3.1 Expression of starch synthesis-related genes
In our previous study, the expressions of starch synthesis-related genes
such as OsGPT2 , OsAGPL1 , OsAGPS1 , OsBEI andOsPho1 were highly correlated with the expression of CRCTin the leaf sheath of CRCT overexpression lines and knockdown
lines (Morita et al., 2015; Morita et al., 2019). There are considerable
differences in the starch content in the leaf sheath among rice
varieties (Goda et al., 2016). Thus, in this study, the relationships
between the expression levels of CRCT and the starch
synthesis-related genes were analyzed in the six rice cultivars,
Nipponbare, Koshihikari, Habataki, Takanari, Momiroman and Leafstar
(Figure 1). Among these cultivars, Nipponbare was used for the
production of transgenic lines. Koshihikari is the most popular cultivar
in Japan. Habataki, Takanari and Momiroman are representative
high-yielding varieties. However, the pattern of changes in the
nonstructural carbohydrate (mainly starch) content in the leaf sheath
and culm during the reproductive stage were different between Takanari
and Momiroman (Yoshinaga et al., 2013). For example, the content of
nonstructural carbohydrate in the leaf sheath and culm were
significantly lower in Takanari than in Momiroman at the grain-filling
stage. Leafstar is characterized as lodging resistant and has a high
starch content in straw (Ookawa et al., 2010). Using the leaf sheath of
these cultivars, the expression levels of CRCT were compared with
the expression levels of starch synthesis-related genes by qRT-PCR. The
expression levels of OsAGPL1 , OsAGPS1 and OsPho1highly correlated with that of CRCT. In contrast, weak or no correlation
with CRCT expression were observed in OsGPT2 and OsBEI .
However, when limited to Nipponbare, Koshihikari and Takanari, theOsBEI expression was correlated with the CRCT expression.
While not apply to all, the expression level of CRCT can be a
major determinant of natural variation in the expression level of
multiple starch synthesis-related genes.
The relationship between the expression level of CRCT and starch
content was also analyzed using these cultivars. In our previous study,
it was clearly shown that starch content was greatly increased in theCRCT overexpression lines and decreased in the CRCTknockdown lines (Morita et al., 2015). However, a significant
correlation was not observed between the starch content and the
expression levels of CRCT in the leaf sheath among these rice
cultivars (Figure 1). This tendency was the same even when limited to
the cultivars showing the correlation between the OsBEIexpression and the CRCT expression.