Abstract
Grain chalkiness is a key quality trait of the rice grain, whereas its underlying mechanism is still not thoroughly understood because of the complex genetic and environmental interactions. We identified a notched-belly (NB) mutant that has a notched-line on the belly of grains. The line dissects the endosperm into two distinct parts, the upper translucent part, and the bottom chalky part in the vicinity of the embryo. Using this mutant, our previous studies clued the negative influence of embryo on the biochemical makeup of the endosperm, suggesting the need for the in-depth study of the embryo effect on the metabolome of developing endosperm. This study continued to use the NB mutant to evolve a novel comparison method to clarify the role of embryo in the formation of a chalky endosperm. Grain samples of the wild-type (WT) and NB were harvested at 10, 20, and 30 days after fertilization (DAF), and then divided into subsamples of the embryo, the upper endosperm, and the bottom endosperm. Using non-targeted metabolomics and whole-genome RNA sequencing (RNA-seq), a nearly complete catalog of expressed metabolites and genes was generated. Results showed that the embryo impaired the storage of sucrose, amino acid, starch, and storage proteins in the bottom endosperm of NB by enhancing the expression of sugar, amino acids, and peptide transporters, and declining the expression of starch, prolamin, and glutelin synthesis-related genes. Importantly, the competitive advantage of the developing embryo in extracting the nutrients from the endosperm, transformed the bottom endosperm into an “exhaustive source” by diverting the carbon (C) and nitrogen (N) metabolism from synthetic storage to secondary pathways, resulting in impaired filling of the bottom endosperm and subsequently the formation of chalky tissue. In summary, this study reveals that embryo-induced metabolic shift in the endosperm is associated with the occurrence of grain chalkiness, which is of relevance to the development of high-quality rice by balancing the embryo–endosperm interaction.
Highlights
High population pressure in rice-consuming regions always demands an increase in production, but the pursuit for a concomitant increase in grain quality is increasing due to the enhanced quest for high-quality food in the developing countries like China
Further classification by Principal component analysis (PCA) analysis shows that the identified metabolites and genes can be divided into two primary groups, each being associated with the embryo (I) and the endosperm (II), respectively (Figure 2D)
This study took advantage of the NB mutant to devise a novel comparison method for quantifying the impact of embryo on metabolic processes in the developing endosperm, unraveling the fact that the embryo has a noticeable effect of diverting C and N metabolism from the biosynthesis of storage compounds to secondary pathways in the chalky endosperm
Summary
High population pressure in rice-consuming regions always demands an increase in production, but the pursuit for a concomitant increase in grain quality is increasing due to the enhanced quest for high-quality food in the developing countries like China Major quality traits of rice grain include the physical appearance, cooking, and sensory and nutritional properties. Chalkiness, the opaque part of rice grain, has been set up as one of the main goals in rice breeding, for it affects the visual appearance quality and the milling recovery of intact grains (Xi et al, 2014). High temperature occurring at the grain-filling stage has been reported as the most influential environmental factor inducing chalkiness, indicating that grain chalkiness would be a major threat to the rice industry in the scenario of global warming (Shi et al, 2017; Wada et al, 2019; Chang et al, 2021; Park et al, 2021; Yang et al, 2021). It is urgent to elucidate the mechanisms responsible for chalkiness formation in rice grain
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