Grain pro-duction is largely influenced by the yield potential of rice varieties.. Therefore, improvement in the yield potential of rice is the major strat-egy to increase world rice p
Trang 1Strategies for increasing the yield potential of cereals: case of rice as an example
GU R D E V S KH U S H
University of California, 39399 Black Hawk Place, Davis, CA 95616-7008, USA; Corresponding author, E-mail: gurdev@khush.org
Received April 30, 2012/Accepted May 30, 2012
Communicated by P Gupta
Abstract
Rice is the most important food crop Worldwide, more than 3.5 billion
people depend upon rice for more than 20% of their calories Global rice
demand is estimated to rise from 676 million tons in 2010 to 763 million
tons in 2020 and to further increase to 852 million tons in 2035 This is
an overall increase of 176 million tons in the next 25 years To meet this
challenge rice production on existing land must be increased Grain
pro-duction is largely influenced by the yield potential of rice varieties.
Therefore, improvement in the yield potential of rice is the major
strat-egy to increase world rice production Various strategies to increase the
yield potential include; (1) conventional hybridization and selection, (2)
ideotype breeding, (3) hybrid breeding, (4) exploitation of wild species
germplasm, (5) enhancement of photosynthesis, (6) genomic approaches
and (7) physiological approaches Rice has become a model plant for
genetics and breeding research Advances in molecular biology and
genomics, proteomics and metabolics have opened new avenues to apply
innovative approaches to rice breeding Molecular-assisted selection
(MAS) has become an integral component of germplasm improvement.
A large number of genes/QTLs for various traits have been tagged with
molecular markers to apply MAS for trait improvement Genome
sequence data have become an important source for detecting allelic
var-iation It is now possible to precisely understand the role of epigenetic
changes in gene expression, thus understanding the stability of various
stresses under changing environments All these advances when
inte-grated with conventional breeding will result in designer rice varieties to
meet the challenges of rice food security in 21th century.
Key words:yield potential — rice breeding — strategies
Rice is the most important food crop and staple food of more than
half of the world’s population Worldwide, more than 3.5 billion
people depend upon rice for more than 20% of their daily calories In
most of the developing world, rice availability is equated with food
security and closely connected to political stability Rice yield
growth has fallen from 2.3% per year during 1970–1990 to 1.5%
dur-ing 1990s and to<1.0% during the first decade of present century
Global rice demand is estimated to rise from 676 million tons
in 2010 to 763 million tons in 2020 and to further increase to
852 million tons in 2035 This is an overall increase of 26% or
176 million tons in the next 25 years However, area planted to
rice in major production countries has been decreasing because
of conversion of land for housing, industries and highways
Hence, increasing rice yields on existing land remains primary
strategy for increasing production Grain production is largely
influenced by the yield potential of rice varieties Therefore,
improvement in the yield potential of rice is the major strategy
to increase world rice production To produce 176 million tons
additional paddy rice by 2035, we need to increase the yield
potential of rice from 10 to 12.3 tons per hectare
Various strategies to increase the yield potential of rice are discussed below
Conventional Hybridization and Selection
Traditional hybridization and selection is still a widely used strategy for developing crop varieties with a higher yield potential In this methodology, the segregating populations derived from crosses between two parents are screened for desirable recombinants and selected lines are evaluated in replicated yield trials This approach is therefore based on the variability created through hybridization between diverse parents and subsequent selection of desirable individuals Based on this strategy, about 1.0% increase has occurred per year in the yield potential of various cereals such as wheat, barley and rice (Peng
et al 2000) This is the time-tested strategy It is basic to all other crop improvement strategies
Ideotype Breeding (New Plant Type)
Ideotype breeding aimed at modifying the plant architecture is also time-tested strategy to increase the yield potential Thus, selection for short-statured cereals such as rice, wheat and sor-ghum resulted in doubling of their yield potential Yield is a function of total biomass and harvest index (HI) Tall and tradi-tional rice varieties could produce a biomass of 13–14 tons per hectares under most conditions, and their HI is 0.3 Their biomass cannot be increased by application of nitrogenous fertilizers as plants grow excessively tall, lodge badly and their yield decreases instead of increasing To increase the yield potential of tropical rice, it was necessary to improve biomass production and HI by increasing their nitrogen responsiveness and lodging resistance and better partitioning of photosynthates As is well known, this was accomplished by reducing plant height through incorporation
