Calmodulin-like protein MdCML15 interacts with MdBT2 to modulate iron homeostasis in apple.

BTB and TAZ domain proteins (BTs) function as specialized adaptors facilitating substrate recognition of the CUL3-RING ubiquitin ligase (CRL3) complex that targets proteins for ubiquitination in reaction to diverse pressures. Nonetheless, knowledge of the molecular mechanisms by which the apple scaffold protein MdBT2 responds to external and internal signals is limited. Here we demonstrate that a putative Ca 2+ sensor, calmodulin-like 15 (MdCML15), acts as an upstream regulator of MdBT2 to negatively modulate its functions in plasma membrane H+-ATPase regulation and iron deficiency tolerance. MdCML15 was identified to be substantially linked to MdBT2, and to result in the ubiquitination and degradation of the MdBT2 target protein MdbHLH104. Consequently, MdCML15 repressed the MdbHLH104 target, MdAHA8's expression, reducing levels of a specific membrane H+-ATPase. Finally, the phenotype of transgenic apple plantlets and calli demonstrated that MdCML15 modulates membrane H+-ATPase-produced rhizosphere pH lowering alongside iron homeostasis through an MdCML15-MdBT2-MdbHLH104-MdAHA8 pathway. Our results provide new insights into the relationship between Ca2+ signaling and iron homeostasis.

Enhanced photosynthetic efficiency by nitrogen-doped carbon dots via plastoquinone-involved electron transfer in apple.

Artificially enhancing photosynthesis is critical for improving crop yields and fruit qualities. Nanomaterials have demonstrated great potential to enhance photosynthetic efficiency; however, the mechanisms underlying their effects are poorly understood. This study revealed that the electron transfer pathway participated in nitrogen-doped carbon dots (N-CDs)-induced photosynthetic efficiency enhancement (24.29%), resulting in the improvements of apple fruit qualities (soluble sugar content: 11.43%) in the orchard. We also found that N-CDs alleviated mterf5 mutant-modulated photosystem II (PSII) defects, but not psa3 mutant-modulated photosystem I (PSI) defects, suggesting that the N-CDs-targeting sites were located between PSII and PSI. Measurements of chlorophyll fluorescence parameters suggested that plastoquinone (PQ), the mobile electron carrier in the photosynthesis electron transfer chain (PETC), was the photosynthesis component that N-CDs targeted. In vitro experiments demonstrated that plastoquinone-9 (PQ-9) could accept electrons from light-excited N-CDs to produce the reduced plastoquinone 9 (PQH2-9). These findings suggested that N-CDs, as electron donors, offer a PQ-9-involved complement of PETC to improve photosynthesis and thereby fruit quality. Our study uncovered a mechanism by which nanomaterials enhanced plant photosynthesis and provided some insights that will be useful in the design of efficient nanomaterials for agricultural/horticultural applications.

Ethylene inhibits malate accumulation in apple by transcriptional repression of aluminum-activated malate transporter 9 via the WRKY31-ERF72 network.

Malic acid accumulation in the vacuole largely determines acidity and perception of sweetness of apple. It has long been observed that reduction in malate level is associated with increase in ethylene production during the ripening process of climacteric fruits, but the molecular mechanism linking ethylene to malate reduction is unclear. Here, we show that ethylene-modulated WRKY transcription factor 31 (WRKY31)-Ethylene Response Factor 72 (ERF72)-ALUMINUM ACTIVATED MALATE TRANSPORTER 9 (Ma1) network regulates malate accumulation in apple fruit. ERF72 binds to the promoter of ALMT9, a key tonoplast transporter for malate accumulation of apple, transcriptionally repressing ALMT9 expression in response to ethylene. WRKY31 interacts with ERF72, suppressing its transcriptional inhibition activity on ALMT9. In addition, WRKY31 directly binds to the promoters of ERF72 and ALMT9, transcriptionally repressing and activating ERF72 and ALMT9, respectively. The expression of WRKY31 decreases in response to ethylene, lowering the transcription of ALMT9 directly and via its interactions with ERF72. These findings reveal that the regulatory complex WRKY31 forms with ERF72 responds to ethylene, linking the ethylene signal to ALMT9 expression in reducing malate transport into the vacuole during fruit ripening.

