Root growth, respiration and beta-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 inoculat ion on plant metabolism in maize and common bean seedlings has been assesse d. Respiration rates have been measured and changes in beta-glucosidase (wh ich may be involved in phytohormone release from conjugates) activity detec ted. A trend of inoculum concentration-dependent increased respiration rate s was detected in inoculated maize seeds in Petri dishes. A similar phenome non was observed for common bean seedlings grown in an hydroponic system, w hen growth promotion and enhanced respiration rates occurred after inoculat ion with Azospirillum at 10(6) cfu/ml. In the hydroponic growth system, Azo spirillum 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 beta-glucosidase activity, in crude extra cts from maize root tips. In vitro inoculation of detached root segments ca used reduction in beta-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 i n Vmax and Km may indicate lower total activity of beta-glucosidase, but hi gher affinity to the substrate of specific beta-glucosidases. Analysis of Z m-p60.1 gene expression, coding for the maize hormones involved in beta-glu cosidase deglycosilation, did not indicate changes in the transcript level following inoculation with Azospirillum.