Review of Literature
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Review of Literature
Rice is a member of grass family. It belongs to genus Oryza of
Gramineae family. The genus posses about 24 species out of which
22 are wild and two species namely Oryza sativa and Oryza
glaberrima are cultivated. While Oryza glaberrima was grown in the
tropical and subtropical Asia but center of domestication is a matter
of discussion. Some believes that simultaneous domestication in
various center extending from the plains below eastern hills of
Himalayas. Another view postulates a more limited center of
origin. Notably inland valleys in Thailand, Burma and Laos ,Oryza
glaberrima is generally considered as a wild specie .It is commonly
found in West Africa in a few secondary centers. As far as production
is concerned China is the top producer with 166,000.000 metric
tonnes, followed by India with 133,513,000metric tonnes. Brazil
posses lowest of production with 10,219,300 metric tonnes
( Anonymous,2009)
From the initial point or points of cultivation, Oryza sativa has
spread over a considerable part of the earth and has become a major
food crop for a larger part of world’s population. Varieties adaptation
generally includes development of the main subspecies i.e “Indica”
and “Japonica” and the improvement of land and water management
practices, which changed and optimizes rice edaphic and climatic
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environment. Rice is essentially a crop of sub-tropic and on the
higher elevations up to 6000 feet above sea level. Rice need
abundant supply of fresh water for irrigation followed by high
temperatures and high atmospheric humidity .Rice thrives over a
wide range of soils ranging from 4.5 to 8.5 pH. Only a few rice
varieties posses tolerance to saline and alkaline condition.
Rice is a staple food for 17 countries and at present it is grown
over more then 100 countries. Archaeological evidences indicated
that the sophisticated rice cultivation system existed in China over
7,000 years ago. Rice is grown in all the continents except Antarctica.
It is grown in 15.294m millions ha . Today rice produces food for
nearly 2/3 of world population about 4/5 of rice is produced by small
scaled farmers and is an source of income for 100 millions
households of Asia and Africa
RICE IN INDIA :- In India, rice is an important staple food for more
than 2/3rd of population. Rice provides a vital role in our national food
security and is a mean of livelihood for millions of rural households. In
India, a number of festivals such as Bihu, Pongal, Onam are
associated with rice harvest .
There is a considerable increase in productivity of rice in India
during the recent past. The productivity of rice was 668 kg/ha in
1950-51 has reached to 2,066 kg/ha during 2000-02. During the
period of 1961 to 2005, the cultivation of rice has increased from 35
m ha to 44m ha followed by increase in production from 54 million
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tonnes to 124 million tonnes and productivity from 1.54 tonnes to
2.93 tonnes/ha. This increase is basically due to cultivation of the
hybrid rice varieties in major areas with advanced rice production
technologies and mechanization ( Fairhust and Doberman, 2002
b). In India rice is the major staple food for 65% of total population.
The rice cultivation area extends throughout India. Rice
cultivation can be taken up in areas lying below sea level (Kerala)
and up to altitude of 2000m (Kashmir) . During last 45 years (1967 –
2008), the rice area has increased by one half times from 115-50
million ha to about 153.26 million hectares. It is estimated that the
rice demand in 2025 will be 140 million tonnes in India. The demand
for food grain is expected to rise not only as a function of population
growth but also as more and more people crosses poverty line.
Rice ranks second in position in terms of area harvested and
cultivated. In terms of importance as a food crop or in terms of
calorific value rice crop occupies first place amongst cereal crops.
The nutritive composition varies with environmental condition, the
germ, pericarp of aleurone layer are rich in endosperm. Carbohydrate
of rice is starch constituting about 72% to 75% with fiber content of
hemicellulose of pentose, Arabinose and Xylose. Protein “Glutelin”, is
also known as “Oryzenin”, rice also contains albumin, globulin and
prolamines. Rice is however , deficient in Lysine and Threonine.
Rice: Improvement Via Tissue Culture.
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Plant tissue culture has become thrust area in all areas of crop
improvement programmes. The beginning of the plant tissue culture
was made early in 1898, when a German botanist, G. Haberlandt
successfully cultured fully differentiated individual plant cells,
isolated from the different tissues in several plant species. During
1934 to 1939, due to discovery of the important hormones like
auxins and B vitamins, the foundations of the plant tissue culture
were laid down by three scientists (Gautheret, White and
Nabecourt.).
