The very-low-fluence and high-irradiance responses of the phytochromes have antagonistic effects on germination, mannan-degrading activities, and DfGA3ox transcript levels in Datura ferox seeds.

Seed germination can be promoted by the modes of action of two of the phytochromes: the low-fluence response (LFR), which is the classical red (R)-far-red (FR) reversible response and the very-low-fluence response (VLFR) that can be saturated by extremely low levels of Pfr, which can be elicited by a saturating FR pulse. The Datura ferox seed population used in this work had acquired the capacity to germinate through a VLFR after pretreatment in a water-saturated atmosphere (WSA) at constant 25 degrees C. After 12 d in WSA germination after a FR pulse was 82%, while it was less than 10% in darkness. It was found that the VLFR of germination is associated with increments in the embryo growth potential (EGP) and in the activity of two enzymes related to the weakening of the micropylar region of the endosperm (ME); endo-beta-mannanase and beta-mannosidase. The FR pulse also significantly stimulated the expression of DfGA3ox, a GA 3beta-hydroxylase, suggesting that the promotion of germination by the VLFR is associated with an increase in the synthesis of active gibberellins. The promotive action of the VLFR on germination is reduced when the FR pulse is immediately followed by a continuous FR treatment for 24 h (FRc). The effect of FRc cannot be reproduced by hourly FR pulses during the same period, showing that the antagonistic effect of FRc is a high-irradiance response (HIR). The action of the HIR in germination is associated with a decrease of both the mannan-degrading enzyme activity and the expression of DfMan in the ME, whereas no changes in the EGP were observed. The HIR also inhibits the accumulation of DfGA3ox in embryos, indicating that its action on germination is mediated, at least in part, through the modulation of active GA contents in seeds. This is the first report of a gene that participates in the VLFR-HIR antagonism in seeds.