1.Introduction
Rice is the staple food for more than 50% of world’s population. With the improvement of people’s living standards, high-quality rice has become more preferred by the rice production and consumption market. However, with the rapid development of industrialization, human activities are estimated to have caused approximately 1.0°C (a likely range of 0.8°C to 1.2°C) of global warming above pre-industrial levels (IPCC, 2018). According to the fifth assessment report (AR5) completed by the IPCC (Intergovernmental Panel on Climate Change), the global temperature is expected to rise by 1.4-5.8°C in 2100 (IPCC, AR5, 2014). This abnormal high temperature would seriously affect the normal growth and development rhythm of rice and ultimately affect yield and quality of rice (Xu et al., 2020; Jagadish, 2020). In particular, results of our 10-year field trials showed that rice quality formation generally exhibits negative response when exposed to high temperature. The increase of temperature leads to the significantly increased chalkiness and decreased milling quality of rice, which could extremely reduce the purchase expectation and the market value of rice (Dou et al., 2017). Therefore, rice production will have to face the challenges of high temperatures exacerbated by the intensification of climate warming in the future.
High temperature usually affects the key processes of rice growth and development, including germination, seedling growth, leaf emergence, tillering, heading and maturity stage (Hakata et al., 2012; Lin et al., 2010; Shi et al., 2016; Tsukaguchi and Iida, 2008). Among them, as the decisive stage of rice quality formation, grain-filling is the most sensitive period to external temperature. Grain-filling is the process of grain development and accumulation of storage materials e.g. starch, storage proteins and lipids, which ultimately determines the rice quality-related indicators. An increase in temperature at this stage would induce the decline in rice quality, including reduced milled rice fraction, significantly increased chalky rate and chalky area (Shi et al., 2013 Jagadish et al., 2015). As the most abundant components in rice grain, starch had been proved to be sensitive to increased temperature. Our previous study showed that the accumulation of total starch and amylose in early grain-filling stage was accelerated under the condition of increased temperature, but the accumulation speed in later stage was significantly decreased, which resulted in the lower content of amylose and total starch in mature grain compared to normal temperature treatment (Tang et al., 2019). Furthermore, high temperature during grain-filling could increase the contents of grain storage proteins, with a significantly increased composition of glutelin and decreased prolamin, and ultimately improved the nutritional quality of rice. However, rice with high protein content is more susceptible to deterioration during storage, and the appearance and eating quality of rice could be further declined (Cao et al.,2017). In addition, high temperature can enhance the activity of protease and accelerate the protein transformation into soluble nitrogen compounds such as amino acids, which would significantly increase the total amount of amino acids and the relative content of each component in rice grain. Overall, high temperature mainly accelerates the rate of grain-filling, but shortens its active duration, resulting in the insufficient accumulation of photosynthetic substances in rice grains (Kim et al., 2011; Wahid et al., 2007). In our previous research, this change was manifested in abnormal grain development caused by high temperature and the accumulation balance change of starch and storage proteins, which coordinately determines the formation of grain quality (Tang et al., 2018; Dou et al., 2017). Although we have obtained the physiological and biochemical evidence of high temperature in regulating grain storage material accumulation through field trials, the regulatory mechanism remains to be further clarified. The anabolism of rice starch and protein is a relatively complex process, including a series of metabolic pathways, synthesis, transport, modification, accumulation and other processes. Therefore, we intend to conduct the investigation and clarification of the key regulatory factors that participate in the above process during the warming process through high-throughput methods, which would help us to further understand the regulatory effects of high temperature on the main pathways of rice quality formation.
In-depth understanding of the regulation mechanism of warming on the synthesis and metabolic pathways of grain storage materials has important practical significance for further establishing high-quality rice cultivation methods under climate warming. Thus, the main purpose of this study is to further evaluate the critical period of temperature regulation of grain-filling and to clarify the key regulatory factors involved in grain quality formation under low-amplitude warming scene in the actual paddy field.