Elongated mesocotyl1, a phytochrome-deficient mutant of maize.

To begin the functional dissection of light signal transduction pathways of maize (Zea mays), we have identified and characterized the light-sensing mutant elm1 (elongated mesocotyl1). Seedlings homozygous for elm1 are pale green, show pronounced elongation of the mesocotyl, and fail to de-etiolate under red or far-red light. Etiolated elm1 mutants contain no spectrally active phytochrome and do not deplete levels of phytochrome A after red-light treatment. High-performance liquid chromatography analyses show that elm1 mutants are unable to convert biliverdin IX alpha to 3Z-phytochromobilin, preventing synthesis of the phytochrome chromophore. Despite the impairment of the phytochrome photoreceptors, elm1 mutants can be grown to maturity in the field. Mature plants retain aspects of the seedling phenotype and flower earlier than wild-type plants under long days. Thus, the elm1 mutant of maize provides the first direct evidence for phytochrome-mediated modulation of flowering time in this agronomically important species.