**Abstract**

Increasing the yield potential through hybrid rice technology was very well proved in nearby countries. Three line hybrid rice technology is encounted with some of the difficulties in seed production. Identification of Environmentally influenced male sterility overcomes the problem encountered in seed production since it is regulated by the temperature it is called temperature regulated male sterility and hybrids produced using this line is called two line rice hybrids. Types of male sterility and methods to identify the new TGMS lines and breeding methods employed for producing the tgms lines were described. Characterization of tgms lines by both conventional method and molecular tools has been enumerated. Seed multiplication of tgms under fertility inducing environment and seed production of two line hybrids has been explained. Seed production location was exclusively identified for seed multiplication of tgms lines. Heterotic potential of two line rice hybrids has been studied. Future prospectes in tgms research and two line heterotic potential was explained for increasing the yield potential in rice.

**Keywords:** rice, TGMS lines, characterization, new line development, two line rice hybrids, yield

## **1. Introduction**

Rice is the staple food for about half of the world's population. The global population is expected to reach 9 billion people by 2050 [1]. This demands for significant efforts to increase grain production, and it is expected to add 44 million tons staple crops per year for ensuring sufficient food production for such huge population [2]. Although rice production has significantly increased from 34.5 million tonnes in 1960–1961 to 117.5 million tonnes in 2020–2021, this significant increase over years was achieved by introduction of semi dwarf varieties, through improved crop management, adoption of hybrid varieties and improved plant protection practices. Rice production needed to be increased 42% by 2050 to feed the demands of an ever-increasing human population globally. Due to the potential of hybrid rice in increasing both rice production and productivity, many countries are focusing on exploiting the benefits of this technology. Success of hybrid rice technology and commercial exploitation was proved in China by the late 1970s marked the second major landmark in the history of rice breeding. It showed commendable breakthroughs in rice production and productivity, made other countries to revive its interest in hybrid rice breeding.

The use of hybrid rice has proved to be an effective and economical way to increase rice production. In rice, the phenomenon of heterosis was reported earlier by Jones [3] and Ramaiah [4]. However, several problems experienced in the production of hybrid seeds discouraged the commercial exploitation of heterosis in India. Later in the year 1976, it was accepted that large scale production of hybrid rice could be achieved through utilization of male sterility (MS) systems [5]. China initiated hybrid rice production in the year 1964 followed by India in the year 1989. Presently around 40 countries are actively involved in commercial hybrid rice production [6].

The over dependence on a single source of cytoplasmic male sterility (CMS) via WA (Wild abortive) and the difficulties in seed production and parental line development warrant the identification of alternate approaches to exploit hybrid vigor in rice. Two line breeding is one such possibility that emerged following the chance discovery of a photoperiod-sensitive genic male sterile plant called Nongken 58S, in the japonica variety Nongken 58 by Prof. Shi Ming Song of China [7–9] which was found to be sterile under longer photoperiods (> 14 hr) and fertile under shorter photo periods (13 hr) subsequently. Temperature sensitive genic male sterility (TGMS) line was identified by Chinese and Japanese scientists which was completely sterile under high temperature (> 32°C) and under low temperature (24°C) it was fertile [10, 11]. Using the PGMS system Yuan [12] put forth a new strategy of hybrid rice breeding which did not involve a maintainer, as the maintenance is taken care off by the shorter photoperiod (13 hr), hence it was called as two line method. During the sterile phase, EGMS plants can be used as a female parent to produce hybrid seed through self-fertilization without the use of a maintainer line as required in the CMS system. Since only two lines are required for the maintenance and multiplication of male sterile lines and production of hybrid seed, the system using this type of male sterility is known as the two-line system of hybrid breeding.