Apple receptor-like kinase FERONIA regulates salt tolerance and ABA sensitivity in Malus domestica.

FERONIA (FER) is a membrane-localized receptor-like kinase that plays pivotal roles in male and female gametophyte recognition, hormone signaling crosstalk, and biotic and abiotic responses. Most reports focus on the functions of FER in model plant Arabidopsis thaliana. However, the functions of FER homologs have not been deeply investigated in apple (Malus domestica), an important economic fruit crop distributed worldwide, especially in China. In this study, we identified an apple homolog of Arabidopsis FER, named MdFER (MDP0000390677). The two proteins encoded by AtFER and MdFER share similar domains: an extracellular malectin-like domain, a transmembrane domain, and an intracellular kinase domain. MdFER was further proven to localize to the plasma membrane in the epidermal cells of Nicotiana benthamiana. MdFER was widely expressed in different apple tissues, but the highest expression was found in roots. In addition, expression of MdFER was significantly induced by treatment with abscisic acid (ABA) and salt (NaCl). Overexpressing MdFER dramatically improved the resistance to salt stress and reduced the sensitivity to ABA in apple callus, while suppressing MdFER expression showed contrary effects. Furthermore, ectopic expression of MdFER in Arabidopsis significantly increased the salt tolerance and reduced the sensitivity to ABA. In addition, under salt stress and ABA treatment, Arabidopsis with highly expressed MdFER accumulated less reactive oxygen species (ROS), and the enzymatic activity of two ROS scavengers, superoxide dismutase and catalase, was higher compared with that of wild type (WT). Our work proves that MdFER positively regulates salt tolerance and negatively regulates ABA sensitivity in apple, which enriched the functions of FER in different plant species.

Ethylene-induced NbMYB4L is involved in resistance against tobacco mosaic virus in Nicotiana benthamiana.

Several MYB transcription factors are known to play important roles in plant resistance to environmental stressors. However, the mechanism governing the involvement of MYBs in regulating tobacco mosaic virus (TMV) resistance in plants is still unclear. In this study, we found that not only is Nicotiana benthamiana MYB4-like involved in defence against TMV, but also that the ethylene pathway participates in MYB4L-mediated resistance. Transcription of NbMYB4L was up-regulated in N. benthamiana infected with TMV. Silencing of NbMYB4L led to intensified TMV replication, whereas overexpression of NbMYB4L induced significant resistance to TMV. Transcription of NbMYB4L was greater in 1-aminocyclopropanecarboxylic acid (ACC, ethylene precursor)-pretreated plants but lower when the ethylene signalling pathway was blocked during TMV infection. Gene expression analysis showed that the transcription of NbMYB4L was largely suppressed in ETHYLENE INSENSITIVE 3-like 1(EIL1)-silenced plants. The results of electrophoretic mobility shift assay and chromatin immunoprecipitation-quantitative PCR (ChIP-qPCR) experiments indicated that NbEIL1 could directly bind to two specific regions of the NbMYB4L promoter. Furthermore, a luciferase assay revealed that NbEIL1 significantly induced the reporter activity of the MYB4L promoter in N. benthamiana. These results point to NbEIL1 functioning as a positive regulator of NbMYB4L transcription in N. benthamiana against TMV. Collectively, our work reveals that EIL1 and MYB4L constitute a coherent feed-forward loop involved in the robust regulation of resistance to TMV in N. benthamiana.

Low nitrate alleviates iron deficiency by regulating iron homeostasis in apple.

