Utilization of tomato microarrays for comparative gene expression analysis in the Solanaceae

The pivotal ripening gene SlDML2 participates in regulating disease resistance in tomato

Fruit ripening and disease resistance are two essential biological processes for quality formation and maintenance. DNA methylation, in the form of 5-methylcytosine (5mC), has been elucidated to modulate fruit ripening, but its role in regulating fruit disease resistance remains poorly understood. In this study, we show that mutation of SlDML2, the DNA demethylase gene essential for fruit ripening, affects multiple developmental processes of tomato besides fruit ripening, including seed germination, leaf length and width and flower branching. Intriguingly, loss of SlDML2 function decreased the resistance of tomato fruits against the necrotrophic fungal pathogen Botrytis cinerea. Comparative transcriptomic analysis revealed an obvious transcriptome reprogramming caused by SlDML2 mutation during B. cinerea invasion. Among the thousands of differentially expressed genes, SlβCA3 encoding a β-carbonic anhydrase and SlFAD3 encoding a ω-3 fatty acid desaturase were demonstrated to be transcriptionally activated by SlDML2-mediated DNA demethylation and positively regulate tomato resistance to B. cinerea probably in the same genetic pathway with SlDML2. We further show that the pericarp tissue surrounding B. cinerea infection exhibited a delay in ripening with singnificant decrease in expression of ripening genes that are targeted by SlDML2 and increase in expression of SlβCA3 and SlFAD3. Taken together, our results uncover an essential layer of gene regulation mediated by DNA methylation upon B. cinerea infection and raise the possible that the DNA demethylase gene SlDML2, as a multifunctional gene, participates in modulating the trade-off between fruit ripening and disease resistance.

Transcriptional regulatory mechanism of a variant transcription factor CsWRKY23 in citrus fruit resistance to Penicillium digitatum

As the most productive fruit in the world, there is a lack of research on disease resistance mechanisms in citrus. WRKY transcription factors are mediators of disease resistance in plants. In this research, CsWRKY23, which could respond to exogenous salicylic acid (SA) was identified. As a variant WRKY, the transient overexpression of CsWRKY23 in citrus peel enhanced the resistance to Penicillium digitatum. The potential targets of CsWRKY23 were identified by using DNA affinity purification sequencing (DAP-seq). In combination with RNA sequencing (RNA-seq) data, 48 direct target genes of CsWRKY23 were identified. In addition, five genes for EMSA and dual-luciferase report system analysis were selected and the results showed that CsWRKY23 activate CsAAE12, CsRbohD, CsSARD1, CsWRKY22 and CsIQM6 expressions. Further analysis revealed that CsWRKY23 contributed to SA synthesis pathway, reactive oxygen species (ROS) accumulation, and cell wall strengthening. This study presented a possible regulatory network for disease resistance involving the CsWRKY23.

CsWRKY25 Improves Resistance of Citrus Fruit to Penicillium digitatum via Modulating Reactive Oxygen Species Production

WRKY transcription factors (TFs) play crucial roles in the regulation of biotic stress. Citrus is the most productive fruit in the world. It is of great value to investigate the regulatory molecular mechanism of WRKYs in improving disease resistance. In this research, the transcription level of CsWRKY25 was upregulated in P. digitatum infected citrus peel, and CsWRKY25 activated the expression of three target genes (RbohB, RbohD, and PR10). Besides, the Agrobacterium-mediated transient overexpression of CsWRKY25 has also been shown to enhance resistance to P. digitatum in citrus, and caused the accumulation of hydrogen peroxide and lignin. The accumulation of ROS also activated the antioxidant system, the catalase (CAT), peroxidase (POD), and cinnamyl alcohol dehydrogenase (CAD) genes were significant upregulated, leading to activation of antioxidant enzymes. In addition, the up-regulated expression of MPK5 and MPK6 genes suggested that the regulatory role of CsWRKY25 might be related to the phosphorylation process. In conclusion, CsWRKY25 could enhance the resistance to P. digitatum via modulating ROS production and PR genes in citrus peel.

