Possible mode of action of Azospirillum brasilense strain Cd on the root morphology and nodule formation in burr medic (Medicago polymorpha)

Abstract

The mechanism(s) involved in the effect of Azospirillum brasilense strain Cd on root susceptibility to nodulation was studied in medic seedlings grown in pouches. The number of nodules above the position of the root-tip mark at the time of inoculation and the position of the uppermost nodule were used as parameters for determining the rate of nodule initiation. Cell-free extracts and culture supernatants prepared from Azospirillum and the cytokinin benzyladenine (10−9 M) significantly increased the number of nodules formed above the root-tip mark when applied together with Rhizobium compared with those formed with Rhizobium alone. The application of indoleacetic acid did not cause an increase in the number of nodules. In the absence of Rhizobium, exposure to Azospirillum at a concentration of 109 cfu/mL or to compounds excreted by the bacteria into the growth medium caused a 40% increase in endogenous ethylene production by the roots. A less concentrated inoculum did not increase ethylene production. Inoculation with Azospirillum significantly increased the specific activity of the enzymes glucose-6-phosphate dehydrogenase, L-phenylalanine ammonia-lyase, and shikimate dehydrogenase compared with roots inoculated with Rhizobium alone. Key words: Azospirillum, Rhizobium, Medicago polymorpha, root morphology, nodule initiation.

Growth stimulation of clusterbean (Cyamopsis tetragonoloba) by coinoculation with rhizosphere bacteria and Rhizobium

“…In earlier studies, promotion of plant growth after inoculation with rhizobacteria has been attributed to indole acetic acid (IAA) production in Rhizobium species (Hirsch and Fang, 1994) and Pseudomonas (Xie et al, 1996;Malik and Sindhu, 2011). Similarly, coinoculation of legumes with Rhizobium and IAA-producers such as Azospirillum brasilense, Bacillus and Pseudomonas have been found to increase the number of nodules, nodule fresh weight and nitrogenase activity in comparison to Rhizobium/Bradyrhizobium-inoculated plants (Yahalom et al, 1990). Inoculation with bacteria having the ability to produce ä-aminolevulinic acid (ALA) and potassium solubilization also resulted in growth improvement of different plants (Hotta et al, 1997;Hu et al, 2006;Basak and Biswas, 2010;Liu et al, 2014).…”

Co-Inoculation of Soybean with <i>Bradyrhizobium</i> and <i>Azospirillum</i> Promotes Early Nodulation

“…[49]. The fact that co-inoculation of legumes with rhizobia and Azospirillum results in increased nodulation of the root crown, when compared to the inoculation of rhizobia alone [50] suggests that Azospirillum may induce earlier nodulation, since the nodules present in the root crown are the first to be formed and are directly related to the inoculated strains of rhizobia [36] It was very important to observe that co-inoculation resulted in earlier nodulation. Precocious nodule formation is very critical for the establishment of the symbiosis and onset of N 2 fixation, particularly in the case of crops with a short growth cycle, as is the case with soybean.…”

Soil Bacteria Support and Protect Plants Against Abiotic Stresses

“…A significant increase of total and upper nodule numbers was observed at different concentrations of Rhizobium inoculum (Dardanelli et al, 2008). Several greenhouse and field experiments demonstrated the potential of co-inoculation to increase grain yield of various legumes (Bashan and Holguin, 1997;Galal et al, 2002;Itzigsohn et al, 1993;Sarig et al, 1986;Yahalom et al, 1989).…”