Figure 8.

A Model of the Light Regulation of Germination in the Grasses and Arabidopsis.

In grasses BL, FR, and R wavelengths are able to regulate germination in dormant grains. In Arabidopsis, R and FR wavelengths have a key role in regulating germination, and a PHYB-mediated BL effect has been also detected, although no relationship with seed dormancy has been reported. Black-dotted line: In wild grasses such as B. distachyon, strong effects of R (promoting germination) and of FR and BL (inhibiting germination) are detectable in dormant grains. Blue-dotted line: In domesticated grasses such as wheat and barley, the existence of R and FR effects has not been reported, while the BL inhibitory effect is conserved. Red-dotted line: In Arabidopsis and other dicots, R and FR had a reversible effect on seed germination driven by changes in the PHYB conformation (Pfr and Pr forms), and BL can partially mimic the FR effect. In cereal grains, BL is perceived by CRY1, and possibly also by other photoreceptors such as PHYB, inducing the expression of the NCED1 gene, while repressing ABA8’OH-1, which stabilizes the ABA content in the embryo, blocking germination. FR and R wavelengths are perceived by PHYB and by other PHY family members. In grasses, the targets of R and FR are not known, but in Arabidopsis, FR induces the expression of NCED6 and NCED9, while R blocks their expression, increasing or decreasing ABA levels, respectively.