Sugars and starch
During embryo and endosperm development, sucrose content gradually
increased, peaking at 15 DAF and then decreased thereafter (Figures 4a,
S6a). Glucose and fructose levels decreased during seed development
(Figures S7a, S7b, S8a, S8b). The ratio of glucose to sucrose mediates
endosperm differentiation (Olsen, 2020). It was higher in the embryo and
endosperm at 5-10DAF, and then decreased gradually (Figures S7c, S8c),
corresponding to the transition from differentiation to storage
accumulation. These sugars were unevenly distributed in rice seed, with
the embryo generally having higher contents (Figures 4a, S6a, S7a, S7b,
S8a, S8b). Genes participating in sugar metabolism were differentially
expressed between embryo and endosperm. For the five genes encoding
sucrose synthase, two of them (OsSUS1 and OsSUS6 ) were
mainly expressed in embryo, while another three (OsSUS2 ,OsSUS3 , OsSUS4 ) in endosperm (Figures 4a, S6a).
During grain filling, starch content increased gradually, peaking at 20
DAF and 30 DAF in the embryo and endosperm, respectively (Figures 4b,
S6b). Compared with embryo, endosperm contained higher starch content,
which is consistent with previous report (Juliano & Bechtel, 1985).
Starch synthesis-related genes were coordinately expressed in the embryo
and endosperm, similar to that in wheat (Xiang et al., 2019). Moreover,
these genes were primarily expressed in endosperm at 5-20 DAF. By
contrast, they displayed a gradual increase in embryo at 5-20 DAF.
T6P and its target protein SnRK1
Trehalose-6-phospate (T6P) signals the availability of sucrose in plant
cells through the feast-famine kinase, SnRK1, which targets the genes
responsible for metabolism, growth, and development (Dingkuhn et al.,
2020; O’Hara, Paul, & Wingler, 2013; Paul, Oszvald, Jesus, Rajulu, &
Griffiths, 2017). To better understand the role of this signaling
pathway in rice seed, we analyzed T6P content and SnRK1 activities
(Figures 4c, 4d, S6c, S6d). T6P level in embryo and endosperm showed a
gradual decrease in the development course. Compared with endosperm,
embryo has higher content at early stage (5-10DAF) (Figures 4c, S6c).
For example, at 5 DAF, T6P was 3.98 and 6.61 nmol g−1DW in endosperm and embryo, respectively. In addition, TPS genes
were expressed in both embryo and endosperm, being predominantly
expressed at 5-20 DAF.
The SnRK1 activity in endosperm maintained a low level at 5-20 DAF, and
then increased rapidly at later stages (25-60 DAF). Different from the
endosperm, SnRK1 in the embryo was at a lower level, being higher only
at 5 DAF (Figures 4d, S6d). SnRK1A was specifically expressed in
the embryo, while two SnRK1B genes (SnRK1B andOSK35 ) were mainly expressed in endosperm. In addition,SnRK1A was broadly expressed across the time points in embryo. By
contrast, SnRK1B and OSK35 were more prevalent in
endosperm at 5-20 DAF. Interestingly, in contrast to the strong
correlations between the actual level of T6P and the activity of its
regulating genes, those for the SnRK1 activity showed no significant
correlation, indicating a strong post-translational regulation.