of recessive gene sd1 for short stature First semi-dwarf or short-statured variety IR8 developed at International Rice Research Institute (IRRI) also had a combination of other desirable traits such as profuse tillering, dark green and erect leaves for good canopy architecture and sturdy stems for lodging resistance It is responsive to nitrogenous fertilizer and can produce higher bio-mass of about 20 tons per hectare It has HI of 0.45 Its yield potential is 8–9 tons per hectare (Chandler 1969) This plant type was accepted widely, and most of the rice breeding programmes world over adopted this plant type To increase the yield potential
of semi-dwarf rices, further IRRI scientists proposed a new plant type (NPT; IRRI 1989) with following characteristics:
wileyonlinelibrary.com
© 2013 Blackwell Verlag GmbH
Trang 21 Reduced tillering (9–10 for transplanted conditions)
2 No unproductive tillers
3 200–250 grains per panicle
4 Dark green and erect leaves
5 Vigorous and deep root system
6 Growth duration of 110–130 days
7 Multiple disease and insect resistance
8 Higher harvest index
Breeding efforts to develop NPT were initiated in early 1990s
Tropical japonica (TJ) varieties called bulus from Indonesia are
known to have low tillering ability, large panicles, sturdy stems
and deep root system Many bulu varieties were crossed with a
semi-dwarf japonica breeding line Sheng Nung 89–366 from
Shenyang Agricultural University, China More than 2000
crosses were made, and over 100 000 pedigree lines were
evalu-ated Breeding lines with desirable ideotype traits were selected,
and more than 500 so-called NPT-TJ lines were evaluated in
yield trials These lines had improved sink size, improved
lodging resistance and no unproductive tillers Grain yield of
these lines, however, was even lower than elite varieties It was
observed that reduced tillering contributed to lower biomass and
lower compensation ability Moreover, these NPT-TJ lines had
very poor grain filling Poor grain filling was attributed to lack
of apical dominance within a panicle, compact arrangement of
spikelets and limited number of large vascular bundles for
assimilate transport to grains Moreover, these NPT-TJ lines
were susceptible to diseases and insects and their grain quality
was not acceptable for consumers in tropical and subtropical
countries These lines were evaluated in several countries Some
of these lines showed good performance in temperate areas
where japonica grain quality is preferred Three of these lines
were released as varieties in Yunnan province of China as
Diancho 1, 2 and 3 (Khush 1995)
To improve the acceptability of these NPT-TJ lines for
tropi-cal conditions and to improve their yield potential, they were
crossed with elite indica lines and varieties with disease and
insect resistance and good grain quality More than 400 lines
designated as NPT-IJ were evaluated in yield trials Several of
these lines out yielded the best improved indica varieties such as
IR 72 by as much as 1.0–1.5 tons per hectare These breeding
lines have been continuously used in the breeding programme
and have contributed to increased genetic diversity through
introduction of japonica germplasm into otherwise indica
breeding materials Before the NPT project, japonica gene pool
was essentially excluded from the breeding programmes in
tropics Four NPT-IJ lines have been released as varieties in the
Philippines and Indonesia Numerous NPT-IJ lines were
distributed to all the breeding programmes throughout the world
through INGER nurseries Many have been used in hybridization
programmes and have contributed to widening of gene pools
For example, IR 66154- 521- 2 -2 a NPT-TJ line has been used
widely in hybridization programmes in China and Vietnam
Stimulated by IRRI’s NPT breeding programme, China
established nationwide mega project on development of super
rice Many super rice breeding lines have been developed by
Chinese breeders, which have been used as parents in hybrid rice
breeding (Peng et al 2008)
Hybrid Breeding
Hybrid breeding exploits the increased vigour or heterosis of F1
hybrids that is generally observed in outcrossing species In
maize, major yield improvement has been associated with the introduction of F1 hybrids on commercial scale, so much so all the maize area in USA and Europe is planted to hybrids Aver-age yield advantAver-age of hybrids vs varieties is approximately 15– 20% (Tollenaar 1994) Rice hybrids were introduced in China during mid-1970s and are now planted to about 13–14 million hectares or 50% of the rice area in that country The average yield advantage of hybrids over varieties is about 10–15% Rice hybrids adapted to tropical conditions have been developed
at IRRI (Virmani 2003) and by the national programmes of most
