Printed in Great Britain © The Company of Biologists Limited 1992

The genetic interactions among mutations that define eight distinct loci involved in light-regulated develop-ment in Arabidopsis thaliana are described. The mu-tations in these eight genes define two distinct phenotypic classes with opposite characteristics. Recessive mutations in either one of two genes, DETL or DET2, result in dark-grown plants that develop as light-grown wild-type seedlings. Mutants in the second class exhibit a reduced response to light Recessive mutations in any one of five genes, HY\, HY2, HY3, HY5, or HY6 cause reduced responses to red-light Four of these genes, HY\, 2,3, and 6, affect the activity of one or all of the red-light photore-ceptors, the phytochromes. The HY4 gene product is involved in blue-light perception or action. The experi-ments described here examine how these eight genes interact to control a particular event, the switch from developmental arrest in the dark (etiolation) to growth in the light (de-etiolation). The phenotypes of doubly mutant strains suggest a hierarchical regulatory network among these genes in the control of the switch from etio-lated to de-etiolated growth strategies. Key words light perception, leaf and chloroplast development, Arabidopsis