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This article provides a case study on transgenic rice for stress tolerance.
Production of agronomically useful transgenic rice has been given top priority in the improvement of crop plants. The loss of rice yield due to stress impact is estimated to cost at least several billion dollars a year. Genetic engineering offers effective way to deal with these problems. Moreover, rice is an important staple food for people from developing countries and its improvement for stress tolerance in particular is highly significant.
Abiotic stress can impose severe constraints in lowering rice yield. Production of transgenic rice tolerance of stresses has been manifested by deploying several genes and involves several components of trait. There have been reports on identification of more than 10 individual genes that are expressed specifically in response to different abiotic stresses like drought, soil salinity and low temperature.
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Transgenic rice plants have been reported to accumulate both low molecular weight osmoprotectants such as mannitol and proline and high molecular weight compounds such as late embryogenesis abundant (LEA) proteins. A group of LEA proteins, HVA, was characterized from barley and introduced into rice plants. There was a concrete evidence on accumulation of the barley HVA1 protein in the vegetative tissue of transgenic rice plants conferred increased tolerance to drought and salt stresses.
The extent of stress tolerance was correlated with the level of HVA1 protein accumulation. A key gene that controls tolerance to drought, salt and cold in rice has been identified. Research groups in Japan and U.S.A. isolated one of the 200 rice genes associated with stress tolerance. This gene is designated as mitogen- activated protein kinase 5 or MAPK5 was used to improve stress tolerance in rice.
The MAPK5 in rice-encoded kinase provides excellent protection against various environmental stresses. This gene is also expressed under biotic stress such as fungal and bacterial phathogens. Therefore, MAPK5 acts as one of the key genetic switch for stress tolerance in rice. Another stress tolerant gene, cor 47, driven by rice actin promoter was deployed into rice grown under drought condition.
In order to support this view, water was withdrawn for few days. The stressed rice plants were then supplied with water for two days for recovery and continued with four cycles. These results consolidate that cor 47 transgenic rice plants are more tolerant to water deficiency than control. The cor 47 gene encoded protein involved in faster growth rate under drought conditions.
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High salinity in rice field presents serious challenge in the objective of productivity. Low molecular weight organic compounds like mannitol and other sugar alcohol provides considerable protection against salt stress in rice.
Transformation of two genes mut1D and gutD gene, which encodes mannitol-1-phosphate dehydrogenase and glucitol-6-phosphate dehydrogenase into the indica rice genome accumulates mannitol and sorbitol, which conferred tolerance against salt stress.
This was confirmed by laboratory experiment where plantlets were grown under different concentrations of salt pool for five days. Production of dehydrin protein; induced by salinity is controlled by a large number of genes. Incorporation of salinity tolerant K+/Na+ selective dehydrin gene into the rice derived from high salt tolerant wheat grass Lophopuron, dramatically improved salt tolerance.
Exploitation of transcription factors like CBF/DRE and ABF related to ABA-dependent stress signalling pathway, in transgenic rice plants become more tolerant to stress. To study stress signalling pathway in transgenic rice, gene constructs were made to contain genes of Arabidopsis CBF3 (DREB1A) and AB1, rice OsCBFs, and barley HUCBFs transcription factors that respond to dehydration.
In an extended study, fusion gene in which two transcription factors CBF and ABF linked by viral 2A sequence induce some stress related rice genes including WS18 and OS dip 1. Several other genes are up regulated by the expression of transcription factors. Hoshida (2000) demonstrated enhanced tolerance to salt stress in transgenic rice that overexpress chloroplast glutamine synthetase, which increase photorespiration and protect against salt stress.
