Sorbitol Modulates Resistance to Alternaria alternata by Regulating the Expression of an NLR Resistance Gene in Apple

The hawthorn CpLRR-RLK1 gene targeted by ACLSV-derived vsiRNA positively regulate resistance to bacteria disease

Virus-derived small interfering RNAs (vsiRNAs) can target not only viruses but also plant genes. Apple chlorotic leaf spot virus (ACLSV) is an RNA virus that infects Rosaceae plants extensively, including apple, pear and hawthorn. Here, we report an ACLSV-derived vsiRNA [vsiR1360(-)] that targets and down-regulates the leucine-rich repeat receptor-like kinase 1 (LRR-RLK1) gene of hawthorn (Crataegus pinnatifida). The targeting and cleavage of the CpLRR-RLK1 gene by vsiR1360(-) were validated by RNA ligase-mediated 5' rapid amplification of cDNA ends and tobacco transient transformation assays. And the CpLRR-RLK1 protein fused to green fluorescent protein localized to the cell membrane. Conserved domain and phylogenetic tree analyses showed that CpLRR-RLK1 is closely related to the proteins of the LRRII-RLK subfamily. The biological function of CpLRR-RLK1 was explored by heterologous overexpression of CpLRR-RLK1 gene in Arabidopsis. The results of inoculation of Pst DC3000 in Arabidopsis leaves showed that the symptoms of CpLRR-RLK1 overexpression plants infected with Pst DC3000 were significantly reduced compared with the wild type. In addition, the detection of reactive oxygen species and callose deposition and the expression analysis of defense-related genes showed that the CpLRR-RLK1 gene can indeed enhance the resistance of Arabidopsis to bacteria disease.

Carbohydrate metabolism and transport in apple roots under nitrogen deficiency

Soluble sugars play important roles in plant development and stress response, and the nitrogen supply level can affect the among-organ distribution and metabolism of sugar in plants and, in turn, plant growth. To explore the adaptive response of apple root growth to nitrogen supply and its relationship with sugar metabolism, we used a hydroponic culture system to study how the nitrogen supply affects soluble sugar concentrations and sugar metabolism in apple roots. In hydroponic seedlings of Malus hupehensis, low nitrogen application caused rapid and vigorous proliferation of lateral roots, and the transcript levels of MdSOT1 and MdSUT3, which are involved in photoassimilate unloading in roots, were upregulated. The accumulation of sorbitol and sucrose in the fine roots was higher, and the activities of sucrose synthase, invertase and sorbitol dehydrogenase, which are involved in the degradation of sucrose and sorbitol, were significantly increased under a low nitrogen supply. Genes involved in sugar degradation, such as MdSDH1, MdSuSy5, and MdNINV3, play important roles in the efficient use of sorbitol and sucrose under nitrogen deficiency. Additionally, the activity of fructokinase and hexokinase, which are involved in hexose phosphorylation, and transcript levels of MdFRK2 and MdHK3 were significantly upregulated under nitrogen deficiency, and the hexose phosphate products F6P and G6P accumulated greatly in the roots. These results showed that the sugar metabolism capability and sink strength of the roots increased under low nitrogen, indicating that low nitrogen promotes the utilization of sugar in the roots to meet the demand for sugar under rapid root growth.

Alternaria Leaf Spot Caused by Alternaria Species: An Emerging Problem on Ornamental Plants in Italy

Serious outbreaks of Alternaria leaf spot and plant decay have recently been recorded on several ornamental plants in the Biella Province (Northern Italy). Twenty-two fungal isolates were obtained from Alternaria infected plant tissues from 13 ornamental hosts. All the isolates were identified morphologically as small-spored Alternaria species. Multilocus sequence typing, carried out by means of ITS, rpb2, tef1, endoPG, Alt a 1, and OPA10-2, assigned 19 isolates as Alternaria alternata, two isolates as belonging to the Alternaria arborescens species complex, and one isolate as an unknown Alternaria sp. Haplotype analyses of ornamental and reference A. alternata isolates from 12 countries identified 14 OPA10-2 and 11 endoPG haplotypes showing a relatively high haplotype diversity. A lack of host specialization or geographic distribution was observed. The host range of the studied A. alternata isolates expanded in cross-pathogenicity assays, and more aggressiveness was frequently observed on the experimental plants than on the host plants from which the fungal isolates were originally isolated. High disease severity, population expansion, intraspecies diversity, and increased range of experimental hosts were seen in the emergence of Alternaria disease on ornamentals. More epidemiological and molecular studies should be performed to better understand these diseases, taking into consideration factors such as seed transmission and ongoing climate changes.

