'Give a detailed discussion and critical analysis of several of the biotechnologies such as tissue culture and seed banking that are being developed and employed to conserve rare and endangered species.'
Conservation has become one of the central themes of modern day biology as a direct response to the alarming decline in the world's biodiversity. Increasingly we hear of endangered species, loss of habitat, extinction and we can see people all over the world becoming more aware of the natural world and its connection to them. The increase in the use of this language however, is a reflection of the changes that are afoot and that there is something wrong.
In 2008, there were 8.457 plant species listed on The International Union of Conservation of Nature and Natural Resources (IUCN) Red List of Threatened Species. This accounts for 70% of the plant species evaluated (12,055) or 3% of the estimated number of the worlds described plant species (298,506) (IUCN, 2008). These numbers do not account for near threatened or data deficient species. Because approximately 4% of the world's estimated plants species are evaluated it is not accurate to say that 3% of the world's plants species are endangered. Contrarily, of the species evaluated, 70% are listed as threatened and this is believed to partially reflect a bias within the botanical community to assess species which are thought to be endangered and overlook species which would be considered least threatened. It is believed the actual global figure for plants listed as endangered lies somewhere in the middle of the two figures (IUCN 2008). There is no doubt that this is an alarming global issue and requires immediate action if we are to protect many of these species from threats which could ultimately lead to their extinction.
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The idea of conservation biology is centred on the long term viability of entire systems and its goal is to provide values and methods for preserving and maintaining biological diversity (Coates and Dixon, 2007). Although the goals of conservation biology remain constant, in the past two decades conservation biology has become more complex and multi-disciplined shifting from the traditional (in situ) methods of habitat management and management of wild populations to (ex situ) methods involving various biotechnologies. Ex situ conservation preserves and maintains samples of living organisms outside their natural habitat in the form of individuals of species, seeds, pollen, vegetative tissue culture, cells or propagules. More often than not, ex situ conservation techniques may be utilized to support in situ techniques and under certain circumstances ultimately may be the only viable choice for certain species (Maunder et al., 1998, Ramsay et al., 2000). Among ex situ conservation the most favoured methods would be cultivation of whole plants in botanic gardens, seed storage and in vitro cultivation (Paunescu, 2008).
The world has a wealth of botanical gardens which are responsible for the cultivation of more than one third of the world's flowering plants. The cultivation of plants in botanic gardens serves as an efficient tool for conservation however this ex situ method can be limited in both time and space and may face potential problems with acclimatisation of plant material. Seed storage seems to be quite a useful method for conservation particularly if the storage term of the material involved is to be of a prolonged nature.
Seed storage is one of the most convenient methods for medium to long term conservation of plant genetic resources. It primarily involves the desiccation of seeds to low moisture contents and the storage of such seeds at low temperatures. Seed conservation of non-crop species has benefitted greatly by the Millennium Seed Bank Project of the Royal Botanic Gardens, Kew which links a host of international partners and emphasises the requirement for further research and development within seed banking (Coates and Dixon 2007). One of the main focus points of the project will be that of species located in arid regions of the world with its aim being to conserve up to 24,000 species of dryland plants which accounts for approximately 10% of the worlds flora (Van Slageren 2003). However, as with any method, it is not suitable for all species as some species produce recalcitrant seeds which lose viability rapidly and therefore would not be suited to ordinary methods of storage. Many crop species face the same difficulties as many do not easily produce seeds or if so the seed is normally strongly heterozygous therefore clonal propagation is the preferred method to preserve any unique or superior genotypes. Examples of such crop species are the potato, banana, cassava, sweet potato and sugar cane which are often stored in field gene banks. Due to the randomness of the natural world field gene banks are quite susceptible to natural disasters, disease or pests and therefore must also be supported using alternative strategies where available.
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The requirement for alternative methods for plant conservation has resulted in advances in biotechnology as a conservation tool that can provide support for methods in place and avenues for potential methods to be explored. Much of the traditional ex situ conservation techniques are now complimented by in vitro conservation through techniques such as Tissue Culture, Cryopreservation and Slow-Growth.