Iron (Fe) is an essential element for plant growth, development and metabolism. Due to its lack of solubility and low bioavailability in soil, Fe levels are usually far below the optimum amount for most plants' growth and development. In apple production, excessive use of nitrogen fertilizer may cause iron chlorosis symptoms in the newly growing leaves, but the regulatory mechanisms underlying this phenomenon are unclear. In this study, low nitrate (NO3- , LN) application alleviated the symptoms of Fe deficiency and promoted lower rhizosphere pH, which was beneficial for root Fe acquisition. At the same time, LN treatment increased citrate and abscisic acid accumulation in roots, which promoted Fe transport from root to shoot and maintained Fe homeostasis. Moreover, qRT-PCR analysis showed that nitrate application caused differential expression of genes related to Fe uptake and transport, as well as transcriptional regulators. In summary, our data reveal that low nitrate alleviated Fe deficiency through multiple pathways, demonstrating a new option for minimizing Fe deficiency by regulating the balance between nutrients.

Phosphate regulates malate/citrate-mediated iron uptake and transport in apple.

The accumulation of iron (Fe) in the apical meristem is considered as a critical factor involved in limiting the elongation of roots under low phosphate (Pi) conditions. Furthermore, the antagonism between Fe and Pi largely affects the effective utilization of Fe. Although the lack of Pi serves to increase the effectiveness of Fe in rice under both Fe-sufficient and Fe-deficient conditions, the underlying physiological mechanism governing this phenomenon is still unclear. In this study, we found that low Pi alleviated the Fe-deficiency phenotype in apples. Additionally, low Pi treatments increased ferric-chelated reductase (FCR) activity in the rhizosphere, promoted proton exocytosis, and enhanced the Fe concentration in both the roots and shoots. In contrast, high Pi treatments inhibited this process. Under conditions of low Pi, malate and citrate exudation from apple roots occurred under both Fe-sufficient and Fe-deficient conditions. In addition, treatment with 0.5 mM malate and citrate effectively alleviated the Fe and Pi deficiencies. Taken together, these data support the conclusion that a low Pi supply promotes organic acids exudation and enhances Fe absorption during Fe deficiency in apples.

Determination of Protein Interactions among Replication Components of Apple Necrotic Mosaic Virus.

Apple mosaic disease is one of the most widely distributed and destructive diseases in apple cultivation worldwide, especially in China, whose apple yields account for more than 50% of the global total. Apple necrotic mosaic virus (ApNMV) is a newly identified ilarvirus that is closely associated with apple mosaic disease in China; however, basic viral protein interactions that play key roles in virus replication and the viral life cycle have not been determined in ApNMV. Here, we first identify an ApNMV-Lw isolate that belongs to subgroup 3 in the genus Ilarvirus. ApNMV-Lw was used to investigate interactions among viral components. ApNMV 1a and 2apol, encoded by RNA1 and RNA2, respectively, were co-localized in plant cell cytoplasm. ApNMV 1a interacted with itself at both the inter- and intramolecular levels, and its N-terminal portion played a key role in these interactions. 1a also interacted with 2apol, and 1a's C-terminal, together with 2apol's N-terminal, was required for this interaction. Moreover, the first 115 amino acids of 2apol were sufficient for permitting the 1a-2apol interaction. This study provides insight into the protein interactions among viral replication components of ApNMV, facilitating future investigations on its pathogenicity, as well as the development of strategies to control the virus and disease.

Phosphorylation of a malate transporter promotes malate excretion and reduces cadmium uptake in apple.

Heavy metal contamination is a major environmental and human health hazard in many areas of the world. Organic acids sequester heavy metals and protect plant roots from the effects of toxicity; however, it is largely unknown how these acids are regulated in response to heavy metal stress. Here, protein kinase SOS2L1 from apple was functionally characterized. MdSOS2L1 was found to be involved in the regulation of malate excretion, and to inhibit cadmium uptake into roots. Using the DUAL membrane system in a screen of an apple cDNA library with MdSOS2L1 as bait, a malate transporter, MdALMT14, was identified as an interactor. Bimolecular fluorescence complementation, pull-down, and co-immunoprecipitation assays further indicated the interaction of the two proteins. Transgenic analyses showed that MdSOS2L1 is required for cadmium-induced phosphorylation at the Ser358 site of MdALMT14, a modification that enhanced the stability of the MdALMT14 protein. MdSOS2L1 was also shown to enhance cadmium tolerance in an MdALMT14-dependent manner. This study sheds light on the roles of the MdSOS2L1-MdALMT14 complex in physiological responses to cadmium toxicity.

