Transcript Plant genetic resources

Plant genetic resources
Plant genetic resources
It is estimated that nowadays only 30 crops provide 95 percent of human
food energy needs and just four of them – rice, wheat, maize and potatoes
– provide more than 60 percent.
Given the significance of a relatively small number of crops for global food
security, it is of pivotal importance to conserve the diversity within these
major crops.
While the number of plant species that supply most of the world’s energy
and protein is relatively small, the diversity within such species is often
immense.
For example, the number of distinct varieties of the rice species Oryza
sativa, is estimated at more than 100 000.
It is this diversity within species that allows for the cultivation of crops
across different regions and in different situations such as weather and soil
conditions.
Plant genetic diversity may also provide valuable traits needed for meeting
challenges of the future, such as adapting our crops to changing climatic
conditions or outbreaks of disease.
A variety of Turkish wheat, collected and stored in 1948 was ignored until the
1980s when it was found to carry genes resistant to many disease-causing
fungi.
Plant breeders now use those genes to breed wheat varieties that are
resistant to a range of diseases.
Wild botanical relatives of our food crops – often found on the periphery of
cultivated lands – may contain genes that allow them to survive under
stressful conditions.
These genes can add important traits to their cultivated relatives, such as
robustness or frost resistance.
Plant genetic diversity is threatened by “genetic erosion”, a term coined by
scientists for the loss of individual genes and of combinations of genes, such as
those found in locally adapted landraces.
The main cause of genetic erosion, according to FAO’s State of the World’s Plant
Genetic Resources for Food and Agriculture, is the replacement of local varieties
by modern varieties.
As old varieties in farmers’ fields are replaced by newer ones, genetic erosion
frequently occurs because the genes found in the farmers’ varieties are not all
contained in the modern variety.
In addition, the sheer number of varieties is often reduced when commercial
varieties are introduced into traditional farming systems.
Other causes of genetic erosion include the emergence of new pests, weeds and
diseases, environmental degradation, urbanization and land clearing through
deforestation and bush fires.
The International Treaty on Plant Genetic Resources for Food and
Agriculture,popularly known as the International Seed Treaty, is a
comprehensive international agreement in harmony with Convention on
Biological Diversity, which aims at guaranteeing food security through the
conservation, exchange and sustainable use of the world's plant genetic
resources for food and agriculture, as well as the fair and equitable benefit
sharing arising from its use.
It also recognises farmers' rights: to freely access genetic resources,
unrestricted by intellectual property rights; to be involved in relevant policy
discussions and decision making; and to use, save, sell and exchange seeds,
subject to national laws.
The treaty was negotiated by the Food and Agriculture Organization (FAO)
Commission on Genetic Resources for Food and Agriculture (CGRFA)and since
2006 has its own Governing Body under the aegis of the FAO.
Gene banks
Gene banks help preserve genetic material, be it plant or animal. In plants,
this could be by freezing cuts from the plant, or stocking the seeds.
In plants, it is possible to unfreeze the material and propagate it, however, in
animals, a living female is required for artificial insemination.
While it is often difficult to utilize frozen animal sperm and eggs, there are
many examples of it being done successfully.
In an effort to conserve agricultural biodiversity, gene banks are used to store
and conserve the plant genetic resources of major crop plants and their crop
wild relatives.
There are many gene banks all over the world, with the Svalbard Global Seed
Vault being probably the most famous one.
Germplasm Conservation, Dissemination, and Evaluation
The dynamic conservation of genetic resources—as a complementary
approach to conserving germplasm in genebanks—aims to promote their
adaptation to the environment through their continuous cultivation under
biotic and abiotic selection pressures in various agroecological conditions.
Through this project, scientists are identifying the opportunities for dynamic
conservation of rice genetic resources based on farmer-managed systems by
improving their understanding of how farmers manage rice diversity and the
genetic consequences of their actions.
Socioeconomic and genetic studies are being conducted in several
agroecological and socioeconomic environments in India, Philippines, and
Vietnam.
The ultimate goal of this project is to define strategies for conservationists
and breeders to promote the adoption and maintenance of rice diversity by
farmers.
