Root growth, respiration and β-glucosidase activity in maize (Zea mays) and common bean (Phaseolus vulgaris) inoculated with Azospirillum brasilense

As Azospirillum is shown to improve plant growth, the influence of inoculation on plant metabolism in maize and common bean seedlings has been assessed. Respiration rates have been measured and changes in β-glucosidase (which may be involved in phytohormone release from conjugates) activity detected. A trend of inoculum concentration-dependent increased respiration rates was detected in inoculated maize seeds in Petri dishes. A similar phenomenon was observed for common bean seedlings grown in an hydroponic system, when growth promotion and enhanced respiration rates occurred after inoculation with Azospirillum at 10⁶ cfu/ml. In the hydroponic growth system, Azospirillum inoculation at 10⁶ cfu/ml, promoted root fresh weight as well as shoot fresh and dry weight of maize seedlings. Treatment with Azospirillum reduced the Km and Vmax values of β-glucosidase activity, in crude extracts from maize root tips. In vitro inoculation of detached root segments caused reduction in β-glucosidase kinetic values. In this case, changes in Km and Vmax were already observed after 5 h and increased up to 24 h. This effect was promoted when inoculum concentration was increased. Reduction in Vmax and Km may indicate lower total activity of β-glucosidase, but higher affinity to the substrate of specific β-glucosidases. Analysis of Zm-p60.1 gene expression, coding for the maize hormones involved in β-glucosidase deglycosilation, did not indicate changes in the transcript level following inoculation with Azospirillum.