Study On Leaf Color Variation Mechanism Of A New Cultivar 'Golden Qilu' In Acer Buergerianum Miq.

Acer buergerianum Miq.is a deciduous tree of Acer in Aceraceae family.It is one of the excellent Acer trees widely distributed in China.It is also often planted in Japan,South Korea and the United States.As an ornamental tree species,Acer buergerianum is usually used in parks,gardens and both sides of the roads.It can also be used as a potted ornamental.Meanwhile,Acer buergerianum grows rapidly and its trunk is straight.It can also be used as wood.The common Acer buergerianum leaves are purple in spring,then gradually turn green,and the ornamental period in spring is short.The Acer buergerianum cultivar ‘Golden Qilu'cultivated by our research group is a yellow leaf mutant.The leaves are golden yellow in the spring and turn green gradually,which greatly improves the ornamental quality of the Acer buergerianum in spring.However,the physiological changes and molecular mechanisms of the Acer buergerianum ‘Golden Qilu' are not clear,and we need further research.In this study,the young leaves,transition leaves,and mature leaves of the Acer buergerianum cultivar ‘Golden Qilu' and the common cultivar ‘2016S4' were used as materials.The ultrastructural changes of the Acer buergerianum mutant were observed by scanning electron microscopy;the relevant pigments content in the leaves were measured;the photosynthesis of leaves was measured;transcriptome of De novo was sequenced and the differential expression gene was analyzed,differential expression gene related to leaf coloration of Acer buergerianum mutants was screened;combined analysis clarified the coloration mechanism of yellow leaf formation and subsequent conversion to green of Acer buergerianum mutants was made.The main results are as follows:(1)Ultrastructural observations revealed that in the mesophyll cells of the leaves of the Acer buergerianum cultivar ‘2016S4',the chloroplast showed a regular oval shape,the structure was clear,the matrix and basal granules were obvious,the arrangement was neat,the number of osmophilic granules was small,and the number of starch granules was big.In the mesophyll cells of the leaves of the Acer buergerianum ‘Golden Qilu',the chloroplasts were small,irregular in shape,and the structure was basically visible.The matrix and grana lamellae were loose,the grana lamellae were broken or missing,the number of osmophilic granules was big,and the staining was shallow.(2)The pigment content measurement results showed that the chlorophyll a,chlorophyllb and total chlorophyll content of the Acer buergerianum ‘Golden Qilu' during the young leaf stage were significantly reduced,and the carotenoid content was increased.The total chlorophyll/carotenoid ratio was only about one third of that of the common variety.The chlorophyll content of Acer buergerianum ‘Golden Qilu' showed an increasing trend with the change of growth period.The chlorophyll content in mature leaves reached 70% of that of common varieties,and the total chlorophyll/carotenoid ratio of mature leaves was slightly lower than that of common varieties.The contents of anthocyanins and total flavonoids in the three periods of ‘Golden Qilu' of Acer buergerianum were significantly lower than those of common varieties.The photosynthesis results showed that the net photosynthetic rate of the leaves of Acer buergerianum ‘Golden Qilu' was lower than '2016S4' from May to September,of which the declines were the highest in May and June,falling by 71.25% and 56.23%,the smallest in July,falling by 32.23%.(3)This experiment used the BGISEQ-500 platform to perform transcriptome sequencing on leaves of the three phases of the Acer buergerianum ‘Golden Qilu' and‘2016S4'.A total of 123.59 Gb data was measured.After assembly and de-redundancy,82,008 Unigenes were obtained.The total length,average length,N50,and GC content were135271472 bp,1649 bp,2243 bp,and 40.56%,respectively.Annotate Unigenes comparison to the seven major functional databases,and finally have 86.15% Unigene obtained functional annotations.57471 coding sequences(CDS)were predicted from Unigene.Transcription factor prediction results showed that a total of 2470 genes were predicted,belonging to 57 transcription factor families.(4)For young leaves(QN),transition leaves(QG),mature leaves(QL)of ‘Golden Qilu'and young leaves(SN),transition leaves(SG),and mature leaves(SL)of ‘2016S4',differential gene analysis was performed.The differential genes were enriched into GO database.According to the detection results of differential expression genes,they were divided into three large functional classes,and 47 small functional classes.The differentially expressed genes were enriched into KEGG database.According to the detection results of differentially expressed genes,they were divided into 5 primary pathways and 19 secondary pathways.(5)Screened and analyzed genes related to leaf coloration.A total of 75 differentially expressed genes were identified from chlorophyll synthesis,carotenoid synthesis,and flavonoid synthesis.In addition,180 down-regulated transcription factor genes were screened through transcription factor prediction.(6)Compared with ‘2016S4',the expression of key genes HEMA,HEMC,CHLM andGLK in the chlorophyll synthesis pathway of ‘Golden Qilu' is down-regulated.The expression of PSY,PDS,CRTZ,CCS genes was up-regulated in the carotenoid synthesis pathway.The down-regulation of C4 H,CHS,F3'H,F3'5'H,LAR and MYB,bHLH genes in the flavonoid synthesis pathway caused a significant decrease in anthocyanin synthesis.The change of these pigment ratios finally made the ‘Golden Qilu' young leaves appear yellow.Compared with young leaves of ‘Golden Qilu',the expression of HEMC and POR genes in the chlorophyll synthesis pathway in mature leaves of ‘Golden Qilu' was up-regulated and the amount of chlorophyll synthesis accumulated.The total chlorophyll / carotenoid ratio increased and the leaf color turned from yellow to green.(7)qRT-PCR verification was performed on 12 genes with significantly different expression.The expression trend of 11 genes among the 12 genes was consistent with the transcriptome data with an accuracy rate of 92%,indicating that the transcriptome data was stable and reliable.To sum up,this experiment combined the physiological measurement and transcriptome sequencing to clarify the mechanism of leaf coloration of Acer buergerianum 'Golden Qilu',which provided a theoretical basis for the further use of this cultivar in the later period,and provides valuable molecular basis for further study on the coloring mechanism of Acer buergerianum leaf.