Genome-wide binding analysis of the tomato transcription factor SlDof1 reveals its regulatory impacts on fruit ripening

The DNA binding with one finger (Dof) proteins are plant-specific transcription factors involved in a variety of biological processes. However, little is known about their functions in fruit ripening, a flowering-plant-specific process that is required for seed maturation and dispersal. Here, we found that the tomato Dof transcription factor SlDof1, is necessary for normal fruit ripening. Knockdown of SlDof1 expression by RNA interference delayed ripening-related processes, including lycopene synthesis and ethylene production. Transcriptome profiling indicated that SlDof1 influences the expression of hundreds of genes, and a chromatin immunoprecipitation sequencing revealed a large number of SlDof1 binding sites. A total of 312 genes were identified as direct targets of SlDof1, among which 162 were negatively regulated by SlDof1 and 150 were positively regulated. The SlDof1 target genes were involved in a variety of metabolic pathways, and follow-up analyses verified that SlDof1 directly regulates some well-known ripening-related genes including ACS2 and PG2A as well as transcriptional repressor genes such as SlIAA27. Our findings provide insights into the transcriptional regulatory networks underlying fruit ripening and highlight a gene potentially useful for genetic engineering to control ripening.

Transcription Factor CsWRKY65 Participates in the Establishment of Disease Resistance of Citrus Fruits to Penicillium digitatum

Penicillium digitatum is the primary pathogen that causes serious yield losses worldwide. In our previous study, CsWRKY transcription factors (TFs) and some genes associated with immunity were identified in citrus fruits after P. digitatum infection, but little information is available in the literature on the mechanisms of TFs in citrus disease resistance. In this study, the possible mechanisms of CsWRKY65 participating in the establishment of disease resistance were investigated. Results show that CsWRKY65 was a transcriptional activator in the nucleus. The dual-luciferase transient assays and electrophoretic mobility shift assays showed that CsWRKY65 bound with CsRbohB, CsRbohD, CsCDPK33, and CsPR10 promoters to activate gene transcription. Besides, the transient overexpression of CsWRKY65 induced reactive oxygen species accumulation and increased PR gene expression in Nicotiana benthamiana leaves. The transient overexpression of CsWRKY65 in the citrus peel enhanced the disease resistance against P. digitatum. In conclusion, CsWRKY65 is likely to be involved in regulating the disease resistance to P. digitatum of citrus fruits by directly activating the expressions of CsRbohB, CsRbohD, CsCDPK33, and CsPR10. This study provides new information for the mechanism of citrus WRKY TFs participating in the establishment of disease resistance.

Ubiquitination of phytoene synthase 1 precursor modulates carotenoid biosynthesis in tomato

Carotenoids are natural pigments that are indispensable to plants and humans, whereas the regulation of carotenoid biosynthesis by post-translational modification remains elusive. Here, we show that a tomato E3 ubiquitin ligase, Plastid Protein Sensing RING E3 ligase 1 (PPSR1), is responsible for the regulation of carotenoid biosynthesis. PPSR1 exhibits self-ubiquitination activity and loss of PPSR1 function leads to an increase in carotenoids in tomato fruit. PPSR1 affects the abundance of 288 proteins, including phytoene synthase 1 (PSY1), the key rate-limiting enzyme in the carotenoid biosynthetic pathway. PSY1 contains two ubiquitinated lysine residues (Lys380 and Lys406) as revealed by the global analysis and characterization of protein ubiquitination. We provide evidence that PPSR1 interacts with PSY1 precursor protein and mediates its degradation via ubiquitination, thereby affecting the steady-state level of PSY1 protein. Our findings not only uncover a regulatory mechanism for controlling carotenoid biosynthesis, but also provide a strategy for developing carotenoid-enriched horticultural crops.

Wang et al. report on the role of a novel E3 ubiquitin ligase, Plastid Protein Sensing RING E3 ligase 1 (PPSR1), during tomato fruit ripening and find that it interacts with phytoene synthase 1 (PSY1) precursor protein and mediates its degradation via ubiquitination. This affects the steady-state level of PSY1 protein, the key rate-limiting enzyme in the carotenoid biosynthetic pathway. This study may provide a strategy for developing carotenoid-enriched horticultural crops.

