Background

About 90% of total rice is produced and consumed in Asia. Since population is increasing in an alarming rate, rice productivity also needs to be produced to meet the global food demand. Hybrid rice, a type of rice which is proved to yield more than the improved rice, play an important role to secure food particularly in Asian countries where rice is the major staple food. It has been established that hybrid rice produce 15-25% more yield than the improved rice varieties when supplied with recommended dose of inputs like fertilizers, irrigation, timely weeding, proper care and maintenance etc (Kushwaha, 2016; Sindhu and Kumar, 2018). Hybrid rice involves two type of seed production techniques: two and three line system. Three line system of seed production techniques is widely known and highly used for seed production of hybrid rice a globally whereas two line system is new over the three line system. 

The discovery of male sterility in rice have been successfully utilized in hybrid rice breeding and this method has created significant contribution to food security in the world. CGMS, PGMS and TGMS have been utilized for practical hybrid production in rice (http://www.nfsm.gov.in/Publicity/HybridRice.pdf). The effective utilization of a cytoplasmic male sterile (CMS) line resulted in a breakthrough for utilization of heterosis in an autogamous crop. The utilization of genetic male sterility (GMS) in hybrid rice production has incredible potential to revolutionize hybrid seed production technique. 

The three-line system (CMS): (A) line, maintainer line (B), and restorer (R) line was developed to produce hybrid seed and proved to be effective for increasing rice yield (Virmani et al., 2003; Sindhu and Kumar, 2018). The two-line breeding framework by using thermo-sensitive genic male sterility (TGMS) has been also discovered and successfully developed as a breeding strategy in rice (Virmani et al., 2003, http://www.nfsm.gov.in/Publicity/ HybridRice.pdf). 

1 Advantages of the EGMS System over CGMS System

• There is no requirement of a maintainer line for seed multiplication, thus saves adequate time;

• EGMS has ability to use a wide range of genotypes as male parents, therefore, the prevalance of heterotichybrids is superior among two-line hybrids than among three-line hybrids. This system is ideal for developing indica/japonica hybrids because most japonica lines don’t have restorer genes;

• Better grain quality and higher yields. It was demonstrated that the TGMS was more effective in increasing grain yield and seed production efficiency (Yuan, 1990);

• Absence of negative impacts of sterility-inducing cytoplasm;

• The TGMS and PGMS trait is usually controlled by one or two major nuclear genes, so it should be comparably easy to transfer to diverse genetic backgrounds, which helps in diminishing potential genetic vulnerability among the hybrids. 

2 Classification of the EGMS System

Depending on the environmental factor affecting expression of the sterility-inducing gene, EGMS is classified in following categories:

• PGMS: Photoperiod-Sensitive Genic Male Sterility

• TGMS: Temperature-Sensitive Genic Male Sterility

• PTGMS: Photothermosensitive Genic Male Sterility 

TGMS lines are delicate to the temperature for the expression of male sterility or fertility (Figure 1). For example, most TGMS lines stay male sterile at high temperature (day temperature > 30°C/night > 24°C) and they revert back to partial fertility at a lower temperature (day < 24°C > 16°C night) (Virmani et al., 2003). 

Figure 1 Rice Knowledge bank IRRI

PGMS lines are responsive to the spell of day length for the expression of sterility or fertility. For example, most PGMS lines show male sterility under long-day (> 13.75 h) conditions and turn back to fertility under short-day (< 13 h) conditions (Virmani et al., 2003; Sindhu and Kumar, 2018). 

PTGMS lines are sensitive to both temperature and photoperiod. Temperature is the key factor since PTGMS lines become absolutely male sterile or fertile beyond a particular temperature range, that is, > 30°C or < 24°C, without any consequence of photoperiod (http://www.nfsm.gov.in/Publicity/HybridRice.pdf). But, inside this temperature range (24-32°C), photoperiod influences the PTGMS lines, that is, longer photoperiod will enhance male sterility at lower temperatures vis a shorter photoperiod (14 h at 30°C will make the PTGMS line sterile in comparison with 13 h at 30°C) (Virmani et al., 2003).

3 Procedure of Two-line (TGMS) Hybrid Rice Seed Production

The production of two-line hybrid involves two major steps (http://www.nfsm.gov.in/Publicity/HybridRice.pdf):

• Multiplication of TGMS lines

• Hybrid rice seed production 

3.1 Multiplication of TGMS lines

• Nucleus seed production of a TGMS line starts within the fertility-inducing environment. Seeding of TGMS lines is organized in such the simplest way that the sensitive stage occurs when the temperature is good for a higher seed set;

• At the duration of flowering, approximately hundred typical plants are chosen from the population of a TGMS line and their panicles are bagged. The selection method should be done within 1 week;

• After the harvest, the chosen plants are scored for spikelet fertility (based on the main panicle) and 50 plants with higher spikelet fertility (> 30%) are selected;

• Progenies of the chosen plants are grown within the sterility-inducing atmosphere. About thirty seeds are taken from every of the chosen plants to grow single-row progenies and the remaining seeds are put away carefully. The balance of the seeds of the progenies that are uniform and completely male sterile must be distinct and bulked to create the nucleus seed;

• Nucleus seed of the TGMS line is utilized for producing breeder seed under strict isolation. Breeder seed for the TGMS line is produced in the fertility-inducing environment;

• The breeder seed produced under the oversight of the plant breeder has high genetic purity and utilized for producing foundation seed of parental lines, which will be used for producing hybrid seed.

3.2 Hybrid rice seed production

In two-line hybrid seed production, the two safe-periods refers to the useful atmospheric conditions that support first the induction of absolute male sterility and second the facilitation of proper pollen movement from the pollen parent and fertilization of the TGMS parent (http://www.nfsm.gov.in/Publicity/HybridRice.pdf). So, the first safe-period determines seed purity, while the second safe-period determines hybrid seed yield (Virmani et al., 2003).

A multidisciplinary approach in developing superior TGMS lines and pollen parents can help to develop two-line rice hybrids suitable for the distinct ecological situations in which rice is grown (Kushwaha, 2016). Despite the promise that two-line hybrid rice technology holds, it would be wise to have a harmonious balance in using three-line and two line hybrids and conventional rice varieties in an appropriate manner in national rice production programs. 

Authors’ contributions

VM wrote and shaped the manuscript, coauthors help to correct whole document. All authors read and approved the final manuscript.

Acknowledgments

I am thankful to Ujjawal Kr. S. Kushwaha for his critical comments to shape this manuscript well.

Reference

Guidelines for seed production of hybrid rice, 2018

Kushwaha U.K.S., 2016, Rice in Black Rice, Research, History and Development, pp.192

https://doi.org/10.1007/978-3-319-30153-2

PMid:27051207

PMCid:PMC4819371

Sindhu J.S., and Kumar I., 2018, Quality seed production in hybrid rice

Virmani S.S., Sun Z.X., Mou T.M., Jauhar A.A., and Mao C.X., 2003, Two-line hybrid rice breeding manual, Los Baños (Philippines): International Rice Research Institute, pp.88

Yuan L.P., 1990, Progress of two-line system hybrid rice breeding, Scientific Agriculture Sin, 3: 1-6