Study On The Qtl For Photosynthetic Traits And Eqtl For Rubisco Activase Genes In Soybean

Soybean is an important food and oil crop. Increasing soybean yield is one of the most important breeding aims. About95%of the crop dry matter comes from the photosynthetic assimilates. Therefore, to increase the light energy use during photosynthetic process is the key for further increasing crop yield and feeding the world’s increasing population. For many years, considerable researches have been carried out, either by conventional breeding methods or by gene transformations, to select and breed for genotypes with superior photosynthetic capacity.Mining germplasms with high photosynthetic efficiency and discovering new genes underlying the photosynthetic process could not only help understanding of the genetic mechanisms of the energy capturing, transferring and converting within photosynthetic apparatus, but also lay a foundation for improving photosynthetic energy use and crop yield. The purposes of this study are:（1） screening for germplasms with high photosynthetic efficiency;（2） mapping quantitative trait loci （QTLs） for photosynthesis related traits in order to develop marker-assisted selection （MAS） breeding programs;（3） cloning soybean Rubisco activase genes and identifying the genetic determinisms of their expression levels; and （4） dissection of the genetic basis of photosynthetic rate.Photosynthetic gas-exchange parameters have been widely used to reflect the photosynthetic capacity of plant. In the present study, pot experiment was conducted to evaluate162soybean varieties （lines） for four gas-exchange parameters, photosynthetic rate, stomatal conductance, internal CO2concentration and transpiration rate, at R6development stage with portable photosynthesis systems （LI-6400）. The results showed:（1） the four gas-exchange parameters correlated with each other positively and significantly;（2） the gas-exchange parameters demonstrated significant difference, wide variation range and quantitative distribution among varieties （lines）, suggesting a larger potential for breeding improvement of these parameters;（3） fifteen varieties （lines） with high photosynthetic rate were selected, which could be used as base materials for future high photosynthetic efficiency breeding.Pot experiments were carried out in two successive years to map QTLs for photosynthetic rate, stomatal conductance, intercellar CO2concentration and transpiration rate in a RILs population （NJRIKY） derived from Kefeng1and Nannong1138-2. As a result, sixteen QTLs located on linkage groups （LG） C1、C2、D2、E、H、I and O, respectively, were identified. The percentage of variation explained by these QTLs ranged from4.80%to12.30%, and with LOD scores from2.25to6.31. Seven QTLs expressed stably across environments, including the QTLs qPNC1.1and qPND2.1for photosynthetic rate, QTL qSCD2.1and qSCI.1for stomatal conductance, QTLs qCiI.l and qCiO.1for intercellular CO2concentration and QTL qTrO.l for transpiration rate. These QTLs might be valuable for soybean breeding. Four genomic regions were detected controlling different parameters:the marker interval sat₃11-sct₁91on LG C1for photosynthetic rate and stomatal conductance, the marker interval sat₂96-sat₂77on LG D2for photosynthetic rate, stomatal conductance and intercellular CO2concentration, the marker interval sat₁72-satt268on LG E for photosynthetic rate, intercellular CO2concentration and stomatal conductance, the marker intervals satt726-satt330on LG I for stomatal conductance and interncellular CO2concentration.Chlorophyll fluorescence parameters can provide qualitative and quantitative information about photosynthetic processes in chloroplasts. JIP-test parameters and fluorescence quenching parameters （FPs） are commonly used chlorophyll fluorescence parameters, determined in transition state （from a dark adapted to a light adapted state） and light adapted state of photosynthetic apparatus respectively. This study was conducted to identify QTLs associated with JIP-test parameters and FPs. Pot and field experiments were conducted to evaluate184recombinant inbred lines （RILs） for five JIP-test parameters （ABS/RC, TRo/ABS, ETo/TRo, REo/ETo, and