Ethylene-related responses in NCED OE grafted plants
ABA signalling enables plants to maintain shoot and root growth in both well-watered and droughted tomato (Sharp et al., 2000, 2004; Dodd et al. 2009) and Arabidopsis (LeNoble et al. 2004) plants by suppressing ethylene production (Sharp et al. 2000; Spollenet al. 2000; LeNoble et al. 2004). Surprisingly, NCED OE rootstocks upregulated genes for biosynthesis of the ethylene precursor ACC (ACC2, Solyc01g095080; ACS1a, Solyc08g081540) and ethylene signalling (several ERFs ), while most genes responsible for the final step in ethylene biosynthetic genes (e.g. ACCO,Solyc07g049550; ACCO-like protein, Solyc12g006380) were down-regulated, especially in SP5 (Figure 7d). Yet, root and leaf phloem ACC concentrations were significantly reduced, as in own-rooted NCED OE plants (Martínez-Andújar et al. 2020b). Since diminished lower (lateral) root development in the NCED OE rootstocks is consistent with the phenotype of the ethylene overproducing mutant epinasticunder control (Negi, Sukumar, Liu, Cohen & Muday 2010) and saline (Ortiz 2017) conditions, higher up-regulation of ERF s in SP5 rootstocks may be involved (Figure 7d). Whether these local changes in ethylene response (and production) are involved in systemic signalling is less clear, as mature reproductive tissues of scions grafted on NCED OE rootstocks had increased ACC levels (Figure 4; Table S1). Overall, the differences existing between SP12 and SP5 lines suggest that complex ABA-ethylene interactions regulate root growth by altering ABA sensitivity and signalling, while long-distance ACC signalling cannot be ruled out.