Rhizobial infection of African landraces of sorghum (Sorghum bicolor L.) and finger millet (Eleucine coracana L.) promotes plant growth and alters tissue nutrient concentration under axenic conditions

Seven strains of infective root-nodule bacteria, ("rhizobia"), namely Rhizobium GHR2,Bradyrhizobium japonicum Tal 110, Sinorhizobium meliloti strain 1, Rhizobium leguminosarum bv. viciae Cn6, R. leguminosarum bv. viciae strain 30, Rhizobium NGR234 and Azorhizobium caulinodans ORS571, were used to study the effects on growth of sorghum and finger millet seedlings cultured aseptically in Leonard jars with 1/2 strength Hoagland nutrient solution containing 1 mM KNO3. The use of scanning electron microscopy on 10-d-old plants revealed the presence of all 6 test strains on root epidermal surfaces as well as inside the tissues of inoculated, but not uninoculated, sorghum and millet roots. Applying root macerate prepared from surface-sterilized inoculated sorghum plant material successfully induced nodule formation and N2 fixation in soyabean seedlings, thus authenticating these internally located root tissue bacteria as rhizobia. Inoculating sorghum seedlings with 4 rhizobial strains (i.e. B. japonicum Tal 110,S. meliloti strain 1, R. l. bv viciae Cn6 and R.l. bv. viciae strain 30) significantly (P<0.05) promoted sorghum shoot growth by 11-51% on fresh weight basis and 8-55% on dry weight basis. There was also 21-32% increase in root length of inoculated sorghum plants compared to uninoculated control. Additionally, root tissue concentrations of P and K were markedly (P<0.05) increased by 17-250% in inoculated sorghum roots relative to uninoculated plants, while in shoots Zn and Cu were significantly (P<0.05) decreased. Bioassays of the test strains for indole acetic acid (IAA) showed that they produced biologically active concentrations of this growth-promoting molecule, which ranged from 0.18 to 2.26 µg IAA per ml culture filtrate. These findings suggest that rhizobial infection of cerals such as sorghum and finger millet can promote an increase in plant growth via improved P and K nutrition and possibly the release of metabolites such as IAA.