Ethylene is a key plant hormone regulating plant development processes, but is often associated with growth inhibition and senescence, particularly under abiotic stress conditions such as salinity. While ethylene is synthesized in different plant tissues, root-derived ethylene plays a critical role as a stress sensor and signalling molecule, mediating roots-to-shoots communication. To examine whether root-sourced ethylene influences salinity responses, a tomato variety ( Solanum lycopersicum L. cv. UniDarkwin) was grafted onto three rootstocks differing in ethylene production: an ethylene-overproducing mutant (epinastic, epi), a root-specific ACC-deaminase overexpressing transgenic line with reduced ethylene synthesis ( ACCD), and the wild-type cv. Micro-Tom (WT). After 100 days of salt treatment (75 mM NaCl), ACCD grafts exhibited reduced concentrations of the ethylene-precursor 1-aminocyclopropane-1-carboxylic acid (ACC) and enhanced plant growth and yield stability, whereas epi and WT grafts were most adversely affected. ACCD rootstocks modulated both local and systemic mechanisms, promoting a larger and more branched root system, altered leaf gene expression and hormone metabolism, diminished Na + toxicity, and improved leaf P, Ca, and S nutrition, photosynthesis, and source-sink relations. Reduced root-sourced ethylene also decreased stress sensitivity by lowering leaf abscisic acid (ABA) concentration through altering its metabolism, limiting jasmonic acid (JA) and Ca translocation from leaves to flowers, and favouring the activity of growth-promoting and anti-senescing hormones cytokinins (CKs) and gibberellins (GAs) under salinity. These findings highlight the role of root-derived ethylene in modulating local and systemic responses to stress and its potential for improving crop yield stability under salinity.
