Phosphorus (P) deficiency has been identified as a major factor limiting grain yield in rice worldwide. In general, rice varieties with tolerance to P deficiency exhibited the root elongation under P deficient conditions, and the relative root length (RRL) were determined as a reliable parameter for tolerance to P deficiency in rice (Shimizu et al., 2004).
In this study, we used a set of recombinant inbred (RI) population from a cross of Asominori with IR24, the molecular data and RFLP map with 375 markers (Tsunematsu et al, 1996), kindly provided by Prof. A. Yoshimura (Kyushu University, Japan) to detect quantitative trait loci (QTLs) for RRL, representing P deficiency response to root-growth seedling. Our experiments were conducted in glasshouse of Shanghai Normal University, Shanghai, China, with three replicates. Six germinated-seeds of each RI line along with its parents were placed into polystyrenefloating plates on plastic containers containing running water on 8 July, 2007. After 5d, all seedlings were placed in a pool containing P-deficient (P=0.5 mg/l) and P-sufficient (P=10 mg/l) nutrient solutions (Yoshida et al., 1976), respectively. The pH value of solutions was adjusted to 5.0 using 1 N NaOH or HCl. The nutrient solution was replaced every five days. After 20d, the root length of each RI line was measured, then, RRL were calculated. QTL analysis was performed with QTL Cartographer software (Wang et al., 2003) version 2.0 through composite interval mapping method. The loci with LOD value greater than 2.0 was declared as indicative of the presence of a QTL.
As a result, there was a clear difference of RRL between Asominori (Non-sensitive to elongation) and IR24 (Relatively sensitive to elongation) and continuous distributions for RRL with some transgressive RI lines were observed, suggesting that RRL is quantitatively inherited trait (Fig. 1). Three QTLs, tentatively designated as qRRL-7, qRRL-11, and qRRL-12, were detected for RRL with LOD value of 2.91 (Chr. 7), 2.16 (Chr.11) and 2.30 (Chr. 12) (Table 1 and Fig. 2) and explained 13.3%, 10.7% and 10.3% of total phenotypic variation, respectively. Comparative mapping has shown that qRRL-12 is allelic to/ tightly linked to the QTL for Pefficiency, dry weight and tiller number (Wissuwa et al., 1998) and relative stem and leaf dry-weight, relative root dry-weight and tiller number (Ni et al., 1998) under low phosphorus stress. Interestingly, qRRL-7 and qRRL-11 were the first time reported as novel QTLs for P deficiency response in rice. From those results, it can be concluded that there maybe exist more novel QTLs for P deficiency response in rice and the tightly linked molecular markers that flank the QTLs might be useful in improving tolerance to P deficiency in rice.
We are greatly indebted to Professor A. Yoshimura (Plant Breeding Laboratory, Agricultural Faculty of Kyushu University, Japan) for kindly providing the RI lines and molecular data. This research was supported by Shanghai Municipal Education Commission of China (No. 06ZZ21), Shanghai Municipal Science and Technology Commission of China (No. 06PJ14074) and the 948 Program from Agricultural Department of China (No.2006-G1).
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