Effects of plant growth regulators on acetylene-reducing associations between Azospirillum brasilense and wheat

Abstract

Treatment of wheat seedlings with the synthetic auxin, 2,4-dichlorophenoxyacetic acid (2,4-D), induced nodule-like structures or tumours (termed para-nodules) where lateral roots would normally emerge.The formation of these structures promoted increased rates of acetylene reduction at reduced oxygen pressure (0.02-0.04 atm) in seedling inoculated with Azospirillum brasilense, compared to seedlings inoculated without auxin treatment. Fluorescent microscopy, laser scanning confocal microscopy and direct bacterial counts all showed that the 2,4-D treatment stimulated internal colonization of the root system with azospirilla, particularly in the basal region of the nodular structures. Both colonization with azospirilla and acetylene-reducing activity were further stimulated by simultaneous treatment with another synthetic auxin, naphthaleneacetic acid (NAA) and, less reliably, with indoleacetic acid (IAA) and indolebutyric acid (IBA). These auxins produced shortening of many initiated lateral roots, although 20 times the concentration of NAA was required to achieve rounded structures similar to those obtained with 2,4-D. Treatment with NAA, IAA or IBA alone also stimulated colonization with azospirilla and acetylene reduction rates at 0.02 atm oxygen, but less effectively than by treatment with 2,4-D. Such exogenous treatments of wheat seedlings with synthetic growth regulators provide an effective laboratory model for studies on the development of a N2-fixing system in cereals.

Colonization of wheat para‐nodules by the N2‐fixing cyanobacterium Nostoc sp. strain 2S9B

“…2). The infection process thus appears similar to that of Azospirillum (Zeman et al, 1992 ;Sriskandarajah et al, 1993). Cyanobacteria were also found in the intercellular spaces of para-nodule tissue, in agreement with findings for Azorhizobium (Yu & Kennedy, 1995) and Azospirillum (Zeman et al, 1992).…”

Mutants with Enhanced Nitrogenase Activity in HydroponicAzospirillum brasilense-Wheat Associations

“…Complete medium was nutrient broth (NB; Difco) for A. brasilense and Luria-Bertani medium for Escherichia coli. Otherwise, Azospirillum strains were grown on minimal lactate medium (15) or nitrogen-free malate medium (NFB) (34), which were supplemented with 40 g of Congo red. The medium for examination of the ability of the strains to swarm was semisolid NB or minimal lactate containing 0.4% agar.…”

“…Natural production of ethylene is insignificant compared to the rates of acetylene reduction observed in hydroponic Azospirillum-wheat systems, and 2,4-D does not enhance ethylene production without added acetylene (34). ARA with Azospirillum strains associated with plant roots as well as ARA in free living cultures were performed as described by Katupitiya et al (18).…”

Azospirilli of Uzbekistan Soils and their Influence on Growth and Development of Wheat Plants

“…The addition of low concentrations of synthetic auxins was shown to stimulate the process of colonization and fixation of molecular nitrogen by Azospirilli (ChristiansenWeniger, 1992;Sriskandarajah et al, 1993;Katupitiya etal., 1995;Fischer et al,2000). The efficiency of associations of plants with bacteria may be estimated at the level of nitrogen fixation and of the synthesis of phytohormones (Barbieri and Galli, 1993;Dobbelaereetal.,1999).…”