Manipulating freezing tolerance in transgenic plants |
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Authors: | Bryan D McKersie Julia Murnaghan Stephen R Bowley |
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Institution: | (1) Department of Crop Science, University of Guelph, Guelph, ON, Canada |
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Abstract: | Winterhardiness is a composite of tolerances to freezing, desiccation, ice-encasement, flooding and diseases. From one point
of view, winterhardiness may not be easily manipulated by genetic engineering technology because many different genes are
involved in the tolerance of these diverse stresses. However, these various stresses have similarities. They promote formation
of activated forms of oxygen, promote membrane lipid and protein degradation, cause similar biophysical changes in membrane
structure, and culminate with increased leakage of cytoplasmic solutes and loss of cellular membrane functions. These similarities
led to the hypothesis that winter injury might be reduced in crop plants if their tolerance of oxidative stress was increased.
Towards that objective we created transgenic alfalfa (Medicago sativa L.) plants that overexpress either Mn-SOD or Fe-SOD cDNA (provided by Dirk Inzé, Universiteit Gent). Petiole explants were
transformed using Agrobacterium tumefaciens and plants were regenerated by somatic embryogenesis. The primary transgenic plants were screened using PCR (polymerase chain
reaction), Southern hybridization and native PAGE for SOD activity. Greenhouse and laboratory studies showed a minimal difference
in stress tolerance between the primary transgenic and non-transgenic plants. In the first field trial, four primary transgenic
plants expressing two forms of the Mn-SOD cDNA had greater survival after two winters than the non-transgenic RA3. Similar
results were obtained in a second field trial, comparing 18 independent transformants with Mn-SOD targeted to the mitochondria,
11 independent transformants with Mn-SOD targeted to the chloroplast and 39 independent transformants with Fe-SOD targeted
to the chloroplast, expressed in three different non-transgenic plants. The transgenic plants averaged over 25% higher survival
than the non-transgenic controls after one winter. There was no effect of subcellular targeting or SOD type on field survival,
but there was variation among independent transformants containing the same SOD construct. Activated oxygen therefore appears
to be one of the possible causes of winter injury, and it should be possible to reduce winter injury in transgenic plants
by constitutive overexpression of SOD. |
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Keywords: | alfalfa Medicago sativa winterhardiness ice-encasement flooding anoxia field trials |
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