*2.6.1 CGMS system*

*Recent Advances in Rice Research*

1 *Rf1a, Rf1b*

6 *Rf6* Chr-10

*Restorer genes in rice plants.*

**Table 2.**

& 8

7 *Rf17* Chr-4 AT10.5–1,

8 *Rf98* Chr-10 UK CMS-

9 *Rf102* Chr-12 UN CMS-

**S. No.** *Rf genes* **Locality Marker CMS** 

2 *Rf2* Chr-2 CAPS42–1 CMS-LD Kasalath,

3 *Rf3* Chr-1 DRRM-Rf3–10 CMS-WA Swarna,

4 *Rf4* Chr-10 RM6100 CMS-WA IR 24, Pusa

SNP 7–16

which in association with hexokinase (*osHXK6*) targets mitochondria and process defective transcript of *atp6-orfH79* at 1238 nucleotide position. Thus, PPR protein family cause editing of aberrant transcript, inhibit their translation, and at the end, fertility restoration. Besides, male fertility in WA-CMS is found to be counteracted by *Rf3* and *Rf4* genes (chrom.-1 and 10, respectively). The genes *Rf3* and *Rf4* encode a pentatricopeptide protein (PPR) where RF4 cleave the abnormal *WA352* transcript and RF3 suppress translation of *WA352* into polypeptide and helps in restoring fertility in WA-CMS. Fertility in LD-CMS is reported to be restored by either *Rf1* or *Rf2.* The *Rf2* gene encodes a glycine-rich protein in mitochondrial; replacement of isoleucine by threonine at amino acid 78 of the *RF2* protein causes functional loss of the *rf2* allele. Moreover, CW-CMS is reported to be restored by a single recessive gene (*Rf17)* which is a retrograde-regulated male

**system**

Chr-10 InDel-Rf1a CMS-BT BTR, IR24,

**Restorer line**

MTC10R; C 9083

Minghui 63

PUSA 33

RT102C, K102-*Oryza rufipogon. T*

RM5373 CMS-HL — — — [16]

CMS-CW CWR PPR2 RNA

33, CRL 22R

5 *Rf5(t)* Chr-10 RM3150 CMS-HL Milyang 23 PPR791 PPR [16]

RT98A

RT102A

**Causative gene**

PPR8–1, PPR791, *Rf1A, Rf1B*

LOC\_ Os02g17380.1 **Encoded product**

Gly. Rich protein

interference

UK UK [19]

RT98C PPR762 PPR [18]

— PPR [15]

PPR782a PPR [15]

PPR [13]

**Reference**

[14]

[17]

Commercial hybrid seed production in rice where natural out-crossing (ranged only 0.3–3.0%) is very low, cumbersome, and an expansive task. To be practical and readily adoptable, it requires some specific parental requirements and agro-management practices. Invention of male sterile lines thus provided unique opportunity to start with the technology in rice. Based on mechanism of male sterility, threshold nature in male sterility expression and number of parental lines used, three types of hybrid seed production system namely three-line system (involving three parents, A, B, and R), two-line system (two parents, A and R), and one-line system (apomictic-based) exist. Among them, CGMS-based three-line system is more suitable,

**30**

sterility (*rms*) gene (**Table 2**) [20].

**2.6 Breeding system**

This system involves three parents such as male sterile line (A-line, cytoplasmic male sterile), B-line (maintainer), and R (restorer) lines and two steps in seed production, that is, CMS multiplication and hybrid seed production under strict isolation (spatial or temporal or physical barrier). Male sterile line (A-line), because of their eliminated manual emasculation needs, served as seed parent and facilitates large-scale seed production. A suitable CMS line to be utilized as seed parent should have complete and stable male sterility, substantial seed producibility, wide compatibility, and good combining ability with minimum CMS load. The wealthy panicle and narrow semi-erect leaf configuration in seed parent has additional impact, assures more seed production. In Indian perspective, hybrid seed production is a major dilemma, generally keen to *Rabi* season, hence, CMS lines should have substantial cold tolerance at seedling stage and heat at flowering stage.

The maintainer (B-line), on the other hand, is an isogenic to the CMS line (differs only for fertility/sterility) in their genetic constitution, able to produce functional pollen and maintain the sterility in male sterile line/seed parent. The maintainer line can maintain 100% male sterility in seed parent thus utilized to perpetuate CMS with their inherent male sterile ability.

In contrast, restorer line can restore male fertility in F1s produced on male sterile parent, thus utilized as pollen parent in hybrid seed production. A good restorer should have substantial genetic distance with seed parent which is prerequisite and major determinant of the extent of heterosis in hybrids (more genetic distance more heterosis and *vice-versa*). Restorer is the major contributor of heterosis in three-line hybrids, hence, should have good combining, strong fertility restoration ability (dominant *Rf* gene(s) responsible for fertility restoration in CMS). Besides, restorer line with ideal plant type, acceptable grain quality parameters, substantial source-sink balance, heavy pollen load, and broad spectrum of resistance/tolerance against multiple biotic/abiotic stresses is imperative in maximization of genetic gain in hybrids.
