4.2. Cold response and the emergence of spatial patterns of ABA signal induction in apple subunits
Unlike mature fruit, early stage developing apple fruit are mostly seedless or have very few incomplete seeds. Assuming that endogenous ABA is mainly generated from seeds during fruit development (Yuan et al., 2003; Zhang et al., 2009), ABA-dependent signals for either cold response or abscission induction may not originate merely from fruit and it is likely that other tissues also contribute to the amplification of abscission induction signals.
Indeed, there was a difference in ABA signal induction among tissue types in cold-stressed apple subunits. Figure 5 indicates the tissue-specific response of ABA signal transduction. As a consequence of early cold stress response, the ABA biosynthesis gene (MdNCED1 ) was activated within 6 hr in the pedicel but not in branch or fruit. After 18 hr, delayed up-regulation of ABA biosynthesis gene were observed in these two tissues, whereas the pedicel showed the down-regulation of the gene. ABA can be transported to neighbouring tissues and possibly amplify its biosynthesis after being delivered from one tissue to another (Nonogaki et al., 2014). Therefore, the up-regulation of MdNCED1 signals in branch and fruit could be stimulated later by ABA which originated from pedicel after 6 hr as a result of the primary cold response.
The branch seems to exhibit a relatively high level of cold response among tissues. Initially, it showed high up-regulation of bothMdCS120-like and MdERF1 . CS120-like is a member of COR signalling (Fu et al., 2018) and ERF1 is also responsive to cold and is associated with accelerated abscission (Gao et al., 2019; Ma et al., 2014; Zhang et al., 2010). The CBF signal was found at a later time point, and there might be a previous signal to activate COR signalling at some time earlier than 6 hr, considering thatMdCS120-like expression was up-regulated at 6 hr in all tissue types.
Furthermore, we observed that the pedicel not only developed AZ cells during 168 hr of incubation but was also severely damaged by desiccation as a result of ex-vivo cold treatment (Figure 3). The response of this cold-driven dehydration had occurred at early time points with the increased expression of responsive genes (Figure 5b). Pedicel showed a significant up-regulation of MdRD22, which is responsive to both ABA and drought stress (Matus et al., 2014) in 6 hr, and MdWRKY57in 18 hr. WRKY57 is also known to activate ABA biosynthesis by binding directly to the NCED promoter (Jiang et al., 2014).