Accumulation of High Molecular Weight Glutenin Subunits in Superior and Inferior Grains of Winter Wheat

The difference in accumulation of high molecular weight glutenin subunits （HMW-GS） in superior and inferior grains results in the non-uniformity of grain quality in winter wheat （Triticum aestivum L.）. The HMW-GS accumulation and glutenin macropolymer （GMP） content were studied in superior and inferior grains during the grain filling period, using the sodium dodecyl sulfate polyacrylamide gel electrophoresis （SDS-PAGE）. Compared to inferior grains, HMW-GS was formed earlier and total accumulation amount was higher in superior grains. The total HMW-GS content was higher in superior grain than that in inferior grain except at maturity. For individual HMW-GS types, the accumulation and content of subunit 7 were the highest, followed by subunit 12, while that of subunit 8 were the lowest, followed by subunit 2 in superior grain. In contrary, the accumulation and content of subunit 7 at maturity were significantly higher than subunit 8, but they were similar between subunit 2 and subunit 12 in inferior grain. Moreover, the accumulation of subunit 7 and 12 in superior grain were significantly higher than that in inferior grain. However, compared to the inferior grain, the GMP accumulation was higher, while content was lower in superior grain at maturity.

Higher Grain-Filling Rate in Inferior Spikelets of Tolerant Rice Genotype Offset Grain Yield Loss under Post-Anthesis High Night Temperatures

Increased nighttime respiratory losses decrease the amount of photoassimilates available for plant growth and yield. We hypothesized that the increased respiratory carbon loss under high night temperatures(HNT) could be compensated for by increased photosynthesis during the day following HNT exposure. Two rice genotypes, Vandana(HNT-sensitive) and Nagina 22(HNT-tolerant), were exposed to HNT(4 ℃ above the control) from flowering to physiological maturity. They were assessed for alterations in the carbon balance of the source(flag leaf) and its subsequent impact on grain filling dynamics and the quality of spatially differentiated sinks(superior and inferior spikelets). Both genotypes exhibited significantly higher night respiration rates. However, only Nagina 22 compensated for the high respiration rates with an increased photosynthetic rate, resulting in a steady production of total dry matter under HNT. Nagina 22 also recorded a higher grain-filling rate, particularly at 5 and 10 d after flowering, with 1.5- and 4.0-fold increases in the translocation of ^(14)C sugars to the superior and inferior spikelets, respectively. The ratio of photosynthetic rate to respiratory rate on a leaf area basis was negatively correlated with spikelet sterility, resulting in a higher filled spikelet number and grain weight per plant, particularly for inferior grains in Nagina 22. Grain quality parameters such as head rice recovery, high-density grains, and gelatinization temperature were maintained in Nagina 22. An increase in the rheological properties of rice flour starch in Nagina 22 under HNT indicated the stability of starch and its ability to reorganize during the cooling process of product formation. Thus, our study showed that sink adjustments between superior and inferior spikelets favored the growth of inferior spikelets, which helped to offset the reduction in grain weight under HNT in the tolerant genotype Nagina 22.

Wetting alternating with partial drying during grain filling increases lysine biosynthesis in inferior rice grain

Lysine content is a criterion of the nutritional quality of rice.Understanding the process of lysine biosynthesis in early-flowering superior grain(SG)and late-flowering inferior grain(IG)of rice would advance breeding and cultivation to improve nutritional quality.However,little information is available on differences in lysine anabolism between SG and IG and the underlying mechanism,and whether and how irrigation regimes affect lysine anabolism in these grains.A japonica rice cultivar was grown in the field and two irrigation regimes,continuous flooding(CF)and wetting alternating with partial drying(WAPD),were imposed from heading to the mature stage.Lysine content and activities of key enzymes of lysine biosynthesis,and levels of brassinosteroids(BRs)were lower in the IG than in the SG at the early grainfilling stage but higher at middle and late grain-filling stages.WAPD increased activities of these key enzymes,BR levels,and contents of lysine and total amino acids in IG,but not SG relative to CF.Application of 2,4-epibrassinolide to rice panicles in CF during early grain filling reproduced the effects of WAPD,but neither treatment altered the activities of enzymes responsible for lysine catabolism in either SG or IG.WAPD and elevated BR levels during grain filling increased lysine biosynthesis in IG.Improvement in lysine biosynthesis in rice should focus on IG.

Removal of superior wheat kernels promotes filling of inferior kernels by changing carbohydrate metabolism and sink strength

Poor filling and low weight of inferior kernels limit the further improvement of wheat yield. Two cultivars, Shuangda 1 and Xinong 538, with different grain weights, were selected to investigate the physiological changes of inferior kernels by removal of superior kernels(RS) at the flowering stage. i TRAQ combined with physiological indexes was used to identify factors limiting the filling of inferior kernels.Removal of superior kernels significantly increased the mean filling rate of inferior kernels and thus increased their weight. A set of 6012 proteins in inferior wheat kernels were differentially expressed between the RS and control. These differentially expressed proteins were involved mainly in carbon metabolism and energy metabolism. The main reason for the promoting effect of RS on the filling of inferior kernels may be that RS downregulated proteins involved in glycolysis and pyruvate metabolism while upregulating proteins involved in carbon fixation and photosynthesis. Consequently, RS greatly increased the ATP content in inferior kernels, supplying energy for them to absorb photosynthetic assimilates. Removal of superior kernels increased the activities of sucrose synthase, soluble starch synthase,adenosine diphosphate glucose pyrophosphorylase, and starch branching enzyme in inferior kernels and promoted starch accumulation in them. Thus, RS promoted the filling of inferior kernels and increased their weight. The promoting effect of RS on starch synthesis in inferior kernels was associated with their endogenous IAA and ABA levels.

Superior grains determined by grain weight are not fully correlated with the flowering order in rice

Rice panicles are composed of many branches with two types of extreme grains,the superior and the inferior.Traditionally,it has been well accepted that earlier flowers result in superior grains and late flowers generate inferior grains.However,these correlations have never been strictly examined in practice.In order to determine the accurate relationship between superior and inferior grains and the flowering order,we localized all the seeds in a panicle in four distinct rice species and systematically documented the rice flowering order,flower locations and the final grain weight for their relationships.Our results demonstrated that the grain weight is more heavily determined by the position of the seeds than by the flowering order.Despite earlier flowering has a positive correlation with the grain weight in general,grains from flowers blooming on the second day after anthesis generally gained the highest weight.This suggests earlier flowers may not result in superior grains.Therefore,we concluded that superior and inferior grains,commonly determined by grain weight,are not fully correlated with the flowering order in rice.Following the order of the grain weight,the superior grains are generally localized at the middle parts of the primary branches,whereas inferior grains were mainly on the last two secondary branches of the lower half part of the panicle.In addition,the weight of inferior grains were affected by spikelet thinning and spraying with exogenous plant growth regulators,indicating that physiological incompetence might be the major reason for the occurrence of the inferior grains.