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.