Revealing mechanistic basis of ameliorating detrimental effects of cadmium in cherry tomatoes by exogenous application of melatonin and brassinosteroids

Increased anthropogenic activities over the last decades have led to a gradual increase in cadmium content in the soil, which, due to its high mobility in soil, makes Cd accumulation in plants a serious threat to the health of animals and humans. Plant hormones including melatonin (Mel) and brassinosteroids (BR) are known to provide tolerance against various abiotic stresses. In this work, the role of combined and separate exogenous application of Mel and BR on Cd stress in cherry tomato plants was examined. Cd stress significantly reduced tomato growth by inducing oxidative stress and reduced K⁺ uptake in roots and shoots. Combined application of Mel and BR reduced detrimental effects of Cd in tomato by (i) reducing Cd accumulation in the shoot; (ii) increasing the activities of different antioxidants (SOD, CAT, APX, GR); (iii) triggering higher expression of genes relating to Cd vacuolar sequestration (Na⁺/H⁺ EXCHANGER, SlNHX1; NATURAL RESISTANCE-ASSOCIATED MACROPHAGE PROTEIN 6, SlNRAMP6), and Cd transport and detoxification (HEAVY-METAL-ASSOCIATED 3, SlHMA3; PLANT CADMIUM RESISTANT 2, SlPCR2); and (iv) improving plant K⁺ homeostasis and contents in root and shoot. The latter trait was associated with the reduced gene expression of K⁺-permeable outward rectifying channel (SlGORK3), and transcriptional upregulation of high affinity potassium transporter 5 (SIHAK5) under Cd stress. A separate application of Mel and BR showed tissue-specific regulation of tomato growth and Cd tolerance by regulating antioxidant activities, K⁺ uptake, Cd uptake, and translocation from root to shoot and their endogenous contents. Melatonin per se was more effective in improving Cd tolerance in shoot while beneficial BR effects were more pronounced in roots, and their combined application was effective in both tissues. Taken together, reported results show tissue-specific regulation of Cd tolerance by Mel and BR in cherry tomato plants and demonstrate the efficiency of combined Mel + BR treatment as a practical tool to reduce Cd accumulation and mitigate its negative effects on plant growth.