Study On The Development And Renewal Characteristics And The Overwintering Dormancy Mechanism Of The Rhizome Bud Of Herbaceous Peony

As a perennial geophytes with persistent rhizome,herbaceous peony(Paeonia lactiflora)demonstrates its significant ornamental and medicinal value.The initial occurrence and subsequent development of rhizome buds are the basis of plant regeneration,which has a direct impact on the ornamental value and survival of the plant.The analysis of rhizome buds for the characteristics of their development and regeneration as well as its dormancy mechanism can shed light on the research into the high-yield cultivation and annual production technology of herbaceous peony seedlings,which is of much significance both theoretically and practically.In this paper,the biological characteristics of the rhizome of herbaceous peony were first studied.Then,morphology,cytology and cultivation methods were applied to analyze the rhizome buds for the characteristics of its occurrence,heterogeneity development,flower bud differentiation,dormancy and growth phase transition,for revealing the characteristics of annual growth and the patterns of development for the rhizome buds of herbaceous peony.In the meantime,the mechanism of dormancy during the overwintering of rhizome buds was analyzed using a combination of morphology,physiology,transcriptome and proteomics methods.The main analytical results as obtained are presented as follows:(1)According to the morphological and anatomical studies on the patterns of development and renewal for the rhizome,the underground tissue of herbaceous peony consists of rhizomes,rhizome buds and roots developing on the rhizome.Derived from the stem,the rhizome is renewed on an annual basis depending on the sympodial branches.The secondary structure of the rhizome is comprised mainly of the periderm,cortex,secondary phloem,cambium,secondary xylem,and pith.In the rhizome are growth rings,the number of which is consistent with the actual growth years of the rhizome.The succulent roots developing on the rhizome stem from fibrous roots,which are classed into adventitious roots.(2)According to an analysis of the rhizome bud for their development,renewal and branching rule,the rhizome bud of herbaceous peony is a combination of renewed and persistent rhizome buds.Despite three different types of lateral buds in renewed rhizome buds,the occurrence of phytomer in each lateral bud follows basically the same pattern.Rhizome bud occurs in the cortex of the axil of the scale,and the developmental process of different rhizome buds shows an evident heterogeneity.Under normal conditions,there is only a certain percentage of scale buds in the lateral buds within the rhizome buds that end up developing into rhizome buds.The life cycle of renewed rhizome buds lasts three years,while that of persistent rhizome buds is the same as the remaining age of rhizomes.The way of renewal and the capabilities of rhizome buds depend on the depth of planting.A reasonable planting depth is effective in improving the quality of both rhizome and rhizome bud.(3)As revealed by the study on the primordium differentiation of rhizome bud and the origin of flower types,the stamen development of single flower is categorized into three types:centrifugal,centripetal and irregular.The differentiation process of the proliferation flower involves two continuous differentiation processes,while that of the lower part involves the differentiation process of bracts,sepals,petals,stamens and pistil primordium.With regard to the upper part,it involves the differentiation process of petals,stamens and pistil primordium.The upper part of the proliferation flower develops based on the proliferation and directional mutation of pistils of single flower types.The development and mutation of stamens for the upper part and lower part in proliferation flower show difference,while the process of development from single flower to proliferation flower makes the irregular mutation of stamens in the lower part of proliferation flower more complicated.(4)According to the studies on the paradormancy release of rhizome buds,there is a paradormancy phenomenon resulting from the apical dominance between rhizome buds in rhizome and different lateral buds within rhizome buds.The removal of buds fails to promote the growth and regeneration of deep rhizomes buds.The apical dominance between the upper and lower parts of the rhizome is effectively eliminated by the cutting of rhizome,and the paradormancy of the rhizome buds is released.(5)The morphological development of rhizome bud and the changes in physiological metabolism of it during overwintering were analyzed,which led to the results suggesting that the early-stage differentiation of flower primordium took place in the underground rhizome buds,while the development and maturation of flower primordium of single flowers as well as the differentiation of flower primordium in the upper part of proliferation flowers took place mostly after germination and excavation.During the overwintering of rhizome buds,starch content declined,while soluble sugar content increased and stayed at a high level.The constant supply of nutrients in rhizome buds stems from rhizomes and succulent roots.Prior to overwintering,the soluble protein content in rhizome buds showed a downward trend as the differentiation of flower bud progressed.At this time,the soluble protein content increased as the temperature dropped.During the overwintering of rhizome buds,proline content surged to a high level,while H₂O₂content fluctuated at a lower level.(6)In this study,transcriptome sequencing and bioinformatics analysis were conducted in four different developmental stages of rhizome buds,including early chilling acclimation,endodormancy,ecodormancy and germination.Ultimately,a total of 41337 unigenes carrying annotation information were obtained.There were 6225 differentially-expressed genes screened,which were mainly enriched in KEGG pathways such as plant hormone signal transduction,starch and sucrose metabolism,the conversion of pentose and glucuronic acid as well as carotenoid biosynthesis.The differentially-expressed genes in relation to photosynthesis were found to be most significantly enriched in the comparison group related to the germination stage.There were multiple genes related to hormone signal transduction pathway that exhibited significant changes in the establishment,maintenance and release of bud dormancy.The genes associated with sugar synthesis and catabolism,such as?-amylase,?-amylase,ADG2,SPS and SUS2,were responsible for the regulation on sugar metabolism at the time of overwintering.Moreover,the genes related to cell cycle and environmental response,such as HSP,LEA,DHN,AQP,were observed to be differentially expressed at different stages of dormancy.This is speculated to be involved in the process of overwintering.(7)In this study,iTRAQ was adopted to sequence the three stages of endodormancy,ecodormancy and germination for the rhizome buds of P.lactiflora�Dafugui',with totally 2321proteins identified.Over the course of dormancy,plenty of the proteins related to carbohydrate and energy metabolism were differentially expressed during the overwintering of rhizome buds of P.lactiflora,especially those associated with respiration and energy metabolism,for example,the proteins involved in pentose phosphate pathway,glycolysis pathway and TCA cycle,which were almost up-regulated from the endodormancy to ecodormancy stage.The proteins involved in hormone synthesis and signal transduction along with other proteins in relation to environmental stress were differentially expressed as well.Moreover,the proteins related to Ca²⁺,cytoskeleton,cell growth and development,as well as protein and amino acid metabolism were differentially expressed.This is speculated to be involved in the regulation of bud dormancy at the time of overwintering.