Production of Transgenic Plants with Increased Salt Tolerance

Rice plants are known not to accumulate glycinebetaine as an osmoprotectant and are very sensitive to salt stress. In order to increase salt tolerance of rice plants, they were transformed with the E. coli betA gene, encoding a bifunctional enzyme (choline dehydrogenase and betaine aldehyde dehydrogenase), using a mitochondrial targeting sequence under the control of CaMV 35S promoter. However, the transformants did not accumulate glycinebetaine. The original betA gene was found to have unfavorable sequences. Accordingly the betA gene was largely modified and then used successfully for transformation of rice plants. The transformants had an increased salt tolerance in terms of both number of seedlings that retained green color after salt treatment (150 mM) for 7 d and that recovered without salt 7 d after the salt treatment, compared with those of control plants. The transformants also acquired an increased drought tolerance. The rice transformed with untargeted versions of either original betA or modified betA did not accumulate glycinebetaine.

To evaluate the effects of glycinebetaine on tolerance against cold and heat, transgenic rice plants overexpressing BADH were produced by the intoduction of a BADH (BBADH1) cDNA from barley. The transgenic BADH rice plants converted exogenously applied betaine aldehyde to glycinebetaine of high levels up to 5 ^moles/g FW more efficiently than wild type plants.30 High levels of glycinebetaine accumulation in the transgenic BADH rice plants conferred significant tolerance against cold and heat stresses as well as salt and drought tolerance. The benefit of glycinebetaine accumulation was much higher under a higher

Table 21.2. Sequence comparison of deduced BADH protein in higher plants

BBADH2

Rice

Sorghum

Atriplex

Spinach

Sugar beet

Amaranthus

BBADH1

70

85

73

70

71

69

71

BBADH2

72

63

71

71

71

73

Rice

77

70

71

69

71

Sorghum

63

63

61

63

Atriplex

89

87

82

Spinach

89

Sugar 83

radiation in rice. The increased stress tolerance against salt, drought, cold or heat was also observed in the rice plants exogenously treated with glycinebetaine.30,31

To produce highly salt-tolerant plants we are planning to use salt-inducible genes obtained by differential display as described above, after each gene is examined for ability to contribute to salt (or other) tolerance, by evaluation using transgenic plants expressing the gene product.

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