3.7. Exogenous ABA application promotes the development of AZ
cells under cold and mediates signals leading to abscission induction
Our results revealed that cold stress impacted different tissue-specific
responses of ABA signalling which may potentially induce AZ formation.
If ABA-dependent signals originating from cold truly contribute to
abscission induction, exogenous ABA treatment on excised apple subunits
would also lead to the responses of AZ development and expression
patterns of relevant genes. To elucidate this, we conducted a secondex-vivo experiment with ABA and cold stress treatments.
First, we measured the expression of MdCS120-like , a member of
COR genes responsible for cold response. The expression ofMdCS120-like was the highest in pedicel with cold and ABA
treatment at 24 hr while the branch maintained its significant
up-regulation of the gene at all time points in the cold-stressed group
without ABA (Figure 7a). Next, we selected ABCG25 as the target
gene, an ABA transporter located at the plasma membrane mediating the
homeostasis of ABA. Previously, Park et al. (2016) demonstrated that the
activity of ABCG25 is controlled by stress conditions. If there
were differences in response between cold-induced ABA signalling and
that from exogenous ABA application, the expression of the ABA
transporter would differ among group as well. Indeed, our data showed
that MdABCG25 expression was significantly up-regulated in
cold-stressed pedicel at all time points, whereas ABA application
maintained its down-regulation regardless of cold stress (Figure 7b).
Then, to investigate the gene expression pattern of ethylene
biosynthesis, we compared the expression of MdACS1 . The
expression was significantly up-regulated in pedicel under cold at all
time points (Figure 7c). Meanwhile, in the branch tissue, the expression
gap was higher in ABA treated group compared to that of cold-stressed
after 12 hr of incubation. In terms of cell wall modification, we
investigated MdLAC7 expression among groups (Figure 7d). Although
the significant up-regulation appeared in the cold-stressed branch early
from 2 hr to 12 hr of incubation, the highest expression was shown after
24 hr in ABA treated group.
Moreover, we examined the development of AZ cortical cells at proximal
part of pedicel in each group (Supporting Information Figure S4).
Interestingly, exogenous ABA application induced the development of
cytoplasmic vesicles at similar level to the one in cold stress. The
cytoplasmic streaming of vesicles was extensively formed in both groups
under cold and ABA treatments. Taken together, these data reflect that
ABA signalling induced by cold would possibly mediate the abscission
induction of early developing apple.