| In order to clarify the molecular mechanism of the formation of walnut bared-nut,to solve the problem of bared-nut in the cultivation and production of walnuts.This study takes the rare'XINLU' walnut as the research object.Firstly,the origins of endocarp development and the dynamic deposition of lignin during the hardening process of 'XINLU' walnut are analyzed based on the differentiation of female flowers and the anatomical structure of fruit development.Secondly,to excavate the physiological and biochemical processes related to the trait of the bared-nut,and to establish the biological function network of the walnut endocarp sclerosis by obtain the transcriptome data of the sclerosis period of walnut endocarp.Finally,the content of intermediate metabolites in the synthesis of lignin during the hardening of walnut endocarp was determined to explore the objective entry point for the study of the bared-nut formation of'XINLU' walnut.The main findings are as follows:(1)The observations for morphological and anatomical of the female flower of 'XINLU'walnut showed that during the maturity of the female flower,the calyx(Cal),bract(Bra)and ovary primordium(Ovap)were clearly distinguished,the cells of peripheral parenchyma of placenta(Ppp)grew rapidly and filled the whole locule,the ovule primordium(Ovp)began to differentiate and formed ovules.During the end of full bloom,the cells of peripheral parenchyma of placenta filled with locule began to degenerate until disappeared completely,and the exocarp,mesocarp and endocarp of the fruit and the vascular tissue distributed in the mesocarp were clearly distinguished under optical microscope.(2)57 days after full bloom,lignin was first observed at the top and base of 'XINLU'walnut fruit in longitudinal direction,and It extended to the middle of endocarp rapidly,and It was later deposited down transverse direction from outer wall of ovary to the inner wall of ovary.92 days after full bloom,the tissues inside the hardened walnut endocarp begins to lose water and becomes loose,and is dyed light red;and these lignin deposition areas are endocarp areas that close to the vascular bundle or spread vascular tissue with a high density usually(3)Transcriptome sequencing and WGCNA analysis were performed on 27 'XINLU'walnut endocarp samples.A total of 6 gene modules with MEblack,MElightcyan,MEdarkviolet,MEantiquewhitel,MEdarkseagreen3,and MEbrown that were significantly associated with the pulpy-hue were obtained.The 2 127 genes in the 6 gene modules involved in 20 important biological processes,including plant-type secondary cell wall biogenesis,material transport,phenylpropanoid metabolism,monolignol biosynthesis,biosynthesis of amino acids,metal ion response and transport,response to light stimulus,response to salt stress,growth and regulation of developmental process.The 177 key genes related to the bared-nut were obtained from 2 127 genes.These enriched biological processes constitute 11 interactive biological function networks(4)The 14 intermediate metabolites related to lignin synthesis during the hardening of the endocarp of 'XINLU' walnut showed great differences in both the scleritic-hue and the pulpy-hue.The cinnamic acid showed an "N"-type change trend in the scleritic-hue.The content reached the maximum value at 64 days after full bloom and fell to a trough at 78 days after full bloom,while the cinnamic acid content in the pulpy-hue has been lower than that in the scleritic-hue from the beginning,and the content fluctuated little throughout the process.The p-coumaric acid showed an inverted "V"-shaped change trend in the pulpy-hue and the "M"-shaped change trend in the scleritic-hue;during these five periods,the content of p-coumaric acid in the pulpy-hue was always higher than that in the scleritic-hue.p-coumaryl alcohol,coniferyl alcohol and sinapyl alcohol are three important monolignols for lignin synthesis These three substances are extremely low in the pulpy-hue;the average content of p-coumaryl alcohol in the scleritic-hue is 19.97 times that in the pulpy-hue.The average content of sinapyl alcohol is 1,842 times that of the pulpy-hueFrom the above results,it is concluded that the endocarp of 'XINLU' walnut is completely developed from the ovary wall,and the outer wall of the ovary contributes the most to the endocarp.The hardening of the walnut endocarp is the transformation of the endocarp from Pulpy-hue to Scleritic-hue Process.During the hardening period of walnut endocarp,when the process is completely completed,a complete walnut is formed,otherwise,a bared-nut walnut is formed.The insufficiency of coumaryl alcohol and sinapyl alcohol in Pulpy-hue prevents the process from be completed completely during the hardening period of the endocarp,thus forming a bared-nut.Walnut bared-nut are mainly involved in biological processes such as phenylpropane metabolism,plant secondary cell wall formation,material transport,metal ion response and transport,and the participation of 177 key genes.The intermediate metabolites of lignin synthesis,cinnamic acid and p-coumaric acid,are compounds in which abnormal lignin synthesis occurs in Scleritic-hue and Pulpy-hue.They are both key intermediates in lignin biosynthesis and important in the process of lignin biosynthesis Regulators,and may be closely related to the formation of ' XINLU' walnut bared-nut. |
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