Genetics and Breeding for High Iron and Zinc Content in Rice Grain

II. REVIEW OF LITERATURES 2.1.

Variation of Iron and Zinc Contents in Rice Grain In 1992 The International Rice Research Institute IRRI began to examine the effect of soil characteristics on the iron content of rice grain. This effort was expanded in 1995 as a part of a program of the Consultative Group for International Agriculture Research CGIAR aimed at improving the iron and zinc contents in rice grains Gregorio et al. 2000. Senadhira 1997 and Htut et al. 2001 reported from preliminary studies at IRRI that variation of grain iron and zinc contents existed in rice and was large enough to undertake breeding for enhanced high iron and zinc content in rice grains. Similar results were also reported by Gregorio et al. 2000, Gregorio 2002 and Welch and Graham 2002 from the evaluation of nearly 7,000 samples. The iron content ranged from 6.3 ppm to 24.4 ppm with a mean value of 12.2 ppm and zinc varied from 13.5 ppm to 58.4 ppm with a mean value of 25.4 ppm in brown rice beras pecah kulit, Indonesian. Popular cultivars contained about 12 ppm iron and about 25 ppm zinc in brown rice. Some traditional varieties have double these amounts. A comparison of aromatic and non aromatic varieties grown under similar conditions showed that aromatic rices were consistently higher in grain iron content and often also in zinc content. Hanarida et al. 2002 also evaluated 251 rice genotypes local varieties, advanced lines, and improved varieties and reported high variability for iron 6.8 ppm – 18.6 ppm and zinc 16.5 ppm – 43 ppm contents in rice grains. Furthermore, Hanarida 2003 reported iron content between 7.1 ppm and 26 ppm and zinc content between 16 and 122 ppm from a set of 440 accessions.

2.2. Genetics and Breeding for High Iron and Zinc Content in Rice Grain

Knowledge on the gene action of a trait is essential to determine the best breeding strategy to incorporate the trait of interest into breeding populations. Htut et al. 2001 reported that the type of genetic variation and its relative importance for grain iron content varied from one breeding population to the another. Dominant and epistatic effects for iron-content in rice grain were observed. Significant environmental variances suggested that rice grain iron- content could be modified, to some extent, by the growing environment. The results showed that the rice grain iron content is a quantitative trait. Therefore, selection in the later generations would be more fruitful. Analysis of combining ability indicated a high and significant general combining ability. This suggests that some donor parents might yield superior progenies with a range of recipient parents possessing high iron content. Specific combining ability is also present indicating specific combination between parents e.g. Azucena x Basmati 370 would produce higher iron content in the F 1 progeny. The presence of reciprocal effects suggest the importance of the choice of female parent. For example, Tong Lang Mo Mi produced higher-iron content in the progenies when used as female parent Gregorio Htut 2003. Furthermore, they reported that grain iron analysis of selected F 1 crosses showed very high iron, suggesting the high potential of these crosses to produce recombinant with high iron content. Gregorio et al. 2000 studied the genetics of iron content using four traditional high iron rice varieties, three advanced lines and three released varieties IR36, IR64, IR72 and reported highly significant differences between the crosses and parents. It clearly indicated that selection among F 1 progenies is possible. The genetic analysis of variance revealed the presence of additive gene action in addition to a significant non-additive genetic variance. Environmental effects were also present, but their magnitude was smaller than the genetic effects. The narrow sense heritability of the traits were found to be moderately low 43 and broad-sense heritability is relatively high 88 which provided further confirmation of the importance of a non-additive type gene action. Based on the above inheritance studies, it is apparent that selection during breeding should be practiced in a later generation such as F 5 , when the dominance effects unfixable genes are minimal. A bulk breeding method is suggested in early generations, during which selection for other agronomic characteristics should be undertaken-without selection yet for the high-Fe trait. Generation of recombinant inbred lines or anther culture developed doubled haploids should be the preferred method of breeding aimed at increasing grain iron content . 4 Because of the influence of environment and cultural practices on iron content, selection should be done in an optimum environment such as application of N and P to maximize genetic variability Gregorio et al. 2000. Breeding can improve the nutritional quality of crops. The philosophy of breeding for nutritional improvement is well developed and perceived. However, an important requirement is that improved varieties with nutritional characteristics must meet farmer’s agronomic criteria. When increasing micronutrients such as iron and zinc in the grains, improvement of both nutritional and agronomic traits should be practiced. High micronutrient content in the seed will certainly permit rapid crop establishment, especially in nutrient- deficient soil. The seed is the main mineral nutrient source for seedlings and the seed iron content is high in plants adapted to soils that are low in available iron Gregorio Htut, 2003. If these micronutrients could be incorporated through breeding in a staple food crop such as rice, expenditures for a micronutrient intervention program could decrease markedly. Although rice is not considered to be a major mineral supplier, any increase in its mineral content could help significantly reduce the iron- and zinc-deficiency problem Gregorio Htut, 2003. A high iron trait can be combined with high yielding traits. This has already been demonstrated by breeders at IRRI. They crossed a high-yielding variety IR72 with a tall, traditional variety Zawa Bonday from India. From which they identified an improved line IR68146-3B-2-2-3 with a high content of grain iron about 21 ppm in brown rice. This elite line has good tolerance to rice tungro virus and acceptable grain quality. The yield is about 10 below than IR72, but in compensation, maturity was earlier. This variety has good tolerance to soil deficient in minerals such as phosphorous, zinc and iron. It has no seed dormancy and has excellent seedling vigour, suggesting that it would be a good direct seeded rice Gregorio et al. 2000. More than 100 crosses were made and advanced in the IRRI breeding nurseries. Early generation selection was done for yield, good plant type, and resistance to diseases, but no selection was done for high micronutrient till the later generation, F or F . The aim in this breeding program is to develop high- 6 7 5 yielding adaptable varieties with enhanced iron and zinc in the grain Gregorio Htut 2003. A previously genotyped rice population was used to tag the genes QTLs for the high-Fe trait in brown rice. A total of 180 polymorphic markers including 146 restriction fragment length polymorphism, RFLP, 8 isozymes, 14 random amplified polymorphic DNA, RAPD, and 12 cloned genes on a linkage map of doubled-haploid-derived lines from the cross between IR64 and Azucena. hree QTLs were located on chromosome 7, 8, and 9 and explaining 19 to 30 variation for iron content. Three QTLs for aroma were also reported on chromosome 3, 7, and 8 explaining 16 to 38 variation. Thus, these QTLs for high iron and aroma have been suggested to be linked Gregorio et al. 2000.None of the above studies mapped genesQTLs in polished rice grains. Quantitative Trait loci for zinc content in germinating seeds was located on chromosome 5 Avendano, 2000 in a study involving 93 F RILs and 31 SSR markers. 8

2.3. Genetic Basis of Mapping GenesQTLs