Rice Genomics and Genetics 2015, Vol.6, No.9, 1-9
5
Several Zn-efficient rice genotypes including Metica
1, Epagri 108, CNA 7550, and CAN 86, with Zn
concentration of 20~25 ppm have also been
identified (Fageria 2001). The Fe and Zn concentrations
varied within the six sets of genotypes (
n
=939) were
7.5
~
24.4 μg/g for Fe, and 13.5~58.4 μg/g for Zn
(Welch & Graham, 2000). Iron and zinc content in
rice varied from 4.3~25.8 mg/kg and 8.6~43.0
mg/kg respectively (Meng et al., 2005). Wide
variations for grain protein and micronutrient levels
were recorded among the tested rice genotypes;
grain protein content ranged from 6.19% to 10.75%,
grain Fe from 4.82 mg/g to 22.69 mg/g and grain Zn
content from 13.95 mg/g to 41.73 mg/g (Banerjee et
al., 2010). The rice genotypes with high grain protein
and micronutrients will provide the basis of
bioavailability assay and will also serve as potential
genetic material for molecular breeding of nutrient
rich rice.
Brar et al. (2011) reported, large variation for iron
and zinc contents in a collection of 220 rice
genotypes; iron content varied between 5.1 (IR6387
2-4-2-2-1) - 441.50 µg/g (HKR95-157) and zinc
content varied between 2.12 (KBR466) – 39.4 µg
per g (Taraori Basmati). Notably, there was about
eighty-fold difference in Fe content and nineteen-
fold difference in Zn concentrations in the present
set of 220 rice genotypes suggesting the existence of
genetic potential to increase the concentrations of
these micronutrients in rice grain. These results also
indicate that there is significant genetic diversity for
Fe and Zn in the available rice germplasm and it
should be feasible to plan a breeding program to
develop high-yielding, mineral-rich rice genotypes.
The large genotypic variation of mineral content
(iron and zinc) in rice grains could be due to tightly
controlled homoeostatic mechanisms that regulate
metal absorption, translocation, and redistribution in
plants
allowing adequate, but non-toxic levels of
these nutrients to accumulate in plant tissues (Welch
and Graham,
2004). Iron and zinc contents in edible
portions also depend on the efficiency of translocation
of minerals from root tissues to edible plant organs
and accumulation thereof. In addition, phloem sap
loading,
translocation and unloading rates within
reproductive organs are important characteristics that
imparts in the variability for iron and zinc contents
in seeds.
4.1 Molecular approaches for developing iron
and zinc dense crops
Molecular markers are valuable tools in both basic
and applied research such as DNA fingerprinting,
analyzing genetic diversity, marker-assisted breeding,
phylogenetic analysis and map-based cloning of
genes (Ni et al
.
, 2002). The microsatellite markers
are supposed to be particularly suitable for
evaluating genetic diversity and relationships among
closely related plant accessions or individuals, such
as different rice cultivars. Genes/QTLs have been
mapped for several agronomically important traits
such as disease and insect resistance, yield, quality
and abiotic stress tolerance traits (drought,
sub-mergence tolerance, salinity tolerance, etc.
(Khush & Brar, 2001). But, there have been only a
few studies to map QTLs for mineral traits. A major
QTL for high-Zn has been identified in rice on
chromosome 5 flanking microsatellite markers, RM
167 and RM 87, using a selective phenotyping and
genotyping approach via microsatellite marker
analysis (unpublished data). Permanent mapping
populations of F
8
recombinant inbred lines were
developed to map high-Fe and high-Zn traits and are
being used to map genes/QTLs for high-micro-
nutrient traits (Fageria, 2001). Bradbury et al. (2005)
reported significant polymorphisms in the coding
region of fragrant rice genotypes relative to
nonfragrant genotypes for a gene with homology to
the gene encoding
betaine aldehyde dehydrogenase 2
.
Garcia-Oliveira et al. (2009) reported substantial
variation for Fe, Zn, Mn, Cu, Ca, Mg, P and K
contents in 85 introgression lines (ILs) derived from
a cross between an elite
indica
cultivar ‘Teqing’ and
the wild rice (
Oryza rufipogon
L.) and all the
mineral elements were significantly positive
correlated or independent except for Fe with Cu.
Zeng et al. (2009) showed that the iron content was
significantly associated with the allele size of
RM225; this marker was also linked with the
nitrogen use efficiency. Cu content was significantly
associated with the allele size of RM81A, RM60,
and RM247, Mn content was correlated with the
allele size of RM60 and RM225 and P content in
brown rice showed a significant correlation with the
allele size of RM81A, RM253, RM232, RM234, and
RM244.
