Project Summary: Functional Genomics of Triterpene Saponin Biosynthesis in Medicago Truncatula

Functional genomics of triterpene saponin biosynthesis in Medicago truncatula

Project Objectives:
Medicago truncatula produces over 30 different triterpene saponins. These exist as various glycosides of five different triterpene aglycones, of which soyasapogenol and medicagenic acid are the most abundant. Saponins possess a range of biological activities, including antimicrobial, anti-palatability, anti-cancer and hemolytic. The biosynthesis of these compounds beyond the initial triterpene cyclization step is not understood. We are taking a functional genomics approach to identify and characterize various cytochrome P450 and glycosyltransferase (GT) enzymes involved in saponin biosynthesis. The strategy is to use DNA micro- and macro-array expression analysis to provide candidate genes for further functional characterization by expression in yeast (P450s) or E. coli (GTs). These genes will then be up- and down-regulated in transgenic Medicago to further assess function and to produce genotypes with altered saponin levels and composition. Crystal structures of the GTs are being determined (by the group of collaborator Xiaoqiang Wang) to facilitate structure-based engineering of GTs for understanding and manipulating substrate specificities.

We have recently characterized the genes encoding the entry point enzymes into the saponin pathway in Medicago (squalene epoxidase and b-amyrin synthase). Transgenic alfalfa is being produced over- or under-expressing b-amyrin synthase. The idea is to increase total saponins (potential use as adjuvants) or to eliminate total saponins (to address function in defense, and to yield alfalfa cultivars that are palatable to poultry). This approach can be further fine tuned using P450s or GTs specific for the individual classes of Medicago saponin.

The promoters of squalene epoxidase and b-amyrin synthase have been isolated. We are determining the cis-elements and trans- factors necessary for their expression.

Present Lab Personnel on Project:

Past Personnel:

Dr Luzia Modolo
Dr Lahoucine Achnine
Dr Kumboo Choi
Dr Hideyuki Suzuki

Past Visiting Scientist:

Dr Neelam Sangwan

Source of Funding:

Noble Foundation
NSF Plant Genome

Collaborators:

Dr Seiichi Matsuda, Rice University, Houston, TX
Dr Lloyd Sumner, Noble Foundation
Dr Xiaoqiang Wang, Noble Foundation

Recent Publications:

Dixon, R.A., Achnine, L., Deavours, B.E. and Naoumkina, M. (2006). Metabolomics and gene identification in plant natural product pathways. In: Biotechnology in Agriculture and Forestry, “Plant Metabolomics”, edited by K. Saito, R. A. Dixon and L. Willmitzer, Springer, pp 243-259.
Achnine, L., Huhman, D.V., Sumner, L.W., Blount, J.W. and Dixon, R.A. (2005) Genomics-based selection and identification of triterpene glycosyltransferases from the model legume Medicago truncatula. Plant Journal, 41, 875-887.
Shao, H., He, X., Achnine, L., Blount, J.W., Dixon, R.A., and Wang, X. (2005). The structure of UGT71G1, a multifunctional triterpene/flavonoid uridine diphosphate glycosyltransferase from the model legume Medicago truncatula. Plant Cell 17, 3141-3154.
Suzuki, H., Reddy, M.S.S., Naoumkina, M., Aziz, N., May, G.D., Huhman, D.V., Sumner, L.W., Blount, J.W., Mendes, P., and Dixon, R.A. (2005). Methyl jasmonate and yeast elicitor induce differential genetic and metabolic reprogramming in cell suspension cultures of the model legume Medicago truncatula. Planta, 220, 698-707.
Suzuki, H., Achnine, L., Xu, R., Matsuda, S.P.T. and Dixon, R.A. (2002). A genomics approach to the early stages of triterpene saponin biosynthesis in Medicago truncatula. Plant Journal 32, 1033-1048.