Genome-wide characterization and expression analyses of the auxin/indole-3-acetic acid (Aux/IAA) gene family in apple (Malus domestica)

Auxin is a necessary phytohormone for fruit development, accompanying the whole process of fruit growth and development. The Aux/IAA gene family is one of the early auxin-responsive gene families. At present, there were few reports involved in Aux/IAA genes in the fruit, especially in apple. In our study, we identified 42 MdAux/IAAs, phylogenetic analysis showed that Aux/IAA proteins from apple, tomato, and strawberry were clustered into 5 groups, 42 MdAux/IAAs randomly distributed on 13 chromosomes. Additionally, a comprehensive analysis of Aux/IAA gene family was completed, including gene structures, conserved motifs, phylogenetic analysis, chromosome mapping, orthologous identification, selection pressure analyses, synteny analysis, and protein interaction. We also tested the expression of MdAux/IAAs in different tissues and fruit development stages using quantitative reverse transcription-polymerase chain reaction (qRT-PCR), we found that the most members of Aux/IAA showed higher expression in seeds compared within stem and leaves, indicating they may play a role in regulating fruit development. We also declared that the expression of Aux/IAA gene was not consistent in the pericarp and seeds at the same developmental stage, 3 MdAux/IAAs of the pericarp were upregulated over 20-fold at 90 d and 5 MdAux/IAAs of the seeds were upregulated over 40-fold at 90 d. It was MdAux/IAA23 that showed extreme up-regulated expression in both pericarp and seeds. This study proved that the Aux/IAA gene families may perform a different function in apple fruit development and ripening, more importantly, it provided a foundation for further exploring the biological function of these MdAux/IAAs.