© 2005 Heron Publishing—Victoria, Canada
Overexpression of mtlD gene in transgenic Populus tomentosa improves salt tolerance through accumulation of mannitol
Lei Hu (1, 2, 3), Hai Lu (1, 2, 3), Qunlu Liu (1, 2, 3, 4), Xuemei Chen (1, 5) and Xiangning Jiang (1, 5)
1. The Tree and Ornamental Plant Breeding and Biotechnology Laboratory of State Forestry Administration, Beijing 100083, P.R.
China / 2. College of Life Sciences and Biotechnology, Beijing Forestry University, Nr 35, Qinghua Donglu, Beijing 100083, P.R. China / 3. These authors contributed equally to this work / 4. The School of Agriculture and Biology of Science, Shanghai Jiaotong University, Shanghai 201101, P.R. China / 5. Corresponding authors (jiangxn@bjfu.edu.cn) / Received April 5, 2004; accepted January 15, 2005; published online August 1, 2005
Summary
The mtlD gene encoding mannitol-1-phosphate dehydrogenase, which catalyzes the biosynthesis of mannitol from fructose, was cloned
from Escherichia coli and transferred to poplar (Populus tomentosa Carr.) through Agrobacterium-mediated transformation. The transgenic plants were screened and selected on Murashige and Skoog (MS) medium containing 30–50
mg l–1 kanamycin and verified by polymerase chain reaction (PCR) and Southern blotting. Expression of the gene led to synthesis
and accumulation of mannitol in the transgenic plants. Gas chromatography and mass spectrometry (GC/MS) and capillary gas
chromatography (GC) showed that transgenic plants accumulated much more mannitol in their tissues than the wild-type plants,
whether cultured in vitro, or grown hydroponically or in the field.
Increased salt tolerance of transgenic plants was observed both in vitro and in hydroponic culture. The transgenic buds rooted
normally on MS medium containing 50 mM NaCl, whereas wild-type buds did not. In the 40-day hydroponic experiments, transgenic
poplar plants survived in a 75-mM NaCl treatment, whereas the wild-type poplar plants tolerated only 25 mM NaCl. Under the
same NaCl stress, stomatal conductance, transpiration rates and photosynthetic rates were all higher in transgenic plants
than in wild-type plants, whereas cellular relative conductivity was lower. We demonstrated that the mtlD gene was expressed in transgenic poplar plants, resulting either directly or indirectly in mannitol accumulation and improved
salt tolerance. The constant mannitol concentrations in transgenic plants during the NaCl treatments indicated that mannitol
accumulation caused by the mtlD gene was not a consequence of NaCl stress.
Height growth was reduced by about 50% in the transgenic plants compared with the wild-type plants in the absence of salt;
however, relative growth rate was much less influenced by salt stress in transgenic plants than in wild-type plants. The stunted
growth of the transgenic plants may in part explain their improved salt tolerance.
Keywords:
GC/MS, physiological characteristics, slower growth, transformant.
