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.