Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

396 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
opposed to cytokinins and GAs that are needed to terminate dormancy (Suttle and Haltstrand,
1994). Recently, receptors for both GA and ABA were identified and cloned from Arabidopsis.
GID1 is a soluble protein receptor of GA and interacts with DELLA protein SLR1 resulting
in degradation of SLR1 through the 26S proteasome pathway. Degradation of DELLA
protein leads to altered responses to GA (Ueguchi-Tanaka et al., 2005). DELLA proteins
are a family of negative regulators of GA responses. ABA receptor FCA is a plant-specific
RNA-binding protein localized in the nucleus. FCA interacts with FY factor (FLOWER-
ING LOCUS Y) and downregulates FLC (FLOWERING LOCUS C). ABA binding prevents
formation of the active FCA-FY complex required for FLC repression (Razem et al., 2006).
Dormancy is regulated largely by both internal (hormones and sugar) and external
signals (light and temperature). The molecular biology of endodormancy has been studied
in potato, poplar (Populus deltoids) (Hsu et al., 2006), and grape (Vitus vinifera) (Or et al.,
2000). GA and ABA play antagonistic roles in the regulation of dormancy. Moreover, the
antagonistic relationship and the ratio between these two hormones may be responsible for
regulation of the transition from dormancy to germination and sprouting in seeds (Razem
et al., 2006). Distinct mechanisms of interaction between GA and ABA are utilized for
different developmental decisions in plants, and these interactions may be organ specific
(Weiss and Ori, 2007). Recent work by Achard et al. (2004, 2007) and Reyes and Chua
(2007) showed the complex interaction between ABA and GA. These hormones act through
a common mediator MYB33. MYB33 promotes ABA responses in seeds and GA responses
in flowers. MicroRNA159 (miR159) induced by both GA and ABA targets MYB33.
19.3.2 Role of ABA
ABA is involved in suppressing α-amylase expression mediated by Ser/Thr protein kinase,
PKABA1 (Gómez-Cadenas et al., 1999). ABA is known as a plant stress hormone
induced during both cold and drought stress (Gilmour and Thomashow, 1991). Cold- or
drought-induced ABA accumulation blocks further growth and development through the
cyclin-dependent kinase inhibitor gene (ICK1) to prevent cell division in buds (Wang et al.,
1997). Inhibitor studies with fluridone using microtuber system resulted in early sprouting.
External application of ABA restored tuber dormancy, and establishes the role of ABA in
induction and maintenance of tuber dormancy (Suttle and Haltstrand, 1994). Three of eight
quantitative trait loci on dormancy also mapped to ABA levels, further confirming ABA’s
role in tuber dormancy (Claassens and Vreugdenhil, 2000). During postharvest storage,
ABA levels decrease as the number of days increases (Suttle, 1995). Sorce et al. (1996)
reported that during dormancy there is an increase in ABA levels in tuber eyes that decrease
once the sprouting process is initiated. Recently, Destefano-Beltrán et al. (2006a) by using
quantitative reverse transcriptase-polymerase chain reaction studied the expression levels
of genes involved in both biosynthetic and catabolic pathways of ABA. As dormancy sets
in, there is an increase in the biosynthesis of ABA. As time progresses and dormancy ends,
degradative enzymes are more active to reduce the ABA concentration in tubers (Destefano-
Beltrán et al., 2006b).
19.3.3 Role of GA
During cereal seed germination, GAs induce the transcription of α-amylases and other
hydrolytic enzymes to hydrolyze starch and proteins, thereby supplying nutrients to the