RNA methylomes reveal the m6A-mediated regulation of DNA demethylase gene SlDML2 in tomato fruit ripening

BACKGROUND

Methylation of nucleotides, notably in the forms of 5-methylcytosine (5mC) in DNA and N6-methyladenosine (m6A) in mRNA, carries important information for gene regulation. 5mC has been elucidated to participate in the regulation of fruit ripening, whereas the function of m6A in this process and the interplay between 5mC and m6A remain uncharacterized.

RESULTS

Here, we show that mRNA m6A methylation exhibits dynamic changes similar to DNA methylation during tomato fruit ripening. RNA methylome analysis reveals that m6A methylation is a prevalent modification in the mRNA of tomato fruit, and the m6A sites are enriched around the stop codons and within the 3' untranslated regions. In the fruit of the ripening-deficient epimutant Colorless non-ripening (Cnr) which harbors DNA hypermethylation, over 1100 transcripts display increased m6A levels, while only 134 transcripts show decreased m6A enrichment, suggesting a global increase in m6A. The m6A deposition is generally negatively correlated with transcript abundance. Further analysis demonstrates that the overall increase in m6A methylation in Cnr mutant fruit is associated with the decreased expression of RNA demethylase gene SlALKBH2, which is regulated by DNA methylation. Interestingly, SlALKBH2 has the ability to bind the transcript of SlDML2, a DNA demethylase gene required for tomato fruit ripening, and modulates its stability via m6A demethylation. Mutation of SlALKBH2 decreases the abundance of SlDML2 mRNA and delays fruit ripening.

CONCLUSIONS

Our study identifies a novel layer of gene regulation for key ripening genes and establishes an essential molecular link between DNA methylation and mRNA m6A methylation during fruit ripening.

Tandem gene duplication and recombination at the AT3 locus in the Solanaceae, a gene essential for capsaicinoid biosynthesis in Capsicum

Capsaicinoids are compounds synthesized exclusively in the genus Capsicum and are responsible for the burning sensation experienced when consuming hot pepper fruits. To date, only one gene, AT3, a member of the BAHD family of acyltransferases, is currently known to have a measurable quantitative effect on capsaicinoid biosynthesis. Multiple AT3 paralogs exist in the Capsicum genome, but their evolutionary relationships have not been characterized well. Recessive alleles at this locus result in absence of capsaicinoids in pepper fruit. To explore the evolution of AT3 in Capsicum and the Solanaceae, we sequenced this gene from diverse Capsicum genotypes and species, along with a number of representative solanaceous taxa. Our results revealed that the coding region of AT3 is highly conserved throughout the family. Further, we uncovered a tandem duplication that predates the diversification of the Solanaceae taxa sampled in this study. This pair of tandem duplications were designated AT3-1 and AT3-2. Sequence alignments showed that the AT3-2 locus, a pseudogene, retains regions of amino acid conservation relative to AT3-1. Gene tree estimation demonstrated that AT3-1 and AT3-2 form well supported, distinct clades. In C. rhomboideum, a non-pungent basal Capsicum species, we describe a recombination event between AT3-1 and AT3-2 that modified the putative active site of AT3-1, also resulting in a frame-shift mutation in the second exon. Our data suggest that duplication of the original AT3 representative, in combination with divergence and pseudogene degeneration, may account for the patterns of sequence divergence and punctuated amino acid conservation observed in this study. Further, an early rearrangement in C. rhomboidium could account for the absence of pungency in this Capsicum species.

The mode of action of remorin1 in regulating fruit ripening at transcriptional and post-transcriptional levels

Remorins are plant-specific and plasma membrane-associated proteins that display a variety of functions in plant growth, development, biotic and abiotic stresses, and signal transduction. However, little information is available for understanding their role in fruit ripening. Here, remorin 1 (SlREM1) is cloned from tomato and its localization is examined by co-localization analysis and immunoblotting. Functions of SlREM1 in fruit ripening are characterized based on gene expression, co-immunoprecipitation coupled with mass spectroscopy and split luciferase complementation imaging assays in SlREM1 overexpression and RNA interference (RNAi) lines. The results indicate that SlREM1 is localized at the plasma membrane. Overexpression of SlREM1 in tomato stimulates fruit ripening with an increase in ethylene production and lycopene accumulation as compared to the wild-type. Consistently, these genes involved in ethylene and lycopene biosynthesis and ripening regulators also are upregulated in SlREM1 overexpression lines. SlREM1 can interact with ethylene biosynthesis proteins SAM1, ACO1 and ACS2 and is degraded by ubiquitin-mediated proteolysis. Our findings reveal that SlREM1 serves as a positive regulator of fruit ripening and provide novel cues for understanding of the molecular regulation network of fruit ripening.