In vitro culture of rice begin in the early 1950 s, it was
“Fujiwara” and “Ojima” (1954) who first of all reported successful
culture of rice roots excised from seedlings germinated aseptically.
However, a major realization in this course of practice is the genetic
variability. This variation in tissue culture raised plants is proving to
be an rich source of crop improvement. Among the techniques anther
culture, protoplast fusion, leaf culture, root culture, immature embryo
culture and seed culture are important in course of crop
improvement.
Rate of callus induction generally depends upon various factors
that includes culture environment genotype, media composition,type
of explant and partial dessication.
( Vasil and Hildebrandt, 1965)
For the rice, the seeds derived embryogenic calli are
considered as most appropriate source of genetic variation (De
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Datta et al., 1990). Embryogenic calli are convenient and large
quantity can be made with absolute uniformity in physiological
characters. They are now widely used for experimental purposes.
The genotype plays a very crucial role in callusing ability of
seeds (Abe and Futsuhara,1986) that requires different types of
medium i.e MS (Murashige, and Skoog,1962 ), N6 ( Chu, 1975),
B6 with different composition of growth hormone. However, for the
rice 2,4-D is found to be only growth regulator for callusing . In
addition to this there are various other chemical like casein
hydrosylate for Japonica rice and Indica Rice. Proline addition in the
medium also induces the callusing in various varieties. Sucrose
concentration however, plays an important role in callusing from the
anther (Guha and Mukherjee., 1964).
Variations induced in Culture :- The variation observed in tissue
culture is due to physiological changes induced by the culture
condition. However, sometimes these variations are temporary and
disappear when culture conditions are removed. However, sometimes
it persists for longer period. The first reports of morphological
variation observed in plants were published by Heinz and Mee
(1971).Since then, several useful variants of sugarcane resistant to
salt stress fungal and viral diseases have now been released. This
variation so produced through tissue culture could be useful for the
development of new cultivars ( Larkin and Scowcroft., 1981)
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Genetic variants selected through tissue couture are referred to
as calli clones, protoclones from protoplast cultures. Somaclonal
variations are used to describe genetic variation regenerated from
the cell cultures . Plants regenerated from cell culture of gametic
origin are termed as “Gametoclones”.
To be of agronomic use, a somaclone must fulfill the basic
requirements i.e It must involves useful characters, it should be
superior from the parent, must be combined with all other desirable
characters of parents and the variations must be inherited stably
through successive generations.
Underlying principle behind the somaclonal variations lies in
quantitative phenotypic variations, activation of transposable
elements and high frequency of sequence changes (Bayliss ,1980).
The “DNA methylation” have been suggested as a possible
mechanism (Kaepler and Phillips, 1993) and patterns of DNA
methylation have been charaterised for many varieties (Selker and
Stevens,1985). The DNA methylation leads to chromatin changes
that ultimately cause disturbed replication timing and base changes.
Essential Role of Phosphorus (P) in plant
Phosphorus is an essential nutrient both as a part of several
key plant structural compound and as a catalyst in the conversion of
numerous key biochemical processes. Phosphorus is a vital
component of DNA, the genetic “Memory unit” of all living things. It is
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also a component of RNA, which reads genetic code. The structures
of both DNA and RNA are liked together by phosphorus bonds.
Phosphorus is highly mobile in plant and when deficiency
occurs it may be translocated from old plant tissue to young actively
growing areas. Thus, early vegetative responses to phosphorus are
often observed .As the plant matures, phosphorus is translocated into
the fruiting areas of the plant where high energy compounds are
needed for the formation of the seeds and fruits.
The percentage of total amount of the each nutrient taken up is
higher for phosphorus late in growing season than for either nitrogen
or potassium.The total phosphorus content of soils is generally very
low i.e 0.6% this compares to an average soil content of 0.14%
nitrogen and 0.83% potassium ( Hoford ,1997) , many factors
influence the content of soil phosphorus. Some of these are : (1)
Type of parent material from which soil is deserved ; (2) Degree of
weathering (3) Climatic conditions. In addition, soil phosphorus levels
are affected by erosion,crop removal and phosphorus fertilization
(Kirk et al., 1998).
