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Progress and issues in development and use of hybrid rice in the tropics - S.S. Virmani

Plant Breeder cum Deputy Head, Plant Breeding, Genetics and Biochemistry Division IRRI, Los Baños, Philippines

INTRODUCTION

Hybrid rice technology aims to increase the yield potential of rice beyond the level of inbred high-yielding varieties (HYVs) by exploiting the phenomenon of hybrid vigour or heterosis. This technology has been successfully developed and widely adopted by farmers in China during the past 25 years. Currently, about 15 million ha (Mha) out of a total of 30 Mha of rice area is covered with hybrid rice in China, producing 103.5 million tonnes (Mt) (17% of world paddy production) - i.e. 22.5 Mt of extra paddy every year. This extra production means that about 6 Mha of riceland is saved in the world (Duvick, 1999). Hybrid rice, therefore, not only contributes to food security in China and the rest of the world, it also plays a part in global environment protection. Food security in China prevents an increase in global rice prices, and poor rice consumers can therefore afford to buy their staple food at reasonable price. Since hybrid rice cultivation requires fresh F1 seed for every cropping season, use of this technology requires the development of hybrid seed production and a processing and marketing infrastructure in the form of seed enterprises (public, private or NGO); this in turn creates additional rural employment opportunities.

Since most of the tropical rice-growing countries in Asia have a high population growth rate and limited land for rice cultivation, there must be an increase in production per unit area per unit time in order to obtain food security. Hybrid rice is one of the time-tested tools for meeting this objective.

IRRI has been developing hybrid rice technology for the tropics since 1979. The Chinese materials were neither adapted nor freely available to tropical rice-growing-countries. Progress over the years has been published (Virmani et al., 1982; Yuan and Virmani, 1988; Virmani, 1994; Virmani, 1998). The elite parental lines, hybrids and the associated seed production technology have been shared in both the public and the private sector, as well as with NGOs working in various rice-growing countries. Virmani (1999) reported the advances made in the development and use of hybrid rice technology for increasing global rice production at the Nineteenth Session of the International Rice Commission held in Cairo in September 1998. The progress made and issues identified since then for the development and use of the technology in the tropics are presented in this paper.

GENERATION OF THE TECHNOLOGY

IRRI has continued to develop new CMS, maintainer and restorer lines in diverse cytoplasmic and nuclear backgrounds over the past 4 years. The increased cytoplasmic diversity (Figure 1) is necessary to overcome the potential genetic vulnerability of rice hybrids caused by the association of cytoplasmic gene(s) with disease/insect susceptibility. Fortunately, the most widely used “WA” cytoplasm has so far not revealed susceptibility to any disease or insect (Virmani et al., 2001; Faiz, 2000). Alloplasmic CMS lines have been developed; they possess the same nuclear genotype but different cytoplasm in order to minimize the risk of future vulnerability of “WA” cytoplasm to diseases or insects in the tropics.

FIGURE 1
Cytoplasmic diversity of CMS lines maintained at IRRI

There is no dearth of restorer frequency among the elite indica inbred lines developed at IRRI as well as in tropical Asian countries, although restorer frequency among tropical japonica lines is negligible. Identification of restorers by conventional test-crossing procedures is quite laborious. Marker-aided selection for Rf 3 gene (located on chromosome 1) using the STS marker RG-140 with PvuII digestion has been quite effective. About 450 lines from the 2001 dry season and 2001 wet season source nursery were characterized for restorer and non-restorer allele. Restorers were identified by a 1.25-kb band while non-restorers possess a 1.15 kb band when resolved in an agarose gel (Figure 2). Using this method, about 50 percent of lines were identified as suspected restorers which were then test-crossed with CMS lines. About 80 percent of these test-crossed lines were confirmed as “WA” CMS restorers. The remaining 50 percent of the suspected non-restorer lines had only 11 percent restorers. These lines may possess some other Rf gene(s). Thus marker-assisted selection was useful for screening restorer lines and for increased breeding efficiency.

FIGURE 2
PCR amplification products using the STS primers RG140L(F) and RG140B(R) and digestion with PvuII

Note: Restorers such as IR 24 and IR 36 exhibit a 1.25-kb band while a 1.15-kb band is produced in non-restorers such as IR 58025A

Since 1998, 19 public and private rice hybrids have been released in Bangladesh, India, Indonesia, Philippines and Viet Nam (Table 1). Several of these combinations were derived from IRRI-bred parental lines; Chinese rice hybrids were also introduced as such in the Red River Delta area of Viet Nam. The yield advantage of the released rice hybrids over inbred HYVs in front-line demonstration trials in the above countries averaged about 1.39 t/ha (Table 2). IRRI continued to provide seeds of new elite hybrids and their parental lines to NARS (national agricultural research systems) for evaluation and utilization.