Germplasm Conservation
Conservation refers to protection of genetic diversity of crop plants from genetic
erosion. There are two important methods of germpalsm conservation or preservation. i) In-situ
conservation and ex situ conservation. These are described below.
i) In - situ conservation:
Conservation of germplasm under natural conditions is referred to as in situ
conservation. This is achieved by protecting the area from – human interference, such an area is
often called natural park, biosphere reserve or gene sanctuary. NBPGR, New Delhi, established
gene sanctuaries in Meghalaya for citrus, north Eastern regions for musa, citrus, oryza and
saccharum. Gene sanctuaries offer the following advantage.
Merits: In this method of conservation, the wild species and the compete natural or seminatural
ecosystems are preserved together.
Demerits:
Each protected area will cover only very small portion of total diversity of a crop species, hence
several areas will have to be conserved for a single species.
The management of such areas also poses several problems.
This is a costly method of germplasm conservation.
ii) Ex - situ conservation:
It refers to preservation of germplasm in gene banks. This is the most practical method of
germplasm conservation. This method has following advantages.
It is possible to preserve entire genetic diversity of a crop species at one place.
Handling of germplasm is also easy.
This is a cheap method of germplams conservation.
Ex - situ conservation: 5 types
1) Seed banks:
Germplam is stored as seeds of various genotypes. Seed conservation is quite
easy, relatively safe and needs minimum space. Seeds are classified, on the basis
of their storability into two major groups.
2. Plant Bank: ( Field or plant bank )is an orchard or a field in which accessions of
fruit trees or vegetatively propagated crops are grown and maintained.
3. Shoot tip banks: Germplasm is conserved as slow growth cultures of shoot-tips
and node segments.
4. Cell and organ banks: A germplasm collection based on cryopreserved (at –
196OC in liquid nitrogen) embryogenic cell cultures, somatic/ zygotic embryos
they be called cell and organ bank.
5. DNA banks: In these banks, DNA segments from the genomes of germplasm
accessions are maintained and conserved
Germplam evaluation
Evaluation refers to screening of gemplasms in respect of morphological,
genetical, economic, biochemical, physiological, pathological and
entomological attributes.
Evaluation of germplasm is essential from following angles.
To identify gene sources for resistance to biotic and abiotic stresses,
earliness, dwarfness, productivity and quality characters.
To classify the germplasm into various groups
To get a clear pictures about the significance of individual germplasm line.
The evaluation of germplasm is done in three different places viz.,
(1) in the field (2) in green house a) 3) in the laboratory.
List of important International Institutes conserving germplasm
Name
IRRI
CIMMYT
Institute
International Rice Research Institute, Los
Banos, Philippines
Centre International de-Mejoramients de
maize Trigo, El Baton, Mexico
Activity
Tropical rice
Rice collection: 42,000
Maize and wheat (Triticale, barely, sorghum) Maize collection –
8000
CIAT
Center International de-agricultural Tropical
Palmira, Columbia
Cassava and beans, (also maize and rice) in collobaration with
CIMMYT and IRRI
IITA
International Institute of Tropical Agriculture,
Ibadan, Nigeria.
Grain legumes, roots, and tubers, farming systems.
CIP
ICRISAT
Centre International de-papa-Lima. Peru
International Crops Research Institute, for
Semi-Arid Tropics, Hyderabad, India
WARDA
West African Rice Development Association, Regional Cooperative Rice Research in Collaboration with IITA
Monrovia, Liberia
and IRRI
IPGRI
AVRDC
International Plant Genetic Research
Institute, Rome Italy
The Asian Vegetable Research and
Development Centre, Taiwan
Potatoes
Sorghum, Groundnut, Cumbu, Bengalgram, Redgram.
Genetic conservation.
Tomato, Onion, Peppers Chinese cabbage.
MODE OF REPRODUCTION
Knowledge of the mode of reproduction and pollination is essential for a plant
breeder, because these aspects help in deciding the breeding procedures to be
used for the genetic improvement of a crop species.
Choice of breeding procedure depends on the mode of reproduction and
pollination of a crop species.
Reproduction refers to the process by which living organisms give rise to the
offspring of similar kind (species). In crop plants, the mode of reproduction is
of two types: viz.
1) sexual reproduction and
2) asexual reproduction
I.
Sexual reproduction
Multiplication of plants through embryos which have developed by fusion of male and
female gametes is known as sexual reproduction. All the seed propagating species
belong to this group.
