1. INTRODUCTION
Light intensity is a key factor affecting plant growth. During the grain-filling period in rice, low light intensity affects the synthesis and transportation of photosynthetic products (Acreche, Ariel Briceño-Felix, Sánchez, & Slafer, 2009; Mauro, Occhipinti, Longo, & Mauromicale, 2011), such as sucrose, which is the main photosynthetic product in plants, the primary raw material for starch synthesis in grains of rice and other grain crops, and the most essential transport form in plants. Such effects of low light intensity influence the yield, seed setting rate, 1000-grain weight, and other yield components of rice. For example, the amylose content of rice grains decreases, and the degree of chalkiness and chalkiness rate increases under low light conditions, which severely affects rice quality (W. J. Ren, Yang, Fan, Zhu, & Xu, 2003; W.J. Ren, Yang, Xu, Fan, & Ma, 2003; L. Wang et al., 2016). Since Sichuan is the largest industrial province in southwestern China, air pollution reduces light availability, which has become one of the main constraints on rice and other crop production here (Deng et al., 2009; Li Wang, Deng, Ren, & Yang, 2013). With the increasing demand for high-quality rice, there is an urgent need to understand the molecular characteristics of the deterioration of rice quality under low light conditions and to provide a theoretical basis for rice breeding.
Circular RNAs (circRNAs) are a class of noncoding RNA that is highly conserved and not easily degraded. In the 1970s, Sanger et al. (1976) first discovered closed circRNA molecules in plant viruses. After the first discovery of circRNA in the roots of Arabidopsis thaliana(Linnaeus) Heynhold in 2014, multiple species of circRNA have been identified (Chen et al., 2018; Darbani, Noeparvar, & Borg, 2016; Z. Wang et al., 2017; Ye, Chen, Liu, Zhu, & Fan, 2015). The primary function of circRNA is to regulate microRNA (miRNA) using the sponge mechanism, and it also regulates variable shearing and transmits signals over long distances (Liu, Zhang, Chen, & Shi, 2017). CircRNA can be obtained by altering the chloroplast and mitochondrial genomes, indicating that circRNA is involved in the regulation of many important life processes, including photosynthesis and respiration (Darbani et al., 2016; Sun et al., 2016; Ye et al., 2015).
CircRNAs of different plants at different growth stages have specific expressions in space and time. During the lifespan of Arabidopsisleaves, circRNAs express differentially at the growth-to-maturation stage of four days and the maturation-to-senescence stage of 16 days (Liu et al., 2017). Analysis of the circRNA-miRNA-mRNA regulatory network has shown that circRNAs might be involved in plant hormone signal transduction and porphyrin and chlorophyll metabolism during leaf senescence (Liu et al., 2017), and in animals it has been found that circRNAs can function as a miRNA sponge (Lasda & Parker, 2014). In cold-treated tomatoes, 102 circRNAs have the potential to act as miRNA sponges based on the predicted miRNA-binding sites for 24 distinct mature miRNAs (Zuo, Wang, Zhu, Luo, & Gao, 2016). In rice, 2354 circRNAs have been found in different tissues, and they have no significant enrichment effect on miRNA targets. This suggests that circRNA and its linear form may be negative regulators of their parent genes (Lu et al., 2015).
Previous reports have shown that circRNAs express differentially under abiotic stresses, such as phosphate, light, chilling, drought, zinc, and iron stress (Chen et al., 2018; Darbani et al., 2016; Ye et al., 2015; Zuo et al., 2016). The expression level of circRNA in Arabidopsisleaves was also different under different light conditions and treatment times (Ye et al., 2015), and 163 circRNAs presented differential expression between the control and chilled plants. Most of the deregulated circRNA in the control plants deregulated in the frozen treatment (Zuo et al., 2016). The authors identified 62 candidate circRNAs that showed differential expression under drought-like stress, with 16 circRNAs that were upregulated and 46 that were downregulated under this condition. This work also indicated the sponge action of circRNAs by showing that 6 out of the 62 circRNAs have miRNA-binding sites that can potentially regulate 26 distinct wheat miRNAs (Y. Wang et al., 2017). In addition, 27 differentially expressed circRNAs were found under conditions of phosphate deficiency in rice, with six circRNAs that were upregulated and 21 circRNAs that were downregulated; moreover, several circRNAs were positively correlated with their parental genes (Ye et al., 2015).
Studies have shown that abscisic acid (ABA) plays a vital role in the regulation of rice grain filling and is involved in multiple biological processes to regulate the resistance of rice to environmental stresses (MingHui, Liu, Lu, Zhao, & Yang, 2009; T. Suzuki et al., 2008; Thoms & Rodriguez, 1994). Abscisic acid-deficient mutants, such asArabidopsis aba1, aba2, aba3, and ABA-deficient mutants in tobacco, tomatoes, and corn mostly grow normally under normal growth conditions, but the plants are stunted. However, these ABA-deficient mutants were more likely to wither and die than wild-type plants under drought and high-salt treatments, while the Arabidopsissupersensitive mutant era1 was more resistant to drought stress, suggesting that ABA plays an essential role in plant stress tolerance (Zhu, 2002).
This study aimed to determine how specific circRNAs are expressed during rice grain filling at different stages of development and under low light conditions, what mechanisms and pathways circRNA uses to regulate grain filling and how this affects grain quality.