Ethylene signals route to ferric foraging
The interface between plant and soil via the root system is very dynamic. In response both to nutrient deficiency and nutrient abundance many plants are able to modify root morphology and/or to alter the allocation of overall resources between root and shoot. A specific and unusual example of this is seen Casuarina glauca, which forms cluster roots under conditions of soil iron deficiency. One of the interesting questions that arise in these situations is how the morphogenic events are controlled within the plant. The primary signal is nutrient deficiency but this signal must be transduced to cause the production and/or activation of meristems. It is this question that has been addressed by Zaid et al. (Rabat, Morocco and Créteil, France, pp. 673-677). They grew casuarina plants in Fe-sufficient and Fe-deficient conditions and then tested the effects of ethylene inhibitors and ethylene precursors on cluster root formation. Iron deficiency led to the formation of four times as many cluster roots compared with plants supplied with iron. Addition of the ethylene precursor ACC (1-aminocyclopropane-1-carboxylic acid) had no effect on cluster root formation in Fe-deficient conditions, but when added to the nutrient solution in Fe-sufficient conditions it led to formation of cluster roots as if the plants were Fe-deficient. This is strong indication that ethylene is involved in cluster root formation. This idea is supported by the results of experiments with inhibitors of ethylene synthesis or of ethylene action: all of these prevented the four-fold increase in cluster root formation normally seen in Fe-deficient conditions. Based on their data, the authors suggest that a very early response to iron deficiency is a marked stimulation of ethylene biosynthesis. This leads, presumably via a 'standard' signal transduction pathway, to the regulation of gene expression and morphogenesis in this developmentally plastic root system.
Professor J.A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
