The root-lesion nematode, Pratylenchus penetrans, affects early growth and physiology of grafted M.9, G.41 and G.935 apple rootstocks similarly under field microplot conditions

The root-lesion nematode, Pratylenchus penetrans, is a ubiquitous parasite of roots of temperate fruit trees. It affects early growth of trees replanted into former orchard sites where populations have built up, and may contribute to decline complexes of older trees. Most British Columbia, Canada apple acreage is planted with M.9 rootstock, but growers are increasingly considering Geneva-series rootstocks such as G.41 and G.935. Among these rootstocks, responses to P. penetrans, specifically, are poorly known. To compare the resistance and tolerance to P. penetrans of G.41, G.935 and M.9 rootstocks ('Ambrosia' scion), a field microplot experiment was established in spring of 2020 at the Summerland Research and Development Centre. The experimental design was a 2 x 3 factorial combination of: P. penetrans inoculation (+/-) and rootstock (G.41, G.935, M.9), with 20 replicate microplots of each of the six treatment combinations arranged in a randomized complete block design. The P. penetrans inoculum was 5400 nematodes per microplot (54 P. penetrans L-1 soil), which is below commonly accepted damage thresholds. Though P. penetrans population densities were lower for the G.41 rootstock by the end of the 2021 growing season, the effects of P. penetrans were similar among rootstocks. In the establishment year (2020), P. penetrans caused significant reductions in aboveground growth. In 2021, shoot growth and root weight were reduced by P. penetrans. The nematode also reduced rates of leaf gas exchange and stem water potential. These data suggest that while G.41 and G.935 may have other horticultural benefits over M.9, they are equally susceptible to P. penetrans at the early stages of tree growth.

Genome-wide analysis of the MdZR gene family revealed MdZR2.2-induced salt and drought stress tolerance in apple rootstock

The DNL-type zinc finger protein constitutes a zinc ribbon protein (ZR) family, which belongs to a branch of zinc finger protein and plays an essential role in response to abiotic stress. Here, we identified six apple (Malus domestica) MdZR genes. Based on their phylogenetic relationship and gene structure, the MdZR genes were divided into three categories, including MdZR1, MdZR2, and MdZR3. Subcellular results showed that the MdZRs are located on the nuclear and membrane. The transcriptome data showed that MdZR2.2 is expressed in various tissues. The expression analysis results showed that MdZR2.2 was significantly upregulated under salt and drought treatments. Thus, we selected MdZR2.2 for further research. Overexpression of MdZR2.2 in apple callus improved their tolerance to drought and salt stress and ability to scavenge reactive oxygen species (ROS). In contrast, transgenic apple roots with silenced MdZR2.2 grew more poorly than the wild type when subjected to salt and drought stress, which reduced their ability to scavenge ROS. To our knowledge, this is the first study to analyze the MdZR protein family. This study identified a gene that responds to drought and salt stress. Our findings lay a foundation for a comprehensive analysis of the MdZR family members.

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.

Appropriate and Constant Potassium Supply Promotes the Growth of M9T337 Apple Rootstocks by Regulating Endogenous Hormones and Carbon and Nitrogen Metabolism

Potassium (K) is an indispensable nutrient element in the development of fruit trees in terms of yield and quality. It is unclear how a stable or unstable supply of K affects plant growth. We studied the root morphology and physiological and molecular changes in the carbon and nitrogen metabolism of M9T337 apple rootstock under different K levels and supply methods using hydroponics. Five K supply treatments were implemented: continuous low K (K_L), initial low and then high K (K_LH), appropriate and constant K (K_AC), initial high and then low K (K_HL), and continuous high K (K_H). The results showed that the biomass, root activity, photosynthesis, and carbon and nitrogen metabolism of the M9T337 rootstocks were inhibited under K_L, K_H, K_LH and K_HL conditions. The K_AC treatment promoted root growth by optimizing endogenous hormone content, enhancing carbon and nitrogen metabolism enzyme activities, improving photosynthesis, optimizing the distribution of carbon and nitrogen, and upregulating the transcription levels of nitrogen assimilation-related genes (nitrate reductase, glutamine synthetase, glutamate synthase, MdNRT1.1, MdNRT1.2, MdNRT1.5, MdNRT2.4). These results suggest that an appropriate and constant K supply ensures the efficient assimilation and utilization of nitrogen and carbon.

