The influence of plant growth–promoting rhizobacteria on growth and enzyme activities in wheat and spinach plants Ramazan Çakmakçı 1,2 *, Mustafa Erat 1 , Ümmügülsüm Erdog ˘ an 2 , and Mesude Figen Dönmez 3 1 Atatürk University, Biotechnology Application and Research Center, 25240, Erzurum, Turkey 2 Atatürk University, Technical Vocational School Ispir, 25900, Erzurum, Turkey 3 Atatürk University, Faculty of Agriculture, Department of Plant Protection, 25240, Erzurum, Turkey Accepted February 7, 2007 Summary A pot experiment in a greenhouse was conducted in order to investigate the effect of different N 2 -fixing, phytohormone- producing, and P-solubilizing bacterial species on wheat and spinach growth and enzyme activities. Growth parameters and the activities of four enzymes, glucose-6-phosphate dehydrogenase (G6PD; EC 1.1.1.49), 6-phosphogluconate dehydrogenase (6PGD; EC 1.1.1.44), glutathione reductase (GR; EC 1.8.1.7), and glutathione S-transferase (GST; EC 2.5.1.18) were determined in the leaves of wheat (Triticum aestivum L., Konya) and spinach (Spinacia oleracea L.), non- inoculated and inoculated with nine plant growth–promoting rhizobacteria (PGPR: Bacillus cereus RC18, Bacillus licheni- formis RC08, Bacillus megaterium RC07, Bacillus subtilis RC11, Bacillus OSU-142, Bacillus M-13, Pseudomonas putida RC06, Paenibacillus polymyxa RC05 and RC14). Among the strains used in the present study, six PGPR exhib- ited nitrogenase activity and four were efficient in phosphate solubilization; all bacterial strains were efficient in indole acetic acid (IAA) production and significantly increased growth of wheat and spinach. Inoculation with PGPR in- creased wheat shoot fresh weight by 16.2%–53.8% and spi- nach shoot fresh weight by 2.2%–53.4% over control. PGPR inoculation gave leaf area increases by 6.0%–47.0% in wheat and 5.3%–49.3% in spinach. Inoculation increased plant height by 2.2%–24.6% and 1.9%–36.8% in wheat and spinach, respectively. A close relationship between plant growth and enzyme activities such as G6PD, 6PGD, GR, and GST was demonstrated. Plant-growth response was variable and dependent on the inoculant strain, enzyme activity, plant species, and growth parameter evaluated. In particular, the N 2 -fixing bacterial strains RC05, RC06, RC14, and OSU-142 and the P-solubilizing strains RC07 and RC08 have great potential in being formulated and used as biofertilizers. Key words: plant growth–promoting bacteria (PGPR) / nitrogen fixation / enzyme activity / wheat / spinach / Bacillus ssp. / Paenibacillus / Pseudomonas  2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1436-8730/07/0204-288 288 DOI: 10.1002/jpln.200625105 J. Plant Nutr. Soil Sci. 2007, 170, 288–295 1 Introduction Improvement or modification of plant growth and develop- ment can be achieved by the direct application of nutrients or plant growth–promoting rhizobacteria (PGPR) to seeds. Large quantities of chemical fertilizes are used to replenish soil N and P, resulting in high cost and environmental contam- ination. Increasing and extending the role of biofertilizers may reduce the need for chemical fertilizers and decrease adverse environmental effects. Apart from fixing N 2 , diazo- trophs can influence plant growth directly by the synthesis of phytohormones and vitamins, inhibition of plant ethylene synthesis, improvement of nutrient uptake, enhanced stress resistance, solubilization of inorganic phosphate, and minera- lization of organic phosphate (Dobbelaere et al., 2003). Trials with Bacillus species indicated yield increases in rice (Khan et al., 2003), wheat (de Freitas, 2000), sugar beet and barley (Çakmakçı et al., 2001), and maize (Pal, 1998). N 2 -fix- ing Paenibacillus species have increasingly been used in nonlegume crop species such as sugar beet, canola, wheat, and conifer species (de Freitas et al., 1997; Çakmakçı et al., 1999; Bent et al., 2001). Inoculation with Pseudomonas has shown significantly increased root dry weight and harvest index of wheat (Walley and Germida, 1997), yield of sugar beet (S ¸ ahin et al., 2004), and growth of spinach (Urashima and Hori, 2003). Some of the bacterial strains such as Bacil- lus, Paenibacillus, and Pseudomonas were able to produce IAA, cytokinins, and gibberellins (Timmusk et al., 1999; Gutierrez-Mañero et al., 2001). Nitrogen assimilation in plants consists of three processes. Initially NO  3 is reduced to NO  2 by nitrate reductase. This is followed by the subsequent reduction of NO  2 to NH  4 by the nitrite reductase. Finally, the resulting NH  4 is assimilated into amino acids by glutamine synthetase/glutamate synthase (Sivasankar and Oaks, 1996). In the oxidative pentose phos- phate pathway (OPPP), glucose-6-phosphate dehydrogen- ase (G6PD) and 6-phosphogluconate dehydrogenase (6PGD) reduce NADP + to NADPH. The pathway also pro- duces the critical precursor for the synthesis of phenolic sec- ondary metabolites, which are important for plant growth and lignifications (McCue et al., 2000). Activities of G6PD and 6PGD provide reducing power for nitrogen assimilation. The capacity of the OPPP is increased in plant tissues during nitrate assimilation, which requires reducing power for nitrate and nitrite reduction (Bowsher et al., 1989). G6PD is able to catalyze the reaction under high NADPH : NADP + ratios, which have been shown during nitrogen assimilation (Wright et al., 1997). Glutathione reductase (GR) is a flavoprotein that catalyzes the NADPH-dependent reduction of the oxidized form of glu- * Correspondence: Dr. R. Çakmakçı; e-mail: rcakmak@atauni.edu.tr