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.