[Carbonized Apple Branches Decrease the Accumulation and Damage of Cadmium on Apple Rootstock by Reducing DTPA-Cd in Soil]

To explore the effects of carbonized apple branches on cadmium(Cd) accumulation and its damage to apple rootstock, the rootstocks of apple(Malus hupehensis Rehd.) in pots containing soil together with 0.5% and 1%(ω) carbonized apple branches were irrigated by a nutrient solution containing CdSO₄. The content of DTPA-Cd(cadmium extracted by diethylenetriamine pentaacetic acid) in the potting soil, and the accumulation of Cd in the roots, stems, and leaves of apple rootstocks, were subsequently monitored. The activities of antioxidant enzymes in roots and leaves, root cell death, and the net photosynthesis rate were further analyzed. The results showed that the concentration of DTPA-Cd in the potting soil with carbonized apple branches was significantly lower than that without carbonized apple branches(Cd-only). Compared with the Cd-only treatment, the concentration of DTPA-Cd in the potting soil decreased by 17.50% and 25.55% in the treatment with 0.5% and 1%(ω) carbonized apple branches for 12 days. The Cd accumulation in roots, stems, and leaves; the accumulations of superoxide anions(·O₂^-), hydrogen peroxide(H₂O₂), and malondialdehyde(MDA) in roots and leaves; and the amount of cell death in the roots of apple rootstock treated by carbonized apple branches were significantly lower compared to the Cd-only treatment. However, the activities of superoxide dismutase(SOD), peroxidase(POD), and catalase(CAT) in the roots and leaves, and the net photosynthesis rate of apple rootstock treated by carbonized apple branches, were significantly higher than under the Cd-only treatment. Compared with the Cd-only treatment, Cd accumulation in roots decreased by 29.49% and 37.18% in the treatment with 0.5% and 1%(ω) carbonized apple branches for 12 days, and the amount of cell death decreased by 22.73% and 29.09%, respectively. Our results show that carbonized apple branches reduce the uptake and accumulation of Cd in apple rootstock by reducing the content of DTPA-Cd in the soil, thereby alleviating the damaging effect of Cd on cells and photosynthesis. Moreover, the use of 1%(ω) carbonized apple branches was more effective than 0.5%(ω).

Apple rootstocks with different phosphorus efficiency exhibit alterations in rhizosphere bacterial structure

AIMS

The purpose of this study was to select phosphorus-efficient apple rootstocks under phosphorus deficiency and to reveal the effects of different apple rootstocks on the rhizosphere bacterial community.

METHODS AND RESULTS

We used 83 hybrid lines of Malus robusta Rehd. × Malling 9 (M.9) to investigate their physiological traits and the phosphorus deficiency phenotypes of leaves in response to phosphorus deficiency (0·1 mmol l^-1 PO₄ ^3- ). All the plants were cultivated in pots in the greenhouse and watered using drip irrigation. In accordance with the results of investigation, we selected the phosphorus-efficient hybrid lines (PE) and the phosphorus-inefficient hybrid lines (PI) to research their root morphology and root hairs (RH). In addition, we used Illumina MiSeq sequencing to determine the bacterial community of the rhizosphere from different rootstocks. The results showed that the PE plants had better growth characteristics and stronger root plasticity than that of the PI plants, and phosphorus deficiency can stimulate the RH growth of PE plants. There was no significant difference in the rhizosphere bacterial diversity, but we found that the bacterial community structure was significantly different at the genus levels; in addition, 89 genera were found to have significant differences between PE and PI plants, especially Bacillus. The PE rhizosphere had more abundant Bacillus compared to the PI. High positive Pearson correlations with the phosphorus concentration in the plantlets of apple rootstocks were detected for the bacterial genera Bacillus (r: 0·776).

CONCLUSIONS

The phosphorus-efficient apple rootstocks adapted to phosphorus deficiency by shaping the root morphology. Notably, different apple rootstocks showed alteration of the microbes in rhizosphere.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study can provide the materials for exploring the mechanism of apple rootstock phosphorus absorption. In accordance with the different bacterial community compositions, we can develop the inoculants to promote nutrient uptake.

Natural variation in cytokinin maintenance improves salt tolerance in apple rootstocks

Plants experiencing salt-induced stress often reduce cytokinin levels during the early phases of stress-response. Interestingly, we found that the cytokinin content in the apple rootstock "robusta" was maintained at a high level under salt stress. Through screening genes involved in cytokinin biosynthesis and catabolism, we found that the high expression levels of IPT5b in robusta roots were involved in maintaining the high cytokinin content. We identified a 42 bp deletion in the promoter region of IPT5b, which elevated IPT5b expression levels, and this deletion was linked to salt tolerance in robusta×M.9 segregating population. The 42 bp deletion resulted in the deletion of a Proline Response Element (ProRE), and our results suggest that ProRE negatively regulates IPT5b expression in response to proline. Under salt stress, the robusta cultivar maintains high cytokinin levels as IPT5b expression cannot be inhibited by proline due to the deletion of ProRE, leading to improve salt tolerance.

