Transcriptome analysis reveals the inhibitory nature of high nitrate during adventitious roots formation in the apple rootstock

Comparative physiological, metabolomic, and transcriptomic analyses reveal mechanisms of apple dwarfing rootstock root morphogenesis under nitrogen and/or phosphorus deficient conditions

Nitrogen (N) and phosphorus (P) are essential phytomacronutrients, and deficiencies in these two elements limit growth and yield in apple (Malus domestica Borkh.). The rootstock plays a key role in the nutrient uptake and environmental adaptation of apple. The objective of this study was to investigate the effects of N and/or P deficiency on hydroponically-grown dwarfing rootstock 'M9-T337' seedlings, particularly the roots, by performing an integrated physiological, transcriptomics-, and metabolomics-based analyses. Compared to N and P sufficiency, N and/or P deficiency inhibited aboveground growth, increased the partitioning of total N and total P in roots, enhanced the total number of tips, length, volume, and surface area of roots, and improved the root-to-shoot ratio. P and/or N deficiency inhibited NO3 - influx into roots, and H⁺ pumps played a important role in the response to P and/or N deficiency. Conjoint analysis of differentially expressed genes and differentially accumulated metabolites in roots revealed that N and/or P deficiency altered the biosynthesis of cell wall components such as cellulose, hemicellulose, lignin, and pectin. The expression of MdEXPA4 and MdEXLB1, two cell wall expansin genes, were shown to be induced by N and/or P deficiency. Overexpression of MdEXPA4 enhanced root development and improved tolerance to N and/or P deficiency in transgenic Arabidopsis thaliana plants. In addition, overexpression of MdEXLB1 in transgenic Solanum lycopersicum seedlings increased the root surface area and promoted acquisition of N and P, thereby facilitating plant growth and adaptation to N and/or P deficiency. Collectively, these results provided a reference for improving root architecture in dwarfing rootstock and furthering our understanding of integration between N and P signaling pathways.

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.

Insights into Factors Controlling Adventitious Root Formation in Apples

Adventitious root (AR) formation is required for the vegetative propagation of economically important horticultural crops, such as apples. Asexual propagation is commonly utilized for breeding programs because of its short life cycle, true-to-typeness, and high efficiency. The lack of AR formation from stem segments is a barrier to segment survival. Therefore, understanding the AR regulatory mechanisms is vital for the prolonged and effective use of biological resources. Several studies have been undertaken to comprehend the molecular and physiological control of AR, which has greatly extended our knowledge regarding AR formation in apples and other crops. Auxin, a master controller of AR formation, is widely used for inducing AR formation in stem cutting. At the same time, cytokinins (CKs) are important for cell division and molecular reprograming, and other hormones, sugars, and nutrients interact with auxin to control excision-induced AR formation. In this review, we discuss the present understandings of ARs’ formation from physiological and molecular aspects and highlight the immediate advancements made in identifying underlying mechanisms involved in the regulation of ARs. Despite the progress made in the previous decades, many concerns about excision-induced AR formation remain unanswered. These focus on the specific functions and interactions of numerous hormonal, molecular, and metabolic components and the overall framework of the entire shoot cutting in a demanding environment.

Transcriptome Analysis Reveals Multiple Genes and Complex Hormonal-Mediated Interactions with PEG during Adventitious Root Formation in Apple

Adventitious root (AR) formation is a bottleneck for the mass propagation of apple rootstocks, and water stress severely restricts it. Different hormones and sugar signaling pathways in apple clones determine AR formation under water stress, but these are not entirely understood. To identify them, GL-3 stem cuttings were cultured on polyethylene glycol (PEG) treatment. The AR formation was dramatically decreased compared with the PEG-free control (CK) cuttings by increasing the endogenous contents of abscisic acid (ABA), zeatin riboside (ZR), and methyl jasmonate (JA-me) and reducing the indole-3-acetic acid (IAA) and gibberellic acid 3 (GA3) contents. We performed a transcriptomic analysis to identify the responses behind the phenotype. A total of 3204 differentially expressed genes (DEGs) were identified between CK and PEG, with 1702 upregulated and 1502 downregulated genes. Investigation revealed that approximately 312 DEGs were strongly enriched in hormone signaling, sugar metabolism, root development, and cell cycle-related pathways. Thus, they were selected for their possible involvement in adventitious rooting. However, the higher accumulation of ABA, ZR, and JA-me contents and the upregulation of their related genes, as well as the downregulation of sugar metabolism-related genes, lead to the inhibition of ARs. These results indicate that AR formation is a complicated biological process chiefly influenced by multiple hormonal signaling pathways and sugar metabolism. This is the first study to demonstrate how PEG inhibits AR formation in apple plants.

Identification of Genes Associated with Nitrogen Stress Responses in Apple Leaves

Nitrogen (N) is an essential macronutrient that regulates diverse physiological processes for plant survival and development. In apple orchards, inappropriate N conditions can cause imbalanced growth and subsequent physiological disorders in trees. In order to investigate the molecular basis underlying the physiological signals for N stress responses, we examined the metabolic signals responsive to contrasting N stress conditions (deficient/excessive) in apple leaves using transcriptome approaches. The clustering of differentially expressed genes (DEGs) showed the expression dynamics of genes associated with each N stress group. Functional analyses of gene ontology and pathway enrichments revealed the potential candidates of metabolic signals responsible for N-deficient/excessive stress responses. The functional interactions of DEGs in each cluster were further explored by protein-protein interaction network analysis. Our results provided a comprehensive insight into molecular signals responsive to N stress conditions, and will be useful in future research to enhance the nutrition tolerance of tree crops.