| Rubber tree(Hevea brasiliensis)is the most important source of natural rubber in the world.The secondary laticifer,a sepecific tissue in the secondary phloem of rubber tree,is the main region to biosynthesize and accumulate the natural rubber.The number of secondary laticifer in the trunk bark of rubber tree is one of the key factors that affect the yield of natural rubber,and the metabolic capacity of latex is positively correlated with the regeneration of latex.So,the structure of bark and metabolism of latex in laticifer always were studied extensively.Tapping panel dryness(TPD)of rubber tree showed that the latex flow at the tapping cut decreased and further stopped completely for the laticifer abnormity,which would lead to shorten the economic life of rubber tree and sharply reduce the output of natural rubber.Therefore,to study the mechanism of TPD happening and find the solution correspondingly are the urgent task for rubber research.Through the methods of pathology,physiology and histology,the TPD were studied and many different viewpoints were presented,but till now,the mechanism of TPD still is unclear.By using the techniques of cytology,biochemistry and molecular biology,the structure of trunk bark and expression of latex metabolism-related genes as well as the synthetic efficiency of natural rubber in latex were researched between the healthy tree and the different grades of TPD tree of RY7-33-97.The results showed the key changes about the bark structure,gene expression of latex and rubber synthesis efficiency during the course of TPD happening,aimed to provide the basis for elucidating the mechanism of TPD,and further provide reference for early evaluation and prediction of TPD.After systematic analysis,the main results showed as following:1.In a healthy rubber tree,the secondary laticifers of trunk bark were arranged in a regular row and dyed into dark brown color by iodine-bromine.Swelling laticifer nearly did not detect in yellow skin and water sac skin,in which the sieve tube had the big diameter about 40μm.Clustered stone cells mainly distributed at the inner sandy skin,only a few single stone cells scattered in yellow skin.Compared with the healthy tree,the bark structure of TPD tree changed obviously.Accompanying the rise of TPD grades,the number of swelling laticifer increased.Swelling laticifer spread from the sandy skin into the water sac skin,with light yellow color even empty.For the disorder of the laticifer row,the number of which was difficult to count.Sieve tube diameter decreased,the clustered stone cells increased and distributed till next to cambia in fifth grade TPD.2.The reddish brown-color tannin cells scattered sparsely in a healthy tree,but obviously increased in TPD trees and even formed in the cambium region.As a secondary metabolic substance with protecting function,tannin cells should play an important role to protect from the adverse condition.3.Fibrous protein-net accumulated largely at the end of the laticifer of tapping cut in a healthy tree,but less or no fibrous protein accumulated in the laticifer of the TPD tree.Far from the tapping cut,discontinuous flocculent protein material could be observed inside laticifer in the TPD tree.4.The expression stability analysis of 20 candidate reference genes showed that UBC2a,UBC2b,and eif2 were relatively stable in healthy trees;UBC3,eifAb and UBC2b were stable in third grade TPD tree.To verify the reference genes,HbDXSl,HbHMGS2,and HbNIN3 were selected.Eventually,UBC2b was selected as the reference gene for this study.5.In this study,the latex metabolism-related genes with the function of defense or stress resistance always up-regulated the expression when the grade of TPD increased.There were about 15 genes that showed the nearly consistent variation between September and January,in which the 9 genes of HbGluc、HbHevein、HbAPX1、HbCuZnSOD、HbRBOHA、HbRBOHB、HbPIP2;5、HbPIP2;7、HbERT2 up-regulated expression as the symptom of TPD aggravated,but 6 genes of Hb44KD、HbChit、HbMYC1、HbHRT2、HbREF、HbSRF down-regulated the expression adversely.6.The latex metabolism-related genes always had higher expression in the first tapping than the subsequent tappings.Usually as the increase of tapping times,the expression of genes down-regulated gradually,only HbPK、HbSAMS、HbCOI1 presented the converse tendency.The major genes related with the phytohormone signaling metabolism of Ethylene and Jasmonates always kept stable expression among different tappings,which may be correlated with the roles of improving resistance and senescence of these two hormones in plant.Both the times of tapping and the degree of TPD increased,the expression of genes always descended.Keeping tapping maybe intensify the TPD,lead to destroying the recovery capability of plants itself.7.The synthetic efficiency of natural rubber in water sac skin always was higher and more stable than that of the yellow skin,which may owe to the laticifers in water sac skin keeping healthy and away from wounding.In the fifth grade TPD tree,the synthetic efficiency of natural rubber in latex descended significantly to the background value,indicating that the latex had lost its capability of rubber biosynthesis. |
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