47 THE MECHANISM INFECTION OF AGROBACTERIUM TUMEFACIENS The Mechanism Infection of Agrobacterium tumefaciens L.Y. SOLÍS-RAMOS 1,3 , T. GONZÁLEZ-ESTRADA 1 , L. C. RODRÍGUEZ ZAPATA 2 ANGELA KU-GONZALES 1 AND E. CASTAÑO DE LA SERNA 1 * 1 Unidad de Bioquímica y Biología Molecular de Plantas, Centro de Investigación Científica de Yucatán. Calle 43, No.130, Colonia Chuburná de Hidalgo C.P. 97200, Mérida, Yucatán, México. 2 Unidad de Biotecnologia, Centro de Investigación Científica de Yucatán. Calle 43, No.130, Colonia Chuburná de Hidalgo C.P. 97200, Mérida, Yucatán, México. 3 Actual address: Escuela de Biologia, Universidad de Costa Rica, P.O. Box 2060, San Pedro, Costa Rica Chapter 3 SUMMARY Genetic engineering in plants has been aided by the use of Agrobacterium tumefaciens, a Gram negative bacterium that allows the introduction of genetic information into cells. This has revolutionized the field, improving cultivars by expressing traits that would not be possible to obtain by classical methods. As such it is a remarkable system that permits to test in vivo the effect of proteins originated from different species in a particular plant. The wild type Agrobacterium carries two types of genes for transformation: oncogenes and those in charge of opinoids biosynthesis that promote the sickness known as crown gall, which consists in a continuous growth due to the lack of control at the cell cycle. The process of transformation by Agrobacterium requires the presence of several genetic elements: (1) The virulent genes from the chromosome of Agrobacterium (chv), (2) The T-DNA which is limited by the left and right borders (3) The virulent genes from the Ti (vir) plasmid that constitute the T-DNA transfer machinery and (4) Proteins coded by the cells to be infected. Here we provide an overview of the mechanisms used by A. tumefaciens to transfer genetic material to a host cell. Key words: Agrobacterium tumefaciens, genes vir, T-DNA, genetic transformation. *Corresponding author: (e-mail:enriquec@cicy.mx)