CDPK6 phosphorylates and stabilizes MYB30 to promote hyperoside biosynthesis that prolongs the duration of full-blooming in okra

The flowers of okra (Abelmoschus esculentus) open and wilt within only a few hours, and this is accompanied by accumulation of hyperoside, a secondary metabolite in the flavonoid pathway. However, little is known about the relationship between flavonoids and flowering. Here, we found that exogenous application of hyperoside extended the duration of the full-blooming period by more than 3-fold, and this was accompanied by a 14.7-fold increase in the expression of CALCIUM-DEPENDENT PROTEIN KINASE6 (AeCDPK6). Gene expression profiling indicated that the transcription factor AeMYB30 was co-expressed with AeCDPK6, and detailed protein interaction and phosphorylation experiments together with yeast two-hybrid and bimolecular fluorescence complementation assays demonstrated an interaction between AeMYB30 and AeCDPK6. AeCDPK6 specifically phosphorylated AeMYB30S191, leading to increased protein stability and prevention of degradation. Furthermore, AeMYB30 directly bound to the promoter of AeUF3GaT1, a key enzyme in the hyperoside biosynthesis pathway. Analysis of transgenic plants showed that AeCDPK6 was required for the hyperoside-induced phosphorylation of AeMYB30 to enhance its stability and transcriptional activity. Ectopic expression of AeCDPK6 promoted hyperoside accumulation and prolonged the full-blooming period in an AeMYB30-dependent manner. Our results indicate the role of AeCDPK6-AeMYB30 in the molecular mechanism by which hyperoside regulates the period of full blooming in okra, a plant with a short duration of flowering.

Time Series RNA-seq in Pigeonpea Revealed the Core Genes in Metabolic Pathways under Aluminum Stress

Pigeonpea is an important economic crop in the world and is mainly distributed in tropical and subtropical regions. In order to further expand the scope of planting, one of the problems that must be solved is the impact of soil acidity on plants in these areas. Based on our previous work, we constructed a time series RNA sequencing (RNA-seq) analysis under aluminum (Al) stress in pigeonpea. Through a comparison analysis, 11,425 genes were found to be differentially expressed among all the time points. After clustering these genes by their expression patterns, 12 clusters were generated. Many important functional pathways were identified by gene ontology (GO) analysis, such as biological regulation, localization, response to stimulus, metabolic process, detoxification, and so on. Further analysis showed that metabolic pathways played an important role in the response of Al stress. Thirteen out of the 23 selected genes related to flavonoids and phenols were downregulated in response to Al stress. In addition, we verified these key genes of flavonoid- and phenol-related metabolism pathways by qRT-PCR. Collectively, our findings not only revealed the regulation mechanism of pigeonpea under Al stress but also provided methodological support for further exploration of plant stress regulation mechanisms.

Physiological and transcriptome analyses of the effects of exogenous dopamine on drought tolerance in apple

Water shortage is one of the main limiting factors in apple (Malus domestica Borkh.) production. Although dopamine is produced in plants and has been linked with response to abiotic stress, the underlying mechanism remains unknown. In this study, physiological analyses revealed that pretreatment with 100 μM dopamine alleviated drought stress in apple seedlings. Dopamine inhibited the degradation of photosynthetic pigments and increased net photosynthetic rate under drought stress. Dopamine also reduced H₂O₂ content, possibly through direct scavenging and by mediating the antioxidant enzyme activity. Seedlings pretreated with dopamine had higher sucrose and malic acid contents but lower starch accumulation in their leaves. RNA-Seq analysis identified 1052 differentially expressed genes (DEGs) between non-treated and dopamine-pretreated plants under drought. An in-depth analysis of these DEGs revealed that dopamine regulated the expression of genes related to metabolism of nitrogen, secondary compounds, and amino acids under drought stress. In addition, dopamine may improve apple drought tolerance by activating Ca²⁺ signaling pathways through increased expression of CNGC and CAM/CML family genes. Moreover, analysis of transcription factor expression suggested that dopamine affected drought tolerance mainly through the regulation of WRKY, ERF, and NAC transcription factors.