Apple ethylene response factor MdERF11 confers resistance to fungal pathogen Botryosphaeria dothidea.

Ethylene response factor (ERF) is a plant-specific transcription factor involved in many biological processes including root formation, hypocotyl elongation, fruit ripening, organ senescence and stress responses, as well as fruit quality formation. However, its underlying mechanism in plant pathogen defense against Botryosphaeria dothidea (B. dothidea) remains poorly understood. Here, we isolate MdERF11, an apple nucleus-localized ERF transcription factor, from apple cultivar 'Royal Gala'. qRT-PCR assays show that the expression of MdERF11 is significantly induced in apple fruits after B. dothidea infection. Overexpression of MdERF11 gene in apple calli significantly increases the resistance to B.dothidea infection, while silencing MdERF11 in apple calli results in reduced resistance. Ectopic expression of MdERF11 in Arabidopsis also exhibits enhanced resistance to B. dothidea infection compared to that of wild type. Infections in apple calli and Arabidopsis leaves by B. dothidea respectively cause an increase in endogenous levels of salicylic acid (SA) followed by induction of SA synthesis-related and signaling-related gene expression. Taken together, these findings illustrate a potential mechanism by which MdERF11 elevates plant pathogen defense against B. dothidea by regulating SA synthesis pathway.

MdGRF11, an apple 14-3-3 protein, acts as a positive regulator of drought and salt tolerance.

The 14-3-3 proteins are a family of highly conserved phosphoserine-binding proteins that participate in the regulation of diverse physiological and developmental processes. In this research, twenty 14-3-3 genes in apples, which contained a highly conserved 14-3-3 domain, were identified and divided into two subgroups. Among them, MdGRF11 was further cloned and investigated. qRT-PCR analyses and GUS staining show that MdGRF11 is expressed in various organs and tissues with the highest expression levels found in the fruit. MdGRF11 was upregulated by polyethylene glycol 6000 (PEG 6000), NaCl, abscisic acid (ABA) and low temperature (4 °C) treatments. MdGRF11-overexpressing transgenic Arabidopsis and apple calli exhibited reduced sensitivity to salt and PEG 6000 treatments. Moreover, the ectopic expression of MdGRF11 improved the tolerance of transgenic tobacco to salt and drought stresses, which grew longer roots, underwent more growth, and presented higher chlorophyll levels than the wild-type control under salt and drought stress conditions. Furthermore, MdGRF11 expression remarkably reduced electrolyte leakage, malondialdehyde content levels, H2O2 and O2- accumulation under salt and drought stress conditions, which relied on the regulation of ROS-scavenging signaling to reduce oxidative damage of cells after salt and drought stress treatment. MdGRF11 also enhanced tolerance to stress by upregulating expression levels of ROS-scavenging and stress-related genes, especially improving responses to drought stress by modifying the water loss rates and stomatal aperture. Moreover, MdGRF11 could interact with MdAREB/ABF transcription factors through yeast two hybrid analyses. In conclusion, our results indicate that MdGRF11 acts as a positive regulator of salt and drought stress responses through regulating ROS scavenging and other signaling systems.

MdHIR4 transcription and translation levels associated with disease in apple are regulated by MdWRKY31.

