Revisiting the Role of Ethylene and N-End Rule Pathway on Chilling-Induced Dormancy Release in Arabidopsis Seeds

Dormant Arabidopsis (Arabidopsis thaliana) seeds do not germinate easily at temperatures higher than 10-15 degrees C. Using mutants affected in ethylene signaling (etr1, ein2 and ein4) and in the N-end-rule pathway of the proteolysis (prt6 and ate1-ate2) we have investigated the effects of cold and ethylene on dormancy alleviation. Ethylene (10-100 ppm) and 2-4 days chilling (4 degrees C) strongly stimulate the germination of wild type (Col-0) seeds at 25 degrees C. Two to four days of chilling promote the germination at 25 degrees C of all the mutants suggesting that release of dormancy by cold did not require ethylene and did not require the N-end-rule pathway. One mutant (etr1) that did not respond to ethylene did not respond to GA(3) either. Mutants affected in the N-end rule (prt6 and ate1-ate2) did not respond to ethylene indicating that also this pathway is required for dormancy alleviation by ethylene; they germinated after chilling and in the presence of GA(3). Cold can activate the ethylene signaling pathway since it induced an accumulation of ETR1, EINI4, and EIN2 transcripts, the expression of which was not affected by ethylene and GA(3). Both cold followed by 10 h at 25 degrees C and ethylene downregulated the expression of PRT6, ATE1, ATE2, and of ABI5 involved in ABA signaling as compared to dormant seeds incubated at 25 degrees C. In opposite, the expression of RGA, GAI, and RGL2 encoding three DELLAs was induced at 4 degrees C but downregulated in the presence of ethylene.