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.