of the countries in tropical Asia but have met with limited success
At present, only about 2 million hectares are planted to hybrid rice outside of China Main reason for lack of success of hybrids on large-scale adoption is limited yield advantage of hybrids over varieties Most of the breeding programmes in tropics have utilized the indica germplasm in hybrid development, which has limited genetic diversity However, magnitude of heterosis is related to genetic diversity In their super hybrid breeding programme, Chinese scientists have developed parental lines with variable amount of japonica alleles in otherwise indica background These hybrids have better heterosis In this context, use of NPT-IJ lines
in the breeding programme for hybrid development should help improve the heterosis In the United States, two-line hybrids have been produced using temperature-sensitive male sterile lines Male sterile female lines are tropical japonicas and restorers are indicas These hybrids are reported to have as much as 25% yield advantage over varieties
Chinese Scientists under the leadership of Professor Yuan Long Peng initiated super hybrid rice breeding programme for improving the level of heterosis Like the NPT breeding programme, they proposed the following ideotype (Yuan 2001)
1 Moderate tillering capacity
2 Heavy (5 g/panicle) and drooping panicles at maturity
3 Plant height slightly taller (about 100 cm)
4 Top three leaves with following characteristics:
• Flag leaf length about 50 and 55 cm for the second and third leaves All three leaves should be above panicle height
• Should remain erect until maturity Leaf angles of flag, 2nd and 3rd leaves should be 5, 10 and 20°, respectively
• Should be narrow and V shaped (2 cm when flattened)
• Should be thick and dark green
• Leaf area index (LAI) of top three leaves should be about 6.0
5 HI of 0.55 Many hybrids of this ideotype have been developed by Chinese breeders
Exploiting the Wild Species Germplasm
Crop gene pools can be widened through hybridization of crop varieties with wild species, weedy races as well as intra-subspe-cific crosses Such gene pools can be exploited for improving many traits including yield For example, Lawrence and Frey at the Iowa State University reported that a quarter of BC2-BC4 segregants between cultivated Avena sativa and wild Avena sterilis crosses were significantly higher in grain yield than the cultivated recurrent parent Nine lines in this study when tested over years and sites had agronomic traits similar to the recurrent parent and 10–29% higher grain yield (Lawerence and Frey
Trang 31976) Xiao et al (1996) reported that some backcross
derivatives from a cross between an Oryza rufipogon accession
from Malaysia and cultivated rice out yielded the recurrent parent
by as much as 18% They identified two QTL from wild species,
which contributed to yield increase NERICA rices developed at
WARDA through interspecific hybridization between tropical
japonica variety ‘Morobrekan’ and an accession of Oryza
glab-errima have higher yield and wide adaption to African
environ-ment Dr Brar and his colleagues at IRRI have developed
numerous breeding lines with improved yield potential from
crosses between elite breeding lines and wild species of Oryza
Enhancing Photosynthesis
In several crop species, incorporation of ‘stay green’ trait or
slower leaf senescence has been a major achievement of
breed-ers In some genotypes with slower senescence, the Rubisco
deg-radation is slower, which results in longer duration of canopy
photosynthesis and higher yields The onset of senescence is
controlled by complement of external and internal factors Plant
hormones such as ethylene and abscisic acid promote
senescence, while cytokinins are senescence antagonists
Therefore, over production of cytokinins can delay senescence
The ipt gene from Agrobacterium tumefaciens encoding
isopentenyl transferase was fused with senescence-specific
pro-moter SAG12 and introduced into tobacco plants The leaf and
floral senescence in the transgenic plants was markedly delayed;
biomass and seed yield were increased, but other aspects of plant
growth and development were normal (Gan and Amasino 1995)
This approach appears to have great potential in improving crop
yields through slowing the senescence and rubsico degradation
and thus improving the canopy photosynthesis
C4 plants such as maize and sorghum are more productive as
compared to C3 rice and wheat C4 plants are 30–35% more
efficient in photosynthesis Professor Matsuoka and colleagues at
Nagoya University initiated a project to explore the possibility of
transferring C4 enzymes from maize to rice Four enzymes are
known to be responsible for C4 photosynthetic pathway in
maize Molecular engineering strategies have been employed to
introduce some of the genes for these enzymes in rice For
example, overexpression of maize PEPC gene in rice has
pro-duced two- to threefold higher activity of this enzyme than that
in maize and the enzyme itself accounted for up to 12% of the
leaf-soluble protein (Matsuoka et al 2001) These results suggest