The dehydration stress of couch grass is associated with its lipid metabolism, the induction of transporters and the re-programming of development coordinated by ABA

Background

The wild relatives of crop species represent a potentially valuable source of novel genetic variation, particularly in the context of improving the crop’s level of tolerance to abiotic stress. The mechanistic basis of these tolerances remains largely unexplored. Here, the focus was to characterize the transcriptomic response of the nodes (meristematic tissue) of couch grass (a relative of barley) to dehydration stress, and to compare it to that of the barley crown formed by both a drought tolerant and a drought sensitive barley cultivar.

Results

Many of the genes up-regulated in the nodes by the stress were homologs of genes known to be mediated by abscisic acid during the response to drought, or were linked to either development or lipid metabolism. Transporters also featured prominently, as did genes acting on root architecture. The resilience of the couch grass node arise from both their capacity to develop an altered, more effective root architecture, but also from their formation of a lipid barrier on their outer surface and their ability to modify both their lipid metabolism and transporter activity when challenged by dehydration stress.

Conclusions

Our analysis revealed the nature of dehydration stress response in couch grass. We suggested the tolerance is associated with lipid metabolism, the induction of transporters and the re-programming of development coordinated by ABA. We also proved the applicability of barley microarray for couch grass stress-response analysis.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4700-3) contains supplementary material, which is available to authorized users.

Post-transcriptional regulation of fruit ripening and disease resistance in tomato by the vacuolar protease SlVPE3

BACKGROUND

Proteases represent one of the most abundant classes of enzymes in eukaryotes and are known to play key roles in many biological processes in plants. However, little is known about their functions in fruit ripening and disease resistance, which are unique to flowering plants and required for seed maturation and dispersal. Elucidating the genetic mechanisms of fruit ripening and disease resistance is an important goal given the biological and dietary significance of fruit.

RESULTS

Through expression profile analyses of genes encoding tomato (Solanum lycopersicum) cysteine proteases, we identify a number of genes whose expression increases during fruit ripening. RNA interference (RNAi)-mediated repression of SlVPE3, a vacuolar protease gene, results in alterations in fruit pigmentation, lycopene biosynthesis, and ethylene production, suggesting that SlVPE3 is necessary for normal fruit ripening. Surprisingly, the SlVPE3 RNAi fruit are more susceptible to the necrotrophic pathogen Botrytis cinerea. Quantitative proteomic analysis identified 314 proteins that differentially accumulate upon SlVPE3 silencing, including proteins associated with fruit ripening and disease resistance. To identify the direct SlVPE3 targets and mechanisms contributing to fungal pathogen resistance, we perform a screening of SlVPE3-interacting proteins using co-immunoprecipitation coupled with mass spectrometry. We show that SlVPE3 is required for the cleavage of the serine protease inhibitor KTI4, which contributes to resistance against the fungal pathogen B. cinerea.

CONCLUSIONS

Our findings contribute to elucidating gene regulatory networks and mechanisms that control fruit ripening and disease resistance responses.