TABLE 1
List of rice hybrids released in the tropical countries

Country

Hybrid

Parentage

Year released

Origin

India

CORH-2

IR 58025A/C 20R

1998

Tamil Nadu

ADTRH-1

IR 58025A/IR66

1998

Tamil Nadu

Sahyadri

IR 58025A/BR827-35

1998

Maharashtra, India/IRRI

Narendra Sankra Dhan-2

IR 58025A/NDR 3026

1998

Uttar Pradesh

PA 6201

Unknown

2000

By HRI Ltd.

Pusa HR-10

Unknown

2001

New Delhi

RH-204

Unknown

2001

By Monsanto

HRI-120

Unknown

2001

By HRI Ltd.

27P-02

Unknown

2001

By Pioneer Co.

Viet Nam

HYT-56

IR 58025A/242 R

1998

Viet Nam

HYT-57

IR 58025A/BR827-35

1999

Viet Nam/IRRI

?

AMS 24A/Que 99

2000

Viet Nam

?

AMS 24A/IR9761

2000

Viet Nam

Philippines

CXRH-5

Unknown

1998

Cargill Co.

Panay

Unknown

1998

AgroSeed

Bangladesh

BRRI Dhan-1 (IR69690H)

IR 58025A/BR827-35

2001

BRRI/IRRI

Indonesia

Intani-1

IR 58025A/K-10R

2001

BISI Co.

Intani-2

IR 58025A/K-10R

2001

BISI Co.

TABLE 2
Yield gains of hybrids released for cultivation in four countries

Country

Hybrid
(t/ha)

Inbred
(t/ha)

Yield gain
(t/ha)

Yield gain
(%)

Bangladesha

7.48

5.79

1.69

29.2

Indiab

6.33

5.22

1.11

21.3

Philippinesc

7.80

6.50

1.30

20.0

Viet Namd

6.30

4.84

1.46

30.2

Mean

6.98

5.59

1.39

24.9

a Hybrid Sonar Bangla over inbred check on sample farms, 1998/99 boro (Figures obtained from Hossain et al., IRRI, 2001).

b Hybrid Sahyadri over best inbred check (Sasyasree) across 15 locations, multilocation trials, 1999/2000 rabi (Figures obtained from Directorate of Rice Research, India, 2000).

c Hybrid Mestizo over best inbred check across five locations, on-farm compact technology demonstration, 1998 dry season (Figures obtained from PhilRice, Philippines, 2001).

d Hybrid HYT 57 over the inbred check (CR203), 1999 summer (Viet Nam).

Work on two-line hybrid rice breeding using the TGMS (thermo-genic male sterility) system has been intensified. Two stable TGMS lines (IR 73827-23S and IR 73834S) were identified for use in the tropics. These lines are stable for male sterility during the wet and dry seasons - an indication that their critical sterility point is lower than in earlier-developed lines. A number of experimental rice hybrids derived from these lines are in the initial evaluation trials. Marker-aided selection (MAS) is being used, with SSR marker RM 11 for TGMS involving tms2 gene located on chromosome 7 to identify heterozygote plants in F2, F3 and F4 generations for advancing the generation of crosses made to breed TGMS lines. This approach increases breeding efficiency. MAS is also being used at IRRI to pyramid various known tms genes to improve stability of TGMS lines.

In order to further enhance heterosis, elite new plant type (NPT) lines are being used as one of the parents in crosses with elite indica lines. Both CMS and TGMS systems are being used to develop female and male parents of the experimental hybrids. Some of the indica/NPT crosses show hybrid sterility; this is being overcome using wide compatibility genes. More than 15 genes for this trait have been reported in literature. The neutral allele has been targeted at S-5 locus, on chromosome 6. STS marker, RG 213, was successfully used to screen WC lines among NPT restorer lines. Six putative heterotic groups in rice have also been identified (Xu et al., unpublished), which should help to breed rice hybrids with enhanced heterosis. The molecular breeding programme at IRRI is identifying heterotic gene blocks in rice for subsequent incorporation into elite parental lines to improve their combining ability. Some hybrid rices appear to perform well in aerobic rice (Table 3) and under inland salinity conditions (Figure 3).

