Characterization of MADS-box gene family and its unique response to drought and nickel stresses with melatonin-mediated tolerance in dragon fruit (Selenicereus undatus L.)

Dragon fruit (pitaya) is an important tropical crop with medicinal and industrial values. However, severe environmental stress factors affect dragon fruit production and development. The MADS-box gene family performs an essential role in the regulation of stress tolerance, reproduction, and fruit ripening in various plants, but relevant studies havn't been conducted on dragon fruit. In this study, the identification and expression analysis of the MADS-box gene family was conducted in response to abiotic stress factors in pitaya seedlings. We identified 67 MADS-box genes distributed across 11 chromosomes of the pitaya genome. All the genes were named from HuMADS-1 to HuMADS-67. The phylogenetic tree of HuMADS-box genes was constructed together with soybean, wheat, and Arabidopsis. Based on the phylogenetic analysis, tree tropology and motif's structure, all the genes were divided into 3 groups as Mα, Mδ, and MIKC. The HuMADS genes showed a conserved intron-exon structure and motif domain organization. The expression patterns of all HuMADS-box genes varied in 15 tissue parts of the pitaya. The analysis of subcellular localization of all proteins predicted that 55.8% of the total gene members were localized in the nucleus. The syntenic analysis showed 28 collinearity gene pairs, in which 4 gene pairs showed tandem duplication and 24 pairs showed segmental duplication on the chromosomes. Cis-acting element analysis predicted their regulatory roles in plant defense processes. RNA-seq analysis of pitaya seedlings under single and combined stresses of drought and nickel exhibited higher expression of HuMADS-box genes. In addition, several HuMADS genes showed enhanced expression in response to melatonin treatment under drought and nickel stresses, thereby enhancing pitaya seedling growth and development. To validate the RNA-Seq data, RT-qPCR analysis was performed on 12 HuMADS genes, revealing that six of them (HuMADS-14/15/22/62/63/67) were predicted to play important roles in abiotic stress tolerance. Overall, our findings offer key insights for future genetic engineering strategies to develop stress-tolerant pitaya genotypes.