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developing embryo. Overexpressing the gibberellin biosynthetic gene GA 20-oxidase re-
sulted in early sprouting in transgenic tubers (Carrera et al., 2000). Suttle (2004a) observed
that there is no difference between endogenous GA levels in the tubers from the beginning
to the end of the dormancy period. But at the end of dormancy during sprout growth, GAs
are rapidly synthesized. KNOTTED-like homeodomain proteins are involved in meristi-
matic growth. The expression of KNAP2 (a KNOTTED-like gene) is upregulated during
dormancy onset, but downregulated during the breaking of dormancy in apple (Brunel et al.,
2002). However, overexpression of a KNOTTED-like gene in potato reduced the level of
GA, resulting in dwarf plants (Faye et al., 2003).
Ethylene and ABA are involved in the senescence process and are also implicated in the
induction of endodormancy in several plant systems (Fedoroff, 2002). The role of ethylene
in endodormancy of potato microtubers is also known (Suttle, 1998a). Synergistic interac-
tions among ethylene, ABA, and phytochrome have been studied in sorghum and duckweed
(Finlayson et al., 1998; Weatherwax et al., 1998). Short-term ethylene treatment breaks dor-
mancy where as continuous treatment of ethylene promotes dormancy (Rylski et al., 1974).
Dormant microtubers when treated with ethylene inhibitors sprouted earlier than controls.
This condition can be reversed with ethylene treatment (Suttle, 2004b). Phytochrome has
long been known to be a key regulator of light responses in plants. Quantitative trait locus
analysis in poplar trees for dormancy induction is mapped to a region of the chromosome
that contains a phytochrome-encoding gene (Frewen et al., 2000).
19.3.4 Role of auxins
The concentration of auxins increases in tuber eyes before the onset of sprouting (Sorce et al.,
2000). Both auxin and GA functions overlap in cell expansion and tissue differentiation in
plants. Auxins positively affect GA signaling in two ways: by inducing the GA biosynthetic
gene (GA20 oxidase) expression in pea and tobacco and by promoting degradation of
DELLA proteins in Arabidopsis roots (Wolbang and Ross, 2001; Fu and Harberd, 2003;
Nemhauser et al., 2006). Increase in auxin concentration before dormancy break may be
upregulating the biosynthesis of GAs to facilitate sprouting in potato tubers.
19.3.5 Role of cytokinins
External application of cytokinins on dormant tubers breaks dormancy. The dormant buds or
eyes in potato tubers can be stimulated by cytokinins to sprout as they promote cell division.
Precocious sprouting was reported when the cytokinin biosynthesis gene expressed in potato
tubers (Ooms and Lenton, 1985). Overexpression of the Sho gene from Petunia hybrida
increased levels of cytokinins, which resulted in the loss of dormancy in tubers (Zubko
et al., 2005). An increased level of cytokinins and cis -zeatin was observed before sprout
initiation in tubers (Suttle, 1998b; Suttle and Banowetz, 2000). Suttle (2004a) suggested that
increasing sensitivity to cytokinins with dormancy progression is a result of an increase in
receptors for cytokinin perception and cytokinin signal transduction pathways. Cytokinins,
along with auxins and GA, act as dormancy-terminating agents in potato tubers.
Apart from above-mentioned hormones, brassinosteroids (Korableva et al., 2002) and
jasmonic acid (Abdala et al., 2000) have shown to be involved, but further research to un-
derstand their specific role in dormancy is needed. Recent successes using genomics-based
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