Agricultural Science Research Journals Vol. 2(11), pp. 574-580, November 2012 Available online at http://www.resjournals.com/ARJ ISSN-L:2026-6073 ©2012 International Research Journals Full Length Research Paper Assessment of plant growth promotion by rhizobacteria supplied with tryptophan as phytohormone production elicitor on Axonopus affinis Shamir. Quiroz-Villareal 1 , Nelly Zavala Hernández 2 , Isaac Luna-Romero 1 , Enriqueta Amora-Lazcano 1 and *Angélica Rodríguez-Dorantes 2 1 Laboratorio de Fitopatología, Departamento de Microbiología, Escuela Nacional de Ciencias Biológicas, Instituto Politécnico Nacional, México, D.F. 11340, México. 2 Laboratorio de Fisiología Vegetal, Departamento de Botánica, Escuela Nacional de Ciencias Biológicas, Instituto Politécnico Nacional, México, D.F.11340, México. *Corresponding Author's Email: rodorantes@yahoo.com.mx, Tel. 55-57-29-60-00, ext. 62332. Abstract One of the principal roles of rhizospheric microorganisms is they direct participation in plants’ hormonal balance as a direct mechanism of plant growth promotion. This study evaluated the in vitro auxin production of eight Pseudomonas fluorescens strains and the selected effect of the bacterization of carpet grass (Axonopus affinis) seeds with these plant growth promoting rhizobacteria, in the presence of tryptophan. From this study only Pseudomonas fluorescens Ba-72 strain, was capable to promote the growth of A. affinis plantlets and successfully colonized the spermosphere of this species seeds at first step of the interaction between the microorganism and plant, after at radicle and epicotyl surface promoting the growth of them. As a fully response, the Vigour Index of these plant species, the IAA production by the rhizobacteria and the supply of Trp conduced to a successfully event that showed a plant growth of this plant species and even more, the great interaction of these three components of the in vitro system. There are further works with these rhizobacteria that complete the effects of them as inoculants for the development of plant-microbe systems. Keywords: plant growth-promoting rhizobacteria, rhizosphere, Axonopus affinis Abbreviations: PGPR (Plant Growth Promoting Rhizobacteria); Trp (Tryptophan); IAA (Indole-3-Acetic Acid) INTRODUCTION Bacteria inhabiting the rhizosphere beneficial to plants are called PGPR (Plant Growth-Promoting Rhizobacteria) (Kloepper et al., 1980; Glick, 1995; Ramos-Solano et al., 2008). Among the diverse bacteria identified as PGPR, the Bacilli and Pseudomonads are the predominant ones (Singh et al., 2011). There are two sources of phytohormones naturally available for the plants: the endogenous production by the plant tissues and the exogenous production by associated microorganisms (Baca and Elmerich, 2007). Ramos-Solano et al. (2009) mentioned that the modification of a plant’s physiology by plant growth regulator production is a very important mechanism, because it is based on the evolutionary development of common metabolic pathways in plants and bacteria. Glick (1995) showed that the promotion of root growth is one of