Abstract
To determine whether root-supplied ABA alleviates saline stress, tomato (Solanum lycopersicum L. cv. Sugar Drop) was grafted onto two independent lines (NCED OE) overexpressing the SlNCED1 gene (9-cis-epoxycarotenoid dioxygenase) and wild type rootstocks. After 200 days of saline irrigation (EC=3.5 dS m-1 ), plants with NCED OE rootstocks had 30% higher fruit yield, but decreased root biomass and lateral root development. Although NCED OE rootstocks upregulated ABA-signalling (AREB, ATHB12), ethylene-related (ACCs, ERFs), aquaporin (PIPs) and stress-related (TAS14, KIN, LEA) genes, downregulation of PYL ABA receptors and signalling components (WRKYs), ethylene synthesis (ACOs) and auxin-responsive factors occurred. Elevated SlNCED1 expression enhanced ABA levels in reproductive tissue while ABA catabolites accumulated in leaf and xylem sap suggesting homeostatic mechanisms. NCED OE also reduced xylem cytokinin transport to the shoot and stimulated foliar 2-isopentenyl adenine (iP) accumulation and phloem transport. Moreover, increased xylem GA3 levels in growing fruit trusses were associated with enhanced reproductive growth. Improved photosynthesis without changes in stomatal conductance was consistent with reduced stress sensitivity and hormone-mediated alteration of leaf growth and mesophyll structure. Combined with increases in leaf nutrients and flavonoids, systemic changes in hormone balance could explain enhanced vigour, reproductive growth and yield under saline stress.
Highlights
Limited water availability is a shared component of drought and salinity stresses that constrains crop growth and yield
Root xylem sap Abscisic acid (ABA) accumulation of SP12 rootstocks grafted to WT scions increased under control conditions compared to the WT rootstocks (22.8 vs. 5.8 ng mlÀ1, p < .01, respectively), but it was stable as salt concentrations increased from 35 to 100 mM NaCl (Figure 6b)
Grafting WT scions onto constitutively ABA-overproducing rootstocks produced local and systemic responses mediated by root-shoot communication
Summary
Root-targeted alteration of hormone metabolism and signalling has been proposed as a biotechnological strategy to overcome the effects of saline soils, and to enable this we must understand the specific adaptive roles of plant hormones (Albacete, MartínezAndújar, & Pérez-Alfocea, 2014; Ghanem et al, 2011) Crops dynamically regulate their root system architecture (RSA) in response to environmental stresses to fulfil their mineral and water requirements. Xylem and leaf ABA accumulation in tomato (Albacete, MartínezAndújar, Pascual, Acosta, & Pérez-Alfocea, 2008b; Li et al, 2018), it is not clear whether it directly controls plant responses, since other hormonal factors (such as the ethylene precursor ACC and the ratio ACC/ABA) co-varied with the productivity (biomass), photosynthetic parameters and WUE (Cantero-Navarro et al, 2016). We hypothesize that grafting a commercial tomato cultivar scion onto ABA over-producing tomato rootstocks would enhance growth and yield under saline conditions, potentially through multiple local and systemic mechanisms
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