Phylogenetic analysis of IDD gene family and characterization of its expression in response to flower induction in Malus

Although INDETERMINATE DOMAIN (IDD) genes encoding specific plant transcription factors have important roles in plant growth and development, little is known about apple IDD (MdIDD) genes and their potential functions in the flower induction. In this study, we identified 20 putative IDD genes in apple and named them according to their chromosomal locations. All identified MdIDD genes shared a conserved IDD domain. A phylogenetic analysis separated MdIDDs and other plant IDD genes into four groups. Bioinformatic analysis of chemical characteristics, gene structure, and prediction of protein-protein interactions demonstrated the functional and structural diversity of MdIDD genes. To further uncover their potential functions, we performed analysis of tandem, synteny, and gene duplications, which indicated several paired homologs of IDD genes between apple and Arabidopsis. Additionally, genome duplications also promoted the expansion and evolution of the MdIDD genes. Quantitative real-time PCR revealed that all the MdIDD genes showed distinct expression levels in five different tissues (stems, leaves, buds, flowers, and fruits). Furthermore, the expression levels of candidate MdIDD genes were also investigated in response to various circumstances, including GA treatment (decreased the flowering rate), sugar treatment (increased the flowering rate), alternate-bearing conditions, and two varieties with different-flowering intensities. Parts of them were affected by exogenous treatments and showed different expression patterns. Additionally, changes in response to alternate-bearing and different-flowering varieties of apple trees indicated that they were also responsive to flower induction. Taken together, our comprehensive analysis provided valuable information for further analysis of IDD genes aiming at flower induction.