Physiological Functions Of Hydrogen Sulfide Signal In Response To Cadmium Stress In Foxtail Millet

Hydrogen sulfide (H2S) always has notorious status for poisoning animals and plants. It is considered as the third gaseous signaling after nitric oxide and carbon monoxide, which means it also has pivotal physiological regulative actions in biont as well. In recent years, many physiological and biochemical functions of H2S have been proved, participating in seed germination, root generation, growth and development, photosynthesis, stomatal movement, programmed cell death and the response processes of (a)biotic stresses including drought, osmosis, heavy metal and even fungal stresses.Multi-layered technologies and methods of pharmacology, biochemistry and physiology were applied to study the creation and change of endogenous F2S and the effects of H2S on seedling generation under Cadmium (Cd) stress in foxtail millet (Setaria italica), with the purpose of further understanding the H2S physiological funtions and acting mechanisms in response to Cd stress in foxtail millet. Here are the results:1. Cd stress could inhibit the seedling growth including the decrease of plant height and root length, the produce of hydrogen peroxide (H2O2) and lipid peroxide (MDA), the decrease of chlorophyll and the increase of electrolyte leakage (EL). With the boosting degree of Cd stress, the activity of superoxide dismutase, ascorbate peroxidase, peroxidase and catalase would increase to withstand the accumulation of ROS.2. Cd stress could enhance the content and the productivity of endogenous H2S in foxtail millet, which suggested the intimate relationship between them. With pretreatment of exogenous H2S, the photosynthesis would be strengthened by increasing net photosynthetic rate (Pn), transpiration rate (E) and stomatal conductance (Gs); the EL and H2O2 content would decrease; the activity of NADPH oxidase and the expression levels of encoding genes rboh D and rboh F induced by Cd would increase, and the transcriptional levels of metallothionein encoding genes MT3 and MT2A would go up at the same time, in order to remit the Cd-related poisonousness.3. Cd stress could increase the content of H2S and Proline (Pro) in foxtail millet and exogenous Pro pretreatment could alleviate the increased degrees of H2O2 and MDA induced by Cd. The additional accumulation of Pro under Cd stress would owe to H2S which could enhance the P5CR activity and reduce the PDH activity through up-regulating P5CR and down-regulating PDH. Otherwise, Cd-induced Pro accumulation could be enhanced by H2S donor (NaHS) pretreatment and reduced by H2S synthesis inhibitors like hydroxylamine (HA), aminooxyacetate, potassium pyruvate, or H2S scavenger like hypotaurine pretreatment. It was suggested that exogenous H2S cooperated with Pro could alleviate the Cd-damage to foxtail millet and increase its tolerance recovering the chlorophyll concentration, lifting the stomatal aperture and cutting the H2O2 content level.4. Cd stress could reduce the content of glucose and sucrose, inhibit the enzymatic activity of ATPase (Ca2+-ATPase、Mg2+-ATPase、H+-ATPase) and induce the gene expressions of HMAs belonging to P1B-ATPase transporters. In other word, the H2S pretreatment was able to increase the glucose content, enhance activities of these ATPase and compartmentalize the intracellular Cd into vacuole or inhibit Cd transfer to overground part, finally lightening the Cd toxicity.5. Cd-induced increases of O2·-, H2O2 and MDA concentrations and inhibition of root growth in foxtail millet could be alleviated by the increase of H2S content because of the exogenous methyl jasmonate (MeJA) pretreatment. MeJA could reduce the Cd absorption in root by means of inhibiting the mRNA levels of Cd-induced SiNRAMPl and S1NRAMP6. Moreover, MeJA could enhance the function of MTPs and CAX2 by means of up-regulating other translocators encoding genes expressions such as MTP1, MTP12, CAX2, HMA3 and ZIP4, which could lead Cd transport into vacuole. The regulation effects of MeJA on Cd detoxication could be influenced by HA or H2S, which also provided evidences for the H2S participation.In conclusion, stress-response signal molecules such as H2S, Pro and MeJA could enhance the vigour of ROS scavenge system decreasing the concentration of ROS and MDA and Cd-absorption as well, which leaded to Cd-tolerance promotion. The studies elaborated the participated pathways and the physiological mechanisms of H2S under Cd stress in foxmail millet; improved the signal functions of H2S; figured out the relationships between Pro and H2S or between MeJA and H2S in process of Cd alleviation; suggested that H2S could regulate HMAs for increasing Cd tolerance. Above all, the research provided much more supporting points for studying the effects of H2S on Cd-tolerance in plants.