Phot1 and phot2 mediate blue light-induced transient increases in cytosolic Ca2+ differently in Arabidopsis leaves.
(33/131)
Phototropins (phot1 and phot2) are blue light (BL) receptors that mediate phototropism, chloroplast movements, and stomatal opening in Arabidopsis thaliana. Physiological studies have suggested that Ca2+ in the cytoplasm plays a pivotal role in these BL-induced responses. A phot1-mediated increase in cytosolic Ca2+ was reported in deetiolated seedlings of A. thaliana; however, the contribution of phot2 remains unknown. We examined a BL-induced transient increase in cytosolic free Ca2+ in leaves of transgenic A. thaliana of WT plants, phot1 and phot2 mutants, and phot1 phot2 double mutants expressing the Ca2+-sensitive luminescent protein aequorin. phot1 and phot2 had different photosensitivities: phot1 increased cytosolic Ca2+ at lower fluence rates (0.1-50 micromol x m-2 x s-1) and phot2 increased it at higher fluence rates (1-250 micromol x m-2 x s-1). By using Ca2+ channel blockers, Ca2+ chelating agents, and inhibitors of phospholipase C, we further demonstrated that both phot1 and phot2 could induce Ca2+ influx from the apoplast through the Ca2+ channel in the plasma membrane, whereas phot2 alone induced phospholipase C-mediated phosphoinositide signaling, which might result in Ca2+ release from internal Ca2+ stores. These results suggest that phot1 and phot2 mediate the BL-induced increase in cytosolic free Ca2+ differently. (+info)
Second positive phototropism results from coordinated co-action of the phototropins and cryptochromes.
(34/131)
Phototropism and hypocotyl growth inhibition are modulated by the coaction of different blue-light photoreceptors and their signaling pathways. How seedlings integrate the activities of the different blue-light photoreceptors to coordinate these hypocotyl growth responses is still unclear. We have used time-lapse imaging and a nontraditional mathematical approach to conduct a detailed examination of phototropism in wild-type Arabidopsis and various blue-light photoreceptor mutants. Our results indicate that high fluence rates of blue light (100 micro mol m(-)(2) s(-)(1)) attenuate phototropism through the coaction of the phototropin and cryptochrome blue-light photoreceptors. In contrast, we also demonstrate that phototropins and cryptochromes function together to enhance phototropism under low fluence rates (<1.0 micro mol m(-)(2) s(-)(1)) of blue light. Based on our results, we hypothesize that phototropins and cryptochromes regulate phototropism by coordinating the balance between stimulation and inhibition of growth of the hypocotyl depending on the fluence rate of blue light. (+info)
From seed to seed: the role of photoreceptors in Arabidopsis development.
(35/131)
As sessile organisms, plants have evolved a multitude of developmental responses to cope with the ever-changing environmental conditions that challenge the plant throughout its life cycle. Of the many environmental cues that regulate plant development, light is probably the most important. From determining the developmental pattern of the emerging seedling, to influencing the organization of organelles to best maximize energy available for photosynthesis, light has dramatic effects on development during all stages of plant life. In plants, three classes of photoreceptors that mediate light perception have been characterized at the molecular level. The phytochromes recognize light in the red portion of the spectrum, while cryptochromes and phototropins perceive blue and UVA light. In this review, we discuss the different aspects of development that are regulated by these photoreceptors in the model plant species Arabidopsis thaliana and how the phytochromes, cryptochromes, and phototropins bring about changes in development seen in the growing plant. (+info)
Blue-light- and phosphorylation-dependent binding of a 14-3-3 protein to phototropins in stomatal guard cells of broad bean.
(36/131)
Phototropins are blue-light (BL) receptor serine (Ser)/threonine kinases, and contain two light, oxygen, and voltage (LOV) domains, and are members of the PAS domain superfamily. They mediate phototropism, chloroplast movement, leaf expansion, and stomatal opening of higher plants in response to BL. In stomatal guard cells, genetic analysis has revealed that phototropins mediate activation of the plasma membrane H+-ATPase by phosphorylation and drive stomatal opening. However, biochemical evidence for the involvement of phototropins in the BL response of stomata is lacking. Using guard cell protoplasts, we showed that broad bean (Vicia faba) phototropins (Vfphots) were phosphorylated by BL, and that this phosphorylation of Vfphots reached to the maximum level earlier than that of the H+-ATPase. Phosphorylation of both Vfphots and H+-ATPase showed similar sensitivity to BL and were similarly suppressed by protein kinase and flavoprotein inhibitors. We found that a 14-3-3 protein was bound to Vfphots upon phosphorylation, and this binding occurred earlier than the H+-ATPase phosphorylation. Vfphots (Vfphot1a and Vfphot1b) were expressed in Escherichia coli, and phosphorylation sites were determined to be Ser-358 for Vfphot1a and Ser-344 for Vfphot1b, which are localized between LOV1 and LOV2. We conclude that Vfphots act as BL receptors in guard cells and that phosphorylation of a Ser residue between LOV1 and LOV2 and subsequent 14-3-3 protein binding are likely to be key steps of BL response in stomata. The binding of a 14-3-3 protein to Vfphot was found in etiolated seedlings and leaves in response to BL, suggesting that this event was common to phototropin-mediated responses. (+info)
Primary inhibition of hypocotyl growth and phototropism depend differently on phototropin-mediated increases in cytoplasmic calcium induced by blue light.
