Study On The Regulation Mechanism Of Leaf Bulb Size Development In Chinese Cabbage

Chinese cabbage(Brassica rapa L.SSP.Pekinensis)originated in China,with a perennial planting area of about 2×106 hm2 and a total yield of about 100 million tons in a year.It is the vegetable crop with the largest planting area and yield,and is known as the "national dish".Leaf bulb is the main edible organ of Chinese cabbage and its size is the key factor to determine the yield of Chinese cabbage.However,the regulation mechanism of leaf bulb size development in Chinese cabbage is still unknown.Therefore,in-depth analysis of the regulation mechanism of leaf size of Chinese cabbage will provide theoretical basis for the next step of improving Chinese cabbage and increasing its yield.In order to analyze the regulatory mechanism of leaf size in Chinese cabbage,this study took Chinese cabbage with large(Y2)and small(Y7)leaf size as experimental materials,and then measured the leaf size,growth rate,augh and gibberellin content,photosynthesis,cell size and other indicators.In addition,we also used rna-seq technology to determine and analyze the transcriptional expression profiles in young leaves of Y2 and Y7,and obtained the following results:1.At 45 days after sowing,the number of leaves of Y2 was significantly higher than that of Y7,and the length and width of its maximum leaves were also significantly higher than that of Y7.In addition,we selected young leaves of basically the same size(about 3cm long)as the material,and then measured the length of leaves every 2 days,and found that Y2 had faster growth rate and longer growth time than Y7.We further observed the epidermal cells of mature leaves with scanning electron microscopy and found that the number of epidermal cells and stomata of Y2 and Y7 in the same leaf area was basically the same,that is to say,there was no significant difference in cell size of Y2 and Y7.Therefore,the reason why Y2 has a larger leaf-bulb and faster growth rate than Y7 is mainly because Y2 has a stronger ability to divide cells than Y7.2.45 days after sowing,we measured the maximum photochemical efficiency(Fv/Fm)of the optical system II of Y2 and Y7,and found that the Fv/Fm values of the two were basically the same,indicating that the growth states of Y2 and Y7 were basically the same.However,the net photosynthetic rate of Y2 and Y7 was significantly lower than that of Y7.However,the effective photosynthetic area of Y2 is significantly higher than that of Y7 because Y2 has stronger cell division ability than Y7,resulting in larger and more leaves.Therefore,although the net photosynthetic rate of Y2 is significantly lower than that of Y7,Y2 has a higher effective photosynthetic area than Y7,which enables Y2 to synthesize more organic materials per unit time to support the rapid growth of Y2.3.At 45 days after sowing,young leaves of Y2 and Y7 that had been fully expanded were selected as materials,and then the auxin and gibellin contents were determined by liquid-mass method.The results showed that the content of auxin and gibberellin in Y2 was significantly higher than that in Y7.Therefore,we believe that the reason why Y2 has larger leaves and faster growth rate than Y7 should be related to the higher auxin and gibberellin content of Y7 in Y2.4.At 45 days after sowing,the fully expanded young leaves of Y2 and Y7 were selected as materials,and then the transcriptional expression profiles of both were determined by rna-seq technology.Through analysis,a total of 4445 genes showed significant differences between Y2 and Y7,among which 3135 genes of Y2 were upregulated and 1310 genes were down-regulated compared with Y7.Through GO and KEGG analysis of the differentially expressed genes,we found that multiple genes were enriched in the items related to plant growth and development.In addition,we further analyzed the differential genes and found that some genes involved in cell division were mainly up-regulated.For example,genes involved in IAA and GAs synthesis,genes encoding cyclin(CYC),and genes such as ANT and GRF involved in cyclin expression.These genes are upregulated in Y2,resulting in larger leaves and faster growth than Y7.In conclusion,this study suggests that the up-regulated expression of genes involved in IAA and GAs synthesis leads to the accumulation of more auxin and gibberline in Y2.The large accumulation of auxin and gibellin further promoted the up-regulated expression of ANT and GRF genes involved in cyclin expression in Y2,which further promoted the up-regulated expression of CYC gene in Y2 and ultimately led to the stronger cell division ability of Y2 than Y7.