Study On The Adaptability Mechanism Of Four Rhododendron Species In The Southwest Plateau Mountains

In plateau mountain ecosystems,the variance of environmental factors caused by altitude gradients are considered to be the epitome of environmental changes on horizontal gradients,which provide a natural laboratory for the research of plant environmental adaptation.In the post-genomic era,the use of multi-omics to study the environmental adaptation mechanisms of species can reveal the relationship between gene expression and life phenomena,metabolic characteristics and internal regulatory mechanisms.Flower,as the most important reproductive organ of plants and evolutionarily unstable genetic feature,is an ideal organization for studying species adaptability.As an important component of the alpine ecosystem,Rhododendron have different petal shapes and diverse flower colors.They are ideal materials for studying the mechanism of flower altitude adaptation.In this study,transcriptomic sequencing,comparative transcriptome analysis,and adaptive evolution analysis of the petals of 4species of Rhododendron with different altitude gradients were conducted to reveal the molecular adaptive mechanisms at the transcriptional level.Non-target metabolomics methods were used to understand the mechanism of the difference in the metabolic level of the petals of 4 Rhododendron species at different altitude gradients.Finally,the transcriptome-metabolism conbined analysis was used to elucidate the altitude adaptation mechanism of 4 plant petals from multiple levels and perspectives.The main research results and conclusions are as follows:1.A total of 68923-87048 and 105932-126123 Unigenes and transcripts were obtained by sequenced and assembled from the petals of 4 Rhododendron species,respectively.6696 single-copy orthologous gene clusters were obtained.1759-2816 significant differentially expressed genes（DEGs）were screened by pairwise comparisions of the 4 Rhododendron species.Comparative transcriptome analysis indicted that genes related to material and energy metabolism,fatty acid metabolism and plant hormone signal transduction were significantly up-regulated in R.fastigiatum,which may be related to its altitude adaptability.Among them,the significantly up-regulated of DEGs related to carbohydrate metabolism and fatty acid metabolism were related to the attraction of pollination and reproduction in plant.The significantly up-regulated of DEGs related to amino acid metabolism,replication and repair,and flavonoid metabolism in the petals of R.lacteum may be related to its environmental adaptability.Genes related to flavonoid biosynthesis are significantly up-regulated in the petals of R.lacteum,which is related to the UV-B radiation protection and color phenotype of the petals.The significantly up-regulated of DEGs related to amino acid metabolism,carbohydrate metabolism,replication and repair in the petals of R.facetum may be related to the altitude adaptability.2.The divergence time and evolution rate estimations showed that the divergence time of the Subgen.Rhododendron and the Subgen.Hymenanthes was about 8.3-41.1mya.The divergence time of R.lacteum and R.facetum in the Subgen.Hymenanthes was about 2.1-21 mya,and the divergence time of R.fastigiatum and R.pachypodum in the Subgen.Rhododendron was about 4.2-23.7 mya.The relationship between the evolutionary rates of the 4 species is: R.lacteum > R.fastigiatum > R.pachypodum >R.facetum.The number of positive selection genes from the selection of 4Rhododendron species was between 170-297.The MEKK1,YODA,PP2 C and Sn RK2 genes in the MAPK pathway in the petals of R.fastigiatum experienced positive selection,and the genes related to DNA repair and DNA damage stimulation also experienced positive selection.This is closely related to the long-term growth of petals in an environment with higher ultraviolet radiation and lower temperature.The lipoic acid synthase gene CIP1,transcription factor FAR1,and propionic acid synthesis-related genes of R.lacteum experienced positive selection.These genes are closely related to the environmental adaptability of R.lacteum,especially the adaptive evolution to oxidative stress.LUT5,a gene related to carotenoid biosynthesis pathway,has a positive selection effect in R.facetum,which is closely related to the pollination of Rhododendron species and the protection of floral organs.The positive selection of the genes PP2 C,CTR1,and EIN3 and the transcription factor b ZIP in R.pachypodum may be related to the long-term response of petals to mechanical damage or the interaction of plant pathogens.3.A total of 440 metabolites in non-target metabolomics research enters downstream analysis.A total of 175-208 significant difference metabolites were obtained by the pairwise comparison analysis of the petals of 4 of Rhododendron species.Studies on the main primary/secondary metabolites have found that amino acid and fatty acid compounds may help the petals of Rhododendron to adapt the highly altitude environments,especially proline,phenylalanine and oleic acid.The relative content of sucrose in the petals of R.fastigiatum is relatively higher than that of other species,which is related to the regulatory effect on the oxidative stress of the petals of R.fastigiatum,and promote the accumulation of anthocyanins and petal coloring.The petals of the 4 species of Rhododendron have more high-terpenoids than low-terpenoids and is related to the response of Rhododendron petal to low temperature,ultraviolet radiation and attracting pollinators.The difference in the content of flavonoids in the petals of the 4 Rhododendron plants with different altitude gradients is one of the reasons for the difference in flower color phenotype.A transcriptome-metabolism conjoint analysis found that the three metabolic pathways of fatty acid biosynthesis,glycerophospholipid metabolism,and pentose and glucuronate interconversions are related to the altitude adaptability of R.fastigiatum;Meanwhile,the flavonoid biosynthesis plays an important role in R.lacteum,purine metabolism and ascorbate and aldarate metabolism also play an important role in the altitude adaptation mechanism of R.facetum.In summary,the present study found that carbohydrates,fatty acids,amino acids,flavonoids and terpenoids are closely related to the altitude adaptability of 4Rhododendron species.The evolution rate of high-altitude species is faster than that of low-altitude species.Genes related to DNA repair,MAPK and ABA signal transduction,lipoic acid and Propanoate metabolism are positively selected in the petals of high-altitude Rhododendrons.The significant differences in gene expression or metabolites related to sucrose,flavonoids and carotenoids at the transcription or metabolism levels of the 4 Rhododendrons may be the cause of the differences in petal color.Our results will not only provide a scientific basis for exploring the adaptive evolution and environmental response of Rhododendron,but also provide theoretical guidance for the effective protection and management of plant germplasm resources under the scenarios of global climate change.