Genome-Wide Identification of the Aux/IAA Family Genes (MdIAA) and Functional Analysis of MdIAA18 for Apple Tree Ideotype

The MdIAA29-MdARF4 complex plays an important role in balancing plant height with salt and drought stress responses

Breeding dwarf apple (Malus domestica) varieties is a recent trend in agriculture because such varieties are easy to maintain and have high yields; however, dwarf apple trees generally have poor stress tolerance. Balancing apple plant height and stress response has been an important breeding goal. In this study, aux/indole-3-acetic acid 29 gene in apple (MdIAA29) overexpression lines (#1, #2, and #3) had reduced plant height by 39%, 31%, and 35%, respectively, suitable for close planting applications. Surprisingly, the dwarf MdIAA29-overexpressing lines also showed increased plant tolerance to salt and drought stresses. Further analysis showed that MdIAA29 inhibited the regulation of auxin response factor 4 (ARF4) on Gretchen Hagen 3.9 (GH3.9) gene and 9-cis-epoxycarotenoid dioxygenase 3 (NCED3) gene in apple and changed the contents of auxin and abscisic acid in different tissues, thus achieving a balance between plant height and stress tolerance. In addition, we also found that MdIAA7 enhanced the inhibitory effect of MdIAA29 on MdARF4. In brief, the MdIAA29-MdARF4 complex significantly impacts the height of apple plants and their ability to respond to salt and drought stress.

A New Function of MbIAA19 Identified to Modulate Malus Plants Dwarfing Growth

The primary determinants of apple (Malus) tree architecture include plant height and internode length, which are the significant criteria for evaluating apple dwarf rootstocks. Plant height and internode length are predominantly governed by phytohormones. In this study, we aimed to assess the mechanisms underlying dwarfism in a mutant of Malus baccata. M. baccata dwarf mutant (Dwf) was previously obtained through natural mutation. It has considerably reduced plant height and internode length. A comparative transcriptome analysis of wild-type (WT) and Dwf mutant was performed to identify and annotate the differentially expressed genes responsible for the Dwf phenotype using RNA-seq and GO and KEGG pathway enrichment analyses. Multiple DEGs involved in hormone signaling pathways, particularly auxin signaling pathways, were identified. Moreover, the levels of endogenous indole-3-acetic acid (IAA) were lower in Dwf mutant than in WT. The Aux/IAA transcription factor gene MbIAA19 was downregulated in Dwf mutant due to a single nucleotide sequence change in its promoter. Genetic transformation assay demonstrated strong association between MbIAA19 and the dwarf phenotype. RNAi-IAA19 lines clearly exhibited reduced plant height, internode length, and endogenous IAA levels. Our study revealed that MbIAA19 plays a role in the regulation of dwarfism and endogenous IAA levels in M. baccata.

Expression Analysis of AUX/IAA Family Genes in Apple Under Salt Stress

Members of the auxin/indoleacetic acid (Aux/IAA) gene family in plants are primary auxin-responsive genes that play important roles in many aspects of plant development and in responses to abiotic stress. Recently, 33 Aux/IAA have been identified in the apple genome. The biological responses of MdIAAs to salt stress are still unknown. In this study, Malus zumi, Malus baccata, and Malus × domestica 'Fuji' plantlets were subjected to salt stress by supplementing hydroponic media with NaCl at various concentrations. M. zumi showed the strongest salt resistance, followed by 'Fuji', and M. baccata was the most sensitive to salt stress. Tissue-specific expression profiles of MdIAAs were determined by quantitative real-time polymerase chain reaction. When apple plantlets were subjected to salt stress, most of salt-responsive MdIAAs were up-regulated by 1 h, 3 h, and 6 h in roots, shoot tips, and leaves, respectively. Highly expressed MdIAAs in roots, especially for M. zumi, consisted with the salt tolerance of apple rootstocks. Transgenic apple calli were tolerant to salt stress when over-expressed salt-responsive genes, MdIAA8, -9, and -25. These results provide clues about salt resistance in these three Malus species, which helps apple breeding of salt tolerance by genetic transformation.

Identification and functional characterization of the Aux/IAA gene VcIAA27 in blueberry

Aux/IAA genes are an important class of players in diverse developmental processes in plants, which generally exert their functions through the auxin signaling pathway. Blueberry is an economically and nutritionally important berry-bearing crop. However, Aux/IAA genes remain unknown in blueberry. In the present study, an Aux/IAA gene (VcIAA27) was identified and characterized in blueberry, and it is most closely related to IAA27 in other plant species. Expression analysis indicated that VcIAA27 transcripts accumulate highly in shoot, flower and fruit. Interestingly, VcIAA27 was highly expressed at early fruit developmental stages, and dramatically decreased from the onset of fruit ripening, implying that VcIAA27 possibly plays important roles during fruit enlargement. Meanwhile, the analysis of promoter activity in Arabidopsis showed that strong GUS signal was detected in the trichome and hydathodes of leaves, receptacle of silique, and lateral roots of seedling. Overexpression of VcIAA27 in Arabidopsis leads to auxin-related defects such as downward-curled leaves, short or sterile siliques, shorter stature, and more shoot branches. Moreover, qPCR analysis indicated that VcIAA27 is able to alter the expression patterns of the auxin-related genes BRU6, SAG13, SAUR26 in Arabidopsis, suggesting that VcIAA27 might be negatively involved in the auxin signaling pathway. The findings will greatly contribute to future investigation of Aux/IAA-mediated mechanisms that control blueberry development, especially fruit development and ripening.