Role of plant growth promoting Bacteria (PGPRs) as biocontrol agents of Meloidogyne incognita through improved plant defense of Lycopersicon esculentum

Background and aims Root-knot nematodes are major constraints among different pathogens with wide host range and cause severe agricultural loss worldwide. The present study was designed to understand the role of plant growth promoting bacteria ( Pseudomonas aeruginosa & Burkholderia gladioli ) on growth and antioxidative potential in nematode infected Lycopersicon esculentum seedlings. Methods An experiment was conducted to assess the levels of superoxide anions, H 2 O 2 and MDA contents generated during nematode infection. Moreover, the contribution of antioxidative enzymes, non-enzymatic antioxidants, total antioxidants and gene expression profiling was also carried out in nematode infected Lycopersicon esculentum seedlings. Results The results of present study revealed that nematode infection reduced the growth of seedlings which upon inoculation of microbes was improved. Moreover, number of galls were reduced upon supplementation of these strains. Nematode infection also caused accumulation of superoxide anion, H 2 O 2 , and malondialdehyde contents along with nuclear damage and loss of cell viability which was reduced upon supplementation of microbes. The oxidative burst generated enhanced various antioxidant enzymes such as SOD (30.6%), POD (3.6%), CAT (18.1%), GPOX (65.9%), APOX (24.8%), GST (5.6%), DHAR (13.9%), GR (11%) and PPO (2.5%) which were further elevated upon application of P. aeruginosa (23.9%, 7.2%, 7%, 66%, 28.9%, 71.3%, 14.5%, 10.6% and 38.3%) and B. gladioli ( 5.1%, 30.6%, 16.2%, 92.1%, 78.5%, 97.5%, 15.5%, 65.7% and 23.2%). The non-enzymatic antioxidants (glutathione, ascorbic acid and tocopherol) and total antioxidants contents (both water soluble and lipid soluble) were also enhanced upon inoculation of microbes. Confocal microscopy revealed the improvement in nuclear damage and cell viability in microbe inoculated roots. Gene expression profiling revealed the enhanced expression levels of SOD, POD, CAT, GR, GPOX, APOX , PPO genes in P.aeruginosa inoculated nematode infected seedlings by 53%, 2.7%, 64.1%, 10.4%,19.7%, 29.2%, 38.4% and B. gladioli inoculated seedlings by 18.3%,144%, 67%, 43%, 308%, 151% respectively. Conclusions The results therefore suggest the favourable aspects of micro-organisms in modulating growth characteristics and antioxidative defense expression of Lycopersicon esculentum to encounter oxidative stress generated under nematode infection.