a possible strategy for introducing the key biochemical
compo-nents of C4 pathway of photosynthesis into C3 rice However,
all C4 domesticated plants have‘Kranz’ anatomy, and to achieve
the goal of converting C3 rice with C4 photosynthetic pathway,
it may be necessary to alter its leaf anatomy Rice germplasm
has been screened to identify some of the components of maize
anatomy For example, maize leaves have narrower vein spacing,
whereas rice has wider vein spacing Wild rice relative Oryza
barthii and Oryza australiensis have narrower vein spacing
IRRI has undertaken a collaborative project involving several
laboratories in Europe and USA funded by Bill and Malinda
Gates Foundation to explore possibility of altering the
photosyn-thetic pathway of rice If successful, there will be major
improvement in the yield potential of rice
Genomic Approaches
Completion of rice genomic sequence has facilitated the
identification and cloning of genes and QTL for yield traits
These genes/QTL can be pyramided in elite varieties to increase their yield potential through molecular marker-assisted selection (Xing and Yang 2010) Rice yield is determined by source and sink size Three grain yield components, for example number of panicles per unit area, number of spikelet per panicle and grain weight determine the sink size Genomic approaches have allowed the identification and cloning of genes for these sink traits (Sakamoto and Matsuoks 2008) These are FC1 and htd1 for tillering, Gn1a and OsSPL14 for panicle architecture, and GS3, GW2 and tgw6 for grain weight (Ashikari and Matsuoka 2006) Development, identification and validation of functional SNP markers for target genes will facilitate the pyramiding of these genes into elite germplasm through molecular marker-assisted selection Yield QTLs are derived from natural variation The use of wide range of germplasm such as wild species can
be exploited to identify genes for yield components For example, 334 introgression lines (INL) derived from crosses between IR64 and 10 donor parents were developed through backcrossing by Dr Kobayashi at IRRI Variations in agronomic characters were characterized, and introgressed segments were determined by SSR markers in each INL Fifty-four regions associated with agronomic traits such as days to heading, culm length, panicle number per plant and grain weight in the genetic background of elite variety IR64 were identified These materials are highly useful for enhancing rice yield potential
Physiological Approaches
Physiological approaches should aim at improving the radiation-use efficiency by improving the performance and regulation of Rubisco, introduction of C4-like traits such as CO2concentrating mechanisms, improvement of light interception and improvement
of photosynthesis at the whole-canopy level Efforts must focus
on identification and utilization of photosynthetically efficient germplasm Genes/QTL for efficient mobilization and loading of photosynthates from source to sink should be identified (Zhong
et al 2000) High throughput protocols for selection of more efficient germplasm in the breeding programme are needed
Prospects
Rice has become a model plant for genetic and breeding research Advances in molecular biology and genomics, proteomics, transcriptomics and metabolics have opened new avenues to apply innovative approaches to rice breeding Molecular-assisted selection (MAS) has become integral component of germplasm improvement A large number of genes/QTLs for various traits have been tagged with molecular markers to apply MAS for trait improvement Map-based cloning has resulted in isolation of several genes for resistance to biotic and abiotic stresses as well as yield-related traits These include tiller number, number of grains per panicle, grain weight and grain filling This has opened the possibility of applying MAS for yield enhancement Genome sequence data have become important resource for detecting allelic variation and genome-assisted breeding programmes Recombinant DNA technology has resulted in production of transgenic rices with new genetic traits and for resistance to biotic and abiotic stresses High throughput transformation protocols for rice, activation tagging and insertional mutagenesis have bearing for enhancing transformation efficiency RNA interference (RNAi) a sequence-specific gene-silencing technology holds promise to modify gene expression for producing germplasm resistant to diseases, insects,
Trang 4nematodes including desired modification in starch and oil
properties It is now possible to precisely understand the role of
epigenetic changes in gene expression, thus understanding the
stability of various stresses under changing environments All
these advances when integrated with conventional breeding will
result in designer rice varieties to meet the challenges of food
security in the 21st century
References
Ashikari, M., and M Matsuoka, 2006: Identification, isolation and
pyramiding of quantitative trait loci for rice breeding Trends Plant
Sci 7, 344 —350.
