Department of Agronomy and Range Science, USDA-ARS, University of California, Davis, CA 95616, USA
Seedling vigor is a desirable trait to breed into both temperate and tropical rice cultivars. It is important in California where rice is sown directly into flooded fields and plants must emerge rapidly through water. Longer shoots are associated with vigor under these conditions (Peterson et al. 1978). Likewise, long roots are important for proper soil anchorage in water-seeded rice. In the southern United States, where drill-seeding is practiced, longer mesocotyls and coleoptiles are important for good seedling emergence and, hence, optimum stand establishment (Dilday et al. 1990). This study identified genes or quantitative trait loci (QTLS) controlling these characters using restriction fragment length polymorphisms (RFLPs).
A mapping population consisting of 204 F`2` plants was developed from a cross between a low-vigor U. S. Japonica cultivar Labelle (LBL) and a high-vigor tropical aus/indica cultivar Black Gora (BG) (Table 1). Parental polymorphism was assessed using 268 clones from Cornell University and five restriction enzymes (EcoRI, EcoRV, HindIII, ScaI, XbaI). One-hundred sixty-eight or 63% of the clones detected RFLPs between the two parents for at least one restriction enzyme. Of these RFLPS, 117 were used as markers in constructing a linkage
Table 1. Means of the two parental cultivars and range of means of the F`3` families for the seedling vigor traits under study _______________________________________________________________________________ Vigor Temp LBL BG LSD F`3` families' trait (deg C) (mm) (mm) (.05) range (mm) _______________________________________________________________________________ Shoot length 18 52 66 21.1 26-119 25 136 180 28.1 70-225 Root length 18 102 121 23.8 56-141 25 148 185 35.8 88-218 Coleoptile length 18 23 29 8.8 14-47 25 23 29 9.2 18-47 Mesocotyl length 18 8 12 9.4 1-30 25 3 9 9.4 1-36 _______________________________________________________________________________ The traits were measured 15 and 10 days after incubation in the 18 deg and 25 deg C slantboard test, respectively.map spanning 1496 cM and encompassing the 12 rice chromosomes. Average distance between markers was 14.2 cM. Seedling vigor traits were measured on 172 F`2`-derived F`3` families in slantboard tests conducted at two temperature regimes (18 deg and 25 deg C). The 18 deg C-slantboard test is a standard laboratory technique used to screen rice breeding materials for seedling vigor in California. Both linkage analysis and interval mapping of QTLs were performed using MAPMAKER (Lander et al. 1987).
A total of 16 QTLS, each accounting from 7.4 to 38% of the phenotypic variance, were identified and mapped in the two environments at a log-likelihood (LOD) threshold of 2.5 (Table 2). Only three QTLs were expressed under both temperature regimes. Several QTLs controlling different traits were mapped to the same interval. Chromosome three, for example, had one QTL each for length of shoot, coleoptile and mesocotyl in the same or adjacent interval. Similarly, shoot and root length QTLs mapped to adjacent intervals in chromosome 1.
Additive as well as dominant and recessive modes of gene action were observed. Positive BG alleles that increased root, coleoptile and mesocotyl
Table 2. Biometrical parameters for QTLs identified for the four traits at two temperatures _______________________________________________________________________________ Vigor Nearest Chrom Temp Peak % Variance Superior market no. (deg C) LOD explained group _______________________________________________________________________________ Shoot length RG536 1 18 2.77 7.4 HET 25 2.78 7.5 HET RZ452 3 18 5.62 17.1 LBL 25 6.00 15.5 LBL IZG435 5 18 3.12 8.0 LBL RZ422 9 18 2.80 9.9 LBL Root length RG222 1 18 5.22 31.1 HET 25 8.26 39.0 HET CDO718 2 18 2.90 9.4 HET Coleoptile length RZ448 3 25 5.31 25.1 BG RG716 6 25 4.72 15.3 LBL Mesocotyl length RZ403 3 18 4.73 17.5 BG RZ448 3 25 3.33 10.3 BG RZ395 7 18 3.36 28.5 BG RG811 1 25 4.21 12.8 BG RZ67 5 25 2.67 12.6 BG _______________________________________________________________________________The marker nearest the peak LOD in a given interval mapping a QTL is listed. LOD scores and percent variance explained by each QTL were computed based on a free genetics model. The group with the highest trait mean at each QTL is shown. LBL and BG indicates homozygosity for Labelle and Black Gora marker genotypes, respectively, and HET indicates heterozygosity.
lengths were idendified. Surprisingly, however, Black Gora (BG) alleles increasing shoot length were not detected. It is possible that positive BG alleles for this trait may have small effects that could not be detected in this analysis. Only 42.4% and 23.0% of the variation of the shoot length were explained by the factors identified at 18 deg and 25 deg, respectively. The Japonica parent Labelle contributed positive alleles for shoot, root and coleoptile lengths but had no alleles increasing the length of mesocotyl.
The stable as well as environment-specific QTLs for seedling vigor traits identified in this study could be useful in rice breeding. These results also indicate that Indica and Japonica cultivars may be utilized to enrich, in a reciprocal fashion, each other's gene pool for seedling vigor alleles. With the marker-aided selection, BG alleles with large effects may be introgressed into U.S. cultivars to increase their seedling vigor under both water- and drill-seeded rice culture.
References
Dilday, R. H., M. A. Mgonja, S. A. Amonsilpa, F. C. Collins and B. R. Wells, 1990. Plant height v. mesocotyl and coleoptile elongation in rice: linkage or pleitropism? Crop Sci. 30: 815-818.
Lander, E. S., P. Green, J. Abrahamson, A. Barlow, M. J. Daly, S. E. Lincoln and L. Newburg, 1987. MAPMAKER: an interactive computer package for constructing primary genetic linkage maps of experimental and natural populations. Genomics 1: 174-181.
Peterson, M. L., D. B. Jones and J. N. Rutger, 1978. Cool temperature of rice lines for seedling vigor. IL Riso 27: 269-274.