Introduction
Cultivated rice, Oryza sativa L., consists of two main subspecies, indica and japonica [1]. Indica rice is grown extensively in tropical and subtropical Asia, regions that have the highest population density in the world, with China and India accounting for over 50% of global rice production [2]. Indica rice is the staple food of more than half the world’s population and thus is the main contender for fulfilling the sustainable development goal of zero hunger [3]. The demand for rice is expected to grow further, with the global population projected to reach 10.6 billion by 2050 [4].
Although much progress has been made in the genetic improvement of indica rice and an array of superb cultivars have been released, vigorous breeding efforts need to continue in order to meet the future challenges of population growth and climate change. Conventional breeding strategies of hybridization and selection need to be augmented by the utilization of new breeding approaches, including marker-assisted selection, genetic modification, and gene editing, with a renewed focus on the development of new cultivars suitable to withstand biotic stresses caused by numerous rice diseases [5] and environmental stresses caused by drought, flooding, or saltwater intrusion [6].
Rice is a short-duration crop of 3–4 months, with usually two cropping cycles per year. It takes six or more years to release a new cultivar through conventional breeding, as inbreeding has to be repeated over several years to achieve purebred lines from an initial cross. Haploids are of major interest to plant breeders because, by doubling their chromosome number, fully homozygous doubled haploids can be produced. The conversion of haploids to doubled haploids ensures fixing homozygosity more rapidly than conventional methods of line development such as selfing and backcrossing [7,8,9,10].
The natural occurrence of haploids was first reported in the weed Datura stramonium [11] and has since been observed in other plant species [7,12]. However, the practical value of such natural spontaneous haploids or doubled haploids is limited as they occur at low frequencies [13]. Therefore, for commercial plant breeding, haploid induction has to be made more efficient. Both in vitro and in vivo systems can be employed for the production of haploids. While in vivo methods have proven to be more efficient than in vitro techniques for haploid induction in crops such as maize [14], a reliable in vivo method is not known for many important crops, including rice. On the other hand, in vitro culture of haploid cells has the potential to be applied to many plant species, even though success will vary among different groups and genotypes.
In this review, we examine the current status of research with regard to the application of in vitro techniques for rice haploid induction and discuss the problems that the indica types are faced with. We also highlight the recent advances in in vivo approaches for haploid induction in rice. Especially with regard to the in vivo methods, the progress made in other cereals is analyzed, and the transferability of these developments to rice is examined, particularly in view of the availability of new tools for gene editing such as the CRISPR/Cas9 system.
References
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Journal: Plants Authors: Mayakaduwa and Silwa
Introduction Cultivated rice, Oryza sativa L., consists of two main subspecies, indica and japonica [1]. Indica rice is grown extensively in tropical and subtropical Asia, regions that have the highest population density in the world, with China and India accounting for over 50% of global rice production [2]. Indica rice is the staple food of more than half the world’s population and thus is the main contender for fulfilling the sustainable development goal of zero hunger [3]. The demand for rice is expected to grow further, with the global population projected to reach 10.6 billion by 2050 [4]. Although much progress has been made in the genetic improvement of indica rice and an array of superb cultivars have been released, vigorous breeding efforts need to continue in order to meet the future challenges of population growth and climate change. Conventional breeding strategies of hybridization and selection need to be augmented by the utilization of new breeding approaches, including marker-assisted selection, genetic modification, and gene editing, with a renewed focus on the development of new cultivars suitable to withstand biotic stresses caused by numerous rice diseases [5] and environmental stresses caused by drought, flooding, or saltwater intrusion [6]. Rice is a short-duration crop of 3–4 months, with usually two cropping cycles per year. It takes six or more years to release a new cultivar through conventional breeding, as inbreeding has to be repeated over several years to achieve purebred lines from an initial cross. Haploids are of major interest to plant breeders because, by doubling their chromosome number, fully homozygous doubled haploids can be produced. The conversion of haploids to doubled haploids ensures fixing homozygosity more rapidly than conventional methods of line development such as selfing and backcrossing [7,8,9,10]. The natural occurrence of haploids was first reported in the weed Datura stramonium [11] and has since been observed in other plant species [7,12]. However, the practical value of such natural spontaneous haploids or doubled haploids is limited as they occur at low frequencies [13]. Therefore, for commercial plant breeding, haploid induction has to be made more efficient. Both in vitro and in vivo systems can be employed for the production of haploids. While in vivo methods have proven to be more efficient than in vitro techniques for haploid induction in crops such as maize [14], a reliable in vivo method is not known for many important crops, including rice. On the other hand, in vitro culture of haploid cells has the potential to be applied to many plant species, even though success will vary among different groups and genotypes. In this review, we examine the current status of research with regard to the application of in vitro techniques for rice haploid induction and discuss the problems that the indica types are faced with. We also highlight the recent advances in in vivo approaches for haploid induction in rice. Especially with regard to the in vivo methods, the progress made in other cereals is analyzed, and the transferability of these developments to rice is examined, particularly in view of the availability of new tools for gene editing such as the CRISPR/Cas9 system.
References