Exogenous ethylene enhanced the cadmium resistance and changed the alkaloid biosynthesis in Catharanthus roseus seedlings

Cadmium (Cd) is not an essential nutrition element to plants and high concentration of Cd in the environment causes severe damage to plants. It is reported that the gas phytohormone ethylene plays important roles in plant responses to Cd stress. However, whether plant secondary metabolism is involved in this process remains to be investigated. Here, the regulations of exogenous ethylene on internal Cd accumulation and terpenoid indole alkaloids (TIAs) biosynthesis in the medicinal plant Catharanthus roseus (L.) G. Don (C. roseus) under Cd stress condition were explored. Our results showed that Cd treatment inhibited biomass accumulation, increased Cd accumulation and H2O2 and malondialdehyde (MDA) productions. Simultaneously, Cd treatment enhanced yields of vindoline, catharanthine and vinblastine, as well as the gene expressions of TIAs pathway enzymes at transcriptional level. Exogenous ethylene application significantly reduced the Cd content in whole plants and the H2O2 and MDA productions in roots and leaves, indicating that Cd stress in C. roseus was effectively alleviated by the ethylene application. It was interesting to find that exogenous ethylene promoted the Cd transport from roots to leaves and the value of Cd TF (transfer factor) was increased from 0.41 to 0.53. In leaves, the transcriptional expression of metallothionein (MT) was up-regulated by exogenous ethylene application, together with the increase of Cd accumulation. Additionally, exogenous ethylene reduced the TIAs productions, with an exception of catharanthine in leaves. The transcriptional expressions of alkaloid transporters, triose phosphate translocator (MDR) and multidrug resistance (TPT) were also up-regulated in leaves by exogenous ethylene, in accordance with the change of catharanthine biosynthesis. All of our results showed that exogenous ethylene could elevate the Cd resistance of C. roseus and effectively influence TIAs biosynthesis in C. roseus. Regulations of ethylene on the transcriptional expressions of MT, pathway enzymes and alkaloid transporters might be involved in this process.