Light-Hormone Crosstalk Coordinates Multiple Stress Adaptation Pathways in Salt-Stressed Tomato Plants

Salt stress represents one of the major agricultural problems, limiting plant growth worldwide. The plant hormone abscisic acid (ABA) plays a crucial role in salt stress adaptation, and its level is regulated by light conditions. Our research was focused on studying the effect of light and ABA as well as their crosstalk, in plant responses to salt stress using the tomato ( Solanum lycopersicum ) photomorphogenic mutant high pigment 1 ( hp1 ), which carries a defect in the UV-DAMAGED DNA BINDING PROTEIN 1 (DDB1) locus. Early tomato seedlings of the wild type (WT) cv. Rutgers and hp1 were stressed with 150 mmol/L NaCl under different light conditions: dark, white light (WL), and blue light (BL). Under BL and compared to the corresponding cv. Rutgers, hp1 shoots exhibited enhanced salt tolerance, while root growth was compromised. Analyses of stable photosynthetic pigments and elevated phenolic content in the mutant also support BL-mediated enhancement of salt tolerance. Additionally, enhanced expression of ELONGATED HYPOCOTYL 5 ( HY5 ), particularly under BL, confirmed amplified light signalling in hp1 plants. Hormonal analysis revealed light-dependent interactions between ABA and 1-aminocyclopropane-1-carboxylic acid (ACC). Under salt stress, BL significantly increased ABA levels in the mutant through upregulated NCED1 and reduced CYP707A3 gene expression, while decreasing ACC content. Moreover, salt-stressed hp1 plants accumulated higher levels of stress-protective compounds (proline, polyamines) under all tested light conditions, correlating with increased expression of biosynthetic genes and precursor availability. Our findings demonstrate that amplified light signalling coordinates multiple stress adaptation pathways through shared metabolic precursors and hormonal interactions, with ABA signalling playing a central role. The pronounced responses under BL highlight its importance in these interactions. The tomato hp1 mutant proves to be an effective model for studying complex plant stress mechanisms contributing to stress-resistant crop development.