NCED OE rootstocks alter cytokinin status in the scion and affect root-shoot signalling
Plants grown on NCED OE rootstocks had lower xylem sap concentrations of bioactive CKs in the leaves and in fruit trusses (Figure 4; Table S1), supporting the operation of an antagonistic interaction with ABA (Gawronska, Deji, Sakakibara & Sugiyama 2003; Ghanem et al.2011a; Peleg & Blumwald 2011), consistent with the finding that NCED OE rootstocks have downregulated expression of CK-metabolic genes (Figure 7a). Despite this inhibited root-to-shoot CK signalling, shoot-to-root CK signalling was activated with phloem iP concentrations increasing, possibly a putative signal to restore root CK status (Hirose, Takei, … & 2008 2008; Matsumoto-Kitano et al. 2008). Moreover, changes in foliar iP accumulation in scions grafted on NCED OE rootstocks correlated with leaf area and AN(r = 0.85 and 0.73; P ≤ 0.01) and could also explain the changes in the leaf mesophyll structure, since this hormone preferentially accumulates in the leaf mesophyll and vascular bundles (Veselov et al. 2018). Indeed, both ABA and iP have been proposed as signalling components of the reticulate leaf phenotype inArabidopsis , where there is altered mesophyll structure and reduced CO2 fixation capacity (Lundquist, Rosar, Bräutigam & Weber 2014). These results suggest that the iP/ABA-mediated mesophyll alteration is favoring CO2 assimilation in this case, probably by facilitating its diffusion to the carboxylation sites into the cells (Flexas et al. 2012, 2013). Moreover, iP-type CKs have been related with xylem development and plant growth vigor and yield in tomato (Qi et al. 2020). Thus, ABA-CK interactions in rootstock-mediated improvement of the scion physiology require further investigation, especially since root-to-shoot CK-mediated plant vigor under salinity (Albacete et al. 2008a, 2009, 2014; Ghanem et al. 2011a) was associated with decreased ABA levels.