A Sugar Transporter Takes Up both Hexose and Sucrose for Sorbitol-Modulated In Vitro Pollen Tube Growth in Apple

Rapid pollen tube growth requires uptake of Suc or its hydrolytic products, hexoses, from the apoplast of surrounding tissues in the style. Due to species-specific sugar requirements, reliance of pollen germination and tube growth on cell wall invertase and Suc or hexose transporters varies between species, but it is not known if plants have a sugar transporter that mediates the uptake of both hexose and Suc for pollen tube growth. Here, we show that a sugar transporter protein in apple (Malus domestica), MdSTP13a, takes up both hexose and Suc when expressed in yeast, and is essential for pollen tube growth on Glc and Suc but not on maltose. MdSTP13a-mediated direct uptake of Suc is primarily responsible for apple pollen tube growth on Suc medium. Sorbitol, a major photosynthate and transport carbohydrate in apple, modulates pollen tube growth via the MYB transcription factor MdMYB39L, which binds to the promoter of MdSTP13a to activate its expression. Antisense repression of MdSTP13a blocks sorbitol-modulated pollen tube growth. These findings demonstrate that MdSTP13a takes up both hexose and Suc for sorbitol-modulated pollen tube growth in apple, revealing a situation where acquisition of sugars for pollen tube growth is regulated by a sugar alcohol.

Analysis of sorbitol content variation in wild and cultivated apples

BACKGROUND

Sorbitol is the major sugar alcohol in apple and its accumulation in fruit is associated with fruit sweetness. However, little is known about variation in sorbitol content in fruits of apple germplasm. In this study, we investigated sorbitol content in mature fruits of 243 apple cultivars and 20 wild relatives using high-performance liquid chromatography (HPLC).

RESULTS

Sorbitol accumulation showed a significant variation in apple germplasm. Overall, cultivated fruits had significantly lower content of sorbitol than wild fruits. Fruit sorbitol concentration was significantly correlated with fruit size and acidity that are extensively domesticated traits. Hence, the variation in sorbitol accumulation between cultivated and wild fruits may be the indirect result of fruit size and acidity selection during domestication. Moreover, sorbitol content was maintained at low levels throughout fruit development, with a dramatic decrease at the middle stage. The SDH1 gene was highly expressed throughout fruit development, and its expression showed a significant correlation with fruit sorbitol concentration, suggesting its potential role in apple fruit sorbitol accumulation.

CONCLUSIONS

The finding that there is a great variation in fruit sorbitol content among apple germplasm will be helpful for genetic improvement of fruit sorbitol content in apple breeding programs. © 2019 Society of Chemical Industry.

Plant immunity in signal integration between biotic and abiotic stress responses

Plants constantly monitor and cope with the fluctuating environment while hosting a diversity of plant-inhabiting microbes. The mode and outcome of plant-microbe interactions, including plant disease epidemics, are dynamically and profoundly influenced by abiotic factors, such as light, temperature, water and nutrients. Plants also utilize associations with beneficial microbes during adaptation to adverse conditions. Elucidation of the molecular bases for the plant-microbe-environment interactions is therefore of fundamental importance in the plant sciences. Following advances into individual stress signaling pathways, recent studies are beginning to reveal molecular intersections between biotic and abiotic stress responses and regulatory principles in combined stress responses. We outline mechanisms underlying environmental modulation of plant immunity and emerging roles for immune regulators in abiotic stress tolerance. Furthermore, we discuss how plants coordinate conflicting demands when exposed to combinations of different stresses, with attention to a possible determinant that links initial stress response to broad-spectrum stress tolerance or prioritization of specific stress tolerance.

