POLLINATION CONTROL SYSTEM FOR HYBRID CROP VARIETIES

The introduction of Hybrid Crop Varieties has enabled spectacular increase in productivity owing to hybrid vigor and increased uniformity, to produce hybrid seeds a pollination Control system is required to prevent unwanted self pollination  or sib-pollination of the female  line  (Katja Kempi, Mario  Gils, 2011).  Pollination control systems are procedures  in plant  breeding or systems adopted by a plant breeder in order to obtain a desired  hybrid cultivars.  In other words the process of breeding hybrid “hybridization “ is  best achieved through pollination control  systems. These systems render the pollen of one parent line non-viable  (male sterile or
female line)  to ensure  pollination  by  the  chosen parent line. The objective is to develop hybrid crop species that are resistant to abiotic stresses, pest and diseases,   viruses,   and also crops with high  agronomic performances  and  discard plants  with undesirable  characters or  traits.       
   Some of these systems includes:
1. Emasculation: This is carried through the mechanical removal of the anthers e.g. in corn, where the male flowers are separated from the female flowers, the process is called  “detasseling” and involves the removal of the male flower from the plant.
Castration can also be carried out through mechanically removing the male flower organs or by spraying chemical hybridizing agents that prevent the development of active pollen. E.g. according to Mendel’s procedures, to prevent self-fertilization in “test” flowers, anthers were removed from those chosen to be seed parents before their pollen-receiving parts (stigma) are fully mature. The stigma of this test flower was protected against any foreign pollen (via insect or air) with the help of its covering by a bag. Pollen from the designated pollen parent was transferred at the appropriate time to the stigma of the seed-parent flower. To be more precise, the pollen at the dehiscence stage is brought from the plant to be used as a male parent and by the help of brush is dusted on the feathery stigma of the emasculated (operated) flower.
At the time of such cross-pollination, the pollen should be mature and stigma be receptive. (Mario Gils, Katja Kempe, 2011).
2. Genetic Method/System: This can be used to generate the derived male sterility in crops that posses full or “perfect” flower (male and female). The system is either based  on mutated  mitochondria DNA  or else nuclear encoded genes or to some other cytoplasmic factors outside the nucleus, resulting in the transformation of the fertile cytoplasm into a sterile one. According to (Srivastva and Sarkission, 1969) as quoted by (Katja, 2011) male sterile conditions may arise by mutation in mitochondrial bodies (mtDNA) of a normal fertile plant. It is known that mitochondrial are the respiratory sites (power house) within the cell. Mutation in mitochondium may reduce the respiratory capacity of the cell and have result in loss of fertility (Rhoade, 1950) as quoted by (Katja, 2011) with two types of cytoplasm (i.e. sterile and fertile) at the most, only two kinds of genotypes are possible. One of them is sterile and the other fertile. E.g. cotton plants that resist pests by incorporation of a bacterial gene encoding a protein toxic to insect.
3. The “Split Gene” System: (Katja, 2011) this is a biotechnological concept for pollination control, in particular it is a novel system suggested for the industrial production of hybrid wheat. It is based on splitting a tapetum-expressed barnase gene, which causes male-sterility by barness ablation into two fragments.
Pollen oblation into two fragments. A precursor-T- DNA (provector) containing two complementary tape-tum-expressed barenase gene fragments controlled by a tapetum-specific promoter is transformed into plants. Site-specific deletions of the T-DNAs during plant development leads to two alternative derivation, with each producing only one of the two complementary barnase precusor protein. crossing those plants that carry the respective complementary loci with each other leads to progeny that carry the two barnase gene fragments in allelic positions. These plants are male-sterile and are used as the female crossing partners for hybrid breeding. The hybrid progeny plants are fertile as the barnase fragments segregate completed in the F1. For the maintenance of the female crossing partner, the heterozygous plant can be crossed to a homozygous line.
4. Male sterility: Male sterility is a reproduction deficiency of some plants where male organs in hermaphrodite flowers are rendered defunct (Sharma, 1994). Male sterility is of great value in the production of hybrid seeds. It is characterized by non-functional pollen grains, while female gametes functions normally. There are two types
i.                    Genetic male sterility. This is ordinarily governed by a single recessive gene, ms, A male sterile line may be maintained by crossing it with heterozygous male fertile plants. Such a mating produces 1:1 male sterile and male fertile plants. E.g progeny from            ms ms x Ms ms crosses one used as female, and interplanted with homozygous male fertile (ms ms) pollinator. Eg Cotton, barley, tomato, smflower, cucurbits etc. while
ii.                  Cytoplasm male sterility. This is determined by the cytoplasm. Since the cytoplasm of a zygote comes primarily from egg cell, the progeny of such male sterile plant would always be male sterile. The male sterile line is maintained by crossing it with the pollinator strain used as the recurrent parent in the backcross programme since its genotype is identical with that of the male sterile line.  Such a male fertile line is known as the maintainer or B as it used to maintain the male sterile line. It is observed in zea mays, Nicotiana, tobacum,  Helianthus annus, oryza sativa, sorghum bicolor, tritium aestivum, gossypium hrisutum. May be utilized in producing hybrid seed in ornamental spp, or spp where a vegetative part is of economic value.

Use of chemical hybridizing agent. This  system prevents the unwanted self-  pollination of the female parental line. It is a castration method carried out in order to prevent the  development of active pollen  in plant crops. 
Chopra (2003)  reported that these are chemicals  used for pollen abortion which make the plants male sterile without  having  any  side effect on ovule fertility. Such  gametocides could be used to produce male sterile versions of any breeding line, thus obviating the need for laborious  backcrossing  as required for the transfer of genic or  cytoplasmic  male sterility. For  using  in  F1 hybrid seed production, chemical hybridizing  agent (CHA) should cause only pollen abortion  and not affect  ouvle fertility, have no  mutagenic effects, be easy and economical to apply and should have no adverse side effects.  
According to (Sneep et al,  1979) as quoted by (Chopra  2003)   in vegetable, positive results were  obtained with GA3 and GA4/7 in  lettuce and oniou, with maleic  hydrazide in tomato and onion, and with sodium 2,3  - dichloroisobutyrate in tomato. Hence for efficient application of CHA heterosis breeding CHA need to be systemic and flowering has to be more determinate. 

REFERENCES
1.      Chopra V.L  (2003) Plant Breeding, Theory  and  Practice,  Oxford and IBH Publishing  Co Ltd Newdelhi, P.261-263)
2.      Sharma j.r (1994)  Principles and Practice of Plant  Breeding Tata McGraw- Hill Publisher  Co Ltd , New Delhi P  47- 49
3.      Katja Kempe and Mario Gils  (2011) Molecular   Breeding (www. Springerlink.com)
4.     Cytoplasmic Male Sterility – Wikipedia the Free encyclopedia
5.      Science Direct. Com  - trend in Plant Science vol  7,  Issue  5,  May  2002 
6.   Indian Agricultural Resources  -Seeds  
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