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Simple Summary: The GRAS gene family plays a critical role in regulation of growth, defense, light and hormone responses. We identified 45 GRAS genes in the pitaya genome and categorized them into nine respective subfamilies: PAT1, SHR, LISCL, HAM, SCR, RGL, LAS, DELLA and SCL-3. Among these 45 HuGRAS family members, we reported nine candidate genes that played key roles in the growth and development of the pitaya plant. The GRAS gene family is one of the most important families of transcriptional factors that have diverse functions in plant growth and developmental processes including axillary meristem patterning, signal-transduction, cell maintenance, phytohormone and light signaling. Despite their importance, the function of GRAS genes in pitaya fruit (Selenicereus undatus L.) remains unknown. Here, 45 members of the HuGRAS gene family were identified in the pitaya genome, which was distributed on 11 chromosomes. All 45 members of HuGRAS were grouped into nine subfamilies using phylogenetic analysis with six other species: maize, rice, soybeans, tomatoes, Medicago truncatula and Arabidopsis. Among the 45 genes, 12 genes were selected from RNA-Seq data due to their higher expression in different plant tissues of pitaya. In order to verify the RNA-Seq data, these 12 HuGRAS genes were subjected for qRT-PCR validation. Nine HuGRAS genes exhibited higher relative expression in different tissues of the plant. These nine genes which were categorized into six subfamilies inlcuding DELLA (HuGRAS-1), SCL-3 (HuGRAS-7), PAT1 (HuGRAS-34, HuGRAS-35, HuGRAS-41), HAM (HuGRAS-37), SCR (HuGRAS-12) and LISCL (HuGRAS-18, HuGRAS-25) might regulate growth and development in the pitaya plant. The results of the present study provide valuable information to improve tropical pitaya through a molecular and conventional breeding program. [ABSTRACT FROM AUTHOR]

• Abiotic stresses (vanadium, drought and heat) adversely affect the growth and production of pitaya. • Genome-wide analysis identified 65 APX genes in the pitaya and were clustered into 13 subfamilies. • Exogenous application of melatonin mitigates the effect of multifactorial stresses. • Melatonin enhanced the expression levels of HuAPX genes, involved in ROS scavenging activities and stress tolerance. Pitaya (Selenicereus undatus L.), an important tropical plant belonging to the Cactaceae family, is rich in antioxidants, dietary fibers and low in calories. Pitaya shows higher tolerance against abiotic stresses; however, various environmental factors lead to excessive production of reactive oxygen species (ROS) persistently that cause injury to the cells. Under vanadium, drought, and heat conditions, plant cells regulate the expression of ascorbate peroxidase (APX , a vital antioxidant enzyme) genes, which play a significant role in detoxifying ROS by catalyzing hydrogen peroxide. In this study, we have identified 65 APX genes (HuAPX) in the pitaya genome and clustered them into 13 subfamilies. Gene structure and motif analysis revealed that HuAPX genes contained well-preserved intron-exon structure and their motif domains also shared the same order within one cluster. Phytohormones-related cis -acting elements including abscisic acid, gibberellin, jasmonic acid, salicylic acid, and auxin were found in abundance in the promoter regions of the HuAPX genes. RNA-seq-based expression analysis of the HuAPX genes unveiled that few genes showed significantly higher expression under single and multifactorial stress combinations of drought, heat, and vanadium. Exogenous applications of melatonin attenuate the vanadium, heat, and drought effects, thus enhancing the growth and development of the pitaya plant, by inducing the HuAPX gene expression efficiently. Moreover, the expression pattern of HuAPX genes were validated through qRT-PCR analysis. Our findings suggest that three candidate genes (HuAPX1/3/6) may play a key role against abiotic stresses by encoding key enzymes related to glutathione metabolism and phenylpropanoid biosynthesis. These results provide valuable information for organizing a new strategy to design the climate-smart pitaya genotypes through conventional and modern biotechnological tools. [ABSTRACT FROM AUTHOR]