Sporogenesis
Production of microspores and megaspores is known as sporogenesis. In anthers,
microspores are formed through microsporogensis and in ovules, the megaspores are
formed through megasporogenesis.
Microsporogenesis
The sporophytic cells in the pollen sacs of anther which undergo meiotic division to form
haploid i.e., microspores are called microspore (MMC) or pollen mother cell (PMC) and
the process is called microsporogenesis. Each PMC produce four microspores and each
microspore after thickening of the wall transforms into pollen grain.
Megasporogenesis
A single sporophytic cell inside the ovule, which undergo meiotic division to form haploid
megaspore, is called megaspore mother cell (MMC) and the process is called
megasporogenesis. Each MMC produces four megaspores out of which three degenerate
resulting in a single functional megaspore.
Gametogenesis
The production of male and female gametes in the microspores and megaspores
is known as gametogenesis.
Microgametogenesis
This is nothing but the production of male gametes or sperm. On maturation of the
pollen, the microspore nucleus divides mitotically to produce a generative and a
vegetative or tube nucleus.
The pollen is generally released in this binucleate stage. The reach of pollen over the
stigma is called pollination. After the pollination, the pollen germinates. The pollen tube
enters the stigma and travels down the style.
The generative nucleus at this phase undergoes another mitotic division to produce two
male gametes or sperm nuclei. The pollen along with the pollen tube possessing a pair of
sperm nuclei is called microgametophyte.
The pollen tube enters the embryo sac through micropyle and discharges the two sperm
nuclei.
Megagametogenesis
The nucleus of the functional megaspore undergoes three mitotic divisions to produce
eight or more nuclei.
The exact number of nuclei and their arrangement varies from one species to
another. The megaspore nucleus divides thrice to produce eight nuclei. Three of these
nuclei move to one pole and produce a central egg cell and two synergid cells on either
side.
Another three nuclei migrate to the opposite pole to develop into three antipodal cells.
The two nuclei remaining in the center, the polar nuclei, fuse to form the secondary
nucleus.
The megaspore thus develops into a mature female gametophyte called
megagametophyte or embryo sac. The development of embryo sac from a megaspore is
known as megagametogeneis.
The embryo sac generally contains one egg cell, two synergids with the apparent
function of guiding the sperm nucleus towards the egg cell and three antipodals which
forms the prothalamus cells and one diploid secondary nucleus.
Fertilization:
The fusion of one of the two sperms with the egg cell producing a
diploid zygote is known as fertilization.
The fusion of the remaining sperm with the secondary nucleus leading
to the formation of a triploid primary endosperm nucleus is termed as
triple fusion.
The primary endosperm nucleus after several mitotic divisions develops
into mature endosperm, which nourishes the developing embryo.
II. Asexual reproduction
Multiplication of plants without the fusion of male and female gametes is
known as asexual reproduction.
Asexual reproduction can occur either by vegetative plant parts or by
vegetative embryos which develop without sexual fusion (apomixis).
Thus asexual reproduction is of two types: viz. a) vegetative reproduction
and b) apomixis.
Vegetative reproduction refers to multiplication of plants by means of
various vegetative plant parts.
Vegetative reproduction is again of two types: viz. i) natural vegetative
reproduction and ii) artificial vegetative reproduction.
Natural vegetative reproduction
In nature, multiplication of certain plants occurs by underground stems,
sub aerial stems, roots and bulbils.
In some crop species, underground stems (a modified group of stems) give
rise to new plants.
Underground stems are of four types: viz. rhizome, tuber, corm and bulb.
Artificial vegetative reproduction
Multiplication of plants by vegetative parts through artificial method is
known as artificial vegetative reproduction. Such reproduction occurs by
cuttings of stem and roots, and by layering and grafting. Examples of
such reproduction are given below:
Stem cuttings: Sugarcane (Saccharum sp.) grapes (Vitis vinifera), roses,
etc.
Root cuttings: Sweet potato, citrus, lemon, etc.
Layering and grafting are used in fruit and ornamental crops.
Apomixis
Apomixis refers to the development of seed without sexual fusion
(fertilization). In apomixis embryo develops without fertilization. Thus apomixis is
an asexual means of reproduction.
Apomixis is found in many crop
species. Reproduction in some species occurs only by apomixis. This apomixis is
termed as obligate apomixis. But in some species sexual reproduction also occurs
in addition to apomixis. Such apomixis is known as facultative apomixis.
