A Spectroscopic View of Some Recent Advances in the Study of Blue Light Photoreception*

Abstract

Abstract:The blue to UV‐A region of the spectrum, spanning the region of about 320–520 nm, strongly influences the growth and development of plants and fungi. Photomorphogenesis in plants is, to a great extent, controlled by phytochrome, but there are unique contributions of the blue region, which cannot be duplicated by any amount of red light. Phototropism is, with few exceptions, a purely blue light response. In fungi, the blue region dominates the photocontrol of growth and development, though some red light effects have been reported. Many blue light action spectra fit the definition of cryptochrome, a pigment class defined by its UV‐A and blue peaks. The action spectrum, if measured to sufficient resolution, displays several minor maxima or shoulders in the blue region which call to mind the vibrational levels of carotenoids and flavins. Recent molecular genetic studies, as well as photobiological work, have shown that some cryptochromes are related to the DNA repair enzyme photolyase, while others appear genetically and spectroscopically distinct. In this review, we have applied established criteria from photobiology, in particular, comparison of action spectra with absorption spectra, to these recent results. It is apparent that photolyase homologs such as CRY1 can explain the blue light portion of the action spectrum for hypocotyl elongation, assuming participation of the oxidized flavin. In fungi, the photoreceptor question remains open. Identification of the nph1 gene in Arabidopsis may soon lead to a photoreceptor for higher plant phototropism. Also, we present a possible solution to the most recent version of the long‐standing flavin‐carotenoid controversy, the zeaxanthin hypothesis for higher plant phototropism. In conclusion, there appear to be at least three classes of cryptochromes.