If ethylene responses all rely upon the same primary signaling pathway, how is it that ethylene can mediate so many different developmental responses? The answer is likely to rely to a large part on the extent to which transcriptional output from the ethylene signaling pathway can be tailored to meet different needs. Multiple mechanisms have been identified by which such regulation can be accomplished. As explained below, these mechanisms include physical interactions among transcriptional regulators, cooperative control of developmental pathways, and the modulation of expression and protein turnover of elements within the transcriptional cascade.
Two recent studies highlight how physical interactions between the EIN3-like transcription factors and other transcriptional regulators can modulate gene expression. The hormones ethylene and jasmonic acid often act synergistically in the control of plant development and pathogen responses, and a mechanistic basis for this synergy was uncovered when it was discovered that JAZ repressor proteins physically interact with and inhibit the activity of the EIN3-like transcription factors [6
]. Signaling by jasmonic acid stimulates the degradation of the JAZ repressor proteins, thereby activating transcription by the EIN3-like transcription factors. The second study identified an interaction between the EIN3-like transcription factors and the transcription factor FIT, a regulator of iron uptake in roots [7
]. Protein levels of both the EIN3-like transcription factors and FIT are regulated by proteasome-mediated degradation. In the presence of ethylene, the EIN3-like transcription factors are stabilized, bind to FIT, and apparently protect FIT from degradation, thereby enhancing its ability to stimulate the expression of genes involved in iron acquisition.
The EIN3-like transcription factors can also function cooperatively with other transcription factors to promote developmental changes. This ability is highlighted in a study examining how the EIN3-like transcription factors and the basic helix-loop-helix (bHLH) transcription factor PIF1 play roles in promoting the greening of seedlings exposed to light [8
]. Both regulate chlorophyll biosynthesis but apparently do so through the targeting of different promoter elements: the EIN3-like transcription factors target the promoters of PORA
while PIF1 targets the promoter of PORC
. These three POR
genes encode different isoforms for a rate-limiting enzyme in chlorophyll biosynthesis. Thus, PIF1 and the EIN3-like transcription factors function in parallel to facilitate the same developmental pathway.
Not all regulation is at the level of the EIN3-like transcription factors, and downstream ERF and EDF transcription factors greatly increase the potential points for interaction and cross-talk of ethylene signaling with other pathways. For instance, overexpression of the MADS-Box gene FYF
inhibits floral organ abscission and senescence in Arabidopsis
, an effect correlating with a decreased responsiveness of the transgenic plants to ethylene [9
]. However, no changes were uncovered in the expression of genes encoding elements of the primary signal transduction pathway for ethylene. Instead, the apparent cause of the phenotype is a decrease in the expression of EDF
s. Furthermore, in monocots, the FRIZZY PANICLE
) gene of rice and the orthologous BRANCHED SILKLESS1
) gene of maize both encode ERF transcription factors, and their loss results in plants that fail to make the transition from inflorescence meristems to floral meristems [10
The study by Lumba et al
] suggests that several of these regulatory mechanisms play roles in how ethylene regulates a developmental pathway for advancing juvenile to adult leaves. The leaves of Arabidopsis
exhibit a gradient of morphological characteristics - involving such attributes as size, shape, and the presence of trichomes - that is dependent on when the leaves are produced during plant development. These characteristic features have facilitated the identification of heterochronic mutations that affect the timing of leaf development, such as mutations in FUSCA3
) resulting in juvenile leaves that have more adult-like traits. FUS3 is a transcription factor and prior work has demonstrated that its effect on leaf phase transitions involves the plant hormones abscisic acid and gibberellin, with abscisic acid retarding and gibberellin advancing the transition from juvenile to adult leaf morphology [3
]. Now a third hormone, ethylene, is thrown into the mix [2
]. Interestingly, ethylene is here found to stimulate a much earlier stage of development than it is normally associated with, accelerating the transition from juvenile to adult in newly formed leaves, but it is nevertheless still acting as an 'aging' hormone. A principal role of FUS3 appears to be to inhibit ethylene-regulated signal output, as its loss results in increased levels of the EIN3 protein as well as increased expression of ERF
s and EDF
s. Part of this effect of ethylene-regulated gene expression is owed to the action of FUS3 on the primary ethylene response pathway. However, at least part of the effect of FUS3 appears to be ethylene-independent as the presence of putative FUS3 binding elements in promoters of ethylene responsive genes raises the possibility that FUS3 also plays a more direct and antagonistic role in controlling transcriptional output from the ethylene signaling pathway. It is likely that similarly complex mechanisms, operating at multiple points in the ethylene signaling pathway, will be uncovered as we begin to explore how ethylene interacts with other plant developmental pathways in more detail.