There are four types of apomixis: viz.
1. Parthenogenesis. Parthenogenesis refers to development of embryo from the
egg cell without fertilization.
2. Apogamy. The origin of embryo from either synergids or antipodal cells of the
embryosac is called as apogamy.
3. Apospory. In apospory, first diploid cell of ovule lying outside the embryosac
develops into another embryosac without reduction. The embryo then develops
directly from the diploid egg cell without fertilization.
4. Adventive embryony. The development of embryo directly from the diploid
cells of ovule lying outside the embryosac belonging to either nucellus or
integuments is referred to as adventive embryony.
Pollination
Transfer of pollen grains from the stamens, the flower parts that produce
them, to the ovule-bearing organs or to the ovules (seed precursors) themselves.
As a prerequisite for fertilization, pollination is essential to the production of fruit
and seed crops and plays an important part in programs designed to improve plants
by breeding.
Types: self-pollination and cross-pollination
Agents of pollen dispersal
Beetles and flies
Bees
Wasps
Butterflies and moths
Wind
Birds
Self-pollination
is a form of pollination that can occur when a flower has both stamen and a
carpel(pistil) in which the cultivar or species is self fertile and the stamens and the
sticky stigma of the carpel contact each other in order to accomplish pollination.
The mechanism is seen most often in some legumes such as peanuts. In another
legume, Soybeans, the flowers open and remain receptive to insect cross
pollination during the day; if this is not accomplished, the flowers self pollinate as
they are closing.
Other plants that can self pollinate are many kinds of orchids, peas, sunflowers,
tridax,etc. Self pollination, or more generally self pollenizing, limits the variety of
progeny and may depress plant vigor.
However, self pollenizing can be advantageous, allowing plants to spread beyond
the range of suitable pollinators or produce offspring in areas where pollinator
populations have been greatly reduced or are naturally variable.
mating systems
Mating system: the mode of transmission of genes from one generation to
the next through sexual reproduction (e.g. maternal selfing rate)
Selfing rate (s): the proportion of seeds that are self fertilized
Outcrossing rate (t=1-s): the proportion of seeds that are outcrossed
Inbreeding depression: the reduction in viability and fertility of inbred
offspring compared with outbred offspring.
Qualitative & quantitative traits
The phenotypic traits of the different organisms may be of two kinds, viz.,
qualitative and quantitative.
The qualitative traits are the classical Mendelian traits of kinds such as form
(e.g., round or wrinkle seeds of pea); structure (e.g., horned or hornless
condition in cattles); pigments (e.g., black or white coat of guinea pigs); and
antigens and antibodies (e.g., blood group types of man) and so on.
We have already discussed in previous chapters that each qualitative trait may
be under genetic control of two or many alleles of a single gene with little or
no environmental modifications to obscure the gene effects.
The quantitative traits, however, are economically important measurable
phenotypic traits of degree such as height, weight, skin pigmentation,
susceptibility to pathological diseases or intelligence in man; amount of
flowers, fruits, seeds, milk, meat or egg produced by plants or animals, etc.
The quantitative traits are also called metric traits. They do not show clear cut
differences between individuals and forms a spectrum of phenotypes which
blend imperceptively from one type to another to cause continuous
variations.
In contrast to qualitative traits, the quantitative traits may be modified
variously by the environmental conditions and are usually governed by many
factors or genes (perhaps 10 or I00 or more), each contributing such a small
amount of phenotype that their individual effects cannot be detected by
Mendelian
methods
but
by
only
statistical
methods.
Qualitative genetics
Quantitative genetics
It deals with the inheritance of traits of kind, viz., It deals with the inheritance of traits of
form, structure, colour, etc.
degree, viz., heights of length, weight,
number, etc.
Discrete phenotypic classes occur which display
A spectrum of phenotypic classes occur
discontinuous variations.
which contain continuous variations.
Each qualitative trait is governed by two
Each quantitative trait is governed by
or many alleles of a single gene.
many non-allelic genes or polygenes.
The phenotypic expression of a gene is
Environmental conditions effect the
not influenced by environment.
phenotypic expression of polygenes
variously.
It concerns with individual matings and their
It concerns with a population of organisms
progeny.
consisting of all possible kinds of matings.
In it analysis is made by counts and ratios.
In it analysis is made by statistical methods.