Comparative genomics of Alternaria species provides insights into the pathogenic lifestyle of Alternaria brassicae - a pathogen of the Brassicaceae family

BACKGROUND

Alternaria brassicae, a necrotrophic pathogen, causes Alternaria Leaf Spot, one of the economically important diseases of Brassica crops. Many other Alternaria spp. such as A. brassicicola and A. alternata are known to cause secondary infections in the A. brassicae-infected Brassicas. The genome architecture, pathogenicity factors, and determinants of host-specificity of A. brassicae are unknown. In this study, we annotated and characterised the recently announced genome assembly of A. brassicae and compared it with other Alternaria spp. to gain insights into its pathogenic lifestyle.

RESULTS

We also sequenced the genomes of two A. alternata isolates that were co-infecting B. juncea using Nanopore MinION sequencing for additional comparative analyses within the Alternaria genus. Genome alignments within the Alternaria spp. revealed high levels of synteny between most chromosomes with some intrachromosomal rearrangements. We show for the first time that the genome of A. brassicae, a large-spored Alternaria species, contains a dispensable chromosome. We identified 460 A. brassicae-specific genes, which included many secreted proteins and effectors. Furthermore, we have identified the gene clusters responsible for the production of Destruxin-B, a known pathogenicity factor of A. brassicae.

CONCLUSION

The study provides a perspective into the unique and shared repertoire of genes within the Alternaria genus and identifies genes that could be contributing to the pathogenic lifestyle of A. brassicae.

Parallel Bud Mutation Sequencing Reveals that Fruit Sugar and Acid Metabolism Potentially Influence Stress in Malus

Apple sugar and acid are the most important traits of apple fruit. Bud sport cultivars can provide abundant research materials for functional gene studies in apple. In this study, using bud sport materials with a rather different sugar and acid flavor, i.e., "Jonathan" and "Sweet Jonathan", we profiled the whole genome variations and transcriptional regulatory network during fruit developmental stages using whole genome sequencing and RNA-sequencing. Variation analysis identified 4,198,955 SNPs, 319,494 InDels, and 32,434 SVs between the two cultivars. In total, 4313 differentially expressed genes among all of the d 44,399 genes expressed were identified between the two cultivars during fruit development, and functional analysis revealed stress response and signal transduction related genes were enriched. Using 24,047 genes with a more variable expression value, we constructed 28 co-expression modules by weighted correlation network analysis. Deciphering of 14 co-expression modules associated with sugar or acid accumulation during fruit development revealed the hub genes associated with sugar and acid metabolism, e.g., MdDSP4, MdINVE, and MdSTP7. Furthermore, exploration of the intra network of the co-expression module indicated the close relationship between sugar and acid metabolism or sugar and stress. Motif-based sequence analysis of the 17 differentially expressed ATP-binding cassette transporter genes and Yeast one-hybrid assay identified and confirmed a transcription factor, MdBPC6, regulating the ATP-binding cassette (ABC) transporter genes and potentially participating in the apple fruit development or stress response. Collectively, all of the results demonstrated the use of parallel bud mutation sequencing and identified hub genes, and inferred regulatory relationships providing new information about apple fruit sugar and acid accumulation or stress response.

Development of an efficient root transgenic system for pigeon pea and its application to other important economically plants

For non-model plants, functional characterization of genes is still hampered by lack of efficient stable transformation procedures. Here, we report a simple, fast and efficient transformation technique with Agrobacterium rhizogenes for generating stable transgenic roots in living plants to facilitate functional studies in vivo. We showed that injection of A. rhizogenes into stems of various plant species lead to stable transgenic root generation, which can sustain plant growth after the original, non-transgenic roots were cut off. A transformation system was established for pigeon pea, a major woody food crop, after optimizing the selection of A. rhizogenes strains, bacterium concentration, injection position and seedling age. RT-PCR and fluorescence observation indicated a transgenic root induction efficiency of about 39% in pigeon pea. Furthermore, induction of hairy roots was achieved in nine out of twelve tested economically important plants at an efficiency of 15-39%. As proof of concept, bimolecular fluorescence complementation (BiFC) assay was applied to test the interaction between CcCIPK14 and CcCBL1/2 in pigeon pea. Additionally, ectopic expression of the bZIP transcription factor MdHY5 from apple confirmed the utility of the transformation technique for engineering anthocyanin synthesis in roots. Taken together, we show that this method allows fast in vivo studies of gene function in a wide range of plant species.

