Preharvest sprouting (PHS) of wheat ( Triticum aestivum L.) is a worldwide problem causing downgrading of grain quality that leads to a significant economic loss for farmers and food produces. White-grained wheat is more preferred nowadays as it produces lighter color in whole grain bread, better color stability in noodles and higher flour extraction. However, white-grained wheat cultivars have long been recognized to be less resistant to PHS than red-grained ones. PHS depends on the level of grain dormancy present at the time of ripening of each genotype. Release of grain dormancy is regulated by complex interaction between environment and genetic factors. This study was carried out with the objective of understanding the genetic status that underlie grain dormancy of white-grained wheat and, to improve white-grained wheat by incorporating molecular markers linked to grain dormancy genes and gluten quality genes.

　Chapter 2 described the introgression effect of previously identified two QTLs QPhs.ocs-3A.1 (QPhs-3AS ) and QPhs.ocs-4A.1 ( QPhs-4AL ) for grain dormancy in red-grained Zenkoujikomugi (Zen), which has extreme tolerance to PHS. Forty-one white-grained recombinant inbred line (RIL) population was selected from a cross between Zen and low dormant white-grained Spica. Randomly selected 20 red-grained RILs were also developed from the same cross and used as a control population. The RILs were grown in the field for three years and in the glasshouse for two years to evaluate the grain dormancy. Grain dormancy was recorded at 15℃ and 20℃ temperatures by germination index (GI). Several molecular markers closely linked to the QPhs-3AS and QPhs-4AL were used to estimate the alleles at the QTLs. Dormancy variation in the RILs was significantly associated with the differences in grain color and the alleles at QPhs-3AS over several years. Although allelic variation was detected in an SSR marker closely linked to QPhs-4AL , there was no difference in germination data between the Zen-allele and the Spica-allele groups. As expected, the red-grained RILs with the Zen allele at QPhs-3AS were the most dormant in the filed. In the glasshouse, the dormancy effect acquired by the grain color was smaller than that of field trial. Some white-grained RILs with the Zen allele at QPhs-3AS showed higher dormancy compared to the red-grained RILs with the alternative allele. These results demonstrated that introgression of the QPhs-3AS gene could contribute to increased grain dormancy in white-grained wheat. Further researches to explore the potential of superior Zen allele of QPhs-3AS for transferring into low dormant wheat cultivars may worth to be experimented.

　White-grained RIL population demonstrated a further variation, which leads to a hypothesis that many genes have involved in maintaining the grain dormancy of Zen x Spica cross. Hence, aiming to be applied in practical breeding, another study described in Chapter 3, was conducted to identify molecular markers linked with novel grain dormancy QTLs as they would allow improving white-grained wheat by MAS. Initially two parents were analyzed with 262 DNA markers, including simple sequence repeats (SSR) and expressed sequenced tag (EST) markers and of them, 135 polymorphic markers were used to screen the white RIL population. In the first step, extremely high dormant and extremely low dormant RILs were genotyped and only the markers those showed an association with GI was selected to analyze with all RILs. At each marker locus GI of Zen and Spica allele groups were compared by the student t test.

　Five novel putative grain dormancy QTLs were identified in the Zen x Spica RIL population, and they were located on chromosomal arms 3BS, 5AL, 5BL, 5DL and 7BL. In all instances, allele contributed to higher dormancy was derived from Zen. The QTL detected on 3BS has accounted for much phenotypic variation and it was significant in 5 environments in the field and 3 environments in the glasshouse. The superior Zen allele of the 3BS QTL can be used to transfer into white-grained wheat by MAS. Five QTLs, detected in this study have led to the foundation for further breeding studies to practice effective MAS for PHS tolerance.

　Nine white-grained lines that carry the grain dormancy QTLs, QPhs-3AS , were selected from Zen x Spica cross and from wRIL42 x Zen backcross. Grain dormancy and flour and starch properties of the selected white-grained lines illustrated in Chapter 4 were examined in order to detect their suitability to be used in the food industry in comparison with several cultivars as a check. Three hard textured lines and six soft textured lines were among them. The protein content of all selected lines seemed either medium or high. Few lines exhibited combination of favorable flour/starch properties, high RVA peak viscosity, low -amylase activity, and proper color index including their grain dormancy. Most of the cultivars developed so far in Japan are suitable for udon noodles and none of white-grained lines is developed for bread making purpose. This study has produced two lines containing 5+10 high molecular weight glutenin subunits (HMW GS) due to Glu-D1d together with other favorable physiochemical quality parameters to reach those objectives. Three lines expressing 2+12 subunits due to Glu-D1a have the potential for noodle making as they demonstrated the best levels of white color and suitable physiochemical qualities.

