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.