MdWRKY100 encodes a group I WRKY transcription factor in Malus domestica that positively regulates resistance to Colletotrichum gloeosporioides infection

Apple (Malus domestica) is an important fruit worldwide; however, the development of the apple industry is limited by fungal disease. Apple bitter rot caused by the pathogen Colletotrichum gloeosporioides is one of the most devastating apple diseases, leading to large-scale losses in apple quality and production. WRKY transcription factors have important functions in the regulation of biotic and abiotic stresses. However, their biological and molecular functions in non-model plants, including apple, remain poorly understood. Here, we isolated MdWRKY100 from 'Hanfu' apple. The MdWRKY100 protein fused to green fluorescent protein localized to the nucleus, and MdWRKY100 in yeast cells displayed transcriptional activation activity, which is consistent with the function of a transcription factor. Additionally, several putative cis-acting elements involved in abiotic stress responsiveness were also identified in the MdWRKY100 promoter. Transcriptional analysis revealed that MdWRKY100 was expressed ubiquitously in all examined apple organs. Overexpression in apple increased resistance to Colletotrichum gloeosporioides, while RNAi silencing transgenic plants were more sensitive to Colletotrichum gloeosporioides. Collectively, our data demonstrate that MdWRKY100 is a positive regulator of Colletotrichum gloeosporioides resistance in apple.

MdUGT88F1-Mediated Phloridzin Biosynthesis Regulates Apple Development and Valsa Canker Resistance

In apple (Malus domestica), the polyphenol profile is dominated by phloridzin, but its physiological role remains largely elusive. Here, we used MdUGT88F1 (a key UDP-glucose:phloretin 2'-O-glucosyltransferase gene) transgenic apple lines and Malus spp. germplasm to gain more insight into the physiological role of phloridzin in apple. Decreasing phloridzin biosynthesis in apple lines by RNA silencing of MdUGT88F1 led to a series of severe phenotypic changes that included severe stunting, reduced internode length, spindly leaf shape, increased stem numbers, and weak adventitious roots. These changes were associated directly with reduced lignin levels and disorders in cell wall polysaccharides. Moreover, compact organization of tissues and thickened bark enhanced resistance to Valsa canker (caused by the fungus Valsa mali), which was associated with lignin- and cell wall polysaccharide-mediated increases of salicylic acid and reactive oxygen species. Phloridzin was also assumed to be utilized directly as a sugar alternative and a toxin accelerator by V. mali in apple. Therefore, after infection with V. mali, a higher level of phloridzin slightly compromised resistance to Valsa canker in MdUGT88F1-overexpressing apple lines. Taken together, our results shed light on the importance of MdUGT88F1-mediated biosynthesis of phloridzin in the interplay between plant development and pathogen resistance in apple trees.

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.

Melatonin increases the performance of Malus hupehensis after UV-B exposure

Melatonin, an evolutionarily conserved molecule, is implicated in numerous physiological processes in plants. To explore the potential roles of melatonin in response to UV-B radiation, we examined the influence of exogenous melatonin on Malus hupehensis Rehd. seedlings under two levels of UV-B radiation. Under UV-B stress, seedlings showed significant reduction in plant growth, biomass production, and root system development. However, 1 μM melatonin solution markedly alleviated these effects, especially at the higher dosage of UV-B radiation. The inhibitory effects of UV-B radiation on photosynthetic parameters, chlorophyll fluorescence parameters, stomatal apertures, chlorophyll levels and leaf membrane damages were also markedly alleviated with melatonin application. Melatonin treatment was also associated with higher activity and expression of genes encoding antioxidant enzymes (ascorbate peroxidase, catalase and peroxidase) and greater decline of H₂O₂ content in leaves exposed to UV-B. Moreover, exogenous melatonin treatment and UV-B stress increased the concentration of endogenous melatonin. The content of several phenolic compounds, including chlorogenic acid, phloridzin and quercetin-3-galactoside, also increased under UV-B stress, and these were further elevated significantly with melatonin addition. This study provides insight into the role(s) of endogenous melatonin in response to UV-B stress, and will facilitate application of exogenous melatonin in agriculture.

