Plant materials and sampling
The notched-belly mutant (NB) was obtained by EMS treatment of a
japonica rice cultivar Wuyujing3 (WT). It possesses a high ratio of
notched-belly grains with a white-belly mainly on the bottom endosperm
(Lin et al., 2014). In 2018, six seedlings of WT and NB were
transplanted into a plastic pot filled with 10 kg paddy soil. The plants
were grown under natural conditions, and at two days before flowering
were transferred to a 31℃/24℃ growth chamber (12h/12h day and night
cycle), with light intensity of 600 µmol photons m−2s−1 and relative humidity 70 ± 5%. Flowering dates
were carefully marked for the sampled caryopsis on the middle primary
rachis. Grains were sampled at eight times, i.e. 5, 10, 15, 20, 25, 30,
45 and 60 DAF, with three biological replicates. Samples were quickly
frozen in liquid nitrogen and then stored at -80 ℃ until analysis. The
developing grains were manually dissected into three subsamples, the
embryo, and the upper and bottom part of the endosperm (Figure 1). It is
noteworthy that the endosperm samples contain the maternal tissues such
as pericarp and seed coat.