The action of ET in the germination process remains unclear.
Possibilities include the promotion of radial-cell expansion and
the increase of the water potential in the embryonic axis, and the increase in seed respiration [42]. Maximal contents of ACC,
PsACO1-mRNA, Ps-ACO1 activity and ET production occur in
Pisum sativum seeds when radicle emergence is just completed
[64]. The implication of the last step of the ET biosynthesis
pathway in protrusion was also confirmed in other seeds
[1,53,60,65]. Notably, the late accumulation of PsACO1-
mRNA in the embryonic axis of pea seeds requires ET
synthesis; however, ET does not affect the expression of this
gene at the beginning of the germination process. According to
Brady and McCourt [66], it would be useful to determine
whether ET stimulates germination by altering GAs biosynthesis
or sensitivity, or whether it acts through its own production
or signalling components. Recently, Achard et al. [67] reported
that at least part of the growth-regulatory action of ET is
mediated via its effects on the DELLA proteins, which repress
growth in response to ET, auxin and GAs signals