TABLE 3
Performance of some hybrids and inbreds grown under aerobic rice (direct seeded under upland conditions with twice weekly irrigations) at IRRI farm, 2001 dry season

Hybrid/inbred

Grain yield
(t/ha)a

Height
(cm)

DTFb

Lodging

AYT

PYT

Magat (IR 64616H)

4.3

5.3

80

83

0

IR 73868H

-

4.8




IR 75207H

-

4.8




IR 75217H

-

4.3




IR 73871H

-

4.2




IR 75585H

-

4.2




IR 73855H

-

4.1




IR 73870H

-

4.1




IR 73860H

-

4.0




IR 75201H

-

3.7




IR 55423-01 (ck)

3.5

-

111

80

0

B6144 (ck)

2.5

3.2

116

75

96

LSD (5%)

0.7

0.6

6

-

3

a AYT = advanced yield trials; PYT = preliminary yield trials.
b DTF = date to flowering.
Source: G. Atlin, personal communication.

FIGURE 3
Standard heterosis of some hybrids in normal and saline soils in Egypt, 2000

Hybrid seed production technology - packaged in the form of a manual (Virmani and Sharma, 1993), a video and on electronic diskette - has been utilized extensively by both public and private seed enterprises in NARS. Seed yields of 1.5 to 2.5 t/ha have been obtained in a large seed-production area in India, Viet Nam and the Philippines. In Viet Nam, average seed yields have increased from 0.30 t/ha in 1992 to 1.6 t/ha in 2001. Similar seed yield improvements have been made in other countries. IRRI is developing new parental lines possessing higher outcrossing rate and working on new strategies to attain synchronization of flowering of female and male parents used in the hybrid seed production plots to further improve seed yields. In India, supplementing GA3 (15 g) with NAA (50 g) was found to be as effective as 200 g of GA3/ha. Considering that the price of NAA is 10 percent lower than that of GA3 (US$1/g), there is potential for a significant reduction in the cost of hybrid rice seed production. As in China, refinement of hybrid rice seed production technology is done by public and private seed enterprises operating in various countries.

IRRI has also developed a two-dimensional DNA sampling strategy for the assessment of hybrid seed purity using molecular markers. This involves bulking tissue samples of rows and columns of 100-hill (10 x 10 rows) hybrid grow-out planted in a seed box, obtaining 20 bulked tissue samples for DNA isolation (Figure 4). DNA fingerprints of the bulked samples were compared with a check hybrid (Figure 5). Specific off-type plants were picked out from the grow-out using the impure rows and columns as coordinates from which percent impurity can be directly derived. Although this strategy is six times more expensive than the conventional field grow-out method, it saves land area and time.

FIGURE 4
A10-row x 10-column (100-hill) hybrid grow-out showing off-type plants (light-coloured hills)

Note: Tissue samples for DNA analysis are bulked by column and by rows.

FIGURE 5
DNA fingerprints of bulked column and row samples as compared with a control hybrid

DISSEMINATION OF THE TECHNOLOGY

IRRI has been working closely with NARS for the dissemination of hybrid rice technology primarily through training and consultancy missions and front-line demonstrations. Both international and in-country training courses were organized in collaboration with NARS and with the assistance of the IRRI-ADB (Asian Development Bank) project on hybrid rice - implemented since 1998 in collaboration with FAO, Asia Pacific Seed Association (APSA) and China in six countries (Bangladesh, India, Indonesia, Philippines, Sri Lanka and Viet Nam). The project helped establish an international hybrid rice network for effective regional collaboration. Coordinated international hybrid rice yield trials began in 1999 at 22 locations in six countries. More than 150 researchers have been trained in hybrid rice breeding and seed production in various training courses at IRRI and in China and India. Nearly 4 000 seed growers and extension workers have been trained locally in hybrid seed production and hybrid rice cultivation in more than 100 in-country training programmes in member countries. Improvement in human resources and facilities accelerated the breeding and release of new hybrids and the development of seed production technologies appropriate to the conditions in member countries. Numerous front-line demonstrations were conducted to create awareness among farmers of the technology.

The IRRI-ADB project also conducted activities (workshops, study tours and studies) to raise awareness among policy-makers of the issues, potentials and constraints in the development and use of hybrid rice technology. The aim was to encourage the adoption of appropriate policy support to national hybrid rice programmes in member countries. A number of policy intervention points were identified for consideration in national programmes to expedite the development and use of hybrid rice; they include:

Following through the above points would determine the rate of adoption of the hybrid rice technology once it is developed and ready for dissemination in a country.

