Cross-Talk Between Ethylene And Heme-Heme Oxygenase Signal System In Primary Root Growth Inhibition Of Etiolated Tomato Seedlings

Ethylene, an important hormone regulating many developmental processes, plays very important roles in controlling the signal transduction under plant root development process. Carbon monoxide (CO), which is produced by heme oxygenase, has been proved to regulate various metabolism processes in animals. Interestingly, recent researches have also demonstrated that HO/CO plays very important roles in plant development and responses to abiotic stresses.In order to investigate the relationship between ethylene signal network and heme-heme oxygenase system, the function of the two signal systems was investigated. In this report, we discovered that ACC inhibits tomato primary roots growth, up-regulates HO transcripts, HO-1 protein expression and HO activity in a dose-dependent manner. ACC oxidase inhibitor CoCl2 was also used to verify that the effect of ACC was due to ethylene action. It was found further that 20μM CoCl2 could partially prevent the transform of ACC to ethylene, thus leading to the alleviation of tomato primary root growth inhibition induced by ACC. Meanwhile, CoCl2 could inhibit endogenous ethylene generation and promote the growth of tomato primary root. These results clearly indicate the negative correlation between ethylene production and tomato primary root growth. Our further results show that 20μM CoCl2 reverses the effects of 10μM ACC on tomato primary root growth, HO transcripts, HO-1 protein expression and HO activity. The same phenomenon was also found in ethephon treatment. Both indicate that there was a negative correlation between ethylene production and HO expression in tomato primary root growth. Furthermore, the application of ethephon, another releasing substance of ethylene, also mimics the effects of ACC on tomato primary root growth and HO transcripts. Our further research found that the application of HO-1 inhibitor ZnPP not only alleviates ACC-, ethephon-, and hemin-induced tomato root growth inhibition, but also extenuates the induction of HO activity. The above results reveal that HO may be involved in ethylene-induced tomato root growth inhibition.In the further step, we investigated the molecular mechanism of ACC- and hemin-induced tomato root growth inhibition. The results show that the up-regulation of expansins might be responsible for root growth inhibition.On the other hand, it was found that 200μM hemin not only promote the production of ethylene, but also significantly inhibits tomato root growth. Meanwhile, the application of HO-1 inhibitor ZnPP, ethylene biosynthesis inhibitor AVG and CoCl2, were able to extenuate the production of ethylene induced by hemin. These results indicated that hemin-induced the production of ethylene was due to HO, and ethylene production induced by hemin was one of the reasons of hemin-induced root growth inhibition. Ethylene receptor, as the common site of ethylene and HO action on tomato root growth was also confirmed. It was found that ACC and hemin were able to up-regulate the transcripts of tomato ethylene receptor, while ethylene action inhibitor AgNO3 could block the effects of ACC and hemin on tomato root growth and ethylene receptor transcripts.Taken together, we propose a conclusion that there might be a cross-talk between ethylene signal and heme-heme oxygenase system leading to primary root growth inhibition.