2. The active role of embryo in grain quality formation
Grain chalkiness is a highly undesirable quality trait that negatively affects not only appearance quality but also milling, eating, and cooking quality. High occurrence of chalky grains has been a major obstacle for rice producers under the likely scenario of global warming (Jagadish, Murty, & Quick, 2015). The physiological and molecular mechanisms responsible for chalky tissue formation are largely elusive. Previously, we employed the NB mutant to compare the transcriptome and proteome profiling of the chalky bottom part and the translucent upper part of the notched-belly grains, and identified some key pathways associated with chalkiness occurrence, such as the shifts between C and N metabolism, and the reduced accumulation of 13 kDa prolamin subunit (Lin et al., 2017). In addition, we uncovered a substantially negative influence of embryo on the chemical storages of endosperm (Lin et al., 2016), highlighting the necessity of discovering the causal factors behind grain chalkiness from perspective of embryo-endosperm interaction.
In this study, we successfully applied a novel comparison method to show the direct effect of embryo on the developmental processes of endosperm. Using WT as a reference, a comparison of the bottom and upper parts of NB grains confirmed the findings of our previous work as well as identified new genes responsible for grain chalkiness formation. In brief, during the critical period of grain filling (5-20 DAF), down-regulation of genes encoding prolamin, while up-regulation of genes related to carbohydrate catabolism and transition metal ion transport play a key role in chalkiness formation in bottom part of NB grains (Figure S10a). In addition, the embryo down-regulated genes in endosperm that relate to cytoskeleton, MCM complex, chromosome segregation, cell cycle and starch metabolic process, while up-regulated those involved in lipid transport, biotic stress resistance and starch hydrolysis (Figure S10b). Notably, the dragging effect of embryo on endosperm provides a fresh perspective of chalkiness formation. As reported by Morita, Yonemaru, and Takanashi (2005) and Shi et al. (2017), heat stress caused rapid proliferation and expansion of the endosperm cells. As a result, high occurrence of chalky tissue was considered to be associated with the incomplete accumulation of starch that can not completely fill the increased endosperm cells. Conversely, our current findings indicate that the slowdown of endosperm development is another cause for chalkiness formation. It is thus inferred that a perfect rice grain without chalky tissue demands a delicate balance between the enlargement of endosperm size and the accumulation of storage products. Hence, the timing of transition from cell differentiation to storage accumulation is critical for the formation of grain chalkiness, and the role of embryo in this transitional process requires extensive investigation.