Uncovering the Chitosan–Melatonin Axis in Sugarcane: First Report of Enhanced Tryptophan Decarboxylase and Tryptamine 5‐Hydroxylase Activities

ABSTRACT

Globally, chitosan and phyto‐melatonin have been explored as versatile bio‐stimulants for the induction of tolerance to multiple stresses and enhancing crop productivity sustainably. Its role has also been reported in eliciting and enhancing the different secondary metabolites in plants. Sugarcane is an efficient bioenergy crop for the production of sugar and different industrial products, as well as biofuel. The present study evaluates the effect of gamma‐irradiated chitosan on improvements in key parameters, viz., germination and physiological and biochemical traits in the sugarcane variety Co 86032 associated with enhanced melatonin biosynthesis at the settling stage. Concomitant with the accumulation of stress‐associated metabolites, application of 100 ppm gamma‐irradiated chitosan significantly enhanced phyto‐melatonin accumulation (2.3 ppm), primarily through the enhanced key enzyme activities in phyto‐melatonin biosynthesis, viz., tryptophan decarboxylase (63.25%) and tryptamine 5‐hydroxylase (36.54%), which further activated the metabolite cascade, resulting in significant enhancements of total auxin content (45.17%), germination percentage, leaf area, vigor index‐I, and vigor index‐II. Photosynthetic pigments also show marked improvements in chlorophyll a , chlorophyll b , total chlorophyll, and carotenoids. Furthermore, there were enhanced enzymatic antioxidants, viz., peroxidase, ascorbate peroxidase, chitinase, and non‐enzymatic antioxidants, viz., reducing sugars, total sugars and phenolics. Collectively, these findings suggest that gamma‐irradiated chitosan is a novel biostimulator that enhances endogenous phyto‐melatonin and auxins levels, settlings vigor, physiological efficiency, and stress tolerance in sugarcane. This study provides the first evidence in sugarcane of gamma‐irradiated chitosan–induced phyto‐melatonin accumulation coupled with upregulation of key phyto‐melatonin biosynthetic enzymes, offering an association of elicitor‐driven, climate‐resilient, and sustainable sugarcane production strategy.