Tissue culture is one of the first steps of ex situ plant conservation and involves initiating plant material in a culture medium after collecting it in the field. Once the plant material has been initiated and the culture can be maintained there are many options available which all aid in the conservation of the plant material in question. Such options would be slow growth storage, seed banking, cryopreservation, propagation for botanic gardens or propagation for re-establishment in the wild. Usually tissue culture occurs in the laboratory and is the 'next-step' to follow from collection in the field but techniques have also been developed to allow tissue culture to be initiated directly in the field (in vitro collection) (Pence, 2005.). Naturally, this type of collection and tissue culture initiation greatly improves the possibility to conserve rare plants or plants which may be located in remote sites.
For successful tissue culture initiation to occur, good quality plant material is required where explants should be without abnormalities, vigorous, free from disease or pathogens and similar to that of the donor plant (Fay, 1992). As with most biotechnological conservation techniques, it is difficult to prescribe a single protocol that would be applicable to a large range of plants, usually it varies between taxa, families and sometimes single species require specific parameters in order for successful initiation into the tissue culture to occur. Depending on the addition of various salts, growth hormones and supplements to the growth media and the nature of the growth media itself be it viscous or solid can greatly influence the successful initiation and growth of many species. The propagation and rooting of the endangered tree Gingko biloba requires endosperm extract from its own seeds in order to allow successful propagation and rooting (Tomassi and Scaramuzzi, 2004). Rooting problems are also noted in other woody species and recalcitrant taxa where specialized protocols are required such as another endangered tree Trochetiopsis ebenus (Sarasan, 2003) or more commonly known as the Saint Helena ebony whose distribution is limited to the island of Saint Helena in the southern Atlantic Ocean.
Contamination poses a large threat and when undertaking in vitro culture the risk of contamination is always higher and can become quite problematic when dealing with endangered species where the source plant may often be limited in its distribution and usually remote in location (Sarasan et al. 2006). Plant tissue culture serves as a good tool for conservation but can also have wider applications such as urban planning where (Sudershan et al, 2003) selected certain genotypes of desert species (Rhanterium epapposum, Ochradenus baccatus, Nitraria retusa and Lysium shawii) and developed tissue culture technology due to their potential for urban planning and desert revegetation.
Cryopreservation involves the storage of in vitro plant material, seeds, spores, and DNA at extremely low temperatures very close to that of liquid nitrogen, approximately -196°C. The metabolic activities of materials stored at these temperatures are essentially arrested or suppressed therefore allowing the selected material to be stored for long periods of time without much alteration. Cryopreservation can be applied to a wide range of vegetative material including shoot-tips, axillary buds, embryonic axes, somatic embryos and gametophytic material of both bryophytes and ferns (Sarasan et al. 2006). It is currently the only technique available that facilitates the safe, long term storage of species with recalcitrant seeds or those species that are vegetatively propagated (Engelmann and Dussert, 2000).
Cryopreservation is ideal for long term storage as it requires minimal space, is quite resistant to the threat of contamination, involves much less labour and maintenance than other biotechnological methods and is generally considered to be quite cost effective. Cryopreservation is currently considered to be the most suitable method for long term storage of genetic resources and has been highlighted by a number of authors as a valuable conservation tool (Engelmann, 2004; Stacey et al., 1999).
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Techniques within cryopreservation are varied but can be divided into two main areas, the 'classic' freeze induced dehydration technique and the more modern vitrification technique. The classic approach involves the gradual freezing of the plant material at a controlled rate (approx. 0.1°C-4°C) down to a temperature of +/- 40°C and immediate submersion in liquid nitrogen once the desired temperature has been reached. This method requires an apparatus which allows the controlled freezing of the material. Vitrification techniques involve the removal of most if not all water which could be frozen by physical or osmotic dehydration of plant material. This is immediately followed by rapid freezing which causes the vitrification of intracellular solutes. It allows the rapid freezing of the plant material without the formation of ice crystals which can be harmful to cell structure integrity. Vitrification is more applicable to complex organs such as embryos and is more suited to basic laboratories as expensive machinery to allow programmed freezing is not required.
