ABSTRACT
Rice (Oryzea sativa L.) is one of the most important food resources for humankind, especially for people in developing countries. The progress in plant genetic engineering provides promising potential for the improvement of rice production and quality. Since the first transgenic rice was reported (1,2), remarkable progress was made in this field during the ensuing decade. Many contributions by various researchers have led to a routine transformation system (3) to obtain transgenic rice plants in many laboratories. Especially in the recent 3-4 years, a number of agronomically important genes have been introduced into rice and have proved to be effective. Engi-neered rice plants with modified Bacillus Thuringiensis (Bt) gene were obtained for insect resistance (4-7), and Cheng and coworkers (7) reported that Agrobacterium species-transformed rice plants expressing cryIA(b) and cryA(c) genes are highly toxic to striped stem borer and yellow stem borer. In order to obtain transgenic rice with resistance to fungal and bacterial diseases, chitinase gene ChiI 1 was transferred into rice protoplast, and the regenerated rice plants showed resistance to sheath blight (8). Song and colleagues (9) cloned the rice blight resistance gene Xa21, and the transgenic rice plants with Xa21 showed high levels of resistance against bacterial blight disease. For virus resistance, transgenic plants were produced with viral coat protein gene or part of another viral gene in order to add resistance against rice stripe virus (10), brome ma-saic virus (11), rice tungro virus (12), and rice dwarf virus (13). For stress tolerance, Xu and colleagues (14) transferred late embryogenesis abundant (LEA) protein gene from barley, Hva 1, into rice by particle bombardment; the transgenic rice plants obtained enhanced tolerance to drought and salinity. In addition, there are studies on rice quality improvements via transgenic approaches (15,16)
In recent years, two significant accomplishments have been achieved in the genetic transformation of cereal plants. First, biolistic technology (microprojectile bombardment) has been successfully developed, allowing routine production of transgenic plants in major cereals including rice. This system has solved many problems that exist in traditional methods of direct gene transfer, such as polyethylene glycol-(PEG)-mediated and electroporation gene transfer into protoplasts. Application of the direct transformation method to various cereal species was limited by some problems, for instance, the somaclonal variation derived from the long period of tissue culture procedure for protoplasts, the need for good protoplast culture, and the regeneration
system, which is time-consuming, laborious, and highly genotype-dependent (17). One of the advantages of the biolistic method is that transgenic plants can be obtained after bombardment of various explants only if they are regenerable. In addition, its short period of tissue culture minimize the possibility of somaclonal variation and thus improves the fertility of transgenic plants. More importantly, it is considered to be variety-independent: transgenic plants of both the Oryzea saliva japonica and O. sativa indica rice varieties have been successfully obtained with relatively high transformation frequency.
