Real-Time PCR Quantification of the Plant Growth Promoting Bacteria Herbaspirillum seropedicae Strain SmR1 in Maize Roots

The plant growth promoting bacteria Herbaspirillum seropedicae SmR1 is an endophytic diazotroph found in several economically important crops. Considering that methods to monitor the plant–bacteria interaction are required, our objective was to develop a real-time PCR method for quantification of PGPB H. seropedicae in the rhizosphere of maize seedlings. Primer pairs were designed, and their specificity was verified using DNA from 12 different bacterial species. Ten standard curves of qPCR assay using HERBAS1 primers and tenfold serial dilutions of H. seropedicae SmR1 DNA were performed, and PCR efficiency of 91 % and correlation coefficient of 0.99 were obtained. H. seropedicae SmR1 limit of detection was 10¹ copies (corresponding to 60.3 fg of bacterial DNA). qPCR assay using HERBAS1 was used to detect and quantify H. seropedicae strain SmR1 in inoculated maize roots, cultivated in vitro and in pots, harvested 1, 4, 7, and 10 days after inoculation. The estimated bacterial DNA copy number per gram of root was in the range 10⁷–10⁹ for plants grown in vitro and it was around 10⁶ for plants grown in pots. Primer pair HERBAS1 was able to quantify H. seropedicae SmR1, and this assay can be useful for monitoring plant–bacteria interaction.