Research article Activity of two catabolic enzymes of the phosphogluconate pathway in mesquite roots inoculated with Azospirillum brasilense Cd Luis A. Leyva a , Yoav Bashan a,b, * a Environmental Microbiology Group, Northwestern Center for Biological Research (CIBNOR), Mar Bermejo 195, Colonia Playa Palo de Santa Rita, La Paz, BCS 23090, Mexico b Department of Soil, Water and Environmental Science, 429 Shantz Building #38, 1200 E. South Campus Dr., The University of Arizona, Tucson, AZ 85721-0038, USA Received 11 December 2007; accepted 22 May 2008 Available online 20 June 2008 Abstract The mesquite amargo (Prosopis articulate), one of the main nurse trees of the Sonoran Desert in Mexico, is responsible for major, natural re- vegetation processes. It exudes gluconic acid in root exudates, a favorite carbon source for the plant growth-promoting bacterium Azospirillum bra- silense. Two enzymes, gluconokinase (EC 2.7.1.12) and 6-phosphogluconate dehydrogenase (EC 1.1.1.44), participating in the phosphogluconate pathway, are active in the bacteria. Bacterial 6-phosphogluconate dehydrogenase is a constitutive enzyme, while gluconokinase is induced upon exposure to gluconic acid. Both enzymes are active in young, non-inoculated mesquite seedlings growing under hydroponic conditions. When A. brasilense Cd bacteria are inoculated on the root system, the roots exhibit much higher activity of gluconokinase, but not 6-phosphogluconate dehydrogenase. Mesquite roots exhibit high levels of root colonization by the inoculating bacteria. At the same time, and also for plants growing under sand culture conditions, the seedlings grew taller, greener, had longer leaves, and were heavier. Ó 2008 Elsevier Masson SAS. All rights reserved. Keywords: Azospirillum brasilense; Gluconic acid; Gluconokinase; 6-Phosphogluconate dehydrogenase; Mesquite; Plant growth-promoting bacteria; Plante bacteria interaction 1. Introduction The mesquite is one of the most important exploited trees of many deserts worldwide [1]. In northwestern Mexico, very large numbers of mesquite are harvested, legally and ille- gally, for the charcoal industry and as material for construction in rural areas [2]. In desert and thorn scrublands, mesquite amargo is a major nurse tree. Below its canopy, a ‘‘resource soil island’’ is created [3]. This area is more fertile, contains more organic matter and clay, and contains more available wa- ter. Here, many desert plants, including long-lived cacti, grow more prolifically during their initial years [3e6]. It also har- bors more soil-dwelling animals, is a nesting location for ro- dents, and is harvested by farmers and gardeners as a soil amendment. Mesquite, as a legume, is normally colonized by two types of rhizobia [7,8] and by arbuscular mycorrhizae (AM fungi) [9], which significantly contributes to nitrogen and carbon accumulation in soils of arid lands [10] and to the gen- eral ecological well-being of deserts [2]. Azospirillum spp. are non-specific plant growth-promoting bacteria (PGPB) that are capable of promoting growth of nu- merous crop plants [11], wild plants [12], and even unicellular microalgae [13]. It successfully promotes growth of the giant cardon cactus, the landmark species of the Mexican Sonoran Desert [14], as well as other cacti species [15] that co-habitat the Sonoran Desert with mesquite. Inoculation of propagated mesquite cuttings with several PGPB, including Azospirillum spp., indicated that inoculation might be beneficial for root * Corresponding author. Environmental Microbiology Group, Northwestern Center for Biological Research (CIBNOR), Mar Bermejo 195, Colonia Playa Palo de Santa Rita, La Paz, BCS 23090, Mexico. Tel.: þ52 612 123 8484x 3668; fax: þ52 612 125 4710. E-mail addresses: bashan@cibnor.mx, bashan@cals.arizona.edu (Y. Bashan). 0981-9428/$ - see front matter Ó 2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.plaphy.2008.05.011 Available online at www.sciencedirect.com Plant Physiology and Biochemistry 46 (2008) 898e904 www.elsevier.com/locate/plaphy