Transgenic rubber tolerant to drought, environment stress

30 January 2013 07:06 pm Views - 5642

Natural rubber is pervasive in modern life with more than 40,000 products and 400 medical devices containing the material. In many strategic and medical applications, no synthetic materials can achieve its unique combination of high performance and cost-effectiveness. Natural rubber also has the increasingly compelling advantage of being a renewable resource that will remain with us long after petroleum-derived polymers have disappeared.

Natural rubber (cis-1,4-polyisoprene) is made by more than 2,500 species of higher plants . Despite this diversity, virtually all natural rubber used commercially for more than a hundred years has been derived from a single species Hevea brasiliensis, Muell. Arg., the Brazilian or para rubber tree. Initially, production was centered in South America, based on harvests of rubber from wild trees naturally dispersed in the native rain forest. Attempts to cultivate the tree in plantations eventually failed because of the devastating disease of leaf blight.

 The commercial enterprise was relocated to Southeast Asia, and this region has been the principle source of production ever since. The advanced lines in production are all very closely related to each other, contain little disease resistance, and are grown as clonal (genetically-identical) scions on seedling root stocks planted in close proximity to each other. This lack of genetic diversity, and the intermingling of roots and branches, puts the industry at serious risk of crop failure through drought and environmental stress.

Thus, it has long been a goal (academic and industrial) to have alternate technologies for natural rubber production. However, except in times of war or high oil prices (and embargoes, and other causes of high price, come and go), such sources can only be developed successfully if they can attain and maintain a commercially-viable position in the global marketplace.




Genetic engineering
The Rubber Research Institute of India and Malaysia have developed genetically modified rubber plants that have better drought resistance and increased environment stress tolerance, few years ago.

In future, they could go a long way towards popularising rubber in non- traditional areas where the climate is not so conducive for plantations.
The major objectives of genetic transformation of rubber trees  was the introduction of genes controlling specific agronomic traits -- such as the genes for resisting diseases, drought and other environmental stress tolerance, enhanced rubber biosynthesis and timber yield and tolerance to tapping panel dryness etc -- to high yielding rubber clones.

The genetic transformation technique involved the introduction of specific genes into single cells and development of whole plants from these cells.
The RRII selected the popular RRII 105 variety for the experiments. Although it is a high-yielding clone, the RRII 105 does not have much drought tolerance. It was found that this clone didn’t perform well in areas such as the North-East of India, the non-traditional areas.

The research identified four genes that would provide draught tolerance, tapping panel dryness tolerance and elevated temperature and light tolerance. These genes were introduced into rubber tissues separately, and transgenic plantlets were developed with the gene coding for `superoxide dismutase,’ (SOD), hardened and transferred to polythene bags. Further, these plantlets were multiplied through bud grafting.

It has been said that preliminary biochemical studies revealed that the SOD transformed tissues over-expressed the gene when subjected to artificial stress conditions. To understand the tapping panel dryness tolerance, extensive field evaluation is needed.

Their work is now on to develop transgenic rubber plants with enhanced rubber production by over-expressing the genes involved in the rubber biosynthetic pathway. Research also is in progress to develop transgenic plants producing pharmaceutically as well as industrially useful recombinant proteins in the latex.

 The Genetic Engineering Approval Committee, under the Union Ministry of Environment and Forests, recently approved field trials of the transgenic plants.

 Transgenic rubber plants offered scope to produce clones that better tolerated the impact of change in climatic conditions, which included breaks in monsoon and protracted periods of drought. Natural rubber being a key industrial raw material, a judicious exploitation of new technology to increase production was warranted.
The transfer of selected genes in a single generation by genetic transformation is especially interesting for the rubber tree, since its improvement through conventional breeding is limited by long breeding cycles and high levels of heterozygosity




Biotechnology
 Biotechnology would play an important role in the future of the rubber industry. Plant regeneration via somatic embryogenesis using a variety of explant sources like, integumental tissues, immature anther, immature inflorescences, and leaf explants are well standardized. Many genes controlling important agronomic traits and tissue-specific promoters have been characterized in rubber. Agrobacterium and biolistic-mediated genetic transformation systems are well established in this crop.
Thus, the basic technology for genetic manipulation of rubber plant at the cellular and molecular levels is available, making rubber a suitable crop for genetic engineering. In different laboratories, rubber plants were genetically transformed for recombinant protein production. Transgenic rubber plants were also produced with Mn.SOD gene to confer tolerance against a variety of environmental stresses and tapping panel dryness. Attempts are also going on to enhance the rubber yield through transgenic approaches. Since, the major harvested products are not used as food material, the biosafety concerns are less for the genetically modified rubber plants.




