Transcriptome analysis reveals the regulatory mechanism by which MdWOX11 suppresses adventitious shoot formation in apple

MdAIL5 overexpression promotes apple adventitious shoot regeneration by regulating hormone signaling and activating the expression of shoot development-related genes

Adventitious shoot (AS) regeneration is a significant factor in the genetic transformation of horticultural plants. It is also a noteworthy approach to their vegetative propagation. AS regeneration remains highly dependent on the genotype or maturity of explants. We here found that the AS regeneration abilities of apple leaves were positively correlated with MdAIL5 expression. MdAIL5 overexpression dramatically increased AS regeneration efficiency. Notably, MdAIL5 overexpression could restore the AS formation ability of explants to a certain extent, which was lost with an increase in maturity. Endogenous hormone detection revealed that MdAIL5 overexpression changed the contents of auxin, cytokinin (CK), and other hormones in apple leaves. Transcriptome analysis revealed that many genes related to auxin, CK, and brassinolide signaling pathways were significantly and differentially expressed between MdAIL5-overexpressing transgenic apple and wild-type apple plants. Yeast one-hybrid assays, the electrophoretic mobility shift assay, and the dual-luciferase reporter assay revealed that MdAIL5 directly binds to MdARF9 and MdHB14 promoters and positively affects their expression. We here established a model of MdAIL5 regulating AS formation, which acts as a theoretical basis for facilitating genotype- or explant maturity-independent AS regeneration in the future.

Cytokinin-responsive MdTCP17 interacts with MdWOX11 to repress adventitious root primordium formation in apple rootstocks

Adventitious root (AR) formation plays an important role in vegetatively propagated plants. Cytokinin (CK) inhibits AR formation, but the molecular mechanisms driving this process remain unknown. In this study, we confirmed that CK content is related to AR formation and further revealed that a high auxin/CK ratio was beneficial to AR formation in apple (Malus domestica). A correlation between expression of CK-responsive TEOSINTE BRANCHED1, CYCLOIDEA, and PCF17 (MdTCP17) and AR formation in response to CK was identified, and overexpression of MdTCP17 in transgenic apple inhibited AR formation. Yeast two-hybrid, bimolecular fluorescence complementation, and co-immunoprecipitation assays revealed an interaction between MdTCP17 and WUSCHEL-RELATED HOMEOBOX11 (MdWOX11), and a significant correlation between the expression of MdWOX11 and AR ability. Overexpression of MdWOX11 promoted AR primordium formation in apple, while interference of MdWOX11 inhibited AR primordium production. Moreover, a positive correlation was found between MdWOX11 and LATERAL ORGAN BOUNDARIES DOMAIN29 (MdLBD29) expression, and yeast one-hybrid, dual luciferase reporter, and ChIP-qPCR assays verified the binding of MdWOX11 to the MdLBD29 promoter with a WOX-box element in the binding sequence. Furthermore, MdTCP17 reduced the binding of MdWOX11 and MdLBD29 promoters, and coexpression of MdTCP17 and MdWOX11 reduced MdLBD29 expression. Together, these results explain the function and molecular mechanism of MdTCP17-mediated CK inhibition of AR primordium formation, which could be used to improve apple rootstocks genetically.

Insights into the complicated networks contribute to adventitious rooting in transgenic MdWOX11 apple microshoots under nitrate treatments

Adventitious root formation is a bottleneck for the mass propagation of microshoots, and nitrate is an essential nutrient regulating adventitious roots. WOX11 is involved in adventitious rooting. But the crosstalk between nitrate and WOX11 is completely unknown. In this study, MdWOX11 transgenic apple microshoots were grown on different nitrate treatments. Low nitrate promotes adventitious rooting in overexpressed microshoots more than wild type and RNA interference microshoots. In contrast, medium nitrate significantly inhibits it in overexpressed and RNA interference microshoots compared with wild type microshoots. Stem anatomy indicated that medium nitrate delays root primordia formation compared with low nitrate. Methyl jasmonate and zeatin riboside played positive and negative roles in adventitious rooting, respectively. Transcriptomic analysis was conducted to understand the molecular mechanisms behind the phenotypes better. Hormone signalling, sugar metabolism, nitrogen metabolism, cell cycle and root development pathway-related genes were selected for their potential involvement in adventitious rooting. Results suggest that nitrogen signaling and MdWOX11 expression affect cytokinin accumulation and response to cytokinin through regulating the expression of genes related to cytokinin synthesis and transduction pathways, which ultimately affect adventitious rooting. This study provided important insights into the complicated networks involved in adventitious rooting in transgenic microshoots under nitrate treatments.

MdWOX4-2 modulated MdLBD41 functioning in adventitious shoot of apple (Malus domestica)

Apple (Malus domestica Borkh.) is not only an important fruit crop distributed worldwide, but also a common model plant. However, the lack of efficient genetic transformation procedures for apples limits the in-depth studies of their gene functions. Although leaf-regenerated adventitious shoots (LRAS) are a prerequisite for successful genetic transformation of apple, little is known about the underlying molecular mechanism of LRAS. Here, we identified the WUSCHEL-related homeobox (WOX) transcription factor in apple, MdWOX4-2, which was a transcriptional activator. Gene expression as well as morphological and histological observations revealed that MdWOX4-2 is involved in the development of LRAS. Overexpression of MdWOX4-2 conferred higher regenerative capacity in transgenic tobacco (Nicotiana tabacum) as compared to the wild type (WT). The combined results of the yeast one-hybrid (Y1H), electrophoretic mobility shift assay (EMSA), dual luciferase assays, and transient transactivation assay, revealed that MdWOX4-2 directly bound to and activated the MdLBD41 promoter. Moreover, transgenic experiments further demonstrated that MdLBD41 could significantly enhance the formation of adventitious shoot in transgenic tobacco. Collectively, our findings demonstrate that MdWOX4-2 is important for regulating the LRAS development by activating MdLBD41.