| Chalkiness, an opaque part in rice kernel, is one of main traits determining rice appearance quality and price. Also, chalkiness has deleterious effect on milling quality and cooking quality. The percentage of chalkiness has been reduced in newly released cultivars, but chalky grains occur most commonly in japonica rice cultivars, especially for those in southern China, such as Jiangsu province. A better understanding of the mechanism and regulation of chalkiness formation will lay the foundation of improvement for rice quality. Using mutants with high occurrence of white-belly and white-core, this study compares the physiochemical properties, endosperm structure, molecular mechanism, and their response to nitrogen between the two main types of chalkiness, white-belly and white core, with the aim of elucidating the underlying foundation of grain chalkiness. The main results are as follows:(1) SEM images showed contrasting differences between white-belly and white-core in endosperm microstructure including the shape of endosperm cell, the size distribution of starch granules, and the amount of protein bodies. Endosperm cells in the ventral part (white-Lelly tissue) were rectangular in shape, while those in the central part (white-core) were relatively slender. In white-belly tissues, starch granules were intact and surrounded by globular protein bodies, with obvious inter-granular air spaces. Conversely, fewer protein bodies were observed in white-core by SEM. For the size distribution of starch granules, white-core tissues had a larger amount of small starch granules than the white-belly tissues. White-belly and white-core also varied markedly in morphological features of the cracked compound starch granules. The single granules for white-belly were regular and generally polygonal in shape, while most of the single granules of white-core were irregular in shape with sharp edges. This may be explained on the base of the size of starch granules. Compound starch granules of white-belly were larger and made up of more single granules. On the contrary, most of the compound starch granules in the white-core were relatively small and were composed of fewer single granules. Our findings indicate that protein has important role in chalkiness formation compared to starch, and further studies should pay more attention to exploring the relationship between nitrogen metabolism and chalkiness.(2) Comparative analysis showed that notable differences were observed in chemical compositions between chalky (white-belly and white-core) and translucent grains. Chalky grains had much lower starch and protein contents than the translucent. Similar results were noted in the majority of the 17 amino acids examined, and contents of Mn, K and Mg. In addition, white-belly rice kernels differed from white-core rice kernels in chemical components. White-belly rice kernels had distinctly higher amylose contents than the translucent, whereas white-core rice kernels displayed no visible difference with the translucent. Zn content in white-belly rice kernels showed significant decrease compared to translucent rice kernels, while it did not for the white-core rice kernels. Additionally, white-core rice kernels exhibited markedly lower contents of oxaloacetate/ phosphoenolpyruvate-derived amino acids than those of the translucent, like phenylalanine, aspartate and threonine. However, no noticeable differences were detected between white-belly and translucent rice kernels. Our findings suggest that it is necessary to separate white-belly and white-core rice in studies on chalkiness, and more attention should be paid to dissecting the role of protein and its accumulation in chalkiness formation from perspective of interactions of carbon and nitrogen metabolism.(3) White-belly and white-core formation were closely associated with carbon and nitrogen metabolism as well ROS homeostasis. But the temporal expression pattern of proteins involved in white-belly and white-core varied. Occurrence of white-belly appeared to be related to lower level of glycolysis and TCA cycle, impaired biosynthesis of starch and protein, repressed development of endosperm cell, the disturbed ROS homeostasis and repressed myo-inositol metabolism. By contrast, occurrence of white-core endosperm was attributed to the down regulated protein involved in central metabolism such as glycolysis, starch and protein biosynthesis, and cell wall biosynthesis. More importantly, white-belly and white-core were related to metabolic disorder within the periods from 4 DAF to 8 DAF and from 12 DAF to 16 DAF, respectively.(4) There were significant differences in response of white-belly and white-core to nitrogen fertilizer. Nitrogen significantly increased the percentages of white-belly and white-core rice kernels, with white-belly being more vulnerable. Averaged across nitrogen treatments, white-belly rice mutants produced grains with similar chemical compostion in comparison with those of the wild type. By contrast, white-core rice mutants had grains with higher level of total starch, amylose and amylopectin contents, especially at higher nitrogen rate. Interestingly, rice kernels from white-belly rice mutant showed significantly elevated contents of storage proteins except for albumin but reduced starch content as nitrogen topdressing rate. In contrast, rice kernels from white-core rice mutant contained significantly higher levels of globulin and glutelin, and lower levels of starch. Nitrogen topdressing increased the contents of the majority of the 17 amino acids, in particular glutamic acid, but decreased histidine and arginine contents for the white-core rice mutant. These findings suggest the N effect on white-belly and white-core be associated with the accumulation of starch and protein during grain filling, indicating the necessity of formulating different strategies for eliminating white-belly and white-core. |
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