Identification of the quantitative trait loci for breaking and bending types lodging resistance in rice

概要

This study aims to identify the genetic factors responsible for strong culm and low lignin in high-biomass, long-culm variety, Leaf Star, and to identify new QTLs for thick and stiff culm, which are associated with lodging resistance.

This dissertation was organized based on four related chapters.

Chapter 1 covers the varietal differences between parental variety, Leaf Star and Koshihikari for traits associated with lodging resistance. In this chapter, to find the varietal differences between Leaf Star and Koshihikari were investigated. As Leaf Star had superior lodging resistance despite low lignin concentration, the physical traits associated with low-lignin characteristics in this variety were clarified, by comparing varietal differences in traits associated with breaking and bending-type lodging resistance between Leaf Star and Koshihikari. The physical parameters of the main culm, which are closely associated with lodging resistance, were used for precise phenotyping. Eight main culms from each replicate were used to determine the average culm length, panicle length, internode length, Young’s modulus, and bending force at maximum force. Bending load at breaking was measured on the 5th internode with a distance of 4 cm between two supporting points by the method of Ookawa and Ishihara (1992) using a universal testing machine. Moreover, for physiological factors to maintain lodging resistance, we focused on cell wall materials such as holocellulose, cellulose, hemicellulose, and starch by comparing their contents and densities. The differences in cell components contribute to high mechanical strength and show that Leaf Star has superior resistance to both breakingand bending-type lodging, based on a combination of culm thickness and culm stiffness.

In chapter 2, we conducted the Quantitative trait loci analysis for physical parameters associated with lodging resistance using recombinant inbred lines. In this chapter, to identify the factors responsible for strong culm and low lignin in Leaf Star and to successfully develop long culm and lignin deficient varieties with superior lodging resistance and high biomass using marker-assisted selection (MAS), starch and cell wall components such as cellulose, hemicellulose, and lignin were investigated and the QTL analysis was conducted, using 94 recombinant inbred lines (RILs), derived from a cross between Leaf Star and Koshihikari. In this chapter, a total of 12 QTLs for breaking-type lodging resistance on Chrs. 1, 2, 3, 5, 10 and 11 were detected in F7 and F8 of 94 RILs. Furthermore, a set of 12 QTLs affecting bending-type lodging resistance was detected in both F7 and F8 on Chrs. 2, 3, 5, 10 and 11. QTLs for traits associated with culm thickness such as SM and SMI on Chrs. 2 and 3 were detected, and these results showed that the detected QTLs for SM and SMI on Chrs. 2 and 3 were consistent with the results from RILs of Chugoku 117 and Koshihikari, the parents of Leaf Star, as reported by Yano et al. (2015). By combining these QTLs, we will be able to produce a variety with thick and stiff culm and increased grain yield.

Furthermore, chapter 3 covers cell wall components and starch evaluation using reciprocal substitution lines (RSLs) on chromosome 5. As in chapter 2, several QTLs for cellulose, hemicellulose, starch contents and densities were detected owing Leaf Star allele especially on Chr. 5, meanwhile, among the 94 RILs, two substitution lines named: TULK-6 carries the Leaf Star chromosome segment on Chr. 5 to the Koshihikari genetic background and TULK-75 carries Koshihikari chromosomal segment on Chr. 5 to the Leaf Star genetic background. Therefore, these substitution lines were selected and further comparisons were conducted to find the effect of chromosome segments of Leaf Star on Chr.5 for cell wall components.

TULK-6 showed higher hemicellulose density, starch content and density than its recurrent parent, Koshihikari due to the effects of Leaf Star alleles on Chr. 5, meanwhile, TULK-75 exhibited a lower cell wall components and starch accumulation than Leaf Star.

In the last chapter of this thesis, in order to narrow down the QTL for cell wall component and starch density backcrossing was performed using the SLs and their parent variety and genetic mapping was constructed to find the QTLs on Chr. 5, Moreover, as a physiological factor to maintain lodging resistance, cell wall materials like holocellulose, cellulose, hemicellulose starch contents and densities were studied. In this chapter, the accumulation of starch in newly developed recombinant homozygous lines for TULK-6 were studied two weeks after heading, to confirm the effects of chromosome segments of Leaf Star on Chr.5 for cell wall components, especially starch.

From our findings in this study, we detected QTLs for traits associated with stiff and strong culm related to bending-and breaking-type lodging resistance contributed from Leaf Star on chr. 5 and we identified new QTLs for cellulose and hemicellulose on chr. 5. These findings suggest that superior lodging resistance could be introduced into long-culm, lignin-deficient rice varieties by precise evaluation and by incorporating rice genes that confer a thick and stiff culm. Further study is required to narrow down the region and to identify the genes responsible for these traits, in order to increase our understanding of the genetic factors controlling the development of rice, wheat and other gramineous crops with improved bending and breaking-type lodging resistance.