4.3 Key regulatory factors contributed to rice quality formation
under elevated temperature
Photosynthesis is the process by which light energy is converted into
chemical energy and stored, and it is also the source of accumulation of
rice grain assimilation. Chlorophyll content and metabolic enzyme
activity are closely related to the strength of photosynthesis.
In our case, chlorophyll a-b
binding protein 1B-21, chlorophyll a-b binding protein P4, and
chlorophyll a-b binding protein 7 were significantly up-regulated, which
induced the acceleration of the synthesis and binding of chlorophyll
(Ballottari et al., 2012). Meanwhile, the expression levels of
photosystem I reaction center subunit VI and oxygen-evolving enhancer
protein 3 from the photosystem I were also increased significantly under
warming conditions, and that may explain the accelerated grain filling
rate during the early grain-filling stage induced by elevated
temperature, and the significant increase in the accumulation rate of
grain materials compared with the normal temperature treatment. However,
the expression of PSB28, which is responsible for water splitting, had a
downward trend throughout the period, obtaining a significant low level
at 12d after flowering under elevated temperature (0.4
folds). That may inhibit electron
transfer and weakens signal transmission, thereby weakening
photochemical reactions and resulting in decreased cell chlorophyll and
photosynthesis (Wada et al., 2019). Previous research shows that the
optical system II (PS II) is the most sensitive element to temperature
in the electron transmission chain
(Zhang et al., 2011). It would be
interesting to further investigate whether PSB28 could be the most
critical component affected by high temperature during the
photosynthesis process.
To our knowledge, the contents and ratio of starch and storage protein
in rice grains are the decisive factors in determining the final rice
quality. Rice starch synthesis is regulated by various enzymes,
including SSS, SBE, DBE and GBSS. GBSS is the main enzyme responsible
for amylose synthesis. Wx protein encoded by the Waxy gene GBSS-I can
tightly bind to the starch granules and promote the synthesis of
amylose. High temperatures can lead to downregulation of gene expression
that regulates GBSS synthesis, resulting in decreased amylose content
and increased amylopectin content (Dian et al. 2005; Fujita et al.
2006). Research by Denver (1996)
shows that GBSS is not only related to the synthesis of amylose, but
also to the extension of amylopectin in starch granules. However, the
exact effect of GBSS on the extension of normal starch granules is still
unclear. Our results showed that the GBSS enzyme was down-regulated at
6d after flowering under elevated temperature. However, enzymes related
to amylopectin synthesis did not change significantly. From 6d to 12d
after flowering, the expression level of granule-bound starch synthase
was significantly lower than that of the control. Under high
temperature, the amylose content of mature rice grains was significantly
lower than that of CK treatment, while the amylopectin content was
significantly increased (Ahmed et al., 2015). Expression levels of the
soluble starch synthase SS4 and SSS2-3, responsible for the synthesis of
amylopectin, were also decreased under high temperatures (Yamakawa,
2012). This change may reduce the activities of granular starch synthase
and soluble starch synthase, and lead to change in the ratio of amylose
and amylopectin, which eventually affected the physical and chemical
properties of starches in rice grain (Tang et al., 2019).
Rice storage proteins include albumin, globulin, glutelin and prolamin.
Prolamin is directly deposited in the endoplasmic reticulum cavity in
the form of intracellular protein particles, and finally buds from the
endoplasmic reticulum in the form of spherical protein bodies (PBIs).
While glutelin is efficiently converted into mature form by vacuolar
processing enzymes, and forms irregular protein bodies II (PBII)
together with α-globulin (Krishnan et al., 1992; Kumamaru et al., 2010).
The results of this study showed that warming had significant up- or
down-regulation effects on the expression of storage protein
family-related regulatory factors at different periods. For example, the
expression of glutelin type-A and type-B proteins were either
significantly up-regulated or down-regulated at 3d and 6d after
flowering, and there is no obvious rule for the regulation mode of these
regulatory factors under warming conditions. Based on our understanding
of glutelin, its synthesis pathway is still unclear, and the presence of
many unknown glutelin genes increases the difficulty in understanding
the expression pattern under warming condition. Therefore, this study
has not been able to essentially find the direct reasons for the changes
in the final grain storage protein content.
However, several types of protein species (ribosomal protein species,
superfamily II DNA and RNA helicase, and molecular chaperone IbpA)
related to biosynthesis and processing of proteins was found to be
affected by high temperature. Ribosomes are the primary sites for
protein synthesis, and different species of ribosomal proteins play an
essential role in translation, ribosome structure, and biogenesis in
protein anabolism (Moin et al.,
2016). In our study, the ribosomal
protein species (25S, 30S, 40S, 50S and 60S) exhibited significant
decreases during the middle stage of grain-filling, which may cause the
reduction in the protein biosynthesis and maintain the balance between
synthesis and degradation of proteins (Moin et al., 2016). The reduction
in protein content related to translation, such as RNA recognition motif
(RRM) domains, eukaryotic initiation factors (eIFs) and elongation
factors (EFs), indicates the adverse effects of high temperature on rice
protein synthesis. Furthermore, a series of molecular chaperone heat
shock proteins (Hsps) were identified to be significantly up-regulated
when exposed to high temperature. Heat shock proteins Heat shock protein
is a highly conserved peptide in structure and could be activated and
produced in large quantities when plants are subjected to abiotic
stress (Timperio et al., 2008). In
this study, the two most sensitive heat shock proteins are HSP70 and
26.7 kDa heat shock protein of the sHSPs (s