PIABS） and three FPs （Fv’/Fm’,ΦPSII, and qP） simultaneously. As a result, eleven and thirteen QTLs were identified for JIP-test parameters and FPs respectively. They were located on LG A1, A2, C1, D2, H, K, M and O respectively, with LOD scores from2.04to9.20, and the percentage of variation explained by these QTLs ranged from2.90%to23.99%. Eight QTLs expressed stably across environments, including QTLs qtrM.l and qtrM.l for TRo/ABS, QTL qetA2.1for ETo/TRo, QTL qreA2.1for REo/ETo, QTL qpiA2.1for PIABS, QTL qfvH.l for Fv/Fm, QTL qqpCl.1for qP, and QTL qΦPSC1.1for Φ PSⅡ. These QTLs might be valuable for soybean breeding. Five genomic regions where different traits were under related genetic control were also detected:the marker interval sat162-AW132402on LG A2for TRo/ABS, ETo/TRo, ABS/RC and PIABS, the marker interval sat₃11-AI794821on LG C1for qP and ΦPSII, the marker interval satt273-sat352on LG K for ETo/TRo, PIABS and ΦPSII, the marker interval satt655-satt210on LG M for TRo/ABS, Fv’/Fm’and qP, and the marker interval sat₂74-sat₁96on LG O for ETo/TRo, REo/ETo, ABS/RC, PIABS, qP, and ΦPSII.Ribulose-1,5-bisphosphate carboxylase/oxygenase activase （RCA） catalyzes the activation of Rubisco in vivo. In many plant species, RCA consists of two isoforms that are produced by alternative splicing of a single pre-mRNA, or encoded by different genes. However, up to now, only in cotton the two forms of RCA were proved to be encoded by separate genes. In the present study, two genes, GmRCAa and GmRCAβ, encoding the longer a and the shorter β isoforms of soybean RCA respectively, were cloned. The cDNAs for GmRCAa and GmRCAβ diverged throughout the translated and3’-untranslated regions. Analysis of genomic DNA confirmed the mRNAs of these two genes are transcribed by separate genes that are located on chromosome GmO2and Gm18respectively. Except these two genes, three other possible RCA genes, GmRCA03, GmRCA14, and GmRCAll, were also isolated. To further examine the function and modulation of RCA genes in soybean, the expression level of GmRCAa, expression level of GmRCAβ, Rubisco initial activity, photosynthetic rate and seed yield were determined in NJRIKY. Association of gene expression levels and the other three traits indicated RCA genes could play an important role in regulating photosynthetic capacity and seed yield. Subsequently, expression quantitative trait loci （eQTL） analysis revealed two trans eQTLs for either of GmRCAa and GmRCAβ. Also, based on the genetic characteristic of the soybean as an allotetraploid, we postulated that the genome duplication and evolution in soybean might have led to multi-family members and different expression pattern of RCA genes. These results could provide a new approach for future modulation of RCA genes to improve photosynthetic rate and plant growth in soybean as well as other plants.QTL analysis not only provides information on the number and position of loci involved in the expression of a trait, it also could test the relationships between physiological traits. Coincidence of QTL for two traits, with allelic differences corresponding to the expected relationship between the traits, is strong evidence that the two traits are causally related. In the present study, four genomic regions were detected where photosynthetic rate and other traits were under related genetic control:the marker interval sat₃11-AI794821on LG C1for photosynthetic rate, stomatal conductance, ΦPSII and qP, the marker interval sat₂96-satt669on LG D2for photosynthetic rate, stomatal conductance, interstellar CO2concentration, Rubisco initial activity and OPSII, the marker interval sat₁72-satt268on LG E for photosynthetic rate, stomatal conductance and intercellular CO2concentration, and the marker interval satt331-sat₁96on LG O for photosynthetic rate, ETo/TRo, REo/ETo, ABS/RC, PIABS, qP and ΦPS II, and in these regions all traits showed the expected additive direction predicted by correlations using phenotypic data, suggesting that the genetic variation of photosynthetic rate was mainly caused by the variation of the stomatal conductance, intercellular CO2concentration, ETo/TRo, REo/ETo, ABS/RC, PIABS, qP, ΦPS II, and Rubisco initial activity.