Physiological and molecular analysis of pitaya (Hylocereus polyrhizus) reveal up-regulation of secondary metabolites, nitric oxide, antioxidant defense system, and expression of responsive genes under low-temperature stress by the pre-treatment of hydrogen peroxide

Low-temperature events are one of the leading environmental cues that considerably reduce plant growth and shift species biodiversity. Hydrogen peroxide (H₂O₂) is a signaling molecule that has a distinguished role during unfavorable conditions and shows outstanding perspectives in low-temperature stress. Herein, we elucidated the protective role and regulatory mechanism of H₂O₂ in alleviating the deleterious effects of low-temperature stress in pitaya plants. Micropropagated pitaya plants were cultured in Murashige and Skoog media supplemented with different levels of H₂O₂ (0, 5, 10, and 20 mM) and then exposed to low-temperature stress (5 °C for 24 h). H₂O₂ at 10 mM, improved low-temperature stress tolerance by relieving oxidative injuries and ameliorating growth parameters in terms of fresh weight (66.7%), plant length (16.7%), and pigments content viz., chlorophyll a (157.4%), chlorophyll b (209.1%), and carotenoids (225.9%). H₂O₂ counteracted the low-temperature stress by increasing amino acids (224.7%), soluble proteins (190.5%), and sugars (126.6%). Simultaneously, secondary metabolites like ascorbic acid (ASA), anthocyanins, phenolics, flavonoids, total antioxidant (TOA), and proline were also up-regulated by H₂O₂ (104.9%, 128.8%, 166.3%, 141.4%, and 436.4%, respectively). These results corresponded to the stimulative role triggered by H₂O₂ in boosting the activities of catalase (22.4%), ascorbate peroxidase (20.7%), superoxide dismutase (88.4%), polyphenol oxidase (60.7%), soluble peroxidase (23.8%), and phenylalanine ammonia-lyase (57.1%) as well as the expression level of HpCAT, HpAPX, HpSOD, HpPPO, and HpPAL genes, which may help to moderate low-temperature stress. In conclusion, our findings stipulate new insights into the mechanisms by which H₂O₂ regulates low-temperature stress tolerance in pitaya plants.