　Finally overall outcome of this study and further research ideas to improve the white-grained lines were discussed in Chapter 5. In this study, using DNA, HMW-GS and flour/starch physiochemical markers, genes for high grain dormancy, glutenin and proper viscosity parameters were pyramid into several genotypes to produce dormant white-grained wheat for bread and noodle making purpose.

（和訳）

　穂発芽は世界のコムギ生産地域でしばしば問題となっている。 穂発芽が加工適性や品質の著しい低下をもたらし、生産者や実需者は大きな経済的損失を受けるためである。 一般に、白粒品種は製粉歩留が高く、色のきれいな品質の優れたパンや麺をつくることができるが、赤粒品種に比べ、穂発芽を起こしやすい。 穂発芽耐性に大きく関わる形質に種子休眠性がある。 種子休眠性は遺伝的要因と環境要因が複雑に影響する。 白粒コムギの種子休眠性の遺伝的メカニズムを理解し、また種子休眠性と連鎖する分子マーカーを用いて白粒コムギを改良し、 実用的な育種に用いることを目的として本研究を行った。

　白粒コムギの穂発芽抵抗性を高めるひとつの方法は、マーカー選抜技術を用いて、 粒色とは独立の種子休眠性QTLを白粒コムギに導入することである。 第2章では以前に種子休眠性極強のゼンコウジコムギ（Zen）の交雑集団で同定された QPhs.ocs-3A.1(QPhs-3AS) と QPhs.ocs-4A.1(QPhs-4AL) の２つの種子休眠性QTLについて、Zen由来の対立遺伝子の効果を検討した。 実験では、Zenと種子休眠性弱白粒品種のSpicaの交雑より得られた白粒41系統、赤粒20系統からなる組換え自殖系統集団（RILs）を、 圃場で3年間、ガラス室で2年間栽培し15℃と20℃の温度条件で発芽指数（GI）を用いて種子休眠性を評価した。 QPhs-3AS と QPhs-4AL に連鎖する近傍のマーカーをいくつか用いて、QTLの遺伝子型を推定した。 粒色、QPhs-3AS の遺伝子型とRILの休眠性は密接な関連性がみられた。 しかし、QPhs-4AL 近傍に連鎖するSSRマーカーでは、Zen型とSpica型の休眠性に差はみられなかった。 QPhs-3AS においてZen対立遺伝子をもつ白粒のRILのうち、赤粒でSpica対立遺伝子をもつ系統よりも高い休眠性を示す系統がみられた。 これらの結果は QPhs-3AS の導入が白粒コムギで種子休眠性を高めるのに寄与していることを証明している。

　RILsの休眠性は連続変異を示し、QPhs-3AS に加えてその他のQTLが種子休眠性に関与することが予想された。 そこで、第3章ではマーカー選抜に有用な新規の種子休眠性QTLの同定をおこなった。 計262の分子マーカーについてZenとSpicaの間の多型解析をおこない、多型の得られた135マーカーについてRILsのスクリーニングをおこなった。 その後、遺伝子型がZen型とSpica型それぞれの平均GIをt検定し比較した。 3B染色体短腕、5A染色体長腕、5B染色体長腕、5D染色体長腕、7B染色体長腕に新規の種子休眠性QTLを同定できた。 これら5つのQTLは、すべてZen型が種子休眠性の向上に貢献していた。 3B染色体短腕のQTLは表現型に大きく寄与した。 そのため、このQTLはマーカー選抜により休眠性向上に有効であると考えられた。

　Zen×Spica とwRIL4×Zen（戻し交雑）で、QPhs-3AS の休眠性を示す白粒９系統を選抜し、 種子休眠性、小麦粉特性、でんぷん特性を実際の栽培品種と比較し第4章に記載した。 選抜した9系統は全て15℃でも20℃でも比較対象とした白粒品種、Spica、AUS1408、AC Karmaと比べて低い発芽指数を示した。 これら9系統のうち3系統は硬質で６系統は軟質であり、タンパク質含量は中程度か高かった。 中には適度な小麦粉でんぷん特性を示し、高いRVAピーク粘度、低いα―アミラーゼ活性で粉色の良い系統もあった。 本研究では Glu-D1 に5+10の高分子量グルテニンサブユニットをもち、優れた物理特性を示す2系統が得られた。 Glu-D1 において2+12をもつ3系統は、粒色や物理特性でも最高の値を示したため、麺品種育成への母材として期待される。 選抜した9系統は QPhs-3AS だけではなく、3BSや7BLにもZen対立遺伝子をもっており、 休眠性の高い赤粒系統から白粒系統に休眠性QTLを導入するのに成功したことを示している。

　最後に第５章では本研究の成果と白粒系統の育種について考察した。 本研究では種子休眠性をもつパン用,麺用の白粒コムギを育種する上で、種子休眠性、HMW GS、小麦粉のデンプン特性、 糊化特性について優れた形質の遺伝子をマーカー選抜技術により集積できることを実証した。