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.