| Wood comes from the secondary vascular tissues differentiated from lateral meristem(cambium),thus it is a hotspot of study on the molecular regulation of differentiation of meristem into vascular tissue.The analysis of Arabidopsis mutants,tracheid elements differentiated from cultured Zinnia mesophyll cells and tension wood induced by gravity has greatly increased our understanding of the maintenance and differentiation of meristems,xylem development.Gravity plays an important role in growth and development,in terms of morphogenesis,physiology,metabolism and other biological process in plant.The change of gravity condition will induce a series of changes in plant growth and orientation.In response to the change of gravity conditions,the activities of cambium in woody dicotyledon plant increase unevenly,leading to the eccentric growth of stem and the formation of reaction wood with the obvious changes of xylem structure and cell wall components.Monocotyledonous have no cambium,and the stem node elongated through the activity of intermediate meristem.The study on the response process and structural change of monocotyledons to gravity,exploration of the similarities and differences of gravimetric response between monocotyledons and dicotyledons as well as the way of their regulation of vascular tissue development,would provide some references for the further research on the mechanism of gravity response about woody plant such as bamboo and trees.The maize inbred line B73 was used as the experimental material in this paper.After the maize seedlings grew to 10 leaf stage,the experimental plants were put down horizontally,and the control plants still kept upright growth.The dynamic changes and time points of maize gravity response process were recorded by delay photography.Through tissue sections and special staining,the changes of tissue structure and the differences of cell wall components of maize stem nodes in response to gravity were analyzed.According to the time node and tissue structure changes in gravity response process,the sampling time and sampling site of transcriptome were determined,RNA was extracted and analyzed by transcriptome sequencing.In addition,the transcriptome data of poplar in this paper are from NCBI transcriptome database(Accessnumber: PRJNA53373).The maize plant began to respond to the gravity from the top,and then curved up from down to the fourth stem node,until the angle between the plant and the horizontal surface was 90 degrees to the end state.The results of repeated experiments showed that only the fourth,fifth and sixth stem nodes had different degree morphological changes.Compared with the control group,the number of proximal vascular bundle cells increased,and the cell wall became significantly thinner without obvious vessel tissue.While the number of cells and the thickness of cell wall in the distal vascular bundles varied with the developmental status.The content of lignin in the cell walls of proximal vascular bundles was low,and the content of cellulose was higher than that of the control.There was no significant content change in the cell wall of the distal vascular bundles compared with the control.Transcriptional analysis was carried out on the proximal samples,distal samples and control samples of plants at 5h,10 h,20h and 30 h in the process of gravity response.The results are as follows:(1)Compared with the control,the number of differentially expressed genes increased first and then decreased with response time changes,and the number of differentially expressed genes was the most at the 10 h.Comparing the proximal and distal sides results,it was found that the difference genes increased with the treatment time,and reached the maximum at 30 h.Functional analysis of up-regulated or down-regulated genes in different parts of different time,it was found that the up-regulated genes in proximal side were mainly involved in cell proliferation,gene expression and gravity signal transduction,while the down-regulated genes were not only involved in gene expression,but also related to plant photosynthesis.The distal side genes are mainly concentrated in material metabolism.(2)Four specific modules were obtained by WGCNA analysis.The four modules were analyzed by go enrichment and KEGG pathway.The results showed that the differential genes were mainly involved in plant hormone(auxin)signal transduction at 5h(pink module),thus the genes in this module might participate in the stress response of plants to gravity.While,the differential genes in cyan and greenyellow modulesat(5-20h)were mainly involved in plant hormone(auxin)signal transduction and other processes playing roles in phenylpropanoid biosynthesis,DNA replication,all kinds of metabolism(amino acid metabolism,carbon metabolism,purine metabolism),cell proliferation and other processes,it is speculated that this process is related to the changes of tissue types in response to gravity in maize.(3)Comparative analysis of gravity response between poplar and maize shows that there are obvious structural differences between the two species in the process of gravity response.By comparing the differentially expressed genes,it can be found that there are more differentially expressed genes in the near ground side than in the far side in both species.The conserved response mechanism of the two plants is that the up-regulated genes are involved in redox reaction and transcriptional regulation and other related biological processes.However,the up-regulated genes in maize were mainly involved in cell proliferation,while those related to vascular activity and cell division were down regulated in poplar.On the far side,the up-regulated genes were mainly related to the synthesis of secondary wall,while the down regulated genes were mainly involved in xylan decomposition in maize.However,the expression of related genes was opposite in poplar.These results are just consistent with the phenotypic differences between the two species.In response to gravity condition,maize can adjust the extension direction of stem node quickly.Compared with the control,the number of vascular bundle cells near the stem node increased significantly,the cell wall became thinner,the lignin decreased and the cellulose increased.Therefore,it is speculated that after responding to gravity,maize may produce tissues similar to tension wood and compressed wood by adjusting the activities of the intercalary meristem,but different from them.By comparing with poplar transcriptome,it was found that the number of differentially expressed genes in the near earth side was more in the process of gravity response.It was speculated that the near earth side was more sensitive to gravity response,and the biological functions of the two genes were similar,indicating that the main biological processes of different plants in the process of response to gravity change were similar,mainly focused on signal transduction and hormone response.On the dista side,the tissue structure changes of plant stem are different,and the functions of different genes are also different. |
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