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Hybrid rice technology has been used for more than 50 years, and eating and cooking quality (ECQ) has been a major focus throughout this period. Waxy (Wx) and alkaline denaturation (ALK) genes have received attention owing to their pivotal roles in determining rice characteristics. However, despite significant effort, the ECQ of restorer lines (RLs) has changed very little. By contrast, obvious changes have been seen in inbred rice varieties (IRVs), and the ECQ of IRVs is influenced by Wx, which reduces the proportion of Wxa and increases the proportion of Wxb, leading to a decrease in amylose content (AC) and an increase in ECQ. Meanwhile, ALK is not selected in the same way. We investigated Wx alleles and AC values of sterile lines of female parents with the main mating combinations in widely used areas. The results show that almost all sterile lines were Wxa-type with a high AC, which may explain the low ECQ of hybrid rice. Analysis of hybrid rice varieties and RLs in the last 5 years revealed serious homogenisation among hybrid rice varieties. [ABSTRACT FROM AUTHOR]

With the improvement of people's living standards and rice trade worldwide, the demand for high-quality rice is increasing. Therefore, breeding high quality rice is critical to meet the market demand. However, progress in improving rice grain quality lags far behind that of rice yield. This might be because of the complexity of rice grain quality research, and the lack of consensus definition and evaluation standards for high quality rice. In general, the main components of rice grain quality are milling quality (MQ), appearance quality (AQ), eating and cooking quality (ECQ), and nutritional quality (NQ). Importantly, all these quality traits are determined directly or indirectly by the structure and composition of the rice seeds. Structurally, rice seeds mainly comprise the spikelet hull, seed coat, aleurone layer, embryo, and endosperm. Among them, the size of spikelet hull is the key determinant of rice grain size, which usually affects rice AQ, MQ, and ECQ. The endosperm, mainly composed of starch and protein, is the major edible part of the rice seed. Therefore, the content, constitution, and physicochemical properties of starch and protein are crucial for multiple rice grain quality traits. Moreover, the other substances, such as lipids, minerals, vitamins, and phytochemicals, included in different parts of the rice seed, also contribute significantly to rice grain quality, especially the NQ. Rice seed growth and development are precisely controlled by many genes; therefore, cloning and dissecting these quality-related genes will enhance our knowledge of rice grain quality and will assist with the breeding of high quality rice. This review focuses on summarizing the recent progress on cloning key genes and their functions in regulating rice seed structure and composition, and their corresponding contributions to rice grain quality. This information will facilitate and advance future high quality rice breeding programs. [ABSTRACT FROM AUTHOR]

Summary: The source–sink relationship determines the overall agronomic performance of rice. Cloning and characterizing key genes involved in the regulation of source and sink dynamics is imperative for improving rice yield. However, few source genes with potential application in rice have been identified. Glucan, Water‐Dikinase 1 (GWD1) is an essential enzyme that plays a pivotal role in the first step of transitory starch degradation in source tissues. In the present study, we successfully generated gwd1 weak mutants by promoter editing using CRISPR/Cas9 system, and also leaf‐dominant overexpression lines of GWD1 driven by Osl2 promoter. Analysis of the gwd1 plants indicated that promoter editing mediated down‐regulation of GWD1 caused no observable effects on rice growth and development, but only mildly modified its grain transparency and seed germination. However, the transgenic pOsl2::GWD1 overexpression lines showed improvements in multiple key traits, including rice yield, grain shape, rice quality, seed germination and stress tolerance. Therefore, our study shows that GWD1 is not only involved in transitory starch degradation in source tissues, but also plays key roles in the seeds, which is a sink tissue. In conclusion, we find that GWD1 is an ideal biotechnological target with promising potential for the breeding of elite rice cultivars via genetic engineering. [ABSTRACT FROM AUTHOR]

Rice is cooked and consumed after a short-term cooling which is associated with the short-term starch retrogradation. The two most important textural attributes (hardness & stickiness) of 28 cooked rice varieties were investigated in this study, based on the causal relations between starch chain-length distributions (CLD) and short-term retrogradation properties. Size-exclusion chromatography (SEC) was used to characterize the starch CLD, while the setback viscosity (SBV) from rapid visco-analyzer and hydrogel firmness (HF) from textural profile analyzer were represented as characteristics of the short-term retrogradation process. Pearson correlations showed that regardless of amylose content, rice starches containing relatively shorter amylose molecules with short to long chains, and more amylopectin medium and long chains with the degree of polymerization (DP) > 69 can all promote the short-term retrogradation rate, resulting in cooked rice with a hard and non-sticky texture. It also suggests that both SBV and HF could be utilized to predict the final cooked rice texture such as the hardness and stickiness. Furthermore, rice starches with a relatively higher amount of medium chains but less short chains within the amylopectin molecules are related to a less sticky texture of the cooked rice. The current study provides important information for rice industry and breeding programs to develop new varieties with targeted eating qualities. Rice starches with a higher amylose content, relatively less amount of amylose swith hort and long chains, while relatively more amylopectin with medium and long chains have a faster short-term retrogradation rate, resulting in a harder but less sticky texture of cooked rice. Furthermore, rice starches with more amylopectin medium chains and relatively less amylopectin short chains could result in a less sticky texture of cooked rice. Image 1 • Cooked rice texture of 28 different rice varieties were investigated. • Setback viscosity and hydrogel firmness can predict cooked rice texture. • Starch short-term retrogradation determines cooked rice texture. • Both amylopectin and amylose molecules are involved in short-term retrogradation. • Starch chain length features controlling cooked rice texture are revealed. [ABSTRACT FROM AUTHOR]