特青是我国育成的一个超高产籼稻常规品种，同时也可作为两系杂交稻的重要亲本，但其直链淀粉含量较高，蒸煮与食味品质较差。采用水稻蜡质基因内的一个分子标记（称为PCR-AccⅠ）进行辅助选择育种，经回交转育向特青品种导入来自中等直链淀粉含量优质籼稻的Wx基因，选育了仍保持原有亲本主要农艺性状的3个优质品系特青TT-1、特青TT-2和特青TT-3。改良品系胚乳中Wx基因表达量较改良前明显下降，直链淀粉合成大幅减少，由改良前的28.5%成功下调至15%左右的中等偏低水平。用改良品系与培矮64S所配两系杂交组合仍能保持较高的结实率及其他优异农艺性状，而杂交稻米的直链淀粉含量明显改善。直链淀粉含量降低后对亲本及杂种稻米中胶稠度等其他蒸煮食味品质的改善也有明显作用。
“Teqing”, both an elite conventional cultivar with high yields and a male parent for two-line indica hybrid rice production, had a great contribution to rice production at the end of last century in China. However, the amylose content (AC) in the endosperm of Teqing seeds is high, resulting in poor cooking and eating quality of the milled rice. The amylose in rice endosperm is synthesized by the granule-bound starch synthase (GBSS), which is encoded by the waxy (Wx) gene. Our previous researches showed that AC was correlated with the ability of excising intron 1 from the leader sequence of the Wx transcript, and a single nucleotide polymorphism (G or T) located at the first nucleotide of splice donor site of Wx intron 1 might be the key factor responding for the alterative splicing. Thus, a CAPS (Cleaved amplified polymorphic sequence) molecular marker, named as PCR-AccI, was subsequently developed to identify whether the nucleotide in this site is G or T, which preferred to easily infer the AC in a genetic or breeding material (Fig.1). In this study, by using the developed molecular marker and its assisted selection, the AC in Teqing was successfully down-regulated for improving its cooking and eating quality by introgressing the Wx locus from an indica rice line (Wx-TT genotype) with intermediate AC and good quality. Three elite improved Teqing lines, Teqing-TT-1, Teqing-TT-2 and Teqing-TT-3, were selected after four backcrosses and two selfinges (Table 1); in each generation the molecular marker-assisted selection was applied to select the individuals carrying Wx locus with GT or TT genotype (Fig.1). In the selected lines, the AC in endosperm was reduced to a relative low level of about 15% from 28.5% of their original (Table 3). Consequently, the two-line hybrids derived from the selected lines and Pei’ai 64S also showed a reduced amylose level (Table 3). The northern blot analysis showed that the reduction of amylose synthesis in selected lines and their hybrids resulted from the lower expression of the introduced Wx gene compared with in the originals (Fig.2). In the field trials, the agronomic performance in the improved lines and their hybrids was examined to be comparable as the originals except for a negative impact on grain weight (Table 1 and Table 2). Simultaneously, the other key factor involved in rice cooking and eating quality, gel consistency (GC), was also improved by the effect of reduced AC in the selected lines and their hybrids (Table 3), but the gelatinization temperature (GT) showed a little increase in the selected lines by comparison with the original one. The selected Teqing lines would have a great potential for the production of both conventional and hybrid indica rice with high quality and yielding.
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