Figure 1.
Effect of increasing intensities of white light on hypocotyl elongation of wild-type, phyA-501 and phyB-9 seedlings.
(a) Seedlings of the indicated genotypes were grown from the day of germination until day 7 under W of increasing intensities (photosynthetic active radiation, PAR, between 4.6 and 72.9 µmol·m−2·s−1; R:FR>2.0). (b) Hypocotyl length of seedlings grown as indicated in a. Values are means ± SE of at least 25 hypocotyls for each light treatment. Asterisks indicate significant differences (*P<0.05, **P<0.01) relative to the control grown under the same light intensity. (c) Representative seedlings, grown as indicated in a, are shown for the three genotypes analyzed.
Figure 2.
Effect of different R:FR on hypocotyl elongation of wild-type, phyA-501 and phyB-9 seedlings under low, medium or high PAR.
(a) Seedlings were germinated and grown for 2 days under W light and then either kept in W or transferred to W supplemented with increasing amounts of FR for 5 more days. Hypocotyl length of seedlings grown as indicated in a under (b) low, (c) medium and high (d) PAR. The amount of PAR is given at the top of each section. The type of R:FR applied (nomenclature provided in Table S1) in the given W+FR treatments is indicated at the top, of the graphs; the R:FR value of each experiment is indicated at the top of each graph. In b, c and d, values are means ± SE of at least 25 hypocotyls for each light treatment. Asterisks indicate significant differences (*P<0.05, **P<0.01) relative to the control (Col-0) grown under the same light conditions.
Figure 3.
Effect of time of W+FR treatment on hypocotyl elongation of wild-type, phyA-501 and phyB-9 seedlings under low PAR.
(a) Hypocotyl length of seedlings germinated and grown for 2, 3, 4 or 7 days under W light and then transferred to W+FR for 5, 4, 3 or 0 more days, respectively. (b) Hypocotyl length of seedlings germinated and grown for 7 days under W light and then either kept in W or transferred to W+FR for 1 more day. PAR was of 15–16 µmol·m−2·s−1 and R:FR of 0.059. Values are means ± SE of at least 25 hypocotyls for each light treatment. Asterisks indicate significant differences (*P<0.05, **P<0.01) relative to the same genotype (Col-0) grown for 2 days under W and 5 days under W+FR.
Figure 4.
Effect of phyA and phyB mutations on the temporal evolution of the hypocotyl length.
(a) Seeds were germinated and grown for 2 days under W (PAR was of 20–25 µmol·m−2·s−1) and then either kept under W (phyB seedlings) or transferred to W+FR (R:FR = 0.038) for 5 more days (Col-0 and phyA seedlings). Circles indicate the days on which hypocotyls were measured. (b) Hypocotyl length of seedlings grown as indicated in a. Values are means ± SE of at least 25 hypocotyls for each light treatment. Asterisks indicate significant differences (*P<0.05, **P<0.01) relative to the wild type seedlings grown under the corresponding light conditions.
Figure 5.
Phytochrome A is stabilized by white light of very low R:FR.
(a) phyA levels were assayed in extracts from Col-0 seedlings grown in darkness for 5 days and then either transferred to W or W+FR for 4 hours. Circles indicate the harvest time of the plant material. (b) Representative steady-state levels of phyA (upper panel) and tubulin (TUB, lower panel) in extracts from seedlings grown as indicated in a,. Bands were detected by immunoblot using the phyA-specific mAb 073D or a TUB-specific mAb. TUB was used as a loading control. (c) Relative levels of phyA normalized to TUB in seedlings differentially grown for 4 h under W or W+FR, as indicated in a; n = 3 independent biological replicas; the P-value between the W and W+FR treated samples was 0.053.
Figure 6.
Model depicting the antagonistic effect of phyA and phyB on the shade-induced hypocotyl elongation.
(a) Under low plant density, the high R:FR induces phyA degradation and stimulates the phyB active Pfr form, which strongly inhibits hypocotyl elongation. (b) In close proximity of vegetation, phyA degradation still occurs, but the low R:FR displaces the photoequilibrium of phyB towards the inactive Pr form, causing hypocotyls to elongate. (c) Under a plant canopy, the low or very low R:FR still displaces the photoequilibrium of phyB towards the inactive Pr form that stimulates hypocotyls to elongate. However, under these conditions phyA is stabilized, particularly at the beginning of the seedling emergence; as a consequence, phyA signaling is enhanced, thereby counteracting the inhibitory effect of the absence of active phyB, so that hypocotyls elongate only moderately. (d) Summary of phenotypes shown by the wild-type, phyA and phyB seedlings growing under the light conditions indicated in a, b and c.