| The surfaces of air-contact organs of terrestrial plants are mostly covered by a waxy layer insoluble in water but soluble in organic solvents.The main components of the waxy layer are VLCFA aliphatic compounds and triterpenes,and the waxy crystals are composed by VLCFA aliphatic compounds.The main function of the wax layer on plants is to retain water in the organs and prevent evaporation.In addition,there are plant protection from ultraviolet radiation,resistance to pests and pathogens infestation characteristics.The formation of plant wax is regulated by some genes related to wax synthesis,and changes in the external environment also affect plant wax.The epidermis of apple fruit is also covered with a layer of wax,and the morphology and composition of wax crystals are different with different varieties.The waxy structure of apple fruit epidermis plays an important role in postharvest storage and preservation.However,little is known about wax biosynthesis during apple growth and development.In this paper,the "Red Delicious" and "Golden Delicious" varieties of apples were studied from three perspectives: the morphology and structure of wax crystals during apple development,the change of main components of wax during development,and the quantitative expression of genes related to wax synthesis during different development stages.The results are shown below:(1)The accumulation of waxy layer is synchronous with the enlargement and ripening of fruit.With the growth of individual fruit,the content of wax per unit area of the epidermis of both kinds of apples increased.Moreover,during the critical period of epidermal wax anabolism of the two kinds of apples,around 90 DAF after anthesis,the total wax content in the apple epidermis increased significantly in this period,which showed that the triterpenes in the inner layer began to be synthesized in large quantities and the content reached the highest.Compared with "Red Delicious",the total wax content of "Golden Delicious" also increased significantly in the 120DAF-150 DAF stage at the end of growth and development.(2)The cuticular wax of apple fruits contains VLCFA aliphatic compounds and triterpenes,etc.GC-MS was used to detect and analyze the extracted wax,and the conclusions were as follows:The VLCFA aliphatic compounds include long-chain alkanes,fatty alcohols,fatty aldehydes and free fatty acids.The triterpenoids were mainly triterpenic acid,followed by lupinol,and the characteristic compound was ursolic acid.The content of long-chain alkanes in VLCFA was very high in both varieties,and the main component was n-Noncosane,which was the main compound of wax crystals in previous studies.10-Noncosane-ol is the most abundant alcohol component in the cuticular wax of "Red Delicious" but the content in "Golden Delicious" is lower than that in "Red Delicious".(3)The epidermal waxy crystals of both types of apples were clearly observed at 90 DAF using SEM low power microscope(X150).At high magnification(X2500),the paraffin arrangement was also observed,and the content of aliphatic compounds in VLCFA,which mainly formed wax crystals,decreased.In the subsequent development process,the growth of waxy crystals became denser and denser,and the content of corresponding components increased gradually.In order to clearly represent this result,we drew a 3D model diagram with reference to the experimental results.(4)We used RT-PCR to detect 10 genes related to apple epidermis wax.The results showed that the expression of all the genes related to wax synthesis and transport changed significantly during apple 90 DAF stage,and the specificity of the expression in the peel tissue was much higher than that in the flesh tissue. |
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