Discovery[ edit ] Although Charles Darwin first documented plant responses to blue light in the s, it was not until the s that research began to identify the pigment responsible. In eukaryotes, cryptochromes no longer retain this original enzymatic activity. The Ramachandran plot  shows that the secondary structure of the CRY1 protein is primarily a right-handed alpha helix with little to no steric overlap. The molecule is arranged as an orthogonal bundle.
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Plant Physiol. Published online Dec 6. PICT to H. Boccalandro, who died suddenly after a traffic accident on December 10, The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors www. Boccalandro moc. Received Sep 15; Accepted Dec 4. All rights reserved. This article has been cited by other articles in PMC. Abstract Leaf epidermal peels of Arabidopsis Arabidopsis thaliana mutants lacking either phototropins 1 and 2 phot1 and phot2 or cryptochromes 1 and 2 cry1 and cry2 exposed to a background of red light show severely impaired stomatal opening responses to blue light.
In leaf epidermal peels, the blue light-specific effect saturates at low irradiances; therefore, it is considered to operate mainly under the low irradiance of dawn, dusk, or deep canopies. Conversely, we show that both phot1 phot2 and cry1 cry2 have reduced stomatal conductance, transpiration, and photosynthesis, particularly under the high irradiance of full sunlight at midday.
These mutants show compromised responses of stomatal conductance to irradiance. However, the effects of phot and cry on photosynthesis were largely nonstomatic. While the stomatal conductance phenotype of phot1 phot2 was blue light specific, cry1 cry2 showed reduced stomatal conductance not only in response to blue light, but also in response to red light. The levels of abscisic acid were elevated in cry1 cry2.
We conclude that considering their effects at high irradiances cry and phot are critical for the control of transpiration and photosynthesis rates in the field. The effects of cry on stomatal conductance are largely indirect and involve the control of abscisic acid levels. The stomata provide a key point of control the exchange of water and CO2 between the plant and the atmosphere. Stomatal conductance depends on the number of stomata per unit area and the aperture of the stomatal pore, and both are affected by the light environment.
High irradiances, typical of open places and seasons with clear skies, increase stomatal density Willmer and Fricker, ; Lake et al. The high red to far-red ratios of open places also increase stomatal density compared to the low ratios of dense vegetation canopies Boccalandro et al. These effects of irradiance Casson et al.
Cryptochrome 1 cry1 and 2 cry2 increase stomatal index Kang et al. In addition to its long-term effects on stomatal density, light has rapid effects on stomatal conductance by enhancing the aperture of the stomatal pore.
This light effect has two components, one sensitive to photosynthetically active radiation and the other responsive specifically to blue light Zeiger and Field, The first component is often called the red light effect Shimazaki et al.
The nature of the receptor involved in the red light effect has not been definitely established Baroli et al. In isolated epidermal peels of Arabidopsis Arabidopsis thaliana leaves, the blue light-induced promotion of the aperture of the stomatal pore is severely impaired in the phototropin1 phototropin2 phot1 phot2 double mutant Kinoshita et al.
Furthermore, stomatal conductance fails to respond to blue light in intact leaves of the phot1 phot2 mutant Doi et al. Since cry Cashmore et al. Cry would primarily function under relatively high fluence rates of blue light, whereas phot would function under both low and high fluence rates of blue light Mao et al.
The proposed function of cry1 and cry2 in blue light-induced stomata opening has been questioned. Ohgishi et al. Shimazaki et al. Differences between the stomatal responses in isolated epidermal peels compared to intact leaves are well documented and could result from the presence of the mesophyll in intact leaves Mott, Therefore, whether cry actually affects stomatal responses in intact leaves remains to be elucidated.
The effect of blue light on stomatal conductance is rapid and would reduce the limitation of photosynthesis by CO2 at dawn, when light reactions reach maximum rates faster than stomatal opening Zeiger and Field, ; Shimazaki et al. The blue light-dependent system would also be important for stomata opening under low irradiances, such as the understory of dense canopies, where light levels might not exceed the threshold for the red-light response Zeiger and Field, This idea is consistent with the promotion of plant growth capacity and photosynthetic rates by phot in low light environments Takemiya et al.
Based on these arguments, one would expect blue light photoreceptors to affect stomatal conductance of fully exposed, nonshaded plants, only at both extremes of the photoperiod. However, this prediction requires experimental evaluation. The aim of this article was to characterize the light response of stomatal conductance in intact leaves of the phot1 phot2 and cry1 cry2 double mutants to elucidate 1 whether cry is a photoreceptor involved in the blue light system that controls stomatal conductance of intact leaves, and 2 , the contribution of phot and cry to the diurnal dynamics of stomatal conductance, transpiration, and photosynthesis in plants grown under full natural radiation.
RESULTS Diurnal Pattern of Stomatal Conductance and Transpiration in cry1 cry2 and phot1 phot2 under Natural Radiation Arabidopsis wild-type and the double mutants cry1 cry2 and phot1 phot2 plants were grown under natural radiation in a glasshouse winter experiment, photoperiod of approximately 10 h; Fig.
The diurnal pattern of stomatal conductance Fig. In the wild type, maximum stomatal conductance and transpiration rate occurred at midday and were higher in summer than in winter. Since the proportion of blue light is higher at the extremes of the photoperiod and previous experiments with epidermal peels had shown saturation of the blue light effect at relatively low irradiances Mao et al.
Contrary to this, both the cry1 cry2 and phot1 phot2 reduced stomatal conductance mainly at midday, when irradiance was high Fig.
Positive phototropism is growth toward a light source, while negative phototropism also called skototropism is growth away from light. Several proteins use blue light to control various physiological processes in the plant. Phototropins and Physiological Responses Phototropins are protein-based receptors responsible for mediating the phototropic response in plants. Like all plant photoreceptors, phototropins consist of a protein portion and a light-absorbing portion, called the chromophore, which senses blue wavelengths of light. Phototropins belong to a class of proteins called flavoproteins because the chromophore is a covalently-bound molecule of flavin. Phototropins control other physiological responses including leaf opening and closing, chloroplast movement, and the opening of stomata. However, of all responses controlled by phototropins, phototropism has been studied the longest and is the best understood.
30.7C: Blue Light Response