Chandler, R F Jr, 1969: Plant morphology and stand geometry in
relation to nitrogen In: J D Eastin, F A Haskin, C Y Sullivan, and
C H M Van Baul (eds), Physiological Aspects of Crop Yield, 265 —
285 ASA Publication, Madison, Wisconsin.
Gan, S., and R A Amasino, 1995: Inhibition of leaf senescence by
auto-regulated production of cytokinin Science 270, 1986 —1988.
International Rice Research Institute (IRRI), 1989: IRRI Towards 2000
and Beyond International Rice Research Institute, Los Banos, Laguna,
Philippines.
Khush, G S., 1995: Breaking the yield barrier of rice GeoJournal 35,
329 —332.
Lawerence, P L., and K J Frey, 1976: Backcross variability for grain
yield in species crosses (Avena sativa L 9 A sterilis L.) Euphytica
24, 77—85.
Matsuoka, M., H Fukayama, M B S Ku, and M Miyao, 2001: High
level expression of C4 photosynthetic genes in transgenic rice In: G.
S Khush, D S Brar, and B Hardy (eds), Rice Genetics IV, 439 —
447 International Rice Research Institute, Los Banos, Laguna, Philippines.
Peng, S., R C Laza, R M Visperas, A L Sanico, K G Cassman, and
G S Khush, 2000: Grain yield of rice cultivars and lines developed in the Philippines since 1966 Crop Sci 40, 307 —314.
Peng, S B., G S Khush, P Virk, Q Tang, and Y Zou, 2008: Progress
in ideotype breeding to increase rice yield potential Field Crops Res.
108, 32 —38.
Sakamoto, T., and M Matsuoks, 2008: Identifying and exploiting grain yield genes in rice Curr Opin Plant Biol 11, 209 —214.
Tollenaar, M., 1994: Yield potential of maize: impact of stress tolerance In: K G Cassman (ed.), Breaking the Yield Barrier Proceedings of a workshop on rice yield potential in favorable environments, 103—109 International Rice Research Institute, Los Banos, Laguna, Manila, Philippines.
Virmani, S S., 2003: Advances in hybrid rice research and development
in the tropics In: S S Virmani, C X Mas, and B Hardy (eds), Hybrid Rice for Food Security, Poverty Alleviation, and Environmen-tal Protection Proceeding of the 4th International Symposium on hybrid rice, May 2002, 14 —17, Hanoi, Vietnam.
Xiao, J., S Grandillo, S N Ahn, S R McCouch, and S D Tanksley, 1996: Genes from wild rice improve yield Nature 384, 1223 —1224 Xing, Y., and Q Yang, 2010: Genetic and molecular bases of rice yield Annu Rev Plant Biol 61, 421 —442.
Yuan, L., 2001: Breeding of super hybrid rice In: S Peng, and B Hardy (eds), Rice Research for Food Security and Poverty Alleviation, 143 —
149 International Rice Research Institute, Los Banos, Philippines Zhong, S., C Lu, L Zhao, L Wang, C Q Dai, and J Zou, 2000: Physiological basis of high yield of an intersubspecific hybrid rice, liangyoupeijiu J Nanjing Agric Tech College 16, 8 —12.