Poster: Plant Pathogen/Symbiont Interactions

Abs # 756: The Melilotus alba – Sinorhizobium meliloti N₂-fixing symbiosis: An expression profile analysis

Presenter:	Lee, Angie , a1lee@yahoo.com
Authors	Lee, Angie  (A)   DeHoff, Peter L (B)   Lin, Peter KC (C)   Suzuki, Akihiro  (A)   Hirsch, Ann M (A) (B)
Affiliations:	(A): University of California Los Angeles, Department of Molecular, Cell and Developmental Biology (B): University of California Los Angeles, Molecular Biology Institute (C): University of California Los Angeles, Department of Organismic Biology, Ecology and Evolution

Melilotus alba is a small, diploid legume with a short life cycle of about 3 months. It forms a N₂-fixing symbiosis with Sinorhizobium meliloti, whose genome has been sequenced (Galibert et al., 2001) and is well studied. The small genome size of M. alba (1254 Mbp per haploid genome) is comparable to that of tomato (ca. 1000 Mbp) and is ca. 6 times the size of Arabidopsis thaliana. Mutagenesis of line U389 has yielded numerous symbiotic mutants (sym), which have been physiologically characterized. Of these mutants, sym3 appears to be defective in the earliest stage of symbiotic development in that it lacks root hair curling, root cortical cell division, and infection thread formation. We have used cDNA-AFLP to analyze the expression profile at this stage of development. Our studies involve the comparison of genes expressed between wildtype (U389) root tips inoculated with S. meliloti strain GMI6032, a Nod factor overproducing strain, and mock-inoculated U389 root tips. U389 root tips and sym3 root tips both inoculated with GMI6032 were also compared. We have found that genes involved in the defense response, for transcription factors, and for components of signaling cascades appear to be important in establishment of the symbiosis. To study these genes further, we are developing Agrobacterium-mediated plant transformation techniques. Several methods have been explored: shoot organogenesis, radicle excision, and vacuum infiltration. Addition of antioxidants and plant defense moderators play a critical role in the efficiency of transformation. A set of conditions for hairy root transformation has been developed for M. alba. _{Support granted by a predoctoral training program in Genetic Mechanisms at UCLA (T32-GM07104) to PLD and a UC-BIOSTAR grant #S98-86 to AMH.}