Photosynthetic Physiological And Transcriptome Basis Of High Altitude Adaptation In Pinus Densata

Hybridization is a prominent process in plant evolution that can lead to phenotypic novelty,heterosis,changes in gene expression,and has important biological significance in the generation of biodiversity.Pinus densata forms extensive forests on the southeastern Qinghai Tibetan Plateau.This species has the highest elevation distribution among all Asian diploxylon pines(or hard pines).Previous studies suggest that P.densata originated from hybridization between Pinus tabuliformis and Pinus yunnanensis,and occupies an environment very different from that of the two parental species.However,the physiological and molecular basis of its adaptation to high plateau habitat have not been investigated to date.This thesis utilizes the interspecific F1 crosses present in a common garden experiment settled on the Tibetan Plateau,and performed large-scale and year-round examinations of the environmental variations,the annual growth,photosynthetic physiology characteristics.The performance and heterosis of P.densata and artificial F1 hybrids were evaluated.Finally,gene expression patterns of P.densata and different F1 hybrids were analyzed.This thesis dissects the overall high-altitude growth adaptation of P.densata at physiological,single gene and pathway levels.The main results are as follows:(1)High plateau environmental conditions were characterized.The common garden site on the plateau had low atmospheric pressure and CO2 concentration,with the mean value were 696.5-717.6 hpa and 190?210 ppm,respectively.With 10? as the lower temperature limit for growth,the number of days that plants can normally grow in the region is 167.The seasonal change of environmental vraiables is large.Spring is a dry season with characteristics of low temperature,drought and high intensity of light radiation.Photosynthetic active radiation and UV-B radiation can reach 1912.4 ?mol m-2 s-1 and 308 ?w m-2,respectively.Summer is a vigorous growing season characterized by high temperature,high humidity and high intensity of light radiation,and the UV-B level is almost the same as the cases in spring.Autumn,however,all the environmental factors are in the middle level.As non-growing season,winter has the lowest temperature,high drought and the lowest intensity of light radiation.Analysis of environmental characteristics can provide a basis for the growth physiology and gene expression analysis of pine trees,provide an important basis for the development of control experiments,and be conducive to the management of genetic resources of pine trees in different habitats.(2)Growth adaptability was revealed.Compared with parental species,P.densata and F1 hybrid showed significant growth advantages in seedling height and ground diameter.No significant difference was detected between the two hybrids;significant quantitative advantages in needle numbers was also detected in two hybrids,no significant difference was detected between the two hybrids.(3)Physiological responses to high plateau environmental conditions were revealed.Compared to P.tabuliformis and P.yunnanensis,both P.densata and interspecific F1 hybrids showed transgressive performance in water-related traits,with higher drought tolerance,long-term water use efficiency,especially for F1 hybrids.P.densata and interspecific F1 hybrids also showed advantages in carbon assimilation,with high ratio of energy allocation,large accumulation of carbon assimilation products,fine control of stomatal,low CO2 concentration adaptation,and effective mesophyll conductance.In high-light adaptation,both P.densata and interspecific F1 hybrids showed low ability in light-harvesting,but superior in light-harvesting efficiency,light energy use efficiency and high-light tolerance,especially in F1 hybrids.Both P.densata and interspecific F1 hybrids had higher levels of total protein and nitrogen content,and P.densata showed higher photosynthetic nitrogen use efficiency and larger distribution ratio to Rubisco and bioenergy.In terms of antioxidant and defense properties,both P.densata and interspecific F1 hybrids had lower levels of antioxidant enzymes and higher flavonoid content.(4)Patterns of heterosis in P.densata and interspecific F1 hybrids were evaluated.Both P.densata and interspecific F1 hybrids showed heterosis that were reflected in many different categories of phenotypic traits.Additionally,qualitative and quantitative differences between the two groups of plants on many traits were detected.F1 hybrids had preliminarily established adaptation to high-altitude habitats.(5)Divergence in gene expression and patterns of inheritance were characterized for this pine species complex.Both P.densata and interspecific F1 hybrids showed a higher level of paternal(P.yunnanensis)dominance;gene expression difference caused by hybridization was 5.7-7%;interspecific F1 hybrids did not show a significant difference from P.densata in gene expression pattern,but an increased difference between F1 hybrids and parental species was detected,especially to P.tabuliformis.Among the differentially expressed genes,down-regulated genes accounted for the largest.Compared to the interspecific F1 hybrids,P.densata showed the highest number and ratio of non-additive genes(1772,77.82%),and a higher ratio of over-or under-dominance genes.(6)Enlarged expression difference was detected between F1 hybrids and parental species.In the formation of homoploid hybrid species,such changes will be conductive to new ecological environments.Small difference were detected between F1 hybrid and P.densata in levels of gene expression.(7)Transcriptomic bases of high altitude adaptation in pines was revealed.GO enrichment analysis showed both F1 hybrids and P.densata showed down-regulated defence response and up-regulated stress response relative to the two parental species.Decreased basal defense level is essential for the formation of heterosis.KEGG enrichment analysis showed that P.densata had rapid defense response,which can relatively increase the energy allocation to growth.Lower glutathione metabolic pathway levels were observed in P.densata,suggesting a lower level of oxidative damage in the species.F1 hybrids showed up-regulation in secondary metabolic pathways such as biosynthesis of cuticle and wax,which is important for high-light adaptation in high-altitude habitats.In this paper,physiological and molecular basis of P.densata was revealed.Substantial ecological transition fueled by hybridization and adaptive selection was revealed.This research has important theoretical significance for further understanding the formation mechanism of homoploid species,as well as providing important research examples for understanding the origin and maintenance of biodiversity in the Tibetan Plateau,and has important value for tree breeding and germplasm innovation.