Million Tagede's Research Interests

Legumes are second only to grasses in economic importance worldwide, and understanding legume molecular genetics is vital to facilitate breeding of important grain and forage legumes. Over the past decade, Medicago truncatula has been selected as a model plant to study biological processes that are unique and pertinent to legumes which cannot easily be studied in Arabidopsis thaliana. The gene rich region of M. truncatula will be soon sequenced which necessitates a concurrent functional genomics approach. 

My current research is focused on generation of large numbers of transposon tagged M. truncatula lines for functional genomics. Because of the absence of standard in planta transformation strategies and the recalcitrant nature of legumes to tissue culture, introduction of genetic mutation in M. truncatula using the common T-DNA tagging approach poses a major challenge. Both the difficulty to regenerate each transformed line and the total number of transformed lines required to saturate the M. truncatula genome make T-DNA tagging impractical, if not, impossible. We resorted to the use of a retrotransposon called Tnt1, initially isolated from tobacco cell culture.

Autonomous long-terminal repeat (LTR) retrotransposons are retrovirus like elements which encode functions required for their own replication and transposition. Retrotransposition involves an RNA intermediate in which the retroelement is first transcribed into a single mRNA and reverse transcribed into a cDNA. The new retroelement then inserts into a new location. Since there is no excision during replicative transposition, mutations generated by retrotransposon insertions are stable. Tnt1 is such a retroelement that has been initially shown to be active in M. truncatula by Pascal Ratet and colleagues at CNRS, France. There are two major advantages in using Tnt1 for insertional mutagenesis in M. truncatula. (I) Tnt1 inserts in more than 15 independent locations in the genome per generation, thus reducing the number of transgenic lines by a factor of 15 compared to that of T-DNA tagging. (II) Since Tnt1 transposition is activated by tissue culture, new transformation is not required every time new insertion is sought. In fact, we regenerate all our mutant population from a single transgenic line. We are currently developing a flanking sequence platform for free access by the legume community. Genomic DNA is also being pooled in a systematic fashion which will soon be available for reverse genetic screens. Figure 1 summarizes our current strategy and plans.

Figure 1. Schematic representation of Tnt1 tagging in Medicago truncatula. The tobacco long-terminal repeat retrotransposon Tnt1 was first introduced into M. truncatula by Agrobacterium-mediated transformation. A homozygous line with low-copy-number Tnt1 inserts (three copies) was grown in soil to bulk up the seeds; these seeds are being used to make all the transposed insertion lines. Seeds are germinated and grown in soil (for R108) or on agar medium (for A17); leaf explants are cultured on auxin-containing medium for five weeks. Calli are formed from individual leaf explants and plants are regenerated from individual calli via somatic embryogenesis. Tnt1 activation and transposition occur during the process of callus formation and also probably during embryogenesis. Thus, each plant derived from individual callus represents an independently transposed Tnt1 line. Seeds from each regenerated plant will be stored for forward genetic screens. DNA extracted from the leaves of each line can be used either to identify flanking plant sequences using Tnt1-specific primers by inverse PCR or can be stored for future use to identify the mutated gene in a desired mutant identified through a forward-genetic screen. DNA from different lines can also be pooled together in a systematic fashion so that it can be conveniently screened for any gene of interest using Tnt1 and gene-specific primers by a reverse genetics approach. 

My further research interests include: The molecular basis of flowering time control in M. truncatula, nodulation signalling, hypoxia mediated oxidative stress, and aerobic fermentation in plants.

Selected publications:

Tadege M, Ratet P, Mysore KS (2005) Insertional mutagenesis: a Swiss Army knife for functional genomics of Medicago truncatula. Trends in Plant Science 10: 222-228. 

Tadege M, Sheldon CC, Helliwell CA, Updaha N, Dennis ES, Peacock WJ (2003) Reciprocal control of flowering time by OsSOC1 in transgenic Arabidopsis and by FLC in transgenic rice. Plant Biotechnology Journal 1:361-369. 

Mellema S, Eichenberger W, Rawyler A, Suter M, Tadege M, Kuhlemeier C (2002) The ethanolic fermentation pathway supports respiration and lipid biosynthesis in tobacco pollen. The Plant Journal 30: 329-336. 

Tadege M, Sheldon CC, Helliwell CA, Stoutjesdijk P, Dennis ES, Peacock WJ (2001) Control of flowering time by FLC orthologues in Brassica napus. The Plant Journal 28: 545-553 (cover) 

Sheldon CC, Finnegan EJ, Rouse D, Tadege M, Bagnall D, Helliwell CA, Peacock WJ, Dennis ES (2000) The control of flowering by vernalization. Curr opin Plant Biol 3:418-422. 

Tadege M, Dupuis I, Kuhlemeier C (1999) Ethanolic fermentation: new functions for an old pathway. Trends in Plant science 4: 320-325. 

Tadege M, Bucher M, Stahili W, Suter M, Dupuis I, Kuhlemeier C (1998) Activation of plant defense responses and sugar efflux by expression of bacterial pyruvate decarboxylase in potato leaves. The Plant Journal 16: 661-671. 

Tadege M, Braendle R, Kuhlemeier C (1998) Anoxia tolerance in tobacco roots: effect of overexpression of pyruvate decarboxylase. The Plant Journal 14: 327-335. 

Tadege M, Kuhlemeier C (1997) Aerobic fermentation during tobacco pollen development. Plant Mol Biol 35: 343-354.

Patent: Kuhlemeier C, Tadege M, Dupuis I, Bucher M (1999) WO9943833A1 Disease resistant transgenic plants