 A unique transgenesis model
The many advantages of transgenic plants for ‘bio-pharming’ notwithstanding, their one significant weakness is the difficulty in recovering the recombinant protein. Unlike transgenic animals where there is continual protein production in the milk, harvesting of the recombinant protein involves destruction of the plant or a portion of it, whether the desired protein is to be found in the seeds, leaves or shoots. After every harvest, it takes time for new growth to take place before the next harvest is possible. As a result, protein recovery is more likely to be batch-wise, rather than a continual process.

 Taking into consideration the strengths of the transgenic animal (continual protein production in the milk) and the transgenic plant (low cost of maintenance, simple clonal propagation) for recombinant protein production, it would obviously be beneficial to have a production system that combines both advantages.

The ideal plant for recombinant protein production would be one that is cheap to maintain and easy to multiply clonally, while allowing for continual harvesting of the protein. This is where the transgenic rubber tree has the distinct advantage when compared with other transgenic crop plants.

In the bark of the rubber tree is a complex network of laticifers, or latex vessels, each vessel merely one-third the thickness of a human hair. These laticifers contain natural rubber latex that is exuded when the bark is cut. Rubber tapping that is routinely practised in estates and smallholdings is essentially the systematic and regulated cutting of the bark to harvest the latex. Since rubber tapping is a non-destructive method of latex extraction and harvesting, the tree can be tapped every alternate day, now even once in 3/4 days, throughout the year without pause.

 Among plants, the rubber tree is unique in its capacity to produce voluminous latex upon tapping and to replenish this supply rapidly in readiness for the next tapping. If Hevea brasiliensis were transformed with a gene encoding a foreign protein, the transgenic Hevea system would allow for continual production of the target protein, a feature lacking in any other transgenic plant system.




Advantages
There are several advantages in using transgenic Hevea for the production of commercially valuable proteins. Among these are:

•  Successful transformation of the rubber tree for a specific gene needs to be achieved only once. Rubber trees are amenable to vegetative propagation and an unlimited number of genetically identical plants (clones) can be generated by conventional horticultural methods.
   
• Production of the target protein is continual through a system of non-destructive harvesting (tapping) of the rubber tree.
   
• The approach is environment-friendly. The process is driven by the sun and is therefore energy-efficient and essentially pollution-free.
   
• The concept is a novel approach to cost-efficient production of high value proteins in the latex of transgenic rubber trees, which essentially serve as production lines.
   
• Rubber trees require no special attention beyond routine horticultural maintenance. Their use is thus highly cost-efficient as compared with conventional bioreactor systems.
   
• The latex that exudes from the rubber tree is free of animal viruses and other contagion vectors. These include pathogenic viruses such as those causing AIDS or hepatitis, and that cause mad cow disease and its human variant.
   
• From the bio-safety viewpoint, the transgenic rubber tree raises far fewer objections as compared with other crops. Hevea  propagation is normally by vegetative means. Hence, it is not expected to have adverse effect on the environment or on the crop. Unlike transgenic food products, recombinant proteins from Hevea are purified from the transgenic elements that are not presented to the consumer.

Conclusion
In simple terms, from the point of view of a practical rubber grower, the main interest in genetic transformation of rubber trees would be the introduction of genes controlling specific agronomic traits-- such as the genes for resisting diseases, drought and other environmental stress tolerance, enhanced rubber biosynthesis and timber yield and tolerance to tapping panel dryness etc -- to high yielding rubber clones. The genetic transformation technique involved is the introduction of specific genes into single cells and development of whole plants from these cells.
(The writer can be contacted at treecrops@gmail.com)