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| Alfalfa |
Alfalfa (Medicago sativa) is the third most widely grown crop in the U.S. Annual U.S. alfalfa production is approximately 84 million metric tons with a total estimated direct value of $7 billion. Alfalfa provides a high quality forage for ruminant animals. In addition, alfalfa plays an important role in sustainable agricultural systems. Alfalfa roots are capable of growing 1.2-2.4 meters/year, an ability that permits them to tap deep water resources and aerate soil which has become compacted from farm machinery traffic. The dense network of alfalfa roots captures nutrient run-off from adjacent crops and soil erosion from adjacent plowed areas. In symbiotic association with the bacterium Sinorhizobium meliloti, alfalfa fixes atmospheric nitrogen. Growing alfalfa in rotation with some crops decreases the required application of N fertilizer up to 100%. In addition, rotation of alfalfa interrupts many pest and pathogen cycles, improving subsequent crop health. Alfalfa clearly plays a major role in protecting human health, farm profitability, and mitigating environmental problems. |
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Genomics
Research |
The barrel medic (Medicago truncatula) is a close relative of alfalfa. It is well-suited for genomics research; it is an annual diploid with a small genome (~450 MB), rapid generation time, and can be transformed efficiently. Gene order and sequence is highly conserved between M. sativa and M. truncatula . In collaborative research funded by NSF Plant Genome grants, we constructed cDNA libraries from diverse tissues of M. truncatula, which have contributed to the large EST database (http://www.tigr.org/tdb/mtgi/), and have developed and used EST microarrays for transcript profiling. We are currently investigating the genes up- and down-regulated in response to foliar pathogens (Colletotrichum trifolii, Erysiphe pisi), a root pathogen (Phytophthora medicaginis), and aluminum treatment. We are utilizing real-time PCR, expression of RNAi and promoter analysis to further characterize genes of interest.
See also: DeHaan, R. L. et al. 2002. Evaluation of annual medic germplasm for upper midwest agroecosystems. J. Agron. Crop Sci. 188:417-425.(PDF file) |
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Pathology
Research |
Yield and persistence of alfalfa depends to a great extent on adaptation to local biotic and abiotic factors including pathogenic microorganisms and soil conditions. Alfalfa yield and persistence is greatly affected by diseases. The disease spring black stem and leaf spot may cause serious yield losses under disease-conducive conditions and contributes to stand decline because it also infects crowns and roots. We have conducted research into the genetic variation in this pathogen and mechanisms of pathogenesis.
A related pathogen, P. sclerotioides , causes brown root rot (BRR) of alfalfa. This disease was first identified in Minnesota and Wisconsin in 2003. We are conducting surveys to determine the distribution of the fungus in Minnesota, investigating genetic diversity in the fungus, and developing sensitive PCR assays to quantify the fungus in plant and soil samples. For Brown Root Rot information click here.
We have previously conducted research on biological control of alfalfa diseases using streptomycetes and bacterial wilt of alfalfa caused by Clavibacter michiganensis subsp. insidiosus
See also: Samac, D. A., Nix, R. J., and Oleson, A. E. 1998. Transmission frequency of Clavibacter michiganensis subsp. insidiosus to alfalfa seed and identification of the bacterium by PCR. Plant Dis. 82:1362-1367. (PDF file)
and Isolation of Clavibacter michiganensis subsp. insidiosus (Cmi) from alfalfa (Medicago sativa). (MS Word file)
For biological control see: Samac, D. A. and Smigocki, A. C. 2003. Expression of oryzacystatin I and II in alfalfa increases resistance to the root-lesion nematode (Pratylenchus penetrans ). Phytopathology 93:799-804.
Samac, D. A. and Kinkel, L. L. 2001. Suppression of the root-lesion nematode (Pratylenchus penetrans) in alfalfa (Medicago sativa) by Streptomyces spp. Plant Soil 235:35-44.
Xiao, K., Kinkel, L. L., and Samac, D. A. 2002. Biological control of Phytophthora root rots on alfalfa and soybean with Streptomyces . Biol. Contr. 23:285-295.
