Quantification of endophyte Serendipita indica in Brassica napus roots by qPCR

Context The fungal endophyte Serendipita indica enhances plant growth and plant resistance to biotic and abiotic stresses. Inoculum concentration greatly impacts the endophyte–plant interaction from mutualism to antagonism. Aims and methods We used both microscopy and qPCR to examine the effect of inoculum concentrations on the extent (%) and density of Brassica napus L. root colonisation by S. indica. B. napus seeds were inoculated with the fungus at five different inoculum concentrations (1–10% w/w basis). Key results Standard curves were constructed using the mean threshold cycle (Ct) and serially diluted gDNA ranging between 4.14×102 and 2.65×105 colony forming units (CFU). The result indicated a linear relationship between Ct and the log of input DNA. Variation in inoculum concentration significantly affected the root colonisation density by the fungus shown by qPCR. However, the percent root colonisation (PRC) measure was not affected and remained the same across all the treatments. Conclusions Our findings show that the qPCR assay developed will determine the colonisation density whereas PRC gives a measure of the incidence of infected roots. Also, we suggest that the optimum quantity of inoculum is a key factor for a successful interaction that impacts the plant–S. indica interaction. Implications To our knowledge, this is the first study that quantitative qPCR has been used to investigate the correlation between inoculum quantities and the corresponding density of root colonisation in S. indica.