EFFECTS OF PLANT-GROWTH REGULATORS ON ACETYLENE-REDUCING ASSOCIATIONSBETWEEN AZOSPIRILLUM-BRASILENSE AND WHEAT

Treatment of wheat seedlings with the synthetic auxin, 2,4-dichlorophe noxyacetic acid (2,4-D), induced nodule-like structures or tumours (te rmed para-nodules) where lateral roots would normally emerge. The form ation of these structures promoted increased rates of acetylene reduct ion 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 micr oscopy 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 colo nization with azospirilla and acetylene-reducing activity were further stimulated by simultaneous treatment with another synthetic auxin, na phthaleneacetic acid (NAA) and, less reliably, with indole-acetic acid (IAA) and indolebutyric acid (IBA). These auxins produced shortening of many initiated lateral roots, although 20 times the concentration o f NAA was required to achieve rounded structures similar to those obta ined with 2,4-D. Treatment with NAA, IAA or IBA alone also stimulated colonization with azospirilla and acetylene reduction rates at 0.02 at m oxygen, but less effectively than by treatment with 2,4-D. Such exog enous treatments of wheat seedlings with synthetic growth regulators p rovide an effective laboratory model for studies on the development of a N2-fixing system in cereals.