Pierik et al. (2006) present a biphasic model to explain the differential responses to ethylene. The authors suggest that internal signals, environmental conditions and species-specific characteristics influence the response to ethy-lene. The authors illustrate that a dose-dependent response to ethylene can be found for different plants/tissues. These plant/tissue-specific ethylene responses can be placed in-between two extremes. One extreme consists of (tissues of) plants showing growth stimulation over a large range of ethy-lene concentrations (with a limited growth-inhibiting range); the other extreme would be (tissues of) plants that show growth inhibition at most ethylene levels. However, it should be noted that none of the dose-response curves shown in Pierik et al. (2006) have more than one data point behaving differently (induction instead of inhibition, or vice versa) from the other. Therefore more detailed analyses are essential to support this point of view.
It is noteworthy that several genes respond differentially to low/high ethylene concentrations (De Paepe et al. 2004a,b). Using a cDNA-AFLP experiment, the authors isolated a group of 30 genes which were more strongly up-regulated when treated with 10 |x.l/l ethylene than when treated with 0.1 ^l/l ethylene, whereas seven genes were more up-regulated by the lower concentration.
Although a gene-specific response to ethylene can in part explain the different ethylene responses, further research is needed in order to discover the factors that modulate these plant- and tissue-specific ethylene responses (including ethylene-responsive tissue-specific promoters, interactions with other hormones, and environmental signals). Finally, ethylene response may not be placed in species-dependent two-dimensional graphs as suggested (Pierik et al. 2006), but could be seen as an «-dimensional function that is dependent on species, environmental conditions, and tissue- and cell-type-specific factors.
Acknowledgements This work was supported by a PhD fellowship to Jasper Dugardeyn from the Institute for the Promotion of Innovation through Science and Technology in Flanders (IWT Vlaanderen).
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