Soil phosphorus is classified in two broad groups (1) Organic
phosphorus, found as plant residues, manure and microbial tissues.
In phosphorus rich soils 50% or more of the phosphorus is in organic
form compaired to 3% in deficient solis (2) Inorganic forms consists
of the apatite i.e original source of all phosphorus.
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Soluble phoshorus either in the form of fertilizer or natural
weathering reacts with clay, iron and aluminium compound in the
soil and is converted readily to less available form by process called
“phosphorus fixation”.
Due to fixation, the phosphorus moves little in the soil (less
then an inch ) a crop absorbs more then 20% of fertilizer as a result
the phosphorus is lost by “Leaching”.
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N P K Ca
Fig.1 Relative movement of nutrients in the soil
Precipitation of the phosphorus as slightly soluble calcium
phosphate occurs in calcareous soils with pH values around 8.0 under
the acidic condition, phosphorus is precipitated as Fe or Al
phosphates of low solubility, maximum availability generally occurs in
a pH range of 6.0 to 7.0 and this pH provide phosphorus as H2PO4-
that is readily absorbed by plant than HPO4-
(9.0) Tricalcium phosphates unavailable
7.0 Mono and dicalcium phosphates maximum available
( 5.5) Fe , Al phosphates unavailable
Fig.2 Effects of pH on phosphorus availability
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Phosphorus Deficiency : Effects on Crop-
The plant obtains phosphorus from the soil. It occurs in both
organic and inorganic forms. It is the most limiting element
after the nitrogen (Jungk , 2001) due to it's fixation in the
soil.
About 5-7 billion ha soil worldwide lacks the sufficient plant
available phosphorus (Bates, 1973) and almost 50% of rice soils are
currently phosphorus deficient. Phosphorus deficiency is widespread
in all major rice ecosystems and is major limiting factor in arid upland
soils where soil P fixation capacity is often very large.
( Raghothama, 1999).
The common causes of P deficiency are: low indigenous soil – P
supply, insufficient supply of mineral P fertilizers, low efficiency of
applied fertilizer, P immobilization ( Kirk et al, 1999), excessive
use of N fertilizers, crop establishment methods ( more likely in
direct- seeded rice where plant density is high and root system are
shallow).
Moderate P deficiency is generally different to recognize. In the
field, P deficiency is often associated with other nutrient disorders.
(Bagyarj and Verma, 1995).
Phosphors deficient plants are stumped with greatly reduced
tillering. leaves are narrow, short, very erect and dirty dark green.
Stems are thin and spindly and plant elongation is retarded. The
number of leaves, panicles and grain per panicle is also reduced red
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and purple colors formation occurs if variety posses property of
anthocynin production
(Doberman and Fairhost,2000b). Other effects include
delayed maturity, large production of empty grain, no response to
application of fertilizer.
Plant mechanism, adaptation for low phosphorus stress -
Plant have developed several mechanism to counter the
problem of low phosphorus availability, so as to acquire more
phosphorus from the surrounding rhizosphere. Such adaptations are
generally at the physiological biochemical and morphological levels.
Morphological adaptation for countering low P stress
General diffusion rates of phosphorus is very low because soil
particles easily binds the phosphorus .If P doesn’t move freely in the
rhizosphere, plant may increase P uptake by the expanding the root
system, thereby exploring more soil volume .The size of root system
is considered as an important factor of plant that tolerate P
deficiency ( Wissuwa,2002). Tolerance however, could be
achieved by the increase in the external P uptake efficiency, defined
as the phosphorus uptake per unit root size (RSA) (Raghothama.,
1999).
Alternatively, plant generally capable of releasing soil bound
phosphorus by excreting the organic compound, P solublization due
to organic ions ( anions) were responsible for the bulk of P uptake by
rice from P deficient soil ( Kirk et.al. 1999). Another response to low
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phosphorus availability is the symbiotic association with the
mycorrhiza. The fungus and plant show symbiosis where fungus helps
in aquitisation of phosphates (Bagyaraj and Verma,1995) . Fungus
so extends the phosphorus depletion zone (Harrison and Van
Burren ,1995. Generally genotypes with the more P uptake
efficiency are highly associated with higher relative root growth. This
is due to fact that additional P allows further biomass accumulation,
including root growth ,this is considered as primary factor for the
genotypic differences in P uptake ( Wissuwa, 2002).