IMPACT

Since 1998, hybrid rice has been commercialized in Bangladesh and the Philippines, and area has increased almost twofold in Viet Nam and by about 50 percent in India. Indonesia has also recently released hybrid rices for commercialization, and Myanmar, Thailand, Iran, Egypt and Pakistan have shown increasing interest in the development and use of the technology. By 2001, the area planted to hybrid rice in Asian countries outside China had increased by 413 000 ha (more than a twofold increase over the 1997 area of approximately 287 000 ha). To plant about 700 000 ha at a rate of 20 kg seed per hectare, around 14 000 tonnes of seed are required, with a value of about US$28 million (at a cost of US$2 per kg). A sizeable seed industry has been developed for the purpose in several Asian countries. Currently, 58 seed enterprises, including private (37), public (14), NGOs (4) and farmers’ seed associations (3), are involved in hybrid rice research and/or seed production and marketing in the six Asian countries. At least 50 person-days additional labour is required to produce F1 seed (with a yield of 1 t/ha), which has led to the creation of employment opportunities (700 000 person-days). With an average 1 t/ha additional yield from hybrids, the 700 000 ha produce an extra 700 000 tonnes of paddy with a value of US$84 million. In this early phase of development and use, hybrid rice is already contributing towards increasing productivity, farmer income and rural employment in Asia.

CHALLENGES TO LARGE-SCALE ADOPTION OF HYBRID RICE

During the past 4 years of experience in commercializing hybrid rice technology in countries outside China, a number of challenges have been identified affecting the large-scale adoption of the technology. These include: inferior grain quality; inadequate disease/insect resistance in the first generation of hybrids; inconsistent and low seed yield; inadequate supply of pure seed of parental lines; and the high cost of seed.

The limited exchange of germplasm among NARS has held back the progress of hybrid breeding. Without a free-sharing policy of IRRI hybrid breeding materials, the development and use of hybrid rice technology would be very limited. Inadequate collaboration of the key public and private sector stakeholders - researchers, seed producers, farmers, processors, millers, traders, consumers and policy-makers - results in a slower rate of adoption of the technology. With the unrelenting efforts of IRRI and NARS, the technical and logistic challenges are being overcome. The IRRI-ADB project, with the support of FAO, APSA and the national programmes, plans to address some of these issues and challenges in its second phase for wide-scale adoption of the technology.

FUTURE DIRECTIONS

Future research priorities in hybrid rice include:

Both conventional and molecular tools will be used to meet the research objectives. The IRRI-ADB project on hybrid rice (phase 2) will:

IRRI’s role in the PETRRA (Poverty Elimination through Rice Research Activities) subproject on hybrid rice in Bangladesh is of an advisory nature. Consultancy services and training and germplasm will continue to be provided so as to assist in the expeditious development and use of hybrid rice technology in this country.

REFERENCES

Duvick, D.N. 1999. Heterosis: Feeding people and protecting natural resources. In Coors, J.G. & Pandey, S. eds. The genetics and exploitation of heterosis in crops, p. 19-29. Madison, Wisconsin, USA, American Society of America, Ind.

Faiz, F.A. 2000. Effect of wild abortive cytoplasm inducing male sterility on biotic/abiotic resistance/tolerance and agronomic and grain quality in some basmati rice hybrids. PhD dissertation, Central Luzon State University, Munoz, Nueva Ecija, Philippines.

Virmani, S.S., Aquino, R.C. & Khush, G.S. 1982. Heterosis breeding in rice, Oryza sativa L. Theor. Appl. Genet., 63: 373-380.

Virmani, S.S. & Sharma, H.L. 1993. Manual for hybrid rice seed production. Manila, Philippines, IRRI.

Virmani, S.S. 1994. Prospects of hybrid rice in the tropics and subtropics. In Virmani, S.S. ed. Hybrid rice technology - new developments and future prospects, p. 7-19. Manila, Philipines, IRRI.

Virmani, S.S. 1998. Hybrid rice research and development in the tropics. In Virmani, S.S., Siddiq, E.A. & Muralidharan, K. eds. Advances in hybrid rice technology, p. 35-49. Proceedings of the Third International Symposium on Hybrid Rice, 14-16 Nov. 1996, Hyderabad, India. Philippines, IRRI.

Virmani, S.S. 1999. Advances in development and use of hybrid rice technology for increasing global rice production. In Proceedings of the Nineteenth Session of the International Rice Commission - Assessment and Orientation towards the 21st Century, p. 86-95. Rome, FAO.

Virmani, S.S., Faiz, F.A., dela Cruz, N. & Ali, A.J. 2001. Has ‘WA’ cytoplasm any genetic liability on rice hybrids? Paper presented in ASA meeting, Oct. 2001, Charlotte, N.C., USA.

Yuan, L.P. & Virmani, S.S. 1988. Organization of a hybrid rice breeding program. In Hybrid rice, p. 33-37. Manila, Philippines, IRRI.


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