Genome-wide identification and expression analysis of calmodulin and calmodulin-like genes in apple (Malus × domestica)

Changes in intracellular calcium (Ca²⁺) levels in response to developmental processes or external stimuli serve as signals in eukaryotic cells. These Ca²⁺ signals are likely perceived through sensor proteins that bind Ca²⁺ by EF-hand (a helix-loop-helix structure) motif. Calmodulins (CaMs), a group of well-characterized Ca²⁺ sensors, and calmodulin-like (CMLs) are implicated in a large number of diverse cellular processes, including plant development and stress responses. In this study, apple (Malus × domestica) genes encoding CaM and CML proteins that only possess EF-hand motifs with no other functional domains were analyzed. A total of 4 MdCaM and 58 MdCML genes were identified, which are spread among 16 out of the 17 apple chromosomes. Bioinformatics analyses, including protein characteristics, conserved domain, evolutionary relationships and chromosomal locations, demonstrated the conservation and divergence of MdCaMs/CMLs. In addition, expression analysis showed that MdCaMs/CMLs are expressed in more than one tissue, including shoot tips, roots, mature leaves, flowers and fruit. Furthermore, the expression of some MdCaM/CML members responded to plant hormones (abscisic acid, jasmonic acid) and salt stress, suggesting a potential role of these genes in responses to biotic and abiotic stress. Overexpression of stress-induced MdCML3 gene significantly improved the tolerance of apple calli to salinity and ABA. The identification and characterization of MdCaMs/CMLs in apple lays a foundation for future functional studies of these genes.

In vitro tissue culture of apple and other Malus species: recent advances and applications

Studies on the tissue culture of apple have allowed for molecular, biotechnological and applied breeding research to advance. In the past 8 years, over 100 papers advancing basic biology, genetic transformation and cryobiology have emerged. Apple (Malus × domestica Borkh.; Rosaceae) is an important fruit crop grown mainly in temperate regions of the world. In vitro tissue culture is a biotechnological technique that has been used to genetically improve cultivars (scions) and rootstocks. This updated review presents a synthesis of findings related to the tissue culture of apple and other Malus spp. between 2010 and 2018. Increasingly complex molecular studies that are examining the apple genome, for example, in a bid to identify the cause of epigenetic mutations and the role of transposable elements in this process would benefit from genetically stable source material, which can be produced in vitro. Several notable or curious in vitro culture methods have been reported to improve shoot regeneration and induce the production of tetraploids in apple cultivars and rootstocks. Existing studies have revealed the molecular mechanism underlying the inhibition of adventitious roots by cytokinin. The use of the plant growth correction factor allows hypothetical shoot production from leaf-derived thin cell layers relative to conventional leaf explants to be determined. This updated review will allow novices and established researchers to advance apple and Malus biotechnology and breeding programs.

Necrotrophic Exploitation and Subversion of Plant Defense: A Lifestyle or Just a Phase, and Implications in Breeding Resistance

Breeding disease-resistant plants is a critical, environmentally friendly component of any strategy to sustainably feed and clothe the 9.8 billion people expected to live on Earth by 2050. Here, I review current literature detailing plant defense responses as they relate to diverse biological outcomes; disease resistance, susceptibility, and establishment of mutualistic plant-microbial relationships. Of particular interest is the degree to which these outcomes are a function of plant-associated microorganisms' lifestyles; biotrophic, hemibiotrophic, necrotrophic, or mutualistic. For the sake of brevity, necrotrophic pathogens and the necrotrophic phase of pathogenicity are emphasized in this review, with special attention given to the host-specific pathogens that exploit defense. Defense responses related to generalist necrotrophs and mutualists are discussed in the context of excellent reviews by others. In addition, host evolutionary trade-offs of disease resistance with other desirable traits are considered in the context of breeding for durable disease resistance.

