4.5. Exogenous ABA application impacted the responses for
abscission induction but in a different manner from those of cold
response
Our ex-vivo experiments with apple subunits demonstrated that
cold stress induced ABA-dependent signals, especially in branch and
pedicel tissues. Although previous studies have shown that ABA
biosynthesis and signalling are controlled under environmental stress
signals, such as cold or drought in plants (Ji et al., 2011; Seiler et
al., 2011), it is worth noting that the responsive pattern of ABA
transporter quite differed between cold and exogenous ABA treatments. In
particular, MdABCG25 expression was significantly up-regulated in
branch and pedicel under cold stress while exogenous ABA treatment
rather maintained its down-regulation (Figure 7b). Perhaps this is
attributed to the ABA homeostasis through its negative feedback
regulation (Liu et al., 2016; Ma et al., 2018). In terms of ethylene
biosynthesis, cold stress contributed to increase MdACS1expression earlier in pedicel whereas exogenous ABA application resulted
in the delayed up-regulation in branch and fruit conversely. On the
other hand, the intensity of AZ development became greater when apple
subunits were treated together with cold stress and exogenous ABA
(Supporting Information Figure S4). These results imply that
cold-induced signals leading to abscission induction may occur in a
different manner than those typically amplified by ABA itself.
Furthermore, the correlation analysis of qRT-PCR genes indicated that
the expression of MdACS1 was consistently positively correlated
to MdLAC7, which mediates lignin polymerization in all branch and
pedicel, under cold stress regardless of exogenous ABA (Supporting
Information Figure S5). Among them, cold-stressed group without
exogenous ABA showed the significant correlations of cold responsive
gene MdCS120- like with other genes, such as MdABCG25 andMdLAC7 , in branch and pedicel. In particular, MdABCG25expression was positively correlated with MdCS120-like andMdLAC7 was rather negatively correlated. Besides, cold-stressed
fruit did not show the significant relationship betweenMdCS120-like and MdABCG25 while there were negative
correlations in fruits from exogenous ABA treatment and control. These
particular patterns of gene correlation may reflect the characteristics
of cold-inducible ABA-dependent signals leading for abscission
induction.
Based on our results, we propose a hypothetical scheme for early
induction of fruit abscission with spatial signals induced by cold
stress (Figure 8). As a primary cold response, ABA biosynthesis is
promoted first in pedicel and its biosynthesis is activated in
neighbouring branch and pedicel tissues, possibly as a consequence of
positive feedback, concurrent with the up-regulation of ABA transporter.
The cold-induced drought responses also appear to contribute to the
up-regulation of abscission signals. The signals for AZ formation are
stimulated largely from branch rather than pedicel, in terms of cell
wall modification. Fruit coordinates to the abscission induction with an
increase in ethylene sensitivity in the pedicel by inhibiting auxin flow
and gibberellin signals.
However, the molecular mechanisms of cold-induced abscission process
under ABA inhibition still remains to be further elucidated.