The Impact Of Exogenous Indole-3-acetic Acid On Tea(Camellia Sinensis(L.) O.kuntze.) Exposed To Cadmium

Land is a very important material basis for plant growth and development,and cadmium pollution in soil can directly affect plant growth.Tea plant(camellia sinensis(L.)O.kuntze.)is one of the most important economic crops in China.The roots of tea tree will absorb and enrich cadmium,and there will be some transport to the upper part of the earth,and cadmium will be enriched in the new shoots.When the soil of the tea garden contains too much cadmium,the concentration of cadmium of tea trees will also increase,which will affect the growth and development of tea trees,and reduce the yield of tea,and influence the quality of tea.Therefore,it is especially important to control the cadmium content during the growth of tea trees and reduce the absorption of cadmium by tea plant.Phytohormone is a signal molecule in plants.Indoleacetic acid(IAA)can promote plant cell division,elongation and growth,and promote lateral root growth,and also have positive effects on plant flowering and fruiting,so it is closely related to plant stress.Studies have shown that under cadmium stress,by adding exogenous IAA,it can promote plant growth,increase growth,reduce plant cadmium absorption,and alleviate cadmium damage to plants.However,IAA is mainly used in model plants.Whether it can improve the resistance of tea trees under cadmium stress and alleviate the damage of cadmium to tea trees is rarely reported.Therefore,the annual seedlings of the national fine species of ‘Xiangfeicui' are used as experimental materials.Heavy metal cadmium as a stress factor,the effects of exogenous IAA on the growth of tea plants under cadmium stress were studied by pot culture method.It was explored whether exogenous IAA could reduce the damage of cadmium to tea trees,and can provide ideas for controlling cadmium content during tea planting.By setting different cadmium concentrations(0,30mg·kg-1)and exogenous IAA concentrations(0,2?M and 10?M),a two-factor orthogonal test was designed to tea plants for 90 days treatment.By measuring phenotypes,physiological and biochemical indicators and transcriptomes sequencing,and obtaining the following findings:(1)When the cadmium concentration in the soil was 30mg·kg-1,the plant height was significantly lower than the control group,and the increase of the ground diameter was also reduced.The antioxidant enzyme activities(SOD,POD,CAT and APX)of stems,old leaves and new-shoot leaves in tea plants were reduced.SOD and POD activities in roots were significantly decreased,while CAT and APX activities were significantly increased.Proline,malondialdehyde and cadmium contents in each tissue increased significantly.The physicochemical components such as water extract,free amino acid,soluble sugar,tea polyphenol and caffeine of one bud and two leaves were significantly decreased,and the root IAA content and root activity were significantly inhibited,indicating that the growth of tea trees was inhibited under this cadmium concentration and it has a toxic effect on tea trees.However,after applying exogenous IAA at concentrations of 2?M and 10?M,the above-mentioned damage was alleviated,the growth was improved.The activities of antioxidant enzymes in roots,stems,old leaves and new-shoot leaves increased.The content of physicochemical components of fresh leaves was increased,and the cadmium content in each tissue was also significantly decreased.Indicating that the exogenous IAA can alleviate the toxic effects of cadmium on tea trees,and the effect of 10?M IAA is better than 2?M IAA.(2)When the concentration of cadmium in the soil was 30mg·kg-1,the chlorophyll content and carotenoid content of the mature leaves decreased significantly,and the chlorophyll fluorescence parameters Fm,Fv/Fm and Fv/Fo decreased significantly compared with the control,while the Fo was markedly increased.The photosynthetic parameters were also different from the control.The ultrastructure of the leaves and roots was observed by transmission electron microscopy,and the chloroplast and/or mitochondrial structure were found to be deteriorated.After application of exogenous IAA,the photosynthetic pigment content,chlorophyll fluorescence and photosynthesis ability of tea leaves were improved,and the chloroplast and/or mitochondrial structure were also restored.(3)According to the results of transcriptome sequencing,598 differentially expressed genes were obtained from the cadmium treatment group,of which 496 were up-regulated genes,102 genes were down regulated,and 820 differentially expressed genes were screened by the hormone repaire group,of which 103 genes were up regulated,717 genes were down regulated.The difference genes were annotated into the GO database for gene function classification.It was found that there were differences in the gene functional classification between the cadmium treatment group and the hormone repair group.The GO function classification of the differential gene in the hormone repair group showed autophagy and negative cell growth regulation,and the genes were down regulated.Through the KEGG metabolic pathway enrichment analysis,the genes in the cadmium treatment group were significantly upregulated in the sulfur metabolism pathway,while the SAUR,a gene involved in auxin synthesis,played a negative regulatory role,and the stress response gene CDPK was significantly up-regulated.After the addition of exogenous hormones,SAUR and CDPK were down regulated.