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Literature review
endogenous gibberellins increase during low temperature treatments (JONES &
STODDART, 1977).
Gibberellins (GA) promote the induction of cell wall hydrolases and thereby promote
endosperm weakening and endosperm rupture (MARION-POLL, 1997). Abscisic acid
(ABA) inhibits the induction of cell wall hydrolases and thereby inhibits this
weakening and rupture (MARION-POLL, 1997).
GA promotes and ABA inhibits the embryo growth potential. ABA, however, also
plays an important role in seed development and germination, acquisition of
desiccation tolerance, accumulation of proteins and lipid reserves, and induction and
maintenance of seed dormancy (MARION-POLL, 1997).
The expression of genes encoding enzymes that mobilise food reserves is induced
by several known GA signalling factors. These food reserves include the starches,
proteins and lipids that are stored in the endosperm (PENG & HARBERD, 2002).
During protein rehydration GA biosynthesis is induced by a phytochrome-mediated
light signal and the newly synthesised GA down-regulates the expression of protein
repressors of germination. These are also activated by rehydration, by protein
degradation, suppression of transcription or mRNA degradation (MARION-POLL,
1997). This newly synthesized GA also initiates signals to induce the expression of
hydrolytic enzymes that modify the cell wall and weaken the endosperm cap, thus
facilitating germination (PENG & HARBERD, 2002).
It has also been proposed that GA could promote the formation of low molecular
weight mono- and disaccharides, which assist the intracellular generation of negative
water potentials, thus aiding radicle emergence (TIAN et al., 2003).
2.8.4.3 Cytokinins and auxins
Cytokinins are involved in cell division and thus both radicle and cotyledon expansion
(EMERY & ATKINS, 2006). Cytokinins accumulates mainly in the endosperm of
seeds (LEUBNER-METZGER, 2006). Auxins are involved in the coordination of
correct cellular patterning after the globular stage in embryogenesis (LEUBNER-
METZGER, 2006).
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