Why is the quality of the rice filled at high temperatures bad

Left: The wild type develops normally at room temperature, and the rice is transparent and full;Right: O3 mutant is opaque in rice flour at high temperatures. Photo courtesy of China Rice

The people take food as the sky, and food is the first priority. Rice is one of the main food crops in China, and more than 60% of the country’s population depends on rice as the staple food. With the improvement of people’s living standards, people’s requirements for the quality of rice are getting higher and higher, not only to eat full, but also to eat well, and pursue health and nutrition.

Recently, the team of academician Hu Peisong of the China Rice Research Institute has made important progress in the research on the regulation mechanism of rice endosperm development and rice quality affected by high temperature, and Plant Communications has published relevant research results online. This study reported that rice OPAQUE3(O3) encodes a bZIP transcription factor (OsbZIP60) with a transmembrane domain, which can simultaneously regulate the biosynthesis of endosperm storage protein and starch, participate in maintaining endosperm endoplasmic reticulum homeostasis at high temperatures, ensure the normal development of rice endosperm, and provide an important theoretical basis for the genetic improvement of rice quality.

Wei Xiangjin, co-corresponding author of the paper and researcher of China Rice Research Institute, introduced that the high temperature during the filling period is the most vulnerable natural stress for rice, especially in the southern indica rice area, and the rice filling process is shortened at high temperature, resulting in insufficient synthesis and accumulation of starch and other storage substances, and changes in the composition and structure of starch, which eventually leads to a decrease in rice yield, deterioration in appearance and taste. At present, the molecular mechanism of high temperature affecting rice quality, leading to chalky or powdery endosperm production is still unclear. Therefore, studying the molecular mechanism that regulates the synthesis of rice starch, protein and other storage substances and is regulated by high temperature is of great significance for the genetic improvement of rice quality and the cultivation of new high-quality and high-yield rice varieties under high temperature stress.

The study found an opaque mutant of the endosperm that is more sensitive to high temperature quality in rice quality, opaque3 (O3). In the natural high temperature environment of Hangzhou, the wild-type medium-flower 11 grains were full and transparent, while the filling rate of mutant O3 grains decreased, the contents of total starch, amylose, total protein and gluten in mature grains were significantly reduced, and the yield of 1,000 grains and single plant was significantly reduced.

After setting stricter artificial high temperature (35°C/28°C) and room temperature environment (28°C/22°C) treatment, it was found that the O3 mature grains under the artificial high temperature environment were more extreme than the abrupt phenotype at room temperature, the grain powder was wrinkled, and the amylose, protein content and grain weight were significantly reduced, while the change of medium flower 11 was small, indicating that the quality of O3 rice was more sensitive to high temperature.

Cytological observation showed that the endoplasmic reticulum of O3 endosperm was abnormal, and a large number of protein precursors were retained in the endoplasmic reticulum and its derived vesicles. Some powder-making bodies in O3 endosperm cells disintegrate early, resulting in the presence of a large number of starch single grains.

After cloning the O3 gene, they found that O3 encodes a transmembrane domain-containing bZIP transcription factor (OsbZIP60), O3 is highly expressed in the developed kernel, and is dual-localized in the endoplasmic reticulum and nucleus, and O3 is mainly localized in the nucleus after adding DTT to induce endoplasmic reticulum stress or remove its transmembrane domain.

Further research found that O3 can directly bind to several related genes and promoters of endosperm storage protein and starch synthesis, and participate in the synthesis of endosperm storage protein and starch.

In addition, O3 can also directly bind to the promoter of the core molecular chaperone gene of endoplasmic reticulum protein processing, and participate in protein processing and secretion.

Studies have proved that O3 plays a central regulatory role in the development of rice endosperm, especially the normal development of endosperm under high temperature stress. It provides an important theoretical basis for the genetic improvement of rice quality and the cultivation of high-temperature passival rice varieties. The significant reduction of gluten in the endosperm of O3 mutant can also provide an important genetic resource for the cultivation of low-gluten special rice suitable for people with kidney disease.

Cao Ruijie, a doctoral student at the China Rice Research Institute, and Zhao Shaolu, a postdoctoral fellow, are the co-first authors of the paper, and Hu Peisong and Wei Xiangjin are co-corresponding authors. The research was supported by the National Natural Science Foundation of China, the Natural Science Foundation of Zhejiang Province, and the Innovation Project of the Chinese Academy of Agricultural Sciences. (Source: Li Chen, China Science Daily)

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