The Japanese winter wheat variety ‘Yumechikara’ shows resistance to wheat yellow mosaic virus (WYMV). A single major QTL for WYMV resistance, Q.Ymym, has been identified in ‘Yumechikara’. Q.Ymym is tightly linked with the high-activity allele of the polyphenol oxidase (PPO) gene, Ppo-D1b, on the long arm of chromosome 2D. The introduction of Q.Ymym into susceptible varieties results in a time-dependent discoloration of wheat end products, especially in noodles, by the cointroduction of Ppo-D1b. The objective of our current study was to break off the tight linkage between Ppo-D1b and Q.Ymym. In this study, we clarified the positional relationship between Ppo-D1 and Q.Ymym. Linkage analysis indicated that Ppo-D1 was mapped on the Q.Ymym region, whereas the reference genome sequence of chromosome 2D showed that the physical distance was 19.7 Mb between Ppo-D1 and Q.Ymym. One recombinant between Ppo-D1 and Q.Ymym could be identified from recurrent back crossed lines between ‘Yumechikara’ and the susceptible variety ‘Tamaizumi’. We successfully developed a line carrying the low-activity Ppo-D1a allele and Q.Ymym from its progenies. The Ppo-D1a/Q.Ymym lines showed resistance to WYMV and low PPO activity. We made noodle sheets of yellow alkaline noodles from the Ppo-D1a/Q.Ymym and Ppo-D1b/Q.Ymym lines and assessed their time-dependent discoloration. The raw noodle sheet discoloration in the Ppo-D1a/Q.Ymym lines was suppressed in comparison to that in the Ppo-D1b/Q.Ymym lines. The Ppo-D1a/Q.Ymym line can solve the problem of noodle discoloration caused by the introduction of Q.Ymym. DNA markers for the selection of Ppo-D1a and Q.Ymym have been developed in previous studies. Breeding for WYMV resistance is expected to improve by combining the Ppo-D1a/Q.Ymym line and DNA markers in marker-assisted selection.
Waxy rice cultivars include fast-hardening and slow-hardening types for rice cakes. Because each type of cultivar is used for different applications, evaluation of the hardening properties of rice cakes is essential in waxy-rice-breeding programs. In the breeding of early generations, in which a large number of samples have to be handled, a simple and efficient evaluation method for screening is required. In this paper, we report an efficient evaluation method for the hardening properties of rice cakes that is applicable to the efficient selection of early generations, using an amylose autoanalyzer, which is widely used in Japanese rice-breeding programs. We were able to efficiently extract the iodine absorption spectrum of amylopectin of waxy rice by calculating the difference between the iodine absorption spectrum in the wavelength range of 400 to 900 nm and the blank iodine absorption spectrum, with double the concentration of I2/KI solution compared to the amylose content measurement of common rice. Investigation of the analytical values derived from the iodine absorption spectrum of waxy rice revealed that the integrated value of the absorbance (IVA) had a high and positive correlation with the b/a value of the bending method from 24 hours later, which is a classical evaluation method for rice cake hardening. This correlation was not inferior to that of the curability indexes such as the pasting temperature and the temperature of maximum viscosity analyzed using a Rapid Visco Analyzer (RVA). The effectiveness of this method was verified by applying it to early generations of a breeding population, and a high correlation was detected between IVA and rice cake hardness. The developed method had the following three advantages compared to RVA: 1) measurable with a small rice flour sample, 2) less than half the measurement time per sample, 3) continuous automatic instrumental analysis is possible. This evaluation method is expected to contribute to the efficient screening of waxy rice varieties in early generations and the efficient breeding of waxy rice cultivars with distinctive hardening properties.