Expression of the ACC Deaminase Gene from Enterobacter cloacae UW4 in Azospirillum brasilense G. Holguin 1,2 B.R. Glick 1 1 Department of Biology, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 2 Department of Microbiology, Centro de Investigaciones Biolo ´ gicas del Noroeste (CIBNOR), La Paz, A.P. 128, B.C.S., 23000, Me ´xico Received: 21 January 2000; Accepted: 13 April 2000; Online Publication: 7 November 2000 A B S T R A C T The ACC deaminase structural gene (acdS) from Enterobacter cloacae UW4 was cloned in the broad host range plasmid pRK415 under the control of the lac promoter and transferred into Azospirillum brasilense Cd and Sp245. A. brasilense Cd and Sp245 transformants showed high ACC deaminase activity, similar to that observed in Enterobacter cloacae UW4. The expression of ACC deaminase improved the existing growth promoting activity of Azospirillum. The roots of tomato and canola seedlings were significantly longer in plants inoculated with A. brasilense Cd transformants than those in plants inoculated with the nontransformed strains of the same bacterium. In the case of wheat seedlings, inoculation with A. brasilense Cd transformants did not promote root growth. The difference in plant response (canola and tomato versus wheat) is attributed to the greater sensitivity of canola and tomato plants to ethylene as compared to wheat plants. Introduction In addition to the well-characterized mechanisms that plant growth promoting bacteria use to stimulate the growth of plants, including solubilization of phosphorus, producing siderophores to sequester iron, fixation of nitrogen, synthe- sis of phytohormones, and decreasing the damage to plants from phytopathogens [9, 16], it was proposed that many plant growth promoting bacteria may promote plant growth through the activity of the enzyme 1-aminocyclopropane-1- carboxylate (ACC) deaminase. This enzyme hydrolyzes ACC, the immediate precursor of ethylene in plant tissues 9, 15]. Ethylene is essential for growth and development of plants, but when it is present in high levels it can be dam- aging for plants, leading to epinasty, shorter roots, and pre- mature senescence, for example. Bacteria that have ACC deaminase can cleave ACC to ammonia and -ketobutyrate, and thus prevent the effects of high levels of ethylene [15, 19]. In nature, ACC deaminase is commonly found in rhi- zosphere bacteria [12–14]. Azospirillum is a plant growth promoting bacterium that colonizes and promotes the growth of numerous plant spe- cies, including cereals, grasses, weeds, annuals, perennials, Correspondence to: B.R. Glick; Fax: (519) 746-0614 E-mail: glick@sciborg. uwaterloo.ca MICROBIAL ECOLOGY Microb Ecol (2001) 41:281–288 DOI: 10.1007/s002480000040 © 2001 Springer-Verlag New York Inc.