Effects Of Heavy Metal Cadmium On The Auxin Dynamic Equilibrium In Root Development Of Arabidopsis

Heavy metal pollution is one of environmental problems in the largest pollution area and the most extensive harm. Heavy metal pollution can reduce crop yield and quality, affect the water environment, and eventually affect human health and life safety through the food chain. Therefore, the research about heavy metal pollution has been the hot spot in the current academic field. Phytohormones play a central role in the developmental processes in plants. In this study, Arabidopsis thaliana, a model plant, was used to investigate the impact of heavy metal cadmium on the dynamic balance of auxin during the root development of Arabidopsis. The results are as follows:1. The heavy metal cadmium stress inhibited the growth of principal root and lateral root, increased lateral root density of Arabidopsis. The root growth of Arabidopsis decreased gradually and the decrease extent reached to 91.3% when the cadmium concentrations increased from 12.5μM to 75.0μM. The number of lateral root primordial at the present of 25.0μM cadmium increased by 21.2%. The hypocotyl elongation was inhibited by the different cadmium concentrations treatments.The hypocotyl length of Arabidopsis in 75.0μM cadmium treatment decreased by 25.5%. The cadmium stress inhibited the formation of chlorophyll. The chlorophyll content under 75.0μM cadmium treatment reduced 44.8%. The starch granule in Arabidopsis root tip was almost completely disappeared in 50.0μM cadmium for 48 h.2. DR5::GUS and DR5::GFP of auxin repotor gene was used as the mark of auxin content. The results indicated that the auxin content in root tip under cadmium stress decreased. The more inhibition on the expression of DR5 was observed in the prolonged time of cadmium treatments. The expression of DR5 in quiescent center of root tip under cadmium treatment for 48 h was significantly reduced.3. Cadmium stresses mould decrease the auxin content. The contents of endogenous auxin in Arabidopsis root at cadmium stress for 48 h and 72 h decreased 16.9% and 28.0%, respectively. The change of indoleacetic acid oxidase activity could affect auxin content. The activity of indoleacetic acid oxidase increased in cadmium stress. The activity of indoleacetic acid oxidase in lower cadmium concentration (12.5 uM) treatment did not increase significantly.The activity of indoleacetic acid oxidase increased by 108.3%-216.6% when cadmium concentration increased to 25.0μM or above.4. Cadmium stress may also affect the expression of genes involved in auxin synthesis. The effects of cadmium on the expressions of PATl-tryptophan synthesis gene and the genes (PAT1、NIT1、N1T2、YUCCA) involved in tryptophan-dependent synthesis of auxin showed difference in root and shoot of Arabidopsis. The expression of PAT 1 in root markedly increased and obviously decreased in shoot under cadmium stress for 6 h and 48 h. The expressions of NIT1 and NIT2 in the indole glucosinolate biosynthetic pathways under cadmium treatment for 6 h began to increase in roots, while NIT1 and NIT2 in shoot began to expression in cadmium treatment for 24 h. YUCCA encoded flavin monooxygenase-like enzymes of the YUCCA family are the key genes in auxin synthesis of tryptophan-dependent pathway. YUCCA 1 expression in Arabidopsis shoot sharply increased at 6 h and 12 h, and then its expression was inhibited by cadmium treatments at 48 h.5. Cadmium stress altered the expression of auxin transporter genes. The data of real time PCR revealed that cadmium treatment stimulated PIN1 expression at 1-6 h and PIN2 expression appeared in cadmium stress at 1-48 h. AUX1 expression of auxin influx carrier gene was markedly induced by cadmium treatment for 1-12 h.