| The early leaf abscission of pear refers to the phenomenon of a large number of leaf shedding in the normal growing period of pear before mid-early October.The occurrence of this phenomenon seriously affected the normal growth of pear trees,reduced the nutrient accumulation of pear trees,and decreased the yield and quality of pear fruits in the second year.This phenomenon occurs to varying degrees in all pear production areas in China,especially in sand pear producing areas in the south of the Yangtze River.The cultivation of sand pear has become the leading industry in some provinces in the south of China.’HosuiHosui ’pear is the representative variety of sand pear system,delicious and juicy,deeply loved by the broad masses of people.As the factors of leaf abscission of pears in southern China are complex and uncertain,general prevention and control measures are not enough to solve the existing problems,and there are few studies on the mechanism of early leaf abscission.In order to study the factors affecting early leaf abscission of pear,this paper takes’ Hosui ’pear and its petiole abscission zone as the research object,which provides a theoretical basis and analyzing the internal mechanism of early defoliation of pear,so as to improve the yield and quality of Hosui ’pear.The main results are as follows:1.By observing the morphological structure of the cells in the abscission zone of the petiole of normal and severely abscission trees,it was found that there was obvious constriction in the abscission zone of ’Hosui’ pear,and the cells in the abscission zone had smaller and denser cytoplasm than the neighboring cells.In the same period,the vascular bundles in the abscission zone of the normal trees were intact,while the vascular bundles of the severely shedding trees tended to break.Through the observation of the tissue morphology of the petiole abscission zone,it was found that with passage of time,the abscission cells gradually became smaller or destroyed by hydrolysis,the width of vascular bundles gradually narrowed,and fracture was formed in the later stage,which promoted the formation of abscission layer.Transcriptome sequencing was carried out on the petiole abscission zone and leaves of normal and severe shedding trees.the data analysis showed that a total of 204 differentially expressed genes were screened in the abscission zone tissues and leaves.These genes included 196 abscission tissue genes（80 up-regulated and 116 downregulated）and 8 leaf genes（7 up-regulated and 1 down-regulated）,and most of them could be enriched in catalytic activity and redox process.Further verification of the gene expression level showed that the expression of genes related to gibberellin synthesis pathway was significantly inhibited,and the genes related to active oxygen scavenging were also inhibited.The above results show that the change of abscission zone during abscission is closely related to gibberellin and reactive oxygen species.2.Using the petiole abscission zone of ’Hosui’ pear as material,by measuring the content of gibberellin,the content of active oxygen（hydrogen peroxide and superoxide anion）,the activity of antioxidant enzymes（catalase,superoxide dismutase and peroxidase）and antioxidants.Compared with the control group,we found that there were physiologically active GA3 and GA4,and the content of GA12,the key intermediate of gibberellin synthesis,decreased significantly.The activities of superoxide dismutase（SOD）,peroxidase（POD）and catalase（CAT）decreased significantly;the content of superoxide anion（OFR）and hydrogen peroxide（H2O2）increased significantly,and the content of H2O2 in OFR was higher than that of the control by 36.29%and 21.00%,respectively,by 22.09%and 99.08%,respectively.The content of reduced ascorbic acid（AsA）decreased significantly,while the content of dehydroascorbic acid（DHA）increased and AsA/DHA decreased significantly,indicating that the degree of oxidation in the petiole detachment zone of early defoliation was higher and the degree of cell damage was more severe.The content of reduced glutathione（GSH）decreased,the content of oxidized glutathione（GSSG）did not change significantly,but the change of GSH/GSSG decreased.Through dynamic determination,it was found that the content of active oxygen（hydrogen peroxide and superoxide anion）increased,and the activities of antioxidant enzymes（CAT,SOD and POD）increased at first and then decreased.The changes of AsA content,GSH content and GSSG content were consistent with the activities of antioxidant enzymes,showing a trend of increasing at first and then decreasing.While DHA content increased continuously and AsA/DHA and GSH/GSSG decreased continuously,indicating that with the progress of growth and development,the antioxidant capacity of cells in the abscission zone of petiole decreased and suffered oxidative damage.The pear tree ’Hosui’ was used as the experimental material to explore the effect of hormone on deciduous leaves by spraying different plant growth regulators.The results showed that exogenous spraying gibberellin could increase the leaf retention rate,and after exogenous spraying gibberellin,it was found that the activity of antioxidant enzymes and the content of reactive oxygen species increased and the content of reactive oxygen species decreased.The above results showed that gibberellin could regulate the production of reactive oxygen species,and the treatment with appropriate concentration of gibberellin could increase the activity of antioxidant enzymes,reduce the content of reactive oxygen species and delay the occurrence of abscission.3.Through transcriptome analysis and fluorescence quantitative PCR verification,two differentially expressed genes PbrBURP and PbrCASP,were screened in the petiole abscission zone to explore the role of these two genes in the abscission process.The functions of PbrBURP and PbrCASP in plant growth and development were verified by subcellular localization experiment and Arabidopsis overexpression transgenic experiment.The role of PbrBURP and PbrCASP in the abscission zone was explored by measuring the content of hydrogen peroxide,superoxide anion,catalase activity and superoxide dismutase activity in transgenic Arabidopsis thaliana.The results showed that the expression of PbrBURP and PbrCASP genes in the early leaf abscission group was significantly lower than that in the control group.It was speculated that the decrease of the expression level of the two genes might lead to leaf abscission.Subcellular localization showed that PbrCASP was located in the cell membrane and nucleus,and PbrBURP observed green fluorescence on the cell membrane;overexpression of PbrCASP in Arabidopsis thaliana could inhibit sepal abscission;similarly,increasing the expression of PbrBURP could delay the abscission of Arabidopsis sepals.In addition,compared with the wild type,the contents of H2O2 and OFR in the leaves of transgenic plants decreased significantly,while the activities of CAT and SOD increased significantly,which improved the antioxidant capacity of transgenic plants.The above results preliminarily confirmed the effect of overexpression of PbrBURP and PbrCASP genes on plant abscission,which provided a molecular basis for early defoliation of’Hosui’ pear. |
PDF Full Text Request