Plant growth promoting rhizobacteria mediated induced systemic resistance against bacterial wilt in ginger

Abstract

The pathogen causing bacterial wilt of ginger was isolated and identified as Ralstonia solanacearum biovar III based on its cultural, morphological and biochemical characters coupled with pathogenicity. Rhizosphere microflora of ginger from different locations of Thrissur, Waynad and Palakkad districts were isolated. Altogether, 163 rhizobacterial isolates were selected from these areas and their antagonistic activity against the pathogen assessed. Out of 163 isolates, only 45 showed antagonistic reaction. Further, study of these antagonists based on zone of inhibition resulted in selection of 20 isolates. The effect of these 20 isolates in promoting the growth of ginger was studied in pot culture in comparison with three reference cultures of P. fluorescens and B. subtilis. Result of this experiment revealed that only 11 isolates including the two reference cultures of P. fluorescens had growth promoting effect as evidenced in terms of yield and yield attributing characters of ginger. Factors which impart growth promotion in ginger by these isolates were assessed by estimating the inhibition zone, vigour index, hydrogen cyanide, indole acetic acid, ammonia production and ‘P’ solubilzation and based on that, PGPR index of the isolates was worked out. In addition to that, production of salicylic acid, antibiotics and siderphore by the isolates, the secondary metabolites which are known to play a role in disease suppression were assessed. The isolates varied in their ability to produce salicylic acid. Isolates RB-22 followed by RB-11, RB-144 and RB-66 produced more number of antibiotics which include pyoluteorin, pyrrolnitrin, 2,4DAPG etc. Similarly, isolate RB-22 and RB-11 produced maximum siderophores.

The potential of these 11 rhizobacterial isolates in imparting resistance against the disease was assessed in another pot culture experiment by estimating phenol, proteins and amino acid content of ginger upon challenge inoculation. Here also, the isolates showed a profound effect on growth and yield of ginger especially by those plants bacterized with RB-11. There was no natural incidence of bacterial wilt in plants treated with RB-11 and RB-22. Upon challenge inoculation also, plants bacterized with RB-11 showed the least incidence. In general, rhizobacterial treated plants contained more amount of phenol, protein and amino acids than untreated ones. Upon challenge inoculation with the pathogen, the rate of increase of these compounds in rhizobacteria treated plants was more than that of control during different intervals of observations. A third pot culture experiment was conducted to assess the effect of rhizobacterial treatments on defense related enzymes of ginger upon challenge inoculation. Here, eight most promising ones including the reference cultures were used. In general, the study revealed more activity of peroxidase (PO), polyphenol oxidase (PPO) and phenylalanine ammonia lyase (PAL) in rhizobacterial treated plants that too after challenge inoculation. Native PAGE analysis revealed six isoforms of PO and four isoforms of PPO in a majority of the rhizobacterial treated plants whereas only three were noticed in control. Similarly, difference in the protein profile of rhizobacterial treated plants and control was noticed. Chlorophyll, NPK and oil and oleoresin content varied among treatments where the highest was observed in rhizobacterial treated plants. An attempt was made to elucidate the molecular mechanism of induced systemic resistance (ISR) in ginger by synthesizing cDNA and was subjected to RAPD assay. However, no conclusive evidence on ISR was observed.

The compatibility of eight rhizobacterial isolates including the two reference cultures with antibiotics, fungicides, insecticides and fertilizers were assessed which revealed variation in their sensitivity. Moreover, mutual compatibility of the rhizobacterial isolates and compatibility with Trichoderma spp. were also studied and it was observed that all bacterial isolates were mutually compatible. However, Pseudomonas aeruginosa, P. fluorescens (RB-66) and the reference culture of P .fluorescens (P.f2) were found incompatible with the Trichoderma spp. The promising six rhizobacteria isolates were identified based on cultural, morphological and biochemical characters and also in comparison with that of reference culture of P. fluorescens. They were tentatively identified as Pseudomonas aeruginosa (RB-22), Pseudomonas fluorescens (RB-82, RB-66, RB-11) and the remaining two, RB-144 and RB-77, as non-fluorescent Pseudomonads.