Root colonisation of Pseudomonas aeruginosa strain UPMP3 and induction of defence-related genes in oil palm (Elaeis guineensis)

Pseudomonas aeruginosa strain UPMP3 labelled with beta-glucuronidase (gusA) and green fluorescent protein (gfp) by electrotransformation yielded ca 1 x 107 transformants mu g-1 DNA. The data obtained from the dilution plate count showed that over 28 days both epiphytic and endophytic populations of P. aeruginosa strain UPMP3 increased from 5.76 log10 [colony forming unit (CFU) + 1] g-1 fresh weight (FW) to 8.19 log10 (CFU + 1) g-1 FW and 4.10 log10 (CFU + 1) g-1 FW to 6.23 (CFU + 1) g-1 FW, respectively. Confocal laser scanning microscopic analysis of oil palm roots treated with gusA:gfp-tagged P. aeruginosa strain UPMP3 showed intense root colonisation over the sampling period. The root surface colonisation by P. aeruginosa strain UPMP3 was followed by a second stage, characterised by cortical infection, and a third stage, which involves xylem ingression. The colonisation of oil palm roots by the gusA:gfp-tagged strain was concentrated on root areas potentially rich in nutrients such as the elongation zones, ridges between epidermal cells and points of secondary adventitious root emergence. Different expression levels of defence-related genes, namely, chitinase and beta-1,3-glucanase in the strain UPMP3host interaction recorded over 28 days, suggested the potential role of P. aeruginosa strain UPMP3 in triggering the defence mechanism in oil palm. This is the first report on root colonisation and upregulation of defence-related genes on oil palm roots by P. aeruginosa strain UPMP3 and shows the potential of this strain to be used as a biocontrol agent in oil palm.