KEY MESSAGE: Here we describe that the regulation of MdWRKY31 on MdHIR4 in transcription and translation levels associated with disease in apple. The phytohormone salicylic acid (SA) is a main factor in apple (Malus domestica) production due to its function in disease resistance. WRKY transcription factors play a vital role in response to stress. An RNA-seq analysis was conducted with 'Royal Gala' seedlings treated with SA to identify the WRKY regulatory mechanism of disease resistance in apple. The analysis indicated that MdWRKY31 was induced. A quantitative real-time polymerase chain reaction (qPCR) analysis demonstrated that the expression of MdWRKY31 was induced by SA and flg22. Ectopic expression of MdWRKY31 in Arabidopsis and Nicotiana benthamiana increased the resistance to flg22 and Pseudomonas syringae tomato (Pst DC3000). A yeast two-hybrid screen was conducted to further analyze the function of MdWRKY31. As a result, hypersensitive-induced reaction (HIR) protein MdHIR4 interacted with MdWRKY31. Biomolecular fluorescence complementation, yeast two-hybrid, and pull-down assays demonstrated the interaction. In our previous study, MdHIR4 conferred decreased resistance to Botryosphaeria dothidea (B. dothidea). A viral vector-based transformation assay indicated that MdWRKY31 evaluated the transcription of SA-related genes, including MdPR1, MdPR5, and MdNPR1 in an MdHIR4-dependent way. A GUS analysis demonstrated that the w-box, particularly w-box2, of the MdHIR4 promoter played a major role in the responses to SA and B. dothidea. Electrophoretic mobility shift assays, yeast one-hybrid assay, and chromatin immunoprecipitation-qPCR demonstrated that MdWRKY31 directly bound to the w-box2 motif in the MdHIR4 promoter. GUS staining activity and a protein intensity analysis further showed that MdWRKY31 repressed MdHIR4 expression. Taken together, our findings reveal that MdWRKY31 regulated plant resistance to B. dothidea through the SA signaling pathway by interacting with MdHIR4.

The MdWRKY31 transcription factor binds to the MdRAV1 promoter to mediate ABA sensitivity.

The phytohormone abscisic acid (ABA) is a major element involved in apple (Malus domestica) production because of its role in seed germination and early seedling development. The WRKY family, which is one of the largest families of transcription factors, plays an important role in ABA signaling in plants. However, the underlying molecular mechanisms of WRKY-mediated ABA sensitivity in apple are poorly understood. A genome-wide transcriptome analysis indicated that MdWRKY31 is a key factor induced by ABA. Quantitative real-time PCR showed that MdWRKY31 is induced by ABA in response to PEG4000, which is used to simulate drought. As a transcription factor, MdWRKY31 is localized in the nucleus. Ectopic expression of MdWRKY31 in Arabidopsis and Nicotiana benthamiana enhanced plant sensitivity to ABA. Overexpression of MdWRKY31 in apple roots and apple calli increased sensitivity to ABA, whereas repression of MdWRKY31 reduced sensitivity to ABA in the roots of 'Royal Gala'. Electrophoretic mobility shift assays, chromatin immunoprecipitation PCR, and yeast one-hybrid assays indicated that MdWRKY31 directly binds to the promoter of MdRAV1. Expression analyses of transgenic apple calli containing MdWRKY31 and pMdRAV1::GUS implied that MdWRKY31 represses the expression of MdRAV1. We also found that MdRAV1 binds directly to the promoters of MdABI3 and MdABI4 and repressed their expression. Our findings reveal a new important regulatory mechanism of MdWRKY31-MdRAV1-MdABIs in the ABA signaling pathway in apple.

Functional identification of apple on MdHIR4 in biotic stress.

