USE OF GREEN FLUORESCENT PROTEIN TO VISUALIZE THE EARLY EVENTS OF SYMBIOSIS BETWEEN RHIZOBIUM-MELILOTI AND ALFALFA (MEDICAGO-SATIVA)

A gene encoding a variant of green fluorescent protein (GFP) of Aequor ea victoria was put under the control of a promoter which is constitut ive in Rhizobium meliloti. The heterologous GFP gene was expressed at high levels during all stages of symbiosis, allowing R. meliloti cells to be visualized as they grew in the rhizosphere, on the root surface , and inside infection threads. In addition, nodules that were infecte d with bacteria which were synthesizing GFP fluoresced when illuminate d with blue light. GFP-tagged bacteria could be seen inside infection threads, providing the opportunity to measure the growth rate and dete rmine the patterns of growth of R. meliloti residing inside its host p lant.