization on maize plants. H. seropedicae strain SmR1 efficiently increased maize root biomass exhibiting its potencial to be used as inoculant in agricultures systems.