The Ethylene Receptor Regulates Typha Angustifolia Leaf Aerenchyma Morphogenesis And Cell Fate

Aquatic plants rely on aerenchyma to transport oxygen to solve the problem of anoxic stress in aquatic environment.Aerenchyma is a number of cavities in plants,which are formed by programmed cell death(PCD).However,cells in the outer edge of Typha angustifolia leaf aerenchyma(palisade cells and wall cells)remained alive and intact,and were not underwent programmed cell death.The PCD-susceptible cells and the PCD-unsusceptible cells are in a same environment,but they have distinct cell fates.The regulation mechanism of this phenomenon is not clear at present.In this paper,Typha angustifolia leaf aerenchyma were studied.To increase and decrease ethylene concentration in Typha angustifolia,ethephon and pyrazinamide(PZA),inhibitors of ACC oxidase,were used to treat the asepsis seedlings,respectively.By measuring the size of aerenchyma,it was found that the formation of aerenchyma was inhibited after inhibiting the synthesis of endogenous ethylene,while the size of aerenchyma cavity increased significantly after ethylene treatment,which confirmed that ethylene is the key hormone regulating PCD in aerenchyma formation.Ethylene receptor inhibitor(1-MCP)was used to block ethylene receptor.It can inhibit the ability of ethylene receptor to bind to ethylene and inhibit ethylene signal transduction.The cell apoptosis test showed negative result,indicating that after blocking ethylene receptors,the cells could not undergo PCD process.Thus aerenchyma formation was inhibited.The expression level of PCD-related genes was also significantly lower than that in the control without inhibiting ethylene receptor,indicating that the expression level of cell death related genes and degradation related genes was regulated by ethylene signal and they were downstream of ethylene receptor.After the receptors were blocked,the expression level of related genes was also inhibited.It is proved that ethylene receptor plays an irreplaceable role in aerenchyma formation.Although blocking ethylene receptors with inhibitors prevents cells from undergoing PCD process,excessive ethylene receptors also prevent cells undergoing PCD process.The reason is that ethylene receptor plays a negative regulatory role in ethylene signaling pathway,and the existence of ethylene receptors can inhibit the corresponding ethylene signaling pathway.However,1-MCP can compete for ethylene receptor with ethylene,blocking the binding of ethylene and ethylene receptor without affecting the inhibitory effect of ethylene receptor,so as to inhibit ethylene signaling pathway.When ethylene molecule binds to ethylene receptor,the inhibition is removed,so that cells can conduct ethylene signal transduction.In this study,the localization of ethylene receptor proteins TaETR1,TaETR2 and TaEIN4 in Typha angustifolia was observed by using polyclonal antibodies and immunofluorescence localization techniques.The results shows that there are a large number of ethylene receptors in palisade cells cells and cells around vascular bundles(ethylene-insensitive cells),but less in spongy cells(ethylene-sensitive cells)which will undergo PCD.The content of ethylene receptor in PCD-unsusceptible cells was 4-14 times higher than that in PCD-susceptible cells.This suggests that the sensitivity of cells to ethylene is closely related to the content of ethylene receptor.Ethylene receptor plays a negative regulatory role in ethylene signaling pathway.In cells with higher content of ethylene receptor,the inhibition of ethylene receptor on ethylene signaling pathway is stronger.Ethylene signaling pathway was blocked,cells can not undergo PCD process,but survived and performed their biological functions.On the contrary,cells with lower ethylene receptor content have smooth ethylene signaling pathway when the ethylene is present.They can undergo PCD procedure and eventually from aerenchyma.