Due to the diverse nature of the plant kingdom and the extreme variability within taxa, it is a challenge to determine a single cryopreservation protocol that could be applied to a wide range of plants. For example some species may be freezing or desiccation tolerant, some species vary in size, even the ecological backgrounds of the various species may need to be investigated in order to determine a suitable cryopreservation method. At the Royal Botanic Gardens, Kew simple encapsulation-dehydration protocols were developed for the cryopreservation of bryophytes as part of an ex-situ project for the conservation of threatened species within the UK. Twenty two species with a broad range of ecological requirements were tested and the results were quite successful with a regeneration rate from frozen of 68% for all species and over half of these species having a 100% regeneration rate from frozen (Ramsay, M. Rowntree, J. 2009). This shows how a particular taxa can be quite suited to a single protocol but how a large number of species within a large number of families with varied ecological requirements can also be suited to a single protocol.
Slow growth as the name suggests involves the reduction in growth rates of in vitro plant collections and is considered to be a useful short-medium term storage method. This method is used in many international laboratories however it can prove to be problematic even with increased time between sub-cultures (Benson 1999). Generally slow growth storage ranges from a few months to a few years and there are a number of recognised protocols for reducing in vitro growth rates including physical, chemical and a combination of the two (Engelmann et al., 2002).
In most instances a decrease in both temperature and light intensity or possibly complete darkness is used to reduce growth rates. The temperature at which species are stored depends mainly on the species in question. For example a temperature of 0-5°C may be adequate to store a cold tolerant species whereas a tropical species may require much higher temperatures in the region of 15-20°C. As with temperature, photoperiod is also species dependent and can range from total darkness to a 12-16 hour photoperiod coupled with a variance in light intensity which can also be species specific (Paunescu, 2009). The endangered Spanish plant Centaurium rigualii was stored for three years with a 90% survival rate (Iriondo and Perez, 1996) by maintaining the temperature at 5°C and a 16 hour photoperiod. In addition to temperature and light, reductions in growth rates can be achieved by altering the culture medium such as reducing mineral and/or sugar concentrations. Another approach which was found to be effective for medium-term storage was to decrease the level of oxygen available to plants by covering explants with a layer of liquid medium or mineral oil (Withers and Engelmann, 1997).
Somaclonal variation is of particular concern for slow growth cultures as often clonal material is used for re-establishment programmes. Continuous sub-culturing can cause genetic changes in plants and lead to progenies with altered genetic characteristics. This is also known as somaclonal variation which is unpredictable in nature and can be both heritable and non-heritable. Slow growth storage is usually used for organized cultures such as shoots as use of undifferentiated tissues such as callus can be more susceptible to somaclonal variation. Although not directly promoted for conservation purposes the propagation of somaclones with superior characteristics may have real potential for reducing stress on natural populations if they were to have similar qualities and compounds.
Biotechnology certainly is an important factor in the conservation or rare and endangered species playing a vital role as a tool for in vitro conservation and as a supporting mechanism for conventional methods of conservation. Biotechnology has helped improve conservation and genetic resources especially with problem species and has provided advanced measures to conserve threatened plant germplasm. In vitro techniques and storage methods have made the establishment of extensive germplasm collections possible. This has multiple benefits by increasing collections size while decreasing the collection pressure on wild populations, allowing continuous supplies of material for re-establishment of wild populations, ecological research and economic uses.
Some of the most critical aspects of these biotechnologies are in vitro collecting, slow growth, and cryopreservation. Tissue culture itself provides a fundamental basis for in vitro germplasm cultivation whilst also being a supporting platform for cryopreservation and other storage methods. The development of these storage methods has provided the possibility of establishing extensive germplasm collections with a vast genetic diversity. Slow-growth storage techniques are slightly more advanced than cryopreservation techniques as more research into species specific protocols is required before cryopreservation can be utilised extensively for a wide range of flora.
While all the technologies mentioned above do provide excellent potential for future conservation and there is a real need for widespread training and collaboration they are expensive and as a large proportion of the worlds threatened taxa are located in developing countries it may pose problems for the future. Networking and exchanging ideas, information and material between in vitro facilities, botanic gardens, research centres and learning institutions at a regional, national and international level will be of particular importance to any large scale conservation effort. Also as people become more aware of man's impact on the planet there is a greater need for the broadening and availability of information and the direct participation of those people be they farmers, tribes, villagers, walkers, hunters etc.