Figure 10.

A model for the control of seed germination under canopy conditions. Upon seed imbibition, canopy conditions (FR conditions) inactivate phytochromes, thus repressing GA synthesis in an unknown manner while stabilizing PIL5, which also represses GA synthesis indirectly. In turn, PIL5 stimulates GAI and RGA transcription, which ensures that sufficient GAI and RGA protein will accumulate to elevate, together with RGL2, endogenous ABA levels. PIL5, GAI and RGA are represented by black ovals to emphasize that they are relevant only under FR conditions. DELLA proteins repress seed germination by (1) blocking testa rupture and (2) stimulating ABA levels, which may involve stimulation of XERICO expression as well as other unidentified factors (X). In turn, ABA blocks endosperm rupture by stimulating ABI3 and ABI5 expression and product activity. RGL2 (white oval) has a central role to repress germination because RGL2 mRNA levels are positively regulated by ABA. This ensures high RGL2 protein levels irrespective of light conditions (i.e. FR or white light conditions).