Enzyme activities and growth promotion of spinach by indole-3-acetic acid-producing rhizobacteria

The objective of this study was to evaluate the effects of twelve plant growth-promoting rhizobacteria (PGPR; Bacillus mycoides FD07, B. sphaericus RC12, B. pumilus RC19, B. cereus RC18, Variovorax paradoxes RC21, Paenibacillus polymyxa RC35, Pseudomonas putida RC06, B. megaterium RC07, B. megaterium M-3, B. licheniformis RC08, B. subtilis RC11, and B. subtilis OSU-142) used as biofertilisers, on various enzyme activities [glucose-6-phosphate dehydrogenase (G6PD); 6-phosphogluconate dehydrogenase (6PGD); glutathione reductase (GR); and glutathione S-transferase (GST)] and on seedling growth in spinach (Spinacia oleracea L.). Enhanced plant growth could result from rhizobacterial production of indole-3-acetic acid (IAA). The highest IAA-producing rhizobacteria (RC35 and RC06) produced the highest root and shoot weights. PGPR improved N and P nutrition in spinach, and therefore stimulated plant growth and key enzyme activities. The responses to inoculation, compared to uninoculated control plants, were: -1.9% to +36.4% for shoot fresh weights (FWs), -5.5% to +30.1% for root FWs, -3.5% to +29.8% for shoot dry weights (DWs), -3.8% to +38.5% for root DWs, and -5.9% to +30.1% for leaf areas. Plant growth responses were variable and dependent on the inoculant strain used, as well as on the enzyme activity and growth parameter being evaluated. Close correlations between plant shoot growth, PGPR inoculation, and G6PD (r = 0.28*), 6PGD (r = 0.55**), GR (r = 0.73**), and GST (r = 0.64**) enzyme activities in spinach have been demonstrated.