(37/131)
The phototropin photoreceptors transduce blue-light signals into several physiological and developmental responses in plants. A transient rise in cytoplasmic calcium (Ca2+) that begins within seconds of phototropin 1 (phot1) excitation is believed to be an important element in the transduction pathways leading to one or more of the phot1-dependent responses. The goal of the present work was to determine whether the Ca2+ response was necessary for (a). the inhibition of hypocotyl elongation that develops within minutes of the irradiation, and (b). hypocotyl phototropism (curved growth of the stem in response to asymmetric illumination). After determining that pulses of light delivering photon fluences of between 1 and 1000 micromol m-2 induced growth inhibition mediated by phot1 without significant interference from other photosensory pathways, the effect of blocking the Ca2+ rise was assessed. Treatment of seedlings with a Ca2+ chelator prevented the rise in cytoplasmic Ca2+ and prevented phot1-mediated growth inhibition. However, the same chelator treatment did not impair phot1-mediated phototropism. Thus, it appears that the early, transient rise in cytoplasmic Ca2+ is an important intermediary process in at least one but not all phot1-signaling pathways. (+info)
A genomic analysis of the shade avoidance response in Arabidopsis.
(38/131)
Plants respond to the proximity of neighboring vegetation by elongating to prevent shading. Red-depleted light reflected from neighboring vegetation triggers a shade avoidance response leading to a dramatic change in plant architecture. These changes in light quality are detected by the phytochrome family of photoreceptors. We analyzed global changes in gene expression over time in wild-type, phyB mutant, and phyA phyB double mutant seedlings of Arabidopsis in response to simulated shade. Using pattern fitting software, we identified 301 genes as shade responsive with patterns of expression corresponding to one of various physiological response modes. A requirement for a consistent pattern of expression across 12 chips in this way allowed more subtle changes in gene expression to be considered meaningful. A number of previously characterized genes involved in light and hormone signaling were identified as shade responsive, as well as several putative, novel shade-specific signal transduction factors. In addition, changes in expression of genes in a range of pathways associated with elongation growth and stress responses were observed. The majority of shade-responsive genes demonstrated antagonistic regulation by phyA and phyB in response to shade following the pattern of many physiological responses. An analysis of promoter elements of genes regulated in this way identified conserved promoter motifs potentially important in shade regulation. (+info)
Light signals, phytochromes and cross-talk with other environmental cues.
(39/131)
Plants have evolved highly complex sensory mechanisms to monitor their surroundings and adapt their growth and development to the prevailing environmental conditions. The integration of information from multiple environmental cues enables the co-ordination of development with favourable seasonal conditions and, ultimately, determines plant form. Light signals, perceived via the phytochrome, cryptochrome and phototropin photoreceptor families, are especially important environmental signals. Redundancy of function among phytochromes and their interaction with blue light photoreceptors enhance sensitivity to light signals, facilitating the accurate detection of, and response to, environmental fluctuations. In this review, current understanding of Arabidopsis phytochrome functions will be summarized, in particular, the interactions among the phytochromes and the integration of light signals with directional and temperature sensing mechanisms. (+info)
High pigment1 mutation negatively regulates phototropic signal transduction in tomato seedlings.
(40/131)
Phototropins and phytochromes are the major photosensory receptors in plants and they regulate distinct photomorphogenic responses. The molecular mechanisms underlying functional interactions of phototropins and phytochromes remain largely unclear. We show that the tomato (Lycopersicon esculentum) phytochrome A deficient mutant fri lacks phototropic curvature to low fluence blue light, indicating requirement for phytochrome A for expression of phototropic response. The hp1 mutant that exhibits hypersensitive responses to blue light and red light reverses the impairment of second-positive phototropic response in tomato in phytochrome A-deficient background. Physiological analyses indicate that HP1 functions as a negative regulator of phototropic signal transduction pathway, which is removed via action of phytochrome A. The loss of HP1 gene product in frihp1 double mutant allows the unhindered operation of phototropic signal transduction chain, obviating the need for the phytochrome action. Our results also indicate that the role of phytochrome in regulating phototropism is restricted to low fluence blue light only, and at high fluence blue light, the phytochrome A-deficient fri mutant shows the normal phototropic response. (+info)