Rice Genetics Newsletter IS (1998)

75

(Molecular Analysis of Rice Genes)

R. Wu, Convener

Section of Biochemistry, Molecular and Cell Biology, Cornell University, Ithaca, NY 14853, USA

The goal of rice biotechnology research is to generate superior transgenic plants that produce either valuable proteins or low-molecular-weight compounds, which result in an increase in yield or value of the plants. Transgenic plants may give higher yields due to more efficient metabolism or synthesis of specific compounds, or due to a decrease in loss caused by biotic or abiotic stresses.

Salt stress and drought stress are the two most important abiotic stresses. Approximately 19% of the world’s agricultural land is subject to salt stress and 5% to drought stress (FAO 1996). Therefore, the focus of this report is to discuss possible ways to reduce loss by generating transgenic rice plants that are tolerant to salt or drought stress.

In recent years, the genes that are responsible for low-molecular-weight metabolites

have been shown to confer increased tolerance to salt or drought stress in transgenic dicot plants (mainly tobacco). These metabolites include mannitol (Tarczynski et al. 1993), proline (Kavi Kishor eta!. 1995), fructan (Pion-Smits and Ebskamp 1995), glycinebetaine (Hayashi eta!. 1997), trehalose (Holmstrom eta!. 1996) and D-ononitol (Sheveleva et al. 1997). AlthOugh work on transgenic plants harboring other genes has been reported, the plants showed either abnormal growth, or their tolerance to abiotic stresses has not been tested.

Since monocots differ from dicots in their physiology, morphology and perhaps response to abiotic stresses, it is important to show how overproduction of a metabolite or protein affects a major monocot cereal plant, such as rice, in response to stresses.

There are very few reports on the effects of different genes in conferring salt or drought tolerance in transgenic rice plants. We demonstrated that transgenic rice plants expressing a barley late-embryogenesis-abundant protein gene, HVAI, have increased salt and drought tolerance (Xu et a!. 1996).

Transgenic plants containing the gene encoding a key enzyme for proline biosynthesis, P5CS, showed increased salt or drought tolerance (Zhu et a!. 1998).

II. Multiple-Gene Approach for Conferring High Levels of Salt and Drought Tolerance in Transgenic Rice Plants

It is known that when a plant is subjected to salt or drought stress, a number of genes are turned on, resulting in increased levels of several osmolytes and proteins, some of which may be responsible for conferring a certain degree of tolerance toward these stresses. Therefore, it will likely be necessary to transfer several potentially useful genes into the same plant in order to obtain a high degree of tolerance to drought or salt stress. We believe that the best approach is to place several useful genes in the same plasmid, with each gene being driven by a different promoter. In this way, the different transgenes will not segregate in the transgenic plants, and gene silencing may be minimized. It is alsc preferable to use different stress-inducible or constitutive promoters to drive the expression of different transgenes so that undesirable affects on the growth or development of th transgenic plants may be avoided.

ifi. Concluding Remarks

Preliminary results have shown that it is possible to use a transgenic approach t confer increased salt and drought tolerance in rice. However, the results have just begun to show promise. Many more experiments are needed to test the single-gene and the multiple-gene approach. Field tests on rice yield under different abiotic stress condition need to be conducted before the usefulness of the transgenic approach can be ascertainec

References

FAO, 1996. Quarterly Bulletin of Statistics 9: 3/4.

Hayashi, H., H. Alia, L. Mustardy, P. Deshnium, M. Ida and N. Murata, 1997. Transformation of Arabidops thaliana with codA gene for choline oxidase: accumulation of glycinebetaine and enhanced tolerance salt and cold stress. Plant j.12: 133-142.

Holmstrozn, K., E. Mantyla, B. Welin,A. Mandal and E.T. Palva, 1996. Drought tolerance in tobacco. Natu 379: 683-684.

Kavi Kishor, P.B., Z. Hong, G. Miao, C.A. Hu and D.P.S. Venna, 1995. Overexpression of Delta’-pyrrolinecarboxylate synthetase increases proline production and confers osmotolerance in transgenic plants. Pla Physiol. 108: 1387-1394. Pilon-Smits, E.A.H. and MJ.M. Ebskamp, 1995. Improved performance of transgenic fructan-accumulating

tobacco under drought stress. Plant Physiol. 107: 125-130.

Sheveleva, E., W. Chimara, H.J. Bohnert and R.G. Jensen, 1997. Increased salt and drought tolerance D-ononitol production in transgenic Nicotiana tabacwn L. Plant Physiol. 115: 1211-12 19.

Tarczynski, M.C., R.G. Jensen and HJ. Bohnert, 1993. Stress protection of transgenic tobacco by production of osmolyte mannitol. Science 259: 508-510.

Xu, D., X. Duan. B. Wang, B. Hong, T.-H.D. Ho and R. Wu, 1996. Expression of a late embryogenesis abun dant protein gene, HVAJ, from barley confers tolerance to water deficit and salt stress in transge~ rice. Plant Physiol. 110: 249-257.

Zhu, B., 1. Su, M. Chang, D.P.S. Verma, Y.-L. Fan and R. Wu, 1998. Overexpression of a Delta‘-pyrro1ine carboxylate synthetase gene and analysis of tolerance to water- and salt-stress in transgenic rice. P1 Science, in press.