Transcriptome Analysis Reveals Multiple Hormones, Wounding and Sugar Signaling Pathways Mediate Adventitious Root Formation in Apple Rootstock

Adventitious roots (AR) play an important role in the vegetative propagation of apple rootstocks. The potential role of hormone, wounding, and sugar signalling pathways in mediating AR formation has not been adequately explored and the whole co-expression network in AR formation has not been well established in apple. In order to identify the molecular mechanisms underlying AR formation in 'T337' apple rootstocks, transcriptomic changes that occur during four stages of AR formation (0, 3, 9 and 16 days) were analyzed using high-throughput sequencing. A total of 4294 differentially expressed genes were identified. Approximately 446 genes related to hormones, wounding, sugar signaling, root development, and cell cycle induction pathways were subsequently selected based on their potential to be involved in AR formation. RT-qPCR validation of 47 genes with known functions exhibited a strong positive correlation with the RNA-seq data. Interestingly, most of the candidate genes involved in AR formation that were identified by transcriptomic sequencing showed auxin-responsive expression patterns in an exogenous Indole-3-butyric acid (IBA)-treatment assay: Indicating that endogenous and exogenous auxin plays key roles in regulating AR formation via similar signalling pathways to some extent. In general, AR formation in apple rootstocks is a complex biological process which is mainly influenced by the auxin signaling pathway. In addition, multiple hormones-, wounding- and sugar-signaling pathways interact with the auxin signaling pathway and mediate AR formation in apple rootstocks.

iTRAQ-Based Proteomic Analysis Reveals Potential Regulation Networks of IBA-Induced Adventitious Root Formation in Apple

Adventitious root (AR) formation, which is controlled by endogenous and environmental factors, is indispensable for vegetative asexual propagation. However, comprehensive proteomic data on AR formation are still lacking. The aim of this work was to study indole-3-butyric acid (IBA)-induced AR formation in the dwarf apple rootstock 'T337'. In this study, the effect of IBA on AR formation was analysed. Subsequent to treatment with IBA, both the rooting rate and root length of 'T337' increased significantly. An assessment of hormone levels in basal stem cuttings suggested that auxin, abscisic acid, and brassinolide were higher in basal stem cuttings that received the exogenous IBA application; while zeatin riboside, gibberellins, and jasmonic acid were lower than non-treated basal stem cuttings. To explore the underlying molecular mechanism, an isobaric tags for relative and absolute quantification (iTRAQ)-based proteomic technique was employed to identify the expression profiles of proteins at a key period of adventitious root induction (three days after IBA treatment). In total, 3355 differentially expressed proteins (DEPs) were identified. Many DEPs were closely related to carbohydrate metabolism and energy production, protein homeostasis, reactive oxygen and nitric oxide signaling, and cell wall remodeling biological processes; as well as the phytohormone signaling, which was the most critical process in response to IBA treatment. Further, RT-qPCR analysis was used to evaluate the expression level of nine genes that are involved in phytohormone signaling and their transcriptional levels were mostly in accordance with the protein patterns. Finally, a putative work model was proposed. Our study establishes a foundation for further research and sheds light on IBA-mediated AR formation in apple as well as other fruit rootstock cuttings.

[Effects of nitrate nitrogen supply on the growth, photosynthetic characteristics and 15N absorption, utilization of Malus hupehensis seedlings]

To explore the effects of different nitrate nitrogen levels (N₀, N₁, N₂, N₃ and N₄ were equivalent to 0, 2.5, 5, 10, 20 mmol·L^-115NO₃^- -N, respectively) on the growth, photosynthetic characteristics and ¹⁵N absorption, utilization and distribution, Malus hupehensis seedlings were grown in cultural liquid Hoagland by using the ¹⁵N-labeled tracer method. The results showed that the leaf chlorophyll content, leaf area and dry mass in different organs were the highest in N₂ treatment. With the increase of ¹⁵NO₃^- -N application rates, the leaf net photosynthetic rate (P_n)significantly increased but tended to decease when the ¹⁵NO₃^--N concentration exceeded N₂ treatment. In the 20th day after treatment, the root activity, root length, root surface area and number of tips of seedlings in N₂ treatment were significantly higher than those in the other treatments. The distribution ratio of ¹⁵N in different organs was significantly different among those treatments. The relatively balanced distribution ratio of ¹⁵N appeared in N₂ treatment, which the ¹⁵N utilization rate also reached relatively higher level. The total N content and ¹⁵N absorption of seedlings increased at low ¹⁵NO₃^--N concentration, reached the highest value in N₂ treatment with 103.77 and 21.57 mg, and then deceased at high ¹⁵NO₃^--N concentration. At the 12th day after treatment, the leaf nitrate reductase (NR) activity was the highest in N₂ treatment and the lowest in N₄ treatment. The leaf nitrate reductase (NR) activity deceased by 84.9% in N₄ treatment compared with N₂ treatment at the 16th day after treatment. Our findings indicated that the photosynthesis and absorption of nitrate nitrogen were inhibited under low ¹⁵NO₃^--N stress, and the assimilation of nitrate nitrogen and root growth were restrained under too much higher ¹⁵NO₃^--N level, which was not good for the growth, nitrogen absorption and utilization of apple seedlings. The appropriate nitrogen level could promote plant growth, enhance the photosynthesis and also increase the absorption, utilization and distribution of nitrogen.