Samac, D. A., Willert, A., McBride, M. J., and Kinkel, L. L. 2003. Effects of antibiotic-producing Streptomyces spp. on nodulation and leaf spot in alfalfa. App. Soil Ecol. 22:55-66.
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| Biotechnology
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Using a highly efficient and rapid transformation and regeneration system, we have developed transgenic alfalfa plants that express a wide array of genes. We have investigated tissue-specific and constitutive promoters, genes for conferring increased aluminum tolerance, genes for increased disease resistance, improving forage quality, use in remediation of atrazine-contaminated soil and water, and for producing valuable co-products.
For more detail see: Agrobacterium -mediated transformation of alfalfa
For new uses for transgenic alfalfa see: Samac, D. A. et al. 2004. Expression of UDP-glucose dehydrogenase reduces cell-wall polysaccharide concentration and increases xylose content in alfalfa stems. Appl. Biochem. Biotechnol. 116:1167-1182.(PDF file)
and Sullivan M. L., et al. 2004. Cloning and characterization of red clover polyphenol oxidase cDNAs and expression of active protein in Escherichia coli and transgenic alfalfa. Plant Physiol. 136:3234-44. (PDF file)
and Saruul, P., et al. 2002. Production of a Biodegradable Plastic Polymer, Poly-ß-Hydroxybutyrate, in Transgenic Alfalfa. Crop Sci. 42:919–927.(PDF file) |
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Aluminum
Tolerance |
Development of acid soils that limit crop production is an increasing problem world-wide. Many factors contribute to phytotoxicity of these soils, however, in acid soils (pH less than 5.5) with a high mineral content, aluminum (Al) is the major cause of toxicity. The target of Al toxicity is the root tip, in which Al exposure causes inhibition of cell elongation and cell division, leading to root stunting accompanied by reduced water and nutrient uptake. Alfalfa is very sensitive to Al and performs poorly in acidic Al-rich soil.
Natural variation for Al tolerance has been identified in many crop species and in some crops tolerance to Al has been introduced into productive, well-adapted varieties. Aluminum tolerance appears to be a complex multigenic trait.
A variety of genes have been identified that are induced or repressed upon Al exposure. Most induced genes characterized so far are not specific to Al exposure but are also induced by other stress conditions. Ectopic over-expression of some of these genes has resulted in enhanced Al tolerance. Expression of genes involved in organic acid synthesis has resulted in enhanced production of organic acids and an associated increase in Al tolerance.
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Secretion of organic acids by alfalfa plants.
Yellow zones indicate acidification of the medium.
Regen-SY: untransformed
MDH20-10: Plant over-expressing an unique malate dehydogenase gene. |
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See also:Samac, D. A. and Tesfaye, M. 2003. Plant improvement for tolerance to aluminum in acid soils-A review. Plant Cell Tiss. Org. Cult. 75:189-207.
Tesfaye, M., et al. 2003. Influence of enhanced malate dehydrogenase expression by alfalfa on diversity of rhizobacteria and soil nutrient availability. Soil Biol. Biochem. 35:1103-1113.
Tesfaye, M., et al. 2001. Over-expression of malate dehydrogenase in transgenic alfalfa enhances organic acid synthesis and confers tolerance to aluminum. Plant Physiol. 127:1836-1844. |
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Forage Quality
and Crop Value |
Alfalfa is known as the “Queen of the Forages” because of the high amount of protein present in alfalfa leaves. Unfortunately, the protein content in alfalfa leaves is poorly utilized because the protein is rapidly digested in the rumen. Although alfalfa stems contain large amounts of cell wall (~70% of stem dry weight), the cellulose and xylan fractions which make up the majority of the cell walls are poorly digested in the rumen. Pectins, found in the primary cell wall and middle lamella, are highly digestible cell-wall polysaccharides.
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Using the EST resources in M. truncatula, we are characterizing genes putatively involved in synthesis of nucleotide sugars, the building blocks of pectin, and modifying expression of these genes in alfalfa. |
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