NCED OE rootstocks have reduced gene expression for ABA receptors
and signalling components
Rootstock SlNCED1 overexpression (Figure 6a) was consistent with
transgene expression level in own-rooted plants (Thompson et al.2007b; Martínez-Andújar et al. 2020b), implying that
shoot-to-root signalling has little effect on constitutive
(root-specific in grafted plants) SlNCED expression. Although
bulk root ABA status did not increase in the adult plants (Figure 3a),
previously ABA in root exudates from approximately 7 week old detopped
plants (Thompson et al . 2007a), in root cultures (Thompsonet al . 2007b) and in bulk root tissue and xylem sap of younger
ungrafted plants (Martínez-Andújar et al . 2020b) was elevated; in
addition, in grafted plants the bulk root ABA was determined by the root
genotype and was elevated in SP5 and SP12 (Thompson et al.2007b). Therefore, the lack of bulk root ABA accumulation in this study
is consistent with increased export (Figure S1) and catabolism of ABA
(Figure 3b).
Many genes were down- or up-regulated in NCED OE rootstocks compared to
the WT grafts (Figure 5). Amongst those genes, 7 PYL ABA
receptors and 3 WRKY factors were downregulated in NCED OE roots,
suggesting decreased sensitivity to ABA, as in own-rooted plants grown
in optimal conditions (Martínez-Andújar et al. 2020b). Several
ABA PYR/PYL receptors are highly expressed in tomato roots
compared to other tissues (González-Guzmán et al., 2014), allowing root
system adaptation to low water potential including via modulation of
osmoregulation and architectural changes (Sharp et al. 2004; Des
Marais et al. 2012; Duan et al. 2013). For example,PYL8 plays an essential role in regulating root
ABA
sensitivity in Arabidopsis (Antoni et al. 2013), promoting
lateral root growth by enhancing MYB77 -dependent transcription of
auxin-responsive genes (Zhao et al. 2014). Loss-of-function of
several pyr/pyl loci impaired
ABA
signalling, causing a robust
ABA-insensitive
root phenotype (Park et al. 2009; González-Guzmán et al.2014). Thus, downregulation of PYL s in NCED OE rootstocks may
account for their limited root system development and sensitivity to
saline stress.
Despite the proposed decreased ABA sensitivity, genes involved in ABA
biosynthesis (FLC/AAO, Solyc07g066480) and signalling
(AREB , Solyc04g078840; ATHB12, Solyc01g096320) were only
slightly induced or not affected in either SP rootstock compared to WT.
Furthermore, transcriptomic and RT-qPCR data of stress-related genes
(TAS14 , Solyc02g084850; KIN2 , Solyc03g095510; LEA,Solyc03g116390 and some MYB s) indicate SP12 was more responsive
than WT rootstocks to low saline stress, with an attenuated response in
SP5. These different transcriptomic responses between SP rootstocks
mimicked the responses of own-rooted plants grown under optimal
conditions (Martínez-Andújar et al. 2020b), implying that each SP
rootstock senses a different stress intensity that optimizes
physiological responses according to both basal and NCED OE ABA levels.