Identification and Functional Characterization of a Tonoplast Dicarboxylate Transporter in Tomato (Solanum lycopersicum)

Acidity plays an important role in flavor and overall organoleptic quality of fruit and is mainly due to the presence of organic acids. Understanding the molecular basis of organic acid metabolism is thus of primary importance for fruit quality improvement. Here, we cloned a putative tonoplast dicarboxylate transporter gene (SlTDT) from tomato, and submitted it to the NCBI database (GenBank accession number: KC733165). SlTDT protein contained 13 putative transmembrane domains in silico analysis. Confocal microscopic study using green fluorescent fusion proteins revealed that SlTDT was localized on tonoplast. The expression patterns of SlTDT in tomato were analyzed by RT-qPCR. The results indicated that SlTDT expressed in leaves, roots, flowers and fruits at different ripening stages, suggesting SlTDT may be associated with the development of different tissues. To further explore the function of SlTDT, we constructed both overexpression and RNAi vectors and obtained transgenic tomato plants by agrobacterium-mediated method. Gas chromatography-mass spectrometer (GC-MS) analysis showed that overexpression of SlTDT significantly increased malate content, and reduced citrate content in tomato fruit. By contrast, repression of SlTDT in tomato reduced malate content of and increased citrate content. These results indicated that SlTDT played an important role in remobilization of malate and citrate in fruit vacuoles.

Comparative transcriptional profiling analysis of developing melon (Cucumis melo L.) fruit from climacteric and non-climacteric varieties

Background

In climacteric fruit-bearing species, the onset of fruit ripening is marked by a transient rise in respiration rate and autocatalytic ethylene production, followed by rapid deterioration in fruit quality. In non-climacteric species, there is no increase in respiration or ethylene production at the beginning or during fruit ripening. Melon is unusual in having climacteric and non-climacteric varieties, providing an interesting model system to compare both ripening types. Transcriptomic analysis of developing melon fruits from Védrantais and Dulce (climacteric) and Piel de sapo and PI 161375 (non-climacteric) varieties was performed to understand the molecular mechanisms that differentiate the two fruit ripening types.

Results

Fruits were harvested at 15, 25, 35 days after pollination and at fruit maturity. Transcript profiling was performed using an oligo-based microarray with 75 K probes. Genes linked to characteristic traits of fruit ripening were differentially expressed between climacteric and non-climacteric types, as well as several transcription factor genes and genes encoding enzymes involved in sucrose catabolism. The expression patterns of some genes in PI 161375 fruits were either intermediate between. Piel de sapo and the climacteric varieties, or more similar to the latter. PI 161375 fruits also accumulated some carotenoids, a characteristic trait of climacteric varieties.

Conclusions

Simultaneous changes in transcript abundance indicate that there is coordinated reprogramming of gene expression during fruit development and at the onset of ripening in both climacteric and non-climacteric fruits. The expression patterns of genes related to ethylene metabolism, carotenoid accumulation, cell wall integrity and transcriptional regulation varied between genotypes and was consistent with the differences in their fruit ripening characteristics. There were differences between climacteric and non-climacteric varieties in the expression of genes related to sugar metabolism suggesting that they may be potential determinants of sucrose content and post-harvest stability of sucrose levels in fruit. Several transcription factor genes were also identified that were differentially expressed in both types, implicating them in regulation of ripening behaviour. The intermediate nature of PI 161375 suggested that classification of melon fruit ripening behaviour into just two distinct types is an over-simplification, and that in reality there is a continuous spectrum of fruit ripening behaviour.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1649-3) contains supplementary material, which is available to authorized users.

Nicotiana benthamiana: Its History and Future as a Model for Plant-Pathogen Interactions

Nicotiana benthamiana is the most widely used experimental host in plant virology, due mainly to the large number of diverse plant viruses that can successfully infect it. Addi- tionally, N. benthamiana is susceptible to a wide variety of other plant-pathogenic agents (such as bacteria, oomycetes, fungi, and so on), making this species a cornerstone of host-pathogen research, particularly in the context of innate immunity and defense signaling. Moreover, because it can be genetically transformed and regenerated with good efficiency and is amenable to facile methods for virus- induced gene silencing or transient protein expression, N. benthamiana is rapidly gaining popularity in plant biology, particularly in studies requiring protein localization, inter- action, or plant-based systems for protein expression and purification. Paradoxically, despite being an indispensable research model, little is known about the origins, genetic variation, or ecology of the N. benthamiana accessions cur- rently used by the research community. In addition to ad- dressing these latter topics, the purpose of this review is to provide information regarding sources for tools and reagents that can be used to support research in N. benthamiana. Finally, we propose that N. benthamiana is well situated to become a premier plant cell biology model, particularly for the virology community, who as a group were the first to recognize the potential of this unique Australian native.

