Cryptochrome as a sensor of the blue/green ratio of natural radiation in Arabidopsis

Abstract

Green light added to blue light has been proposed to shift cryptochromes from their semireduced active form to the reduced, inactive state. Whether the increased proportion of green light observed under leaf canopies compared to open places reduces cryptochrome-mediated effects remained to be elucidated. Here we report that the length of the hypocotyl of Arabidopsis (Arabidopsis thaliana) seedlings grown under controlled conditions decreased linearly with increasing blue/green ratios of the light within the range of ratios found in natural environments. This effect was stronger under higher irradiances. We developed a model, parameterized on the basis of field experiments including photoreceptor mutants, where hypocotyl growth of seedlings exposed to different natural radiation environments was related to the action and interaction of phytochromes and cryptochromes. Adding the blue/green ratio of the light in the term involving cryptochrome activity improved the goodness of fit of the model, thus supporting a role of the blue/green ratio under natural radiation. The blue/green ratio decreased sharply with increasing shade by green grass leaves to one-half of the values observed in open places. The impact of blue/green ratio on cryptochrome-mediated inhibition of hypocotyl growth was at least as large as that of irradiance. We conclude that cryptochrome is a sensor of blue irradiance and blue/green ratio.

Figure 1.

Photoreceptor redundancy increases with the irradiance of natural radiation environments. The length of the hypocotyl of 8-d-old seedlings of Arabidopsis is plotted against the average midday irradiance (400–800 nm) to which they were exposed in field experiments (expressed as a percentage of full sunlight under the experimental conditions). Note that the difference between the wild type and the phyA phyB or the cry1 cry2 double mutants decreases under well-illuminated conditions, indicating that the photoreceptors that remain in each double mutant are almost sufficient to produce wild-type inhibition of hypocotyl growth under strong irradiance. Data are means and se of six replicate boxes. Ler, Landsberg erecta. [See online article for color version of this figure.]

Figure 2.

The contribution of phyA and phyB to the inhibition of hypocotyl growth under different red/far-red ratios. The length of the hypocotyl of 8-d-old seedlings of Arabidopsis is plotted against the red/far-red ratio to which they were exposed in field experiments under sunlight in combination with selective filters. Note that the phyA phyB double mutant is unaffected by light, indicating that no other phy contributes significantly to the inhibition of hypocotyl growth by red and far-red light. The difference between the phyA and phyA phyB mutants reveals the contribution of phyB, which increases with red/far-red ratios above 0.3. The difference between the phyB and phyA phyB mutants reveals the contribution of phyA, which is largely unaffected by red/far-red ratio in the range tested here. Data are means and se of 16 replicate boxes. Ler, Landsberg erecta. [See online article for color version of this figure.]

Figure 3.

Broad-band green light inhibits hypocotyl growth of phyA phyB mutant (A–C) or wild-type (D–F) seedlings when compared to darkness but blue plus green mixtures are less effective than predicted by the actions of blue and green alone. A, Hypocotyl length of 4-d-old seedlings of phyA phyB (the mutant is used to reveal cry action) against the log photon irradiance of continuous blue light (P < 0.0001) or green light (P < 0.05) under controlled conditions. B, Hypocotyl length against the log photon irradiance of continuous blue light or green light transformed into blue light equivalents (i.e. green light divided by the ratio between the slope for blue and for green light in A). Note that while data from seedlings treated with either green or blue light alone fall on the same line, seedlings exposed to blue plus green mixtures tend to show longer hypocotyls. C, Hypocotyl length of phyA phyB seedlings exposed to blue plus green mixtures plotted against the blue/green ratio of the mixture (P < 0.0001). D, Hypocotyl length of the wild type against the log photon irradiance of continuous blue (P < 0.0001) or green light (P < 0.05). E, Hypocotyl length of the wild type against the log photon irradiance of continuous blue light or green light transformed into blue light equivalents. F, Hypocotyl length of the wild type plotted against the blue/green ratio of the mixture (P < 0.0001). Data are means and se (omitted in B and E for clarity) of three replicate boxes. The slopes and their se are indicated. Ler, Landsberg erecta. [See online article for color version of this figure.]

Figure 4.

Inhibition of hypocotyl growth by high, compared to low blue/green ratios within the range of natural radiation. Four-day-old seedlings of the phyA phyB mutant (A) or of the wild type (B) grown under different irradiances of continuous light of two contrasting blue/green ratios in controlled conditions. Hypocotyl length of the phyA phyB cry1 triple mutant under log blue light equivalent of 0.8 was 0.99 ± 0.01 and 0.89 ± 0.02 for blue/green ratios of 1.1 and 0.5, respectively. Data are means and se (whenever larger than the symbols) of eight replicate boxes. Ler, Landsberg erecta. [See online article for color version of this figure.]

Figure 5.

Blue light irradiance and blue/green ratio are not correlated. R² = 0.05, P > 0.3. Data correspond to the 20 field stations (Supplemental Table S1; Supplemental Fig. S1). Photon irradiances are expressed as a percentage of sunlight values under unshaded conditions. Data are means and se of four measurements. [See online article for color version of this figure.]

Figure 6.

Parametization, analysis of the goodness of fit, and validation of the model stage II. A, Observed versus predicted values of hypocotyl length for the data used to parametize the model. The embedded table shows the estimated coefficients of the model and their significance. B, Validation: observed versus predicted values for an independent set of data. The observed-predicted regression lines and the 1:1 lines (dashed) are included. Ler, Landsberg erecta. [See online article for color version of this figure.]

Figure 7.

Response of the blue/green ratio to increasing degrees of shade by a green canopy of P. dilatatum. The blue/green ratio, the log photon irradiance of blue light expressed as a percentage of the values above the canopy and the red/far-red ratio were calculated from the same scans and are plotted against the leaf area index above the sensor. [See online article for color version of this figure.]