In plants, hypersensitive-induced reaction (HIR) proteins are involved in stress responses, especially biotic stress. However, the potential molecular mechanisms of HIR-mediated biotic resistance in plants are rarely reported. We found that apple (Malus domestica) MdHIR4 was localized in the cell nucleus and membrane similar to AtHIR1 in Arabidopsis. Moreover, salicylic acid and the bacterial flagellin flg22 (a conserved, 22-amino acid motif), which are relevant to biotic stress, could induce MdHIR4 expression. Additionally, the transcription level of MdHIR4 was increased by Methyl jasmonate treatment. Ectopic expression of MdHIR4 in Arabidopsis and Nicotiana benthamiana reduced sensitivity to Methyl jasmonate and enhanced resistance to the bacterial pathogen Pst DC3000 (Pseudomonas syringae tomato DC3000). The interaction between MdHIR4 and AtJAZs proteins (AtJAZ3, AtJAZ4, and AtJAZ9) implied that MdHIR4 participated in the jasmonic acid (JA) signaling pathway. We found the expression of JA-related genes and PRs to change in transgenic plants, further demonstrating that MdHIR4 mediated biotic stress through the JA signaling pathway. Repressing the expression of MdHIR4 in apple leaves and calli increased resistance to Botryosphaeria dothidea by influencing the transcription of resistance-related genes. Our findings reveal the resistant function to biotic stress of MdHIR4 in transgenic plants, including Arabidopsis, tobacco, and apple, and identify the regulating mechanism of MdHIR4-related biotic resistance.

MdbHLH93, an apple activator regulating leaf senescence, is regulated by ABA and MdBT2 in antagonistic ways.

The molecular mechanism of leaf senescence in apple (Malus domestica) is still not fully understood. We used gene expression analysis and protein-protein interactions to decipher the relationships of abscisic acid (ABA) and two proteins, MdbHLH93 and MdBT2, in the senescence process. We found that MdbHLH93 promoted leaf senescence and the expression of senescence-related genes, which exhibited similar effects to ABA on leaf senescence. MdbHLH93 activated directly the transcription of MdSAG18. We also found that an ABA-responsive protein, MdBT2, interacted directly with MdbHLH93, and induced the ubiquitination and degradation of the MdbHLH93 protein, and thus delayed leaf senescence. Our findings provide new insights into the regulatory network of leaf senescence through the functional interactions among ABA, MdbHLH93 and MdBT2.

EIN3-LIKE1, MYB1, and ETHYLENE RESPONSE FACTOR3 Act in a Regulatory Loop That Synergistically Modulates Ethylene Biosynthesis and Anthocyanin Accumulation.

Ethylene regulates climacteric fruit ripening, and EIN3-LIKE1 (EIL1) plays an important role in this process. In apple (Malus domestica), fruit coloration is accompanied by ethylene release during fruit ripening, but the molecular mechanism that underlies these two physiological processes is unknown. In this study, we found that ethylene treatment markedly induced fruit coloration as well as the expression of MdMYB1, a positive regulator of anthocyanin biosynthesis and fruit coloration. In addition, we found that MdEIL1 directly bound to the promoter of MdMYB1 and transcriptionally activated its expression, which resulted in anthocyanin biosynthesis and fruit coloration. Furthermore, MdMYB1 interacted with the promoter of ETHYLENE RESPONSE FACTOR3, a key regulator of ethylene biosynthesis, thereby providing a positive feedback for ethylene biosynthesis regulation. Overall, our findings provide insight into a mechanism involving the synergistic interaction of the ethylene signal with the MdMYB1 transcription factor to regulate ethylene biosynthesis and fruit coloration in apple.

An apple NAC transcription factor enhances salt stress tolerance by modulating the ethylene response.

It is known that ethylene signaling is involved in the regulation of the salt stress response. However, the molecular mechanism of ethylene-regulated salt stress tolerance remains largely unclear. In this study, an apple NAM ATAF CUC transcription factor, MdNAC047, was isolated and functionally characterized to be involved in ethylene-modulated salt tolerance. MdNAC047 gene was significantly induced by salt treatment and its overexpression conferred increased tolerance to salt stress and facilitated the release of ethylene. Quantitative real-time-PCR analysis demonstrated that overexpression of MdNAC047 increased the expression of ethylene-responsive genes. Electrophoretic mobility shift assay, yeast one-hybrid and dual-luciferase assays suggested that MdNAC047 directly binds to the MdERF3 (ETHYLENE RESPONSE FACTOR) promoter and activates its transcription. In addition, genetic analysis assays indicated that MdNAC047 regulates ethylene production at least partially in an MdERF3-dependent pathway. Overall, we found a novel 'MdNAC047-MdERF3-ethylene-salt tolerance' regulatory pathway, which provide new insight into the link between ethylene and salt stress.

