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.
IMPORTANCE
OF POLLINATION CONTROL SYSTEM IN PLANT BREEDING
A pollination
control system and effective transfer of
pollen from one parent to the
other offers improved plant breeding and
an effective method for producing hybrid seeds. The adoption
of pollination control system by plant breeder will lead to the production of hybrid spp/
verities that one tolerant / resistant
to abiotic stress factors, multiple
rests, disease and insects which cause severe yield losses in agricultural production. (chopra, 2003) stated that breeding for tolerance to various
biotic stresses not only ensure stable production but also safe guards the environment
and health. such as
i.
Hybrid for abiotic stress
tolerance. By breeding me seek to develop genotypes that combine superiority
in yield with tolerance to various kinds of stresses eg high or low temperature, salinity, acidity, lack of nutrients and atmospheric and soil pollutants, are the
kinds of abiotic stresses inhibiting
crop growth.
ii.
Hybrids of high quality and quantity yield, tolerance to diseases, pests, insects,
virus, bacteria and pathogen may be achieved through the use of pollination control systems.
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.
www.ScienceDirect.Com - trend in Plant Science vol 7,
Issue 5, May
2002
6.
Indian Agricultural
Resources -Seeds