Two dominant photomorphogenic mutations of <i>Arabidopsis thaliana</i> identified as suppressor mutations of <i>hy2</i>

<jats:p>By screening suppressor mutants of the <jats:italic>hy2</jats:italic> mutation of <jats:italic>Arabidopsis thaliana</jats:italic>, two dominant photomorphogenic mutants, <jats:italic>shy1‐1D</jats:italic> and <jats:italic>shy2‐1D</jats:italic>, for two genetic loci designated as <jats:italic>SHY1</jats:italic> and <jats:italic>SHY2</jats:italic> (<jats:styled-content>s</jats:styled-content>uppressor of <jats:italic><jats:styled-content>hy</jats:styled-content>2</jats:italic> mutation) have been isolated. Both of these non‐allelic, extragenic suppressor mutations of <jats:italic>hy2</jats:italic> are located on chromosome 1 of the <jats:italic>Arabidopsis</jats:italic> genome. Both mutations suppress the elongated hypocotyl phenotype of <jats:italic>hy2</jats:italic> by light‐independent inhibition of hypocotyl growth as well as by increasing the effectiveness of light inhibition of hypocotyl elongation. The <jats:italic>shy1‐1D</jats:italic> mutation is partially photomorphogenic in darkness with apical hook opening and reduced hypocotyl elongation. The <jats:italic>shy2‐1D</jats:italic> mutant displays highly photomorphogenic characteristics in darkness such as true leaf development, cotyledon expansion, and extremely reduced hypocotyl growth. In regard to hypocotyl elongation, however, the <jats:italic>shy2‐1D</jats:italic> mutation is still light sensitive. Examination of red/far‐red light responses shows that the <jats:italic>shy1‐1D</jats:italic> mutation suppresses the hypocotyl elongation of the <jats:italic>hy2</jats:italic> mutation effectively in red light but not effectively in far‐red light. The <jats:italic>shy2‐1D</jats:italic> suppresses hypocotyl elongation of the <jats:italic>hy2</jats:italic> mutation effectively in both red and far‐red light. Both mutations can also suppress the early‐flowering phenotype of <jats:italic>hy2</jats:italic> and have a distinct pleiotropic effect on leaf development such as upward leaf rolling. The data obtained suggest that SHY1 and SHY2 represent a novel class of components involved in the photomorphogenic pathways of <jats:italic>Arabidopsis</jats:italic>. This is the first report on the identification of dominant mutations in the light signal transduction pathway of plants.</jats:p>