Plant breeders are always in search of new breeding tools to produce high-yielding crop varieties with superior grain and nutritional quality, which are also resistant to diseases and insects, and tolerant of environmental stresses (drought, flooding, salinity, cold, etc.). Of the many tools available, anther culture-derived doubled haploids (haploid cells having two copies of the set of chromosomes) have been considered the most desirable method to shorten the breeding cycle in the varietal development process and to map genes/quantitative trait loci (QTLs) for agronomic traits.
The anther culture technique is a friendly tool for plant breeders. It provides a link between conventional plant breeding and genomics. Haploids were first produced from anther culture of the ornamental herb Datura innoxia by scientists at Delhi University, India, in 1964. Since then, haploids have been produced in several plant species, including cereals. In fact, several rice varieties have been developed and released, particularly in Korea and China, using this technique. These varieties are japonica type, which are very responsive to anther culture. The potential of this technique in indica rice breeding, however, has not been realized because of the recalcitrant anther culture response of indica varieties, and also because it often produces albino plants that eventually die.
In addition to increasing rice yield potential, plant breeders are aiming to improve the grain’s nutritional quality. Breeders are trying to enhance micronutrient content, particularly iron and zinc, through biofortification to overcome the problem of malnutrition. Iron deficiency alone affects more than 3 billion people in the developing world. Lack of this nutrient during childhood and adolescence impairs physical growth, mental development, and learning capacity. In adults, it reduces the capacity to perform physical tasks. Moreover, according to the World Health Organization, zinc deficiency ranks fifth among the most important health risks in developing countries, and eleventh worldwide. In children, zinc deficiency is commonly associated with diarrhea, pneumonia, and stunted growth, and it can cause death.
To overcome zinc and iron deficiency among people where rice is a staple, the International Rice Research Institute (IRRI) has embarked on an ambitious project supported by the HarvestPlus Challenge Program to produce nutritious rice. This requires increasing the grain micronutrient content of existing and future highyielding indica varieties using highzinc, high-iron japonica donors, which are known to be highly responsive to anther culture. We used anther culture to produce doubled-haploid (DH) lines from crosses between indica and japonica lines. Mega-varieties such as IR64, IR36, PSBRc82, and BR29 were selected as one of the parents in diverse crosses with japonica donors. IR64 and IR36 are internationally popular varieties, while PSBRc82 is a popular Philippine variety and BR29 is a mega-variety in Bangladesh. These two countries are the target areas where zinc and iron deficiencies are most prevalent.
Fortunately, we have been successful in producing more than 1,500 DH lines through anther culture. These lines are being evaluated for their agronomic potential and for high iron and zinc contents. Some of the lines may perform better than the parents. This technique has opened more opportunities to map genes/QTLs governing high iron and high zinc since little is known about the genetics of these traits, and to search for elite DH lines possessing high-yield traits, along with high iron and zinc. This information will be useful to design efficient breeding strategies for improving the nutritional quality of rice.
This technique is important in developing true breeding lines in the next generation from any segregating population; hence, the DH plants can be multiplied and analyzed just like pure breeding lines. This shortens the breeding cycle, as traits get fixed in the homozygous state. The other advantage is that DH populations can be used as permanent mapping populations because they are stable and constant. The DH lines offer a unique opportunity to improve selection efficiency for various traits because the haploid method is based on gametophytic instead of sporophytic selection. One such example is IRRI’s use of a DH mapping population derived from a cross between IR64 and Azucena. This population was shared worldwide, and rice researchers have used it to map QTLs for several agronomic traits.
This technology is expected to support conventional breeding, especially for value-added traits.
As mentioned earlier, it has been challenging to use the DH technique in indica rice breeding. Consequently, this technique has not been deployed on a large scale to become an integral component of breeding programs for indica rice.
Nevertheless, IRRI has developed an indica-type variety through anther culture and it has been released for salt-affected areas in the Philippines. The difficulty will soon change, however, as we continue to explore and search for genes for high anther culturability, and eventually transfer these genes into recalcitrant indica varieties. Once such genes become available, this would make this technology more effective in adding new genetic properties into the breeding programs of indica rice, for which they are highly needed.
Dr. Grewal is a postdoctoral fellow on plant breeding and genetic transformation at IRRI.