HO participates in ABA regulation of grafting-induced chilling tolerance in cucumber.

Rootstock provides more abscisic acid (ABA) content to scions to increase the chilling tolerance of seedlings. HO is involved in ABA regulation of grafting-induced chilling tolerance of cucumber. Here we examined the role of ABA in the response of grafted cucumber to chilling stress. The data showed chilling induced an increase in leaf and root ABA content and there was a positive correlation between ABA content and the chilling tolerance of the varieties. The increase of ABA content and NCED mRNA abundance in the leaf of both Cs/Cs (self-root) and Cs/Cm (grafted with pumpkin as rootstock) showed a delay under aerial stress compared with those under whole plant and root-zone stress. Intriguingly, an increase in ABA in xylem was found under whole-plant and root-zone chilling stress but was not detected under aerial stress, implying the increases in ABA content in leaves were mainly from root ABA transportation. Compared to Cs/Cs, a higher ABA content and NCED mRNA abundance were observed in Cs/Cm, which showed that Cm could output more ABA than Cs. The removal of endogenous ABA decreased the difference in chilling tolerance induced by Cm, as evidenced by the observed similar oxidative stress levels and photosynthetic capacity between Cs/Cs and Cs/Cm after chilling stress. Moreover, we found that the HO signal in grafted cucumber could respond to chilling stress earlier than the HO signal in self-rooted cucumber. The inhibition of endogenous HO decreased the chilling tolerance of grafted cucumber induced by ABA by reducing photosynthesis and the mRNA abundance of CBF1 and COR. Thus, our results indicate that HO, as the downstream signal, participated in the rootstock-induced chilling tolerance of grafted seedlings induced by ABA.