It is also reported that under low phosphorus availability the
root to shoot ratio generally increases followed by increase in root
diameter with increase in absorptive surface area. The root
assimilates more root hairs as to enhance uptake of Pi. (Gahhonia
and Nielson. 1998).
Molecular and Biochemical mechanisms involved under phosphorus limited environment :-
Efforts have been directed for improvement of tolerance to low
phosphorus stress. Since, there is considerable variation at genotypic
level for the ability to take up P from a highly fixing soil exists in gene
bank accession.
Genetic studies on tolerance to P deficiency in rice have
identified a major quantitaitve trait locus ( QTL) for Pi uptake
(Senanayake, 1984) which has subsequently been mapped to a
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small marker interval on the long arm of chromosome 12.
( Wissuwa et al., 1998).
Two OsPHR genes from rice Oryza sativa .L were isolated and
designated as OSPHR-1 and OSPHR-2 based on the amino acid
sequences homology to AtPHR 1 ( Bari et.al., 2006) that plays an
crucial role during the Pi starved condition in Arabidopsis
(Hamberger et al ,2002). OsPHr – 1 and 2 are involved in Pi
starvation signaling pathway by regulation of the express of Pi-
starvation.
In E.coli there exists a high affinity phosphate transporter Pst
CAB – this is generally active transport system, more specifically on
ATP binding cassette ( ABC) transporter. It is generally activated
under the low phosphate levels. Similarly, OSP-11 have been
characterized (Wasaki et al., 2003 ). It is responsible for the
adaptation to early stages of adaptation to low phosphorus
environment ( Liu et al., 1997 ).
Recently OSPTF–1 (transcriptional factor) was introduced into
Nipponbare i.e a phosphorus deficiency sensitive, derived from
Kasalth (Keke et. al, 2005). The transformed cultivars showed an
(over expressing OSPTF–1) considerable tolerance to phosphorus
deficiency. Possibility of manipulation of the expression of
phosphorus transported gene may be involved in the mobilization of
phosphorus within soil.
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In E.coli and Brewer's yeast there are reports of some groups
that are activated under phosphorus starved condition (Torriani
and Ludtke, 1990). Psi genes are useful in the enhanced uptake of
pi from soil under p starved condition. The PHO-regulon is now well
known aggregation of gene ( Sacchromycese cerevisiae) that helps in
scavenging any available Pi from soil under starving condition ( Abel
et al, 2002).
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The PHO regulon is generally regulated by the two component
system. namely PHO. B – Pho- R.
Pho R
(Autophosphorylation under P starved condition)
PhoB Enhanced uptake
Binds Pho–Box of Pho- regulon Activation of operon
Fig . Mechanism of Pho -regulon working (Torriani, 1990)
Similar to Pho ragulon of sacchromycese cerevisae, Pho.1 and
Pho. 2 and also Pho.3 have been isolated (Hamberger et al.,2002).
During the phosphorus starved state the common enzymes
associated with phosphate metabolism gets activated, (Green,
1994). This includes “Phytase ”, RNAase” and some phosphate
transporters (Raghothama, 1999). These enzymes plays
crucial role in phosphate nutrition during the stress by performing
phosphorylation and dephosphorylation thereby causing activation
and deactivation of many proteins specially transporters necessary
for phosphorus uptake ( Liu et. al., 1997).
Acid phosphatase is the most crucial enzyme that is involved in
the release of the Pi from organic compounds, ions, soils and tissues
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( Tabatabai and Bremner,1969). The activity of acid phosphatase
generally increase under the low phosphorus stress state. Acid
phosphatase is responsible for moblisation of Pi from phosphate
esters.
It is now well know that the low phosphorus stress leads to
accumulation of the polyamines specially “putrescine” ( Bertoldi et
al, 2004). It was found that common precursor of putrescin i.e.
Argenine decarboxylase complex (ADC) generally increases in various
stresses including P. This in turn increases synthesis of the putrescin
production. The decarboxylated form of SAM i.e S. adenosyl
methionine is precursor of the putrescin and ethylene. Thus, under
low P condition ethylene production is also elevated. Polyamines are
general indication to stresses like acid stresses and somatic stresses
( Flores and Galston, 1982).
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