Increased activity of MdFRK2, a high-affinity fructokinase, leads to upregulation of sorbitol metabolism and downregulation of sucrose metabolism in apple leaves

To investigate the functions of fructokinase (FRK) in apple (Malus domestica) carbohydrate metabolism, we cloned the coding sequences of MdFRK1 and MdFRK2 from the 'Royal Gala' apple. The results showed that MdFRK2 expression was extremely high in shoot tips and young fruit. Analyses of heterologously expressed proteins revealed that MdFRK2 had a higher affinity for fructose than did MdFRK1, with Km values of 0.1 and 0.62 mM for MdFRK2 and MdFRK1, respectively. The two proteins, however, exhibited similar Vmax values when their activities were significantly inhibited by high concentrations of fructose. MdFRK2 ectopic expression was associated with a general decrease in fructose concentration in transgenic lines. In leaves, increased FRK activity similarly resulted in reduced concentrations of glucose and sucrose but no alterations in sorbitol concentration. When compared with those in the untransformed control, genes involved in sorbitol synthesis (A6PR) and the degradation pathway (SDH1/2) were significantly upregulated in transgenic lines, whereas those involved in sucrose synthesis (SPS1) and other degradation processes (SUSY4, NINV1/2, and HxK2) were downregulated. The activity of enzymes participating in carbohydrate metabolism was proportional to the level of gene expression. However, the growth performance and photosynthetic efficiency did not differ between the transgenic and wild-type plants. These results provide new genetic evidence to support the view that FRK plays roles in regulating sugar and sorbitol metabolism in Rosaceae plants.

Genome-Wide Identification and Characterization of CIPK Family and Analysis Responses to Various Stresses in Apple (Malus domestica)

In the CIPK family, the CBL-interacting protein kinases have shown crucial roles in hormone signaling transduction, and response to abiotic stress in plant developmental processes. The CIPK family is characterized by conserved NAF/FISL (Asn-Ala-Phe) and PPI (protein-phosphatase interaction) domains in the C-terminus. However, little data has been reported about the CIPK family in apple. A total of 34 MdCIPK genes were identified from the apple genome in this study and were later divided into two groups according to the CIPK domains, characterized by gene structure and chromosomal distribution, and then mapped onto 17 chromosomes. All MdCIPK genes were expressed in the four apple tissues (leaf, root, flower, and fruit). In addition, the MdCIPK gene expression profile showed that five members among them revealed enhanced expression during the pollen tube growth stages. The MdCIPK4 was the most expressive during the entire fruit development stages. Under stress conditions 21 MdCIPK genes transcript levels were up-regulated in response to fungal and salt treatments. This suggested the possible features of these genes’ response to stresses in apples. Our findings provide a new insight about the roles of CIPK genes in apples, which could contribute to the cloning and functional analysis of CIPK genes in the future.

Sugar metabolism and accumulation in the fruit of transgenic apple trees with decreased sorbitol synthesis

Both sorbitol and sucrose are synthesized in source leaves and transported to fruit for supporting fruit growth in tree fruit species of the Rosaceae family. In apple (Malus domestica), antisense suppression of aldose-6-phosphate reductase, the key enzyme for sorbitol synthesis, significantly decreased the sorbitol concentration but increased the sucrose concentration in leaves, leading to a lower sorbitol but a higher sucrose supply to fruit in these plants. In response to this altered carbon supply, the transgenic fruit had lower concentration of sorbitol and much higher concentration of glucose but similar levels of fructose, sucrose, and starch throughout fruit development relative to the untransformed control. Activities of sorbitol dehydrogenase, fructokinase, and sucrose phosphate synthase were lower, whereas activities of neutral invertase, sucrose synthase, and hexokinase were higher in the transgenic fruit during fruit development. Transcript levels of MdSOT1, MdSDHs, MdFK2, and MdSPS3/6 were downregulated, whereas transcript levels of MdSUC1/4, MdSUSY1-3, MdNIV1/3, MdHKs, and MdTMT1 were upregulated in the transgenic fruit. These findings suggest that the Sucrose cycle and the sugar transport system are very effective in maintaining the level of fructose and provide insights into the roles of sorbitol and sucrose in regulating sugar metabolism and accumulation in sorbitol-synthesizing species.