| Ethylene is a simple gaseous phytohormone that is involved in numerous plant processes such as development and environmental responses. Two enhanced ethylene response mutants (eer), eer3 and eer5, were found to give an increased response to ethylene in the hypocotyls at the seedling stage of growth, yet do not regulate ethylene responsive genes properly at the molecular level since they do not increase mRNA levels of a subset of ethylene regulated genes such as PDF1.2, chiB and AtEBP. EER3 encodes AtPHB3 which is part of the family of prohibitin proteins found in many other organisms and have several diverse functions such as transcriptional regulation of cell cycle proteins, mitochondrial biogenesis, and even regulating membrane proteases. eer3-2, a more severe T-DNA mutation in AtPHB3 displays a constitutive severely-inhibited growth phenotype. Double mutant analyses between eer3 and ctr1-3, ein3-1 or ein2-5 mutants indicate that AtPHB3 functions downstream of CTR1 and EIN2 and may act independently or downstream of EIN3 and EIL1. Interestingly, an interaction between the carboxy-terminal end of EIN2 and AtPHB3 was found, indicating that EIN2 controls the function of AtPHB3. AtPHB3 also interacts with EER4, a TFIID associated factor, and may serve to induce a subset of genes required for inhibiting or resetting the ethylene response. EERS contains a PCI/PINT domain and encodes a putative subunit of the COP9 Signalosome (CSN). The CSN is a multisubunit complex and one of its functions is to interact with specific E3 ubiquitin ligase complexes to regulate the degradation of target proteins via the 26S proteasome. Double mutant analysis between eer5 and ctr1-3 indicates that EER5 functions below CTR1. The carboxy-terminal end of EIN2 also interacts with EER5, as well as CSN3, CSN6A and CSN6B, indicating that the COP9 Signalosome may regulate ethylene signal transduction. EER5 likely represents a CSN subunit that is not required for several CSN related phenomena, but may regulate a specific function of the CSN related to ethylene signal transduction. The CSN complex may involve the targeted degradation of factors repressing a subset of genes required for inhibiting or resetting the ethylene response. |
