| Gnetum is aunique group of gymnosperms with a controversial phylogenetic position which has high scientific and economic value.As one of important species widely distributedinthe south of China,G.parvifoliumhas been used as a traditional Chinese medicinefor more than one thousandyears.The previous studies primarily show that Gnetum has specific characteristics in stilbene content and photosynthesis ability,their significance,however,in evolutionary adaptation remains unclear.In this study,stilbenes metabolic pathway,photosynthetic trait and its comparative evolution(positive selection and chloroplast phylogenomics)were carried out to explore evolutionary and ecological adaptation questions,which would lay the foundation for the study of phylogenetic evolution,introduction and cultivation.The main conclusionswere as follows:(1)The contents of total stilbenes in G.parvifolium reached 28.0 mg·g-1,higher than that of mulberry and grape.Its contents were different inthe different tissues,and ranked top in the roots of young seedlings,and then in the seed embryos of mature trees.Moreover,four specific stilbene components(resveratrol,piceatannol,isorhapontigenin,and gnetol)were identified in the selected tissues.Gnetol was only found in fruit flesh(890.38 μg·g-1·DW),the content of resveratrol in endosperm was the highest(763.1 μg·g-1·DW),piceatannol and isorhapontigenin were the richest in the roots of seedlingswith 2189.3μg·g-1·DW and 2569.2 μg·g-1·DW,respectively.Likewise,these compounds to some extent were induced by environmental stressesof UV and high temperature.(2)The comparative study on photosynthesis between Gnetum(G.parvifolium,etc.)and other plant speices showed that low photosynthetic trait,including photosynthetic rate(1.6-3.6 μmol·CO2m-2·s-1),stomatal conductance(0.015-0.060 mol·H2O m-2·s-1)and water transport capacity(0.48-1.70 mmol·H2O m-2·s-1)was inherent in Gnetum.But there was a bit difference between liana and arbor species,the light compensation point(LCP)and light saturation point(LSP)of tree Gnetum were higher than that of liana Gnetum;besides,the photosynthetic characteristics inthe seedling was in accord with that in the mature.(3)RNA-sequencing was used to generate 94816 unigenesfrom a pool of different tissues in G.parvifolium.The candidate genes involved in photosynthesis and stilbene metabolism pathways were identified through KEGG(18 and 178).The qRT-PCR results showed that UV-C and high temperature significantly induced the expression of genes invloved in stilbene biosynthetic pathways,includingPAL,4CL,C4 H,STS and CYP.Our results provide important genetic resources for Gnetum.(4)The positive selectionanalysis of G.parvifolium,G.montanum,and G.gnemondemonstrated that thesame selection genes were found in these three species,indicating that the same traits were under selection in the process of adapting to the environment.The selected genesenriched in “chloroplast”,“stromule”,“envelope” etc,indicated that photosynthesis and other traits were affected by environmental selection during their adaptation to outside stimuli.Additionally,the positive selectionanalysisof G.parvifolium,Ephedra intermedin and Welwitschia mirabilisshowed that Ca2+ signal transduction in plant cells was closely related to the W.mirabilis adaption of extreme drought environment,and the actin,which was associated with pollen tube growth,gravity signal transduction and chloroplast movement,was closely related to the insect-borne and climbing of Gnetum.(5)The comparison of chloroplast genomesin the selected Gnetum and other related species(or belonged to the same genus)showed that 17 coding genes were missing in Gnetum chloroplast genome,which could be related to its low photosynthetic trait.The phylogenetic tree,inferred from the chloroplast genomes of38 land plants,strongly supported “Gnecup” hypothesis: Gnetales is placed within conifers,and sister to Pinaceae. |
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