A New Family of Plant Photoreceptors: The Phototropins

The Briggs group recently presented convincing evidence that the Arabidopsis protein nph1 functions as a photoreceptor for phototropism—-the phenomenon whereby plants grow in the direction of a blue-light source. This response is ubiquitous among higher and many lower plants and assures that growing shoots can orient to a patch of blue sky when direct sunlight is not available. The orientation capacity guarantees maximizing light capture for photosynthesis and is particularly valuable in forest environments where the majority of light comes through open patches in the overhead canopy.

The nph1 protein is a serine/threonine kinase that binds two molecules of flavin mononucleotide as the light-absorbing chromophores within two regions designated the LOV domains (found in proteins sensing Light, Oxygen, or Voltage). Each domain undergoes a unique photochemical reaction involving the light-activated formation of a covalent bond between the flavin and a neighboring cysteine residue. The flavin-cysteine complex decays within minutes in the dark. Nevertheless, formation of the complex almost certainly brings about a protein conformational change that results in the activation of the signal transduction pathway leading to phototropic curvature.

Another Arabidopsis gene, designated NPL1 (for NPH1-Like), encodes a protein npl1 that shares about 70% of its identity with the nph1 amino acid sequence. Like nph1, npl1 contains two flavin-binding LOV domains and a protein kinase domain. In collaboration with Professor K. Okada’s group in Kyoto, postdoctoral fellow John Christie expressed the NPL1 gene in insect cells and demonstrated that recombinant npl1, like nph1, undergoes blue light-dependent autophosphorylation. Postdoctoral fellow Masahiro Kasahara demonstrated that the LOV domains of npl1 also bind flavin mononucleotide and exhibit the same photochemistry as the nph1 LOV domains.

The Okada group and a group from M. Wada’s laboratory from the Tokyo Metropolitan University have recently found that npl1 is the photoreceptor for the avoidance response of chloroplasts to excessive light. Moreover, both nph1 and npl1 appear to be photoreceptors for the opposite response--the accumulation of chloroplasts in regions of dim light. The Okada group has also shown that npl1 is essential for phototropic responses to very high-light intensities.

Thus, higher plants contain at least two members of the nph1 photoreceptor family--nph1 and npl1--and the scientists named them phototropins. As they fall into two categories, both in terms of sequence similarity and function, they are designated them phot1 and phot2. Phot1 plays an important role both in phototropism and chloroplast movement in relatively dim light and phot2 plays a role in both processes in bright light.

In collaboration with Professor A. Nagatani’s group in Kyoto, the Briggs researchers recently investigated the properties of a nph1 homologue from the unicellular green alga Chlamydomonas reinhardtii. As before, Christie was able to express the full-length gene in insect cells and demonstrate light-activated autophosphorylation. Kasahara also demonstrated that the flavin-binding LOV domains undergo the same photochemistry as those of nph1. As Chlamydomonas shows neither phototropism nor light-activated chloroplast movement, the role of this photoreceptor is presently unknown.

Postdoctoral fellow Koji Sakamoto has made significant progress in introducing phot constructs that yield a highly fluorescent product into the phototropically null Arabidopsis mutant, nph1-5. He has also introduced constructs that express glucuronidase activity, driven by the phot1 promoter, which is another way of marking the distribution of a protein. These transformants will enable him to determine both the subcellular and tissue distribution of phot1 and phot2.

To date, the phototropins represent the only plant blue-light photoreceptors whereby a photochemical cycle can be linked to signal transduction. That they can be studied at the cellular, biochemical, and molecular levels gives the laboratory powerful tools to unravel a long-elusive signal transduction pathway.