Abstract
Indolacetic acid (IAA) production was evaluated under in vitro conditions by five P-solubilizing fungi (PSF), identified as Aspergillus sp. and four P-solubilizing bacteria (PSB) of the Enterobacteriaceae family. The bacteria and fungus isolates were incubated in a liquid medium at 28ºC for two and four days, respectively. Moreover, two PSB isolates of the Enterobacteriaceae family (PSB 8 and PSB 56) were evaluated in relation to their ability to stimulate or inhibit spore germination and hyphal growth of two arbuscular mycorrhizal fungi (AMF) species under in vitro conditions. Two assays were carried out in Petri dishes with agar (0.8%), the first using the AMF Gigaspora margarita and the second with the AMF Scutellospora sp. The treatments were: inoculation of PSB 8; PSB 56 and uninoculated control. P-solubilizing isolates produced different amounts of IAA and most PSF isolates produced higher IAA than PSB. From the 18th day of incubation of Scutellospora sp., a significant increase in hyphal growth in the treatment inoculated with PSB 8 was observed. In contrast, PSB 56 inhibited the hyphal growth of Gigaspora margarita from the 24th day of incubation.
Highlights
Several edaphic microorganisms are able to mineralize organic and solubilize inorganic P sources, playing a key role in plant nutrition
P-solubilizing fungi (PSF) 7, PSF 9, P-solubilizing bacteria (PSB) 8, PSB 9 and PSB 50 were isolated from the rhizoplane and rhizosphere of Mimosa caesalpiniifolia grown in Argisol collected from an Atlantic Forest area in Paraty, Rio de Janeiro, Brazil
The PSF 19 and PSB 55 isolates were obtained from the rhizoplane and rhizosphere of Mimosa caesalpiniifolia grown in Cambisol from Pontevedra, Galicia, Spain
Summary
Several edaphic microorganisms are able to mineralize organic and solubilize inorganic P sources, playing a key role in plant nutrition. Beyond releasing organic acids solubilizing P, some microorganisms produce auxins (Leinhos and Vacek, 1994; Gutiérrez-Mañero et al, 1996; Yasmin et al, 2004), giberellins (Gutiérrez-Mañero et al, 2001) and cytokinins (Timmusk et al, 1999). These microorganisms are commonly classified as “Plant Growth Promoting Rhizobacteria” (PGPR) (Kloepper and Schroth, 1978). They are called “Plant Growth Promoting Fungi” (Reyes et al, 2002). Freeliving as well as symbiotic PGPR can enhance plant growth directly by providing bio-available P for plant uptake, fixing nitrogen for plant use, absorbing trace elements like iron for plants from siderophores, producing phytohormones and lowering plant ethylene levels (Glick et al, 1999). The effect of inoculants on bacterial and fungal native populations in the rhizosphere is decisive for maximizing plant nutrient availability (Medina et al, 2003)
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