BREEDING, GENETICS, AND PHYSIOLOGY Hybrid Rice Breeding

BREEDING, GENETICS, AND PHYSIOLOGY
Hybrid Rice Breeding
Z.B. Yan, W.G. Yan, C.W. Deren, and A. McClung
ABSTRACT
Over the past decade, Arkansas rice acreage planted in F 1
hybrids increased from
less than 1% to 28%. In 2010 the University of Arkansas began a hybrid rice breeding
program at the Rice Research and Extension Center in Stuttgart. Two hundred three accessions
of diverse germplasm from 30 countries were used to develop male-sterile lines.
This germplasm was also screened to be used as parents in the development of restorer
and maintener lines. Out of these crosses, male-sterile lines were identified and will be
tested in matings with restorers for seed production and F 1
hybrid productivity.
INTRODUCTION
Since the 1970s, the development of hybrid rice has progressed from an experiment
to becoming a major source of rice production. China has led the way, and now
many Asian countries are engaged in hybrid development as well as growing hybrids
on farms. In 2008 China had over 15,000,000 ha planted in hybrids, which was over
50% of the total rice acreage in the country. In the 35 years since 1976, hybrid yields
in China went from 3,470 kg/ha to 6,610 kg/ha (IRRI World Rice Statistics, 2010). In
Arkansas, hybrids were planted on less than 1% of the rice land in 2003, but that had
increased to 28% by 2010 (Wilson and Branson, 2004; Wilson et al., 2010).
In the U.S., hybrid seed has come from only one source, RiceTec, Inc., Alvin,
Texas. Their germplasm originated in China, and through two decades of breeding it has
been adapted to the rice production environment of the mid-South. In response to their
success and the increased popularity of their varieties, the public breeding programs in
Texas, Louisiana, and Arkansas began working on hybrid development.
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PROCEDURES
Without access to male-sterile lines, the Arkansas program began making wide
crosses between genetically distant lines in order to develop genetic male sterility, both
two-line and three-line. Additionally, known restorer and maintainer lines in the USDA
rice world collection were crossed with U.S. and other adapted germplasm to transfer
restoring genes and maintainer traits into the adapted plant type. Adapted lines which
had restoring ability were crossed with male-sterile lines to create F 1
hybrids. About
1,278 crosses were made initially, which were advanced to 3,395 F 3
panicle rows and
then to 1,565 F 4
panicle rows.
Most parent lines came from the U.S. Small Grains Collection located in Aberdeen,
Idaho, which has more than 18,000 accessions. From that collection, a subset
or core collection was developed which captured most of the variability of the larger
collection (Yan et al., 2007). Core collection accessions were selected as parental lines
based on their known genetic distance as revealed by molecular markers, agronomic
characteristics in the Arkansas environment, and their countries of origin. Crosses whose
progeny exhibited male-sterility were identified in the field by their erect panicles and
blank florets. To ascertain that these plants were indeed male-sterile and not blank
from other causes (insects, disease, etc), they were dug up, potted, and taken to the
greenhouse to be observed in ratoon growth. Those that were fertile in the autumn
environment of shorter day length and cooler temperatures were determined to be 2-
line male-steriles, also known as environmental male sterility (EMS). The EMS plants
will be repeatedly backcrossed to U.S. adapted cultivars and other selected lines to
introgress the EMS into a suitable plant type. Progeny will be tested for male-sterility
under appropriate day length and temperature regimes. Those plants that continued to
express male-sterility under short, cool days and were clearly healthy were possibly
cytoplasmic male-steriles (CMS), where sterility was controlled by genes in both the
cytoplasm and the nucleus not affected by environment (IRRI, 1997). The CMS lines
will also be backcrossed to elite U.S. lines and other parents to introgress male sterility
into a suitable background.
Crosses were also made between known restorers and maintainers with germplasm
that had desirable plant type and grain characteristics, including some Arkansas varieties.
The aim was to create new restorers and maintainers that had the improved, adapted
plant traits in addition to the genes for restoring fertility and good combining ability.
The important traits for the new maintainers are large, exerted stigmas that remain out
of the spikelets after flowering for effective cross pollination for production of hybrid
seed, in addition to adapted plant traits such as short height and early maturity. Progeny
resulting from these crosses would then be crossed with male-sterile lines to evaluate
their maintaining or restoring capability as expressed in the subsequent generations.
Some of these crosses could also become conventional inbred lines as well.
Test crosses were made on selected male-steriles to evaluate seed production and
combining ability. Ten male-steriles were mated to 11 restorers to produce 110 hybrid
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lines. These lines were grown in isolation in the field to prevent outcrossing with other
pollinators. Seed yield and milling quality were evaluated and the F 1
seed will be planted
next year to observe grain production and agronomic traits of the hybrids.
RESULTS AND DISCUSSION
Final evaluation of most crosses will require a few seasons to assess their potential
and identify useful parent lines for F 1
hybrid production. The test for seed production
successfully produced seed on 6 of the 10 male-sterile lines. Each restorer successfully
mated with each male-sterile except one. Milling quality and amylose will be assessed
on these hybrids. Seed produced will be grown in 2011 to evaluate grain production
and quality.
SIGNIFICANCE OF FINDINGS
The identification of stable male-sterile lines, both EMS and CMS, is a promising
step in developing a major component of a hybrid rice breeding program. The transfer
of stable male sterility into an adapted plant type and the mating with various restorers
will assess their utility and identify potential hybrid combinations.
ACKNOWLEDGMENTS
Funding for this project was provided by the taxpayers of Arkansas through
general revenues and by the Rice Research and Promotion Board.
LITERATURE CITED
IRRI. 1997. Hybrid rice breeding manual HR2-01. International Rice Research Institute,
Los Baños, Philippines.
IRRI. World rice statistics. 2010. http://www.irri.org.media/achievements
Wilson, C.E. and J.W. Branson. 2004. Trends in Arkansas rice production. In: R.J.
Norman, J.-F. Meullenet, and K.A.K. Moldenhauer (eds.). B.R. Wells Rice
Research Studies 2003. University of Arkansas Agricultural Experiment Station
Research Series 517:15-21. Fayetteville, Ark.
Wilson, C.E., S.K. Runsick, and R. Mazzanti. 2010. Trends in Arkansas rice production.
In: R.J. Norman, J.-F. Meullenet, and K.A.K. Moldenhauer (eds.). B.R.
Wells Rice Research Studies 2009. University of Arkansas Agricultural Experiment
Station Research Series 581:11-21. Fayetteville, Ark.
Yan, W.G., J.N. Rutger, R.J. Bryant, H.E. Bockelman, R.G. Fjellstrom, M.H. Chen,
T.H. Tai, and A.M. McClung. 2007. Development and evaluation of a core subset
of the USDA rice (Oryza sativa L.) germplasm collection. Crop Sci. 47:869-878.
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