Ubiquitination-Related MdBT Scaffold Proteins Target a bHLH Transcription Factor for Iron Homeostasis.

Iron (Fe) homeostasis is crucial for plant growth and development. A network of basic helix-loop-helix (bHLH) transcription factors positively regulates Fe uptake during iron deficiency. However, their up-regulation or overexpression leads to Fe overload and reactive oxygen species generation, thereby damaging the plants. Here, we found that two BTB/TAZ proteins, MdBT1 and MdBT2, interact with the MbHLH104 protein in apple. In addition, the function of MdBT2 was characterized as a regulator of MdbHLH104 degradation via ubiquitination and the 26S proteasome pathway, thereby controlling the activity of plasma membrane H+-ATPases and the acquisition of iron. Furthermore, MdBT2 interacted with MdCUL3 proteins, which were required for the MdBT2-mediated ubiquitination modification of MdbHLH104 and its degradation. In sum, our findings demonstrate that MdBT proteins interact with MdCUL3 to bridge the formation of the MdBTsMdCUL3 complex, which negatively modulates the degradation of the MdbHLH104 protein in response to changes in Fe status to maintain iron homeostasis in plants.

Overexpression of MdbHLH104 gene enhances the tolerance to iron deficiency in apple.

Fe deficiency is a widespread nutritional disorder in plants. The basic helix-loop-helix (bHLH) transcription factors (TFs), especially Ib subgroup bHLH TFs which are involved in iron uptake, have been identified. In this study, an IVc subgroup bHLH TF MdbHLH104 was identified and characterized as a key component in the response to Fe deficiency in apple. The overexpression of the MdbHLH104 gene noticeably increased the H(+) -ATPase activity under iron limitation conditions and the tolerance to Fe deficiency in transgenic apple plants and calli. Further investigation showed that MdbHLH104 proteins bonded directly to the promoter of the MdAHA8 gene, thereby positively regulating its expression, the plasma membrane (PM) H(+) -ATPase activity and Fe uptake. Similarly, MdbHLH104 directly modulated the expression of three Fe-responsive bHLH genes, MdbHLH38, MdbHLH39 and MdPYE. In addition, MdbHLH104 interacted with 5 other IVc subgroup bHLH proteins to coregulate the expression of the MdAHA8 gene, the activity of PM H(+) -ATPase and the content of Fe in apple calli. Therefore, MdbHLH104 acts together with other apple bHLH TFs to regulate Fe uptake by modulating the expression of the MdAHA8 gene and the activity of PM H(+) -ATPase in apple.

MdVHA-A encodes an apple subunit A of vacuolar H(+)-ATPase and enhances drought tolerance in transgenic tobacco seedlings.

Vacuole H(+)-ATPases (VHAs) are plant proton pumps, which play a crucial role in plant growth and stress tolerance. In the present study, we demonstrated that the apple vacuolar H(+)-ATPase subunit A (MdVHA-A) is highly conserved with subunit A of VHA (VHA-A) proteins from other plant species. MdVHA-A was expressed in vegetative and reproductive organs. In apple in vitro shoot cultures, expression was induced by polyethylene glycol (PEG)-mediated osmotic stress. We further verified that over-expression of MdVHA-A conferred transgenic tobacco seedlings with enhanced vacuole H+-ATPase (VHA) activity and improved drought tolerance. The enhanced PEG-mimic drought response of transgenic tobacco seedlings was related to an extended lateral root system (dependent on auxin translocation) and more efficient osmotic adjustment. Our results indicate that MdVHA-A is a candidate gene for improving drought tolerance in plants.