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Hormone responses shed light on photomorphogenesis
In our desire to reach an understanding of gene expression there is a tendency to simplify the effects of genes and to try to link complex phenomena with the activity of single genes. However, it is usually more complex than that. This is illustrated in work by Oono et al. (University of Tokyo, pp. 77-82) with a mutant of the SHY2 gene of Arabidopsis. This gene was originally identified as being involved in photomorphogenesis; the mutants show many features of normal lightdriven development when grown in the dark. However, the sequence of SHY2 shows it to be the same as IAA3, a primary auxin response gene. The IAA genes are expressed within minutes of auxin application; they encode proteins that form homo and heterodimers amongst each other and with proteins called auxin response factors (ARFs). These protein dimers recognize auxinresponse elements in the promoters of auxinregulated genes and thus act as positive or negative transcription factors. The effect of the mutant IAA3/SHY2 gene was analysed by hybridization of specific mRNAs and by investigating expression of a reporter gene (GUS) placed under the control of a promoter containing an auxinresponse element. The authors describe a fascinating array of effects on the expression of the IAA/ARF genes themselves and on the expression of the genes regulated by IAA/ARF transcription factors. However, the following result is of particular interest in relation to photomorphogenesis: in the IAA3/SHY2 mutants, ectopic expression of the reporter gene occurred in several organs, including hypocotyls and cotyledons. It is in these organs that the effects of light on development are first seen, suggesting that in wildtype plants auxin may suppress expression of particular genes but when these genes are not downregulated (as in IAA3/SHY2 mutants), darkgrown plants behave as if grown in the light.
Professor J.A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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