High impact information on Phototropism

• Phototropin, the protein product of the NPH1 gene in Arabidopsis, likely serves as the photoreceptor for phototropism and appears to have no other role [1].
• We conclude that cryptochrome is one of the photoreceptors mediating phototropism in plants [2].
• These responses--which in higher plants include phototropism, inhibition of hypocotyl elongation, and stomatal opening--are in many cases thought to be mediated by flavin-type photoreceptors [3].
• The positive phototropism and the negative geotropism of grass seedling coleoptiles are believed to result from lateral movement of auxin from the irradiated to the shaded side and from the upper to the lower side, respectively [4].
• Here, Arabidopsis thaliana mutants defective in the avoidance response were isolated, and the mutated gene was identified as NPL1 (NPH-like 1), a homolog of NPH1 (nonphototropic hypocotyl 1), a blue light receptor used in phototropism [5].

Biological context of Phototropism

• These results, along with those for Arabidopsis, demonstrate that NPH3/CPT1 is a key signal transduction component of higher plant phototropism [6].
• Blue light regulates photoactivation of carotenoid synthesis, conidiation, phototropism of perithecia and circadian rhythms [7].
• The genetic and physiological basis for signal integration was investigated with a set of phototropism mutants (genotype mad) of the UV- and blue-light-sensitive fungus Phycomyces blakesleeanus, which responds also to gravity, ethylene and nearby obstacles (autochemotropism or avoidance response) [8].
• A total of 134 sexual crosses between strains carrying mutations affecting phototropism (madA-madE), carotenoid biosynthesis (carA), auxotrophy (ribA-ribD, purA-purC, lysA and lysB, nicA and nicB, and leuA) and resistance to 5-fluorouracil (furA and furB) were studied; mating type (sex) was also included as a marker [9].

Associations of Phototropism with chemical compounds

• Phototropin, a major blue-light receptor for phototropism in seed plants, exhibits blue-light-dependent autophosphorylation and contains two light, oxygen, or voltage (LOV) domains and a serine/threonine kinase domain [10].
• Phototropins (phot1 and phot2), the plant blue-light receptors for phototropism, chloroplast movement, and stomatal opening, are flavoproteins that contain two approximately 12 kDa FMN-binding domains, LOV1 and LOV2, at their N-terminus, and a serine/threonine protein kinase domain at their C-terminus [11].
• This mutant showed abnormal phototropism, which was correlated with the altered sterol composition [12].
• Feeding with the tetrapyrroles biliverdin, the proposed precursor of the phytochrome chromophore, or phycocyanobilin, which may replace the phytochrome chromophore, resulted in the rescue of ptr116 phototropism [13].
• CONCLUSIONS: The data suggest that Ca(2+) participates in the signalling stage of the BL-induced phototropism, whereas the phototropic bending response is linked to changes in the transport of H(+), K(+) and Cl(-) [14].

Gene context of Phototropism

• Arabidopsis NPH3: A NPH1 photoreceptor-interacting protein essential for phototropism [15].
• We found that mutations in the ARG1 locus of Arabidopsis thaliana alter root and hypocotyl gravitropism without affecting phototropism, root growth responses to phytohormones or inhibitors of auxin transport, or starch accumulation [16].
• A brassinosteroid-hypersensitive mutant of BAK1 indicates that a convergence of photomorphogenic and hormonal signaling modulates phototropism [17].
• Thus, PHYA and PHYB play a key role in mediating red-light-dependent positive phototropism in roots [18].
• Type 1 phototropin, one of the blue light receptors responsible for phototropism, is encoded in peas by at least two genes, PsPHOT1A and PsPHOT1B (formerly PsPK4 and PsPK5), both of which are more similar to Arabidopsis PHOT1 than to Arabidopsis PHOT2 [19].

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