4.4. Spatial responses leading to complex downstream signal
transduction for AZ formation
Phytohormone signal transduction affects the abscission process which
includes ethylene, ABA, jasmonic acid as positive regulators and auxin,
gibberellin as negative regulators (Cin et al., 2005; Estornell et al.,
2013). We found different signal responses among tissue types in terms
of abscission induction by cold, but the gene expression data were
difficult to understand clearly. Overall, the initial key activator for
the cold-inducible abscission signal seems to be ABA as its biosynthesis
was mainly amplified after cold stress (Figure 4). Given that ABA
hormone, being cold-inducible (Knight et al., 2004), mediates early
abscission induction of apple fruitlet (Giulia et al., 2013), it is no
wonder that ABA-dependent signalling may contribute to abscission
induction of early developing fruit as a result of cold response.
Initially, we found different responsive time for the expression of
phytohormone signals, such as ABA and ethylene, in the pedicel. Though
ABA biosynthesis (MdNCED1 ) was up-regulated in the early stage,
it was soon inhibited (Figure 4) and the signals for ethylene
biosynthesis (MdACS1 ) and reception (MdETR2 ) increased
significantly in the later stage (Figure 6a). This may imply that change
from ABA to ethylene signalling in the pedicel explains the abscission
induction signals and the fact that AZ cells also acquire the competence
to respond to ethylene signal during AZ development (Meir et al., 2019).
After cold stress, fruit showed an overall down-regulation of gene
expressions including both GA biosynthesis (MdGA20ox ) and polar
auxin transport (MdPIN1 ), whereas the other branch and pedicle
tissue did not exhibit such change (Figure 6). We believe that this
down-regulation may also contribute to signals of AZ formation by
potentially increasing the sensitivity to ethylene in the pedicel,
considering that the auxin signal in fruit affects the ethylene
sensitivity for AZ formation (Botton et al., 2011). Finally, the branch
showed more pronounced changes in gene expression after cold compared
with the other two tissues with regard to AZ formation as well as cold
response. Although all tissues might potentially respond to cold stress,
it seemed that the branch showed maintenance of the up-regulation of
downstream cold responses. After 18 hr of cold shock, the expressions ofMdCBF2 and MdLAC7 were the highest in branch than in other
tissues. The expression of these genes increased significantly from 6 to
18 hr in cold-stressed branch compared with that in the control.