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.