Research review paperAdvances in development of transgenic pulse crops
Introduction
Despite significant political and regulatory barriers, genetically modified crops represent one of the most rapidly adopted technological innovations to have been commercialized in the history of agriculture (Dunwell, 2000, Fernandez-Cornejo, 2005). It is almost a decade since the first transgenic crop moved from laboratory to large farm lands. Pulse crops, well known for their inherent ability to fix nitrogen and the pivotal role they play as the major source of protein for the population of developing countries, ranking third in world food production after cereals and oil seed crops need further genetic improvement by incorporation of alien genes using transgenic technology to meet the increase in demand for food along with improved quality (Christou, 1997, Popelka et al., 2004). Crop yields can be improved by manipulating the physiological processes and the ability to withstand biotic and abiotic stresses. Although conventional plant breeding methods in the past have contributed to the improvement of pulse crops, transgenic technology has immense potential to achieve this objective as an additional/ supplementary technology. Despite the development of transgenic technology in 1980's and the first report on development of a transgenic pulse crop-Vigna aconitifolia in the same decade (Eapen et al., 1987, Kohler et al., 1987a, Kohler et al., 1987b), the progress achieved is not significant compared to their counterpart crops-namely cereals. Cereals were considered to be recalcitrant for regeneration in 1970's and for transformation in 1980's. However, concentrated efforts and free flow of funding support for cereal transformation have pushed transgenic cereals to the forefront of transgenic success stories (Shrawat and Lorz, 2006), while transgenic pulse crop research ,although have moved forward, is still confined to the four walls of the laboratory. Since these tropical grain legumes are of prime importance to developing countries, less efforts have been put forward compared to cereals. Besides, different factors like recalcitrance of pulses for regeneration, low competency of regenerating cells for transformation and lack of a reproducible in planta transformation system have been pointed out as reasons for non-development of transgenic pulse crops with high efficiency (Somers et al., 2003, Popelka et al., 2004, Dita et al., 2006). However — soybean, a leguminous oil crop and a source of protein is a successful example in commercialization of transgenics.
The major pulse crops of the world are bean (Phaseolus vulgaris L.), pea (Pisum sativum L), broadbean (Vicia faba L), chickpea (Cicer arietinum L), pigeonpea (Cajanus cajan L Millsp), blackgram (Vigna mungo L), green gram (Vigna radiata L Wilczek), grasspea (Lathyrus sativus L), lupin (Lupinus spp), lentil (Lens culinaris Medik L Walsp), cowpea (Vigna unguiculata) and winged bean (Psophocarpus tetragonolobus L) and majority of them are grown in tropical and subtropical regions of the world.
Section snippets
Development of transgenic pulse crops
Transgenic pulse crops have been produced using Agrobacterium — mediated (Eapen et al., Eapen et al., 1987, Krishnamurthy et al., 2000, Sharma et al., 2006), by particle gun bombardment (Kamble et al., 2003, Indurker et al., 2007), by electroporation of intact axillary buds (Chowrira et al., 1996) and by electroporation and PEG mediated transformation using protoplasts (Kohler et al., 1987a, Kohler et al., 1987b). Of all the methods, Agrobacterium mediated transformation of explants is the most
Competent cells for regeneration
Plant regeneration in pulses like in other plants can occur through three pathways namely de novo organogenesis, somatic embryogenesis or through proliferation of shoot meristems from areas surrounding a shoot bud. (See Jaiwal and Singh, 2003). Among the three modes of regeneration as target cells for transormation, meristematic areas of cotyledonary nodes are the most preferred explant source as in C. cajan (Dayal et al., 2003, Thu et al., 2003), C. arietinum (Sarmah et al., 2004), V. mungo (
Improving the efficiency of gene transfer
A quick method of delivering foreign gene with high transformation efficiency is required for the development of transgenic pulse crops. Agrobacterium mediated gene transfer is the most common method of transformation of tropical grain legumes, although particle gun bombardment is also used to develop transgenic plants. Among the pulse crops, V. aconitifolia is the only pulse crop reported to have regeneration of complete plants from isolated protoplasts (Eapen et al., 1987) which was exploited
Selectable marker genes
Selectable marker genes have been extremely useful in selection of transgenic cells and plants. Neomycin phosphotransferase (nptII) gene conferring resistance to kanamycin is the most widely used selectable marker gene for pulse crop transformation. Other markers like hpt gene (Puonti-Kaerlas et al., 1990), herbicide resistance bar gene (Richter et al., 2007) and anthranalate synthase gene (asa2) (Chen, 2004) have also been used for selection of genetic transformants. Genes of plant origin -atwc
Chloroplast transformation for preventing horizontal gene transfer
One of the primary concerns about transgenic plants is that they will hybridize with wild relatives, permitting transgenes to escape to the environment. One of the strategies to prevent escape of transgenes to other related plants is to target the foreign gene into chloroplast. Transgene targeting to chloroplast has not only the advantage of high level expression, but also transgene containment by maternal inheritance. It will also reduce gene silencing and pleiotropic effects caused by foreign
Possible targets for transgenic plant development
Inspite of the importance of pulse crops as a key to sustainable agriculture, increases in yield through breeding over the last few decades have lagged behind cereals (Graham and Vance, 2003). Biotic impediments like soil salinity, acidity, drought, nutrient limitations, diseases and pests limit the yield of pulse crops. The main targets for improvement are the following:
- 1.
Pulse crops are susceptible to several insect pests and they bring about great losses in yield. The genes which can be
Integration and utilization of information on genomics and functional genomics from model legumes
Legume research, both fundamental and applied is undergoing a revolution in the field of genomics and functional genomics, ever since, legumes like Medicago truncatula (www.noble.org, www.ncgr.org/research.mgi) (Bell et al., 2001, Tadege et al., 2005) and Lotus japonicus (Udvardi et al., 2005) were adopted internationally as model legumes. Availability of genomic sequences and DNA markers have accelerated the process of identification and isolation of genes responsible for legume specific
Conclusions and future prospects
At present, transgenic pulse crops are produced mostly up to T0 levels only and rarely upto T1 and T2. For successful commercialization, first, the stability and inheritance of transgene have to be ascertained under field conditions. Preferably transgenes have to be contained in the chloroplast by producing transplastomic plants to prevent horizontal gene transfer to related species. The post-genomic era offers opportunities for better understanding of metabolic pathways and identification of
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