| Salinity decreases plant growth and production, but the underlying mechanisms of tolerance to salinity are poorly understood. The capacity of plants to achieve successful germination and early seedling establishment under high salinity is crucial for tolerance of plants to salt. Ethylene is the only known gaseous plant hormone and has long been known to affect development process and fitness responses such as germination, flower and leaf senescence, fruit ripening and leaf abscision. To better understand the role of the ethylene signal transduction pathway during germination and seedling establishment, an ethylene insensitive mutation (ein2-5) and an ethylene sensitive mutation (ctrl-1) of Arabidopsis were analyzed under saline conditions and compared with the wild type plant (Col-0) as control. Effects of addition of the ethylene precursor1-aminocyclopropane-1-carboxylic acid (ACC) or inhibitors of ethylene to salinity stressed seeds were also analysed. Furthermore, the effects of nitric oxide (NO) on seed germination under salinity and its interaction with ethylene during this process were also investigated, To underlying mechanisms of ethylene regulates seed germination under salinity, Endogenous concentration of hydrogen peroxide (H2O2) in germinating seeds as well as apoptosis and lipid breakdown in adjacent transition zone and lower hypocotyl in geminating seeds under salinity were also determined in this study. As the model species, Arabidopsis thaliana is a member of the Brassicaceae, to which many vegetable and oil crops belong, understanding the mechanisms by which their germination and early growth respond to salinity is of ethylene regulates seed germination in A. thaliana under salinity is of great importance for plant breeding research and saline cropland utilization.Findings of this study can be summarized as follows:1. Inhibitory effect of salt on seed germination is related to NaCl stress rather than osmotic stress. Seed germination of Col-0were analysed under NaCl (50mM,100mM,150mM and200mM) and mannitol (100mM,200mM,300mM and400mM) with same osmotic potential, and the results showed that inhibitory effects of NaCl were more severe than those of mannitol, indicating that germination is not mainly inhibited by osmotic stress but by the ion stress from NaCl.2. Ethylene was essential in regulating seed germination under salinity in A. thaliana. High salinity (>100mM NaCl) inhibited and delayed germination. These effects were more severe in the ethylene insensitive mutants (ein2-5) and less severe in the constitutive ethylene sensitive plants (ctrl-1) compared with Col-0plants. Addition of the ethylene precursor ACC or inhibitors of ethylene action implied that ethylene was essential for early seedling establishment under normal and saline conditions. Besides, early development and growth of seedlings after germination also can be promoted by presence of ethylene.3. Involvement of NO in modulating seed germination under salinity and its relationship with ethylene during this process. Application of exogenous sodium nitroprusside (SNP, an NO donor) largely overcame the inhibition of germination induced by salinity. The effects of SNP were decreased by2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide (cPTIO), a specific NO scavenger, indicating that NO promoted seed germination under salinity. The results indicated that salinity increased the production of ethylene as well as NO, and each one promoted the production of the other, by which a feedback network was established and ethylene and NO cooperate in enhancing seed germination of Arabidopsis under salinity.4. Relationship of ethylene, NO and H2O2in germinating seeds under salinity. Application of exogenous ACC increased germination under oxidative stress induced by H2O2while SNP had smaller effects on Col-0and no effect on the ethylene insensitive mutant (ein2-5) seeds. This shows that NO increased germination under salinity indirectly through H2O2acting via the ethylene pathway. The expression profiles of the genes which code for ROS scavenging (antioxidant) enzymes SOD, CAT and POD were severely decreased in ein2-5under salinity compared with Col-0. Furthermore, activities of the enzymes SOD, POD and CAT were also decreased significantly in ein2-5under salinity when compared with Col-0plants. Taken together, these results demonstrate that ethylene signaling was of vital importance for maintaining redox homeostasis, and this was hypothesized to be the underlying mechanism by which ethylene promoted seed germination under salinity and oxidative stress.5. Effects of ethylene signaling on apoptosis and lipid breakdown in adjacent transition zone and lower hypocotyl in geminating seeds under salinity. The radicle growth and elongation of ctrl-1was less affected by salinity than that of Col-0and ein2-5. Exaggerated ethylene signaling in ctrl-1resulted in lateral growth of cells and their size decreased so they were arranged more densely in the adjacent transition zone and lower hypocotyl. Furthermore, the results showed that lipid breakdown in ein2-5was inhibited under salinity, the number of oil bodies decreased drastically whereas the size of them were bigger. Besides, more cells of ein2-5were undergoing apoptosis compared to those of Col-0and ctrl-1. Conversely, the exaggerated ethylene response accelerated lipid breakdown in ctrl-1cells as the number of oil body increased as well as its oil bodies size decreased compared to those of Col-0. This stimulated radicle growth, maintained cell viability and decreased the occurrence of apoptosis, and finally inhibition of seed germination was prevented under salinity. |
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