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BIOTECHNOLOGY
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Mango Biotechnology
Dr. Jafar, Yosuf & Haider
Enthusiasm for planned hybridization effected by means of hand pollination. However, in recent years, improvements in pollinating techniques and more rapid screening of hybrid populations have enabled the release of many hybrid mango cultivars of commercial value. Because of the world market's demand for mangoes with specific qualities, the synthesis of new cultivars has become imperative. Moreover, problems such as irregular bearing, poor fruit set, insect pests, sensitivity to cold, pathological diseases and poor post-harvest storage are serious problems in mango industry.
Rapid strides in molecular biology and in other aspects of biotechnology have opened up new approaches in plant breeding. The recent development of genetic markers for mango and their application to classical breeding offer tremendous potential for mango improvement. The introduction of specific genes for disease resistance from cultivated varieties and wild species into popular cultivars should soon be a reality.
Although genetic transformation of mango with selectable and scorable marker genes is not limiting factor (Mathews and Litz, 1992), very few genes have actually been isolated from mango. Genes coding for such horticulturally important traits as tree size, yield and fruit quality are not yet available.
In Vitro Selection and Regeneration
In-vitro selection have the potential to select mango types that have positive mutation/variation through somaclonal variation. Various regeneration protocols have been developed including callus induction, somatic embryogenesis and organogenesis using different explants including cotyledons, nucellus, leaf disks and shoot tips of mango. Somatic embryos has been recovered from nucellus tissues of young mango fruit but standardization of media for maturation and germination of somatic embryos is very substantial.
The Mangifera spp. have their center of origin in southeast Asia. Vast areas have been completely or partially deforested for expanding agriculture. This has caused great genetic erosion within many species and genera including mango. Because of the loss of natural habitat, the establishment of ex situ and in situ germplasm collections of Mangifera species was considered to be imperative. Sources of resistance to mango malformation, anthracnose, powdery mildew are urgently needed and can be exploited from other species of Mangifera.
Preservation of Mangifera spp. germplasm for long-term conservation should also be feasible by means of cryopreservation of embryogenic cultures (Engelmanns, 1991). Of the hundreds of mango cultivars, only some 25 to 40 are of commercial importance. There is considerable confusion regarding cultivar nomenclature, since similar cultivars grown in different areas are known by different names (Campbell, 1992).
For example 'Alphonso', probably the best mango cultivar grown in India, has been referred to as Badami, Gundu, Patnam, Jathi, Khader, Aphus, Hapus etc. (Lakshminarayana, 1980). Furthermore, different cultivars sometimes appear under the same name (Gangolly et al., 1957).
Mango cultivars are currently identified on the basis of morphological traits such as leaf and fruit characteristics. Recently, reliable genetic markers have been developed and introduced for mango cultivar identification. These include isozymes, random amplified polymorphic DNAs (RAPDs) and variable number tandem repeats (VNTRs). Genetic markers are not only to be useful for cultivar identification but also for the purpose of mango breeding.
Mango trees are very large because they evolved as part of the canopy layer of tropical forests. Hence attempts have been focused on obtaining hybrids with dwarf tree framework. The Indian cultivars that could be useful as a source for dwarfness include Kerala Dwarf, Amarpalli, Creeping, Manjeera (Iyer and Subramanyan, 1986) and Nileswar Dwarf (Singh, 1990), otherwise fruit quality of these selections is generally considered to be inferior to that of existing commercial cultivars.
Alternate bearing is a serious production problem in mango. Mango flowering is influenced by various factors, but most importantly by the interaction between genotype and the environment . Tota Pari (Banglora), Fazli and Neelum are regular bearers and have been extensively used as one of the parents in a hybridization program to transfer the regular bearing habit to hybrids. The regular bearing Florida cultivars, i.e. Tommy Atkins, Keitt etc.
also have potential as parents. Also, it could be possible to overcome the problem of alternate bearing in mango by genetic engineering. The recently discovered flower-meristem-activity LEAFY gene from Arabidopsis thaliana (Weigel and Nilsson, 1995) causes flower initiation in transgenic poplar (Populus sp.). Under the control of an appropriate development promoter, it could be possible to overcome the problem of alternate bearing in mango by genetic engineering.
In postharvest physiology, one of the main concerns for prolonging the shelf life of fresh fruits is to stop ethylene action and this is usually achieved by the use of controlled atmospheres or ethylene absorbents like potassium permanganate or activated charcoal/vanadium oxide (Maekawa, 1990).
Mango is a climacteric fruit and is highly perishable due to over ripening & increased susceptibility of mature fruit to development of anthracnose. Genetic transformation of mango has potential for prolonging fruit shelf life. The mango ACC synthase and ACC oxidase genes are currently being used in mango transformation experiments.
These are the first mango genes that have been cloned in antisense strategy to stop ethylene production and extend the fruit shelf life. With regard to insect pests, considerable variation is also known to occur among mango cultivars with respect to their susceptibility to attack and injury. Transgenic plants provide better insect control It is evident that mango suffer from a range of fungal and bacterial infections and result in an overall loss of fruit yield.
Plants also respond to microbial infection by the activation of genes that produce antimicrobial compounds, e.g. phytoalexins (Anderson, 1978), protease inhibitors, defensins (Osborn et al., 1995) and the release of oligosaccharide elicitors (Broglie and Broglie, 1993).
Dr. Jafar Jaskani
Incharge Plant Tissue Culture Laboratory,
Department of Horticulture, University of Agriculture, Faisalabad
Haider Abbas
Horticulturist
University of Karachi, Karachi
Mr. Yousaf
M.Sc Student
Department of Horticulture, University of Agriculture, Faisalabad
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