Genetic engineering of rice for improved agronomic characteristics

Abstract

This thesis describes the production of three populations of transgenic rice plants using particle bombardment method altered in two main traits: (1) polyamine content and (2) insect pest resistance. The expression of antisense heterologous oat arginine decarboxylase (ADC) cDNA in transgenic rice plants suppressed endogenous ADC enzyme activity, and decreased putrescine and spermidine levels in a tissue/organ dependent manner, with no concomitant changes in the expression of other polyamine biosynthetic genes. The second population of transgenic rice plants engineered with a homologous spermidine synthase (SpdSyn) transgene, and observed through two generations, showed increased expression of both endogenous and transgene mRNAs. However, no significant accumulation of spermidine level in transgenic rice plants when compared to wild type control plants was observed. Putrescine levels were significantly increased in these transgenic plants. The study suggested the possible presence of an inter-conversion process from spermidine to putrescine in transgenic plants, triggered by over-expression of SpdSyn mRNAs. Novel insect resistance gene constructs encoding fusion proteins, including (1) rice thioredoxin h fused with snowdrop lectin-GNA (TRX-GNA), (2) the first domain of Bt toxin gene-Cry 1 Ac fused with GNA (Ac-GNA) and (3) Cry 1 Ac fused with ricin B chain-RTB (Ac-RTB) were assembled. When expressed in transgenic plant, these fusion proteins displayed an additive effect as insect toxins by maintaining the functional properties of the individual proteins. Artificial diet bioassays against insect pests showed that using these fusion proteins could enhance toxicity, insecticidal spectrum and possibly durability of resistance to insect pests. Our results clearly showed that transgenic rice plants expressing these fusion protein genes are resistant to brown planthopper, an important insect pest in tropical rice growing areas. These rice plants behave as horizontally resistant cultivars that are suitable for integrated pest management (IPM) networks.