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.