Breeding of Fruit and Plantation Crops 2 + 1

Biotechnological application

a. Micro propagation

• Superior selections and hybrids developed at various research centers failed to reach the orchardists due to lack of sufficient planting material. It leads to non-realization of the potential of improved cultivars, thus making the efforts of fruit improvement programme unfruitful. In this case, micropropogation can be a powerful tool for large scale propagation of fruit crops.
  


• This is also an ideal system for production of disease free plants. Among the fruits, micro propagation has been most successful in banana, papaya and date palm multiplication. Long term micro propagation of passion fruit by formation of multiple shoot primordial initiated from leaf explants has been reported (Kawate et al., 1995). In vitro propagation of grape vine is also possible (Heloir et al., 1997. Gray and Fisher, 1985).



b. Conservation of germplasm

• The potential importance of natural gene pool to meet the future need of crop improvement by introducing specific characters such as abiotic stress resistance can not be under estimated. However, the number of wild species and their natural habitats are disappearing rapidly, as a result of introduction of highly bred modern varieties, growing urbanization and an increased pressure on land for cultivation. This leads to the erosion of the natural germplasm to such extent that there is a general fear that potentially valuable germplasm is being lost irretrievable. In plant improvement, it is necessary to facilitate the assimilation of germplasm collection in working for the use of the breeders.


• The process of genetic erosion necessitates measure that germplasm must be conserved in such a manner that there should be minimal losses of genetic variability of a population. The conventional methods of germplasm conservation may be vulnerable to losses due to
(i) Attack by pest and pathogens
(ii) Climatic disorders
(iii) Natural disasters and
(iv) Political and economic causes. Besides this, the seeds of many important fruit plants such as mango, litchi etc, may loose their viability in a short time under conventional storage system.


• National Bureau of Plant Genetic Resources, New Delhi is maintaining large in-vitro germplasm collection of banana, phalsa, bael, jackfruit, pomegranate etc. There are two basic approaches followed to maintain the germplasm in-vitro.Conservation of germplasm through biotechnology is a more efficient tool for short and medium term storage. It can be achieved by reduced temperature and light, incorporation of sub lethal levels of growth retardants, induction of osmotic stress and maintenance of culture of a reduced nutritional status particularly reduced carbon and reduction of gas pressure over the culture.


• Advantage of this approach is that culture can be readily brought back to normal culture conditions to produce plants on demand. But the disadvantage is that culture may be subjected to contamination and somaclonal variation.Cryopreservation at ultra low temperature of liquid nitrogen at -190ºC offers the possibility for long term storage with maximum phenotypic and genotypic stability.

c. Anther culture

• In-vitro androgenesis holds a myriad of possibilities for improvement of horticultural crops. This technology has been extended for a number of horticultural crops. The purpose of anther and pollen culture is to produce haploid plants by the induction of embryogenesis from repeated divisions of monoploid spores, either microspore or immature pollen grains


• .The major interest in haploids is based upon the production of homozygous plants as an alternative for repeated cycles of inbreeding in self pollinated crops. In cross pollinated species, double haploids are more to be used as parents in the production of single or double cross hybrids which are as follows.


• As a result of haploid induction, chromosome homozygosity is attained in a very short time. This is particularly useful in heterozygous and self incompatible crops like mango, etc.With the use of homozygous parents in crossing programme, the production of pure F1 hybrids become possible.Haploid cell lines have great advantages in studies on mutant selection in-vitro.

d. Overcoming crossing barriers (embryo culture)

• This technique pertains to the cultivation of excised plant embryo in artificial medium. Embryo culture technique has found its application both in the applied and basic research. In the conventional plant breeding programme, breeder often faces problem in transferring resistance from wild species to the cultivated species and getting the desirable interspecific hybrids (Yeung et al., 1981).
  


• Application of embryo rescue can overcome some of the pre and post-fertilization barriers in fruit crops. Further, most useful and popular application of zygotic embryo culture has been used in raising hybrids. Embryo culture technique has important role in haploid production, shortening of breeding cycle (Lammerts, 1942) rapid seed viability test and propagation of rare plants.

e. Somaclonal variation

• Somaclonal variation explores the naturally occurring or in-vitro induced variability of somatic cells following plant regeneration. Somaclonal variation is an excellent method for shortening breeding programmes. Somaclonal variation may be due to variation in chromosome number, structural variation of chromosomes due to deletions, duplication, translocation, genetic and cytoplasmic mutation etc.


• Hwang and Ko (1987) identified Somaclonal variation in the cultivars Giant Cavendish with putative field resistance to Fusarium wilt (race 4) but inferior in agronomic characters. A somaclonal variant of Cavendish banana expressing resistance to Yellow Sigatoka Leaf Spot disease with satisfactory yield has been reported (Chandha and Sahiram, 2000).

f. Somatic hybridization

• It is an approach of immense value in the area of fruit breeding. Somatic hybridization provides a method where sexual incompatibility in the plants can be by-passed. Protoplast culture includes a series of operation such as isolation of the protoplasts from cells, culturing them in a suitable medium, inducing them to divide and then regenerating plantlets from them. Fusion of protoplasts may occur spontaneously or they may be induced to fuse in the presence of fusigenic agent. The polyethylene glycol (PEG) is the most widely used fusigenic agnet (Chandha et al., 2000)

Important fruit plants in which protoplast fusion is practised are as under

Name	Method of fusion
Citrus (Tangelo)+Murrya paniculata	Electrofusion
(Citrus reticulate x Citrus paradisi)+ Citrus jambhiri	Electrofusion
Citrus sinensis+Citrus reticulata	Peg mediated

Molecular markers

• Morphological characters, chemical composition and cytological information have been used over the years for the classification of plants. However, these techniques have certain limitation as they could be influenced by environmental and developmental effects. The molecular markers are now being increasingly used in the areas of plant classification and breeding. Polygenic characters which are very difficult to analyse using traditional plant breeding methods can be easily analysed using molecular markers.

h. Genetic engineering

• The advent of recombinant DNA technology has opened tremendous possibilities for transforming almost any plant by transferring any gene from any organism across, taxonomic barriers. The recombinant DNA technology involves the following major steps.
• Isolation of gene of desired characters.
• Insertion of the isolated gene in a suitable vector (making it a recombinant vector).
• Transformation – Insertion of the recombinant vector into a suitable host (organism /cell).
• Selection of the transformed host.
• Multiplication followed by expression of the introduced gene into the host.