Contribution Of Photosynthetic Capacity Of Capsule Walls And Maternal Effect Of Grain Weight In Sesame

Grain weight plays a crucial role in the formation of plant yield in sesame（Sesamum indicum）.90%to 95%of crop biomass derives from photosynthetic products.After photosynthetic assimilates were produced by photosynthetic organs,they were transported into the developing seeds,and transformed to seeds to produce lipids,proteins and other substances during grain development,which ultimately determined grain weight.Previous studies found that so-called non-leaf organs containing chlorophyll,such as pod/capsule of plant in addition to green leaves,also had photosynthetic ability,and maternal effects in grain-related traits in many crops.However,it was not well-known in sesame about the capsule photosynthetic capacity and its contribution to sesame grain weight,as well as the maternal effect on grain weight.In this paper,the photosynthetic performance of sesame capsule walls and the transport and transformation characteristics of photosynthetic products were studied to reveal the photosynthetic capacity of sesame capsules and its contribution to grain weight.Then the maternal effects on sesame grain weight were also studied based on the comparison of the grain weight of F1 hybrids with their parents and the transcriptome analysis of RNA-sequencing data of the developmental seeds.The results would enrich the understanding of the source-sink-flow relationship of sesame,provide scientific information for in-depth analysis of grain weight formation mechanism,and provide theoretical support for high-yield breeding in sesame.The main conclusions were as follows:1.Characteristics of photosynthetic characteristics of sesame capsules at different development stagesThe variation trends of chlorophyll content,Rubisco activity and the fluorescence parameters of sesame capsule photosystem II（PSⅡ）in sesame capsule walls of different sesame accessions were generally consistent.The chlorophyll content,Rubisco activity increased significantly from 5 to 25 DAP and peaked at 25 DAP,and then decreased significantly.Moreover,the chlorophyll content and Rubisco activity of the capsule walls of L1（with large seeds）were higher than those of S1（with small seeds）at capsule developmental stages.TheΦ_PSII of sesame capsule walls of two accessions kept stable during 5 to 35 DAP and then sharply decrease,which decreased more sharply in S1 than in L1.As for the r ETRmax,it increased till 15 DAP reaching a peak value and then deceased till 40 DAP to the bottom.Theαvalue increased to 20 DAP reaching a peak point and then down slowly.And the Ik maintained a high level during 5 to 15 DAP and then decreased till 40 DAP reaching a bottom point.The results showed that the sesame capsule has photosynthetic capacity which might affect the formation of sesame seed weight.2.Characteristics of nutrient content in sesame capsule walls and seeds at different development stagesThe variation trends of nutrient content in sesame accessions L1 and S1 were basically the same.During the development of sesame capsules,the soluble sugar content in the capsule walls,was maintained at a high level of synthesis from 5 to 20 DAP,but decreased sharply after 25 DAP.Meanwhile,the soluble sugar content in the seeds increased rapidly from 5 to 15 DAP,and then decreased rapidly after reaching a peak at15 DAP.During sesame capsule development and grain filling,the contents of starch,protein and fat in capsule walls were low,but the contents of fat,starch and soluble protein in seeds began to increase sharply at 10 DAP（fat）or 20 DAP（starch and protein）,and reached peak values at 25 DAP（fat）and 35 DAP（starch and protein）,respectively.The15DAP point might be a key stage for seeds filling,at which point the soluble sugar was transported from capsules into seeds and then transformed into nutrient substances in seeds.3.Correlation analysis between capsule photosynthetic indexes and the physiological indexes and dry weight of the capsule walls and seedsThe correlation analysis showed that theΦ_PSIIof capsule walls was significantly and positively correlated with the chlorophyll content in capsule walls,which suggested that the changes of chlorophyll content in capsule walls had important effect on the actual photosynthetic efficiency(Φ_PSII).grain dry weight,starch,protein and fat content.Moreover,Φ_PSIIof capsule walls was significantly and positively correlated with the changes of grain dry weight and the content of fat,protein and starch;and r ETRmax was significantly and positively correlated with the soluble sugar content of capsule wall and grain,which indicated the photosynthetic capacity of capsules play important role in seed filling and formation of seed weight in sesame.And the chlorophyll content in capsule walls was significantly and positively correlated with the fat content in seeds which suggested the increase of chlorophyll content in capsule walls would help to the fat accumulation in the seeds.4.Contribution of photosynthesis of sesame capsules to grain weightTo further clarify the effect of sesame capsule photosynthesis on capsule development and grain weight,girdling and shading experiment on sesame capsules of different accessions which differed in seed weight were performed.The results showed that the contribution of sesame capsule photosynthesis to dry weight of capsule ranged from 13.81%to 43.21%in different accessions.And as to to he contribution to dry weight of capsule walls and seeds ranged from 24.26%to49.94%,and 3.63%to 16.03%,respectively.It was further proved that the photosynthetic capacity of capsules has an important effect on capsule development and grain weight formation.5.Maternal effect on sesame grain weight bases on phenotypic dataThe maternal effect analysis was conducted by analyzing the grain weight of the two accessions which differed in seed weight and their F₁ seeds which derived from the hybrid combinations of the parents.Overall,the F1 grain weights in any combinations were biased toward the maternal parent.The maternal effect values of grain weight measured in different hybrid combinations ranged from 0.69 to 1.05,which demonstrated maternal effect on grain weight in sesame.6.Analysis of maternal effects based on transcriptomic dataBy comparing the transcriptome data of seeds at different developmental periods of FL1（F₁on large-grain parent）and FS2（F₁on small-grain parent）,40 and 41 differentially expressed genes（DEG）were identified as up-and down-regulated,respectively.These genes may be related to the maternal effect on grain weight in sesame.We analyzed the expression of the reported maternal effect genes in the sesame seed transcriptome and found that AP2,DA1,and ANT may be related to the maternal effect on the seed-related traits in sesame,and they may function at different developmental periods.7.Analysis of grain weight-related genes based on transcriptomic dataBy comparing the transcriptomic data of the large-grain parent L1 and the small-grain parent S2 differed in seed weight at different developmental stages（5 DAP,10 DAP and 20 DAP）,253 DEGs were detected in the seeds of the three developmental stages of L1 and S2,including 123 up-regulated genes and 130 down-regulated genes.These genes may be related to grain weight formation.