The Pathway And Efficiency Of Phloem Loading In Camellia Oleifera Leaves At Different Ages

Camellia(C.)oleifera,an evergreen small arbor,is one of the major woody oil species in the world,the oil extracted from its seeds contains a large amount of unsaturated fatty acids,which serves as a kind of high-quality edible oil.However,with the growing demand for high-quality edible oil,the low production of tea oil can no longer meet the market demand,so increasing the output of C.oleifera has always been one of the most urgent problems to be solved in the research of this species.The yield of plant is closely related to leaf senescence and phloem loading,but the physiological characteristics of leaf senescence,the underling mechanisms,the cytological pathway of phloem loading and the differences in leaf sugar transport efficiency in leaves at different ages in C.oleifera are still unknown.In present study,the physiological differences related to senescence in the leaves at different ages were analyzed,and the main loading pathway in C.oleifera was identified by using many cytological research methods.In addition,the differences in leaf loading efficiency between these two kinds of leaves were determined and the underlying mechanisms were detected.Furthermore,by transcriptome analysis and q RT-PCR,the potential regulatory genes of leaf senescence and assimilate loading were identified.The specific results are as follows:By measuring leaf senescence-related physiological indexes,the results found that the senescence of C.oleifera leaves is accompanied by a series of physiological changes such as the degradation of chlorophyll soluble protein,reduction in antioxidant enzyme activity,and accumulation of malondialdehyde and proline.Transcriptome analysis showed that there are significant differences in senescence-associated genes which participate in chlorophyll degradation,hormone signaling and oxidation pathways.And by further analysis,77important senescence-associated genes were identified,such as NOL,ATAF1,MDAR and SAG12,and there were certain expression correlations among these genes.The ultrastructural observation of the leaf veins showed that there was a certain density of plasmodesmata at each cell interface.The result of fluorescence tracer found that there were obvious fluorescence signals in the vein and the mesophyll cells around the vein.The functional cell coupling was identified by FLIP assay.The results suggested that the cell coupling between the vascular bundle sheath cells and the sieve element-companion cell complex was much stronger.The results of apoplast inhibition experiments showed that C.oleifera leaves were not sensitive to the sucrose transporter inhibitor and H~+-ATPase inhibitors,indicating that C.oleifera mainly adopts symplastic loading strategy.The Photosynthetic characters and sucrose biosynthesis related enzyme activities of two kinds of leaves were measured.The results showed that the photosynthetic capacity and sucrose synthesis capacity of senescent leaves decreased significantly.The results of accumulation of soluble sugar,starch and activities of starch-related enzymes showed that more soluble sugar was accumulated in senescent leaves.These results suggested that during the process of senescence,the loading efficiency of C.oleifera dropped.By analyzing the vein structure,callose and lignin accumulation,the results identified that the decreased loading efficiency of aging leaves was mainly due to the decreased density of plasmodesmata at the VPC:CC interface and BSC:CC interface in phloem.Through transcriptome analysis of these two kinds of leaves,the key differently expressed genes which may participate in various assimilate loading processes were detected,such as Co SUT1,Co SWEET12 and Co PDCB1-1.The further analysis showed that the expression of plasmodesmata-related regulatory genes and sugar transporter regulatory genes were correlated with that of leaf senescence-associated genes.