Unexpected roles for cryptochrome 2 and phototropin revealed by high-resolution analysis of blue light-mediated hypocotyl growth inhibition

Isolation and characterization of a Chlamydomonas gene that encodes a putative blue-light photoreceptor of the phototropin family

In the search for a Chlamydomonas reinhardtii photoreceptor that may mediate blue-light-induced responses we identified a gene that encodes a protein with a structure typical for that of members of the phototropin family, i.e. two LOV domains that may function in flavin mononucleotide binding and a ser/thr kinase domain. The amino acid sequences of these domains are closely related to those of higher plant phototropins. This single-copy gene (Phot) encodes a protein with a calculated molecular mass of 81.4 kDa which is distinctly smaller than the homologous proteins of higher plants that exhibit molecular masses around 120 kDa. Expression analyses revealed rather constant levels of Phot mRNA and Phot protein in vegetative cells incubated in the dark and in cells undergoing gametogenesis. Only vegetative cells in the light showed a reduced expression of the Phot gene. Cell fractionation studies revealed that the protein is membrane-associated. In higher plants, phototropins were shown to be bound to the plasma membrane. However, the expression of a Phot-GFP gene fusion in tobacco protoplasts revealed an association of the fusion protein with the endogenous membrane network of the cell.

Action spectrum for cryptochrome-dependent hypocotyl growth inhibition in Arabidopsis

Cryptochrome blue-light photoreceptors are found in both plants and animals and have been implicated in numerous developmental and circadian signaling pathways. Nevertheless, no action spectrum for a physiological response shown to be entirely under the control of cryptochrome has been reported. In this work, an action spectrum was determined in vivo for a cryptochrome-mediated high-irradiance response, the blue-light-dependent inhibition of hypocotyl elongation in Arabidopsis. Comparison of growth of wild-type, cry1cry2 cryptochrome-deficient double mutants, and cryptochrome-overexpressing seedlings demonstrated that responsivity to monochromatic light sources within the range of 390 to 530 nm results from the activity of cryptochrome with no other photoreceptor having a significant primary role at the fluence range tested. In both green- and norflurazon-treated (chlorophyll-deficient) seedlings, cryptochrome activity is fairly uniform throughout its range of maximal response (390-480 nm), with no sharply defined peak at 450 nm; however, activity at longer wavelengths was disproportionately enhanced in CRY1-overexpressing seedlings as compared with wild type. The action spectrum does not correlate well with the absorption spectra either of purified recombinant cryptochrome photoreceptor or to that of a second class of blue-light photoreceptor, phototropin (PHOT1 and PHOT2). Photoreceptor concentration as determined by western-blot analysis showed a greater stability of CRY2 protein under the monochromatic light conditions used in this study as compared with broad band blue light, suggesting a complex mechanism of photoreceptor activation. The possible role of additional photoreceptors (in particular phytochrome A) in cryptochrome responses is discussed.

Phototropins 1 and 2: versatile plant blue-light receptors

Blue and ultraviolet-A light regulate a wide range of responses in plants, including phototropism, chloroplast migration and stomatal opening. However, the photoreceptors for these light responses have been identified only recently. The phototropins (phot1 and phot2) represent a new class of receptor kinases that appear to be exclusive to plants. Recent genetic analysis has shown that phot1 and phot2 exhibit partially overlapping functions in mediating phototropism, chloroplast migration, and stomatal opening in Arabidopsis. Although significant progress has been made in understanding the early photochemical and biochemical events that follow phototropin excitation, the details of how this excitation activates such different responses remain to be elucidated.

Opposing roles of phytochrome A and phytochrome B in early cryptochrome-mediated growth inhibition

The cryptochrome 1 (cry1) photoreceptor is responsible for the majority of the inhibitory effect of blue light on hypocotyl elongation, but phytochrome photoreceptors also contribute to the response through a phenomenon known as coaction. In Arabidopsis thaliana the participation of phytochromes A and B (phyA and phyB) in the early phase of cry1 action was investigated by determining the effects of phyA, phyB and hy1 mutations on a cry1-dependent membrane depolarization, which is caused by the activation of plasma-membrane anion channels within seconds of blue light treatment. High-resolution growth measurements were also performed to determine the timing of the requirement for phytochrome in cry1-mediated growth inhibition, which is causally linked to the preceding anion-channel activation. A null mutation in PHYA impaired the membrane depolarization and prevented the early cry1-dependent phase of growth inhibition as effectively and with the same time course as mutations in CRY1. Thus, phyA is necessary for cry1/cry2 to activate anion channels within the first few seconds of blue light and to suppress hypocotyl elongation for at least 120 min. This finding furthers the notion of an intimate mechanistic association between the cry and phy receptors in mediating light responses. The absence of phyB did not affect the depolarization or growth inhibition during this time frame. Instead, double mutant analyses showed that the phyB mutation suppressed the early growth phenotypes of both phyA and cry1 seedlings. This result is consistent with the emerging view that the prevailing growth rate of a stem is a compromise between light-dependent inhibitory and promotive influences. It appears that phyB opposes the cry1/phyA-mediated inhibition by promoting growth during at least the first 120 min of blue light treatment.

Photocontrol of stem growth

Rapid and measurable growth rate changes that occur in seedling stems upon illumination serve as an excellent means to analyze signal transduction. Growth kinetic studies have shown how red, far-red and blue light signals are transduced via the solitary and/or coordinated action of known plant photoreceptors. These reports are consistent with current findings describing light-induced photoreceptor interaction and compartmentation.