Identification of microRNAs in six solanaceous plants and their potential link with phosphate and mycorrhizal signaling

To date, only a limited number of solanaceous miRNAs have been deposited in the miRNA database. Here, genome-wide bioinformatic identification of miRNAs was performed in six solanaceous plants (potato, tomato, tobacco, eggplant, pepper, and petunia). A total of 2,239 miRNAs were identified following a range of criteria, of which 982 were from potato, 496 from tomato, 655 from tobacco, 46 from eggplant, 45 were from pepper, and 15 from petunia. The sizes of miRNA families and miRNA precursor length differ in all the species. Accordingly, 620 targets were predicted, which could be functionally classified as transcription factors, metabolic enzymes, RNA and protein processing proteins, and other proteins for plant growth and development. We also showed evidence for miRNA clusters and sense and antisense miRNAs. Additionally, five Pi starvation- and one arbuscular mycorrhiza (AM)-related cis-elements were found widely distributed in the putative promoter regions of the miRNA genes. Selected miRNAs were classified into three groups based on the presence or absence of P1BS and MYCS cis-elements, and their expression in response to Pi starvation and AM symbiosis was validated by quantitative reverse transcription-polymerase chain reaction (qRT-PCR). These results show that conserved miRNAs exist in solanaceous species and they might play pivotal roles in plant growth, development, and stress responses.

Expression Comparison of Oil Biosynthesis Genes in Oil Palm Mesocarp Tissue Using Custom Array

Gene expression changes that occur during mesocarp development are a major research focus in oil palm research due to the economic importance of this tissue and the relatively rapid increase in lipid content to very high levels at fruit ripeness. Here, we report the development of a transcriptome-based 105,000-probe oil palm mesocarp microarray. The expression of genes involved in fatty acid (FA) and triacylglycerol (TAG) assembly, along with the tricarboxylic acid cycle (TCA) and glycolysis pathway at 16 Weeks After Anthesis (WAA) exhibited significantly higher signals compared to those obtained from a cross-species hybridization to the Arabidopsis (p-value < 0.01), and rice (p-value < 0.01) arrays. The oil palm microarray data also showed comparable correlation of expression (r² = 0.569, p < 0.01) throughout mesocarp development to transcriptome (RNA sequencing) data, and improved correlation over quantitative real-time PCR (qPCR) (r² = 0.721, p < 0.01) of the same RNA samples. The results confirm the advantage of the custom microarray over commercially available arrays derived from model species. We demonstrate the utility of this custom microarray to gain a better understanding of gene expression patterns in the oil palm mesocarp that may lead to increasing future oil yield.

Brassica villosa, a system for studying non-glandular trichomes and genes in the Brassicas

Brassica villosa is a wild Brassica C genome species with very dense trichome coverage and strong resistance to many insect pests of Brassica oilseeds and vegetables. Transcriptome analysis of hairy B. villosa leaves indicated higher expression of several important trichome initiation genes compared with glabrous B. napus leaves and consistent with the Arabidopsis model of trichome development. However, transcripts of the TRY inhibitory gene in hairy B. villosa were surprisingly high relative to B. napus and relative transcript levels of SAD2, EGL3, and several XIX genes were low, suggesting potential ancillary or less important trichome-related roles for these genes in Brassica species compared with Arabidopsis. Several antioxidant, calcium, non-calcium metal and secondary metabolite genes also showed differential expression between these two species. These coincided with accumulation of two alkaloid-like compounds, high levels of calcium, and other metals in B. villosa trichomes that are correlated with the known tolerance of B. villosa to high salt and the calcium-rich natural habitat of this wild species. This first time report on the isolation of large amounts of pure B. villosa trichomes, on trichome content, and on relative gene expression differences in an exceptionally hairy Brassica species compared with a glabrous species opens doors for the scientific community to understand trichome gene function in the Brassicas and highlights the potential of B. villosa as a trichome research platform.

Tomato nuclear proteome reveals the involvement of specific E2 ubiquitin-conjugating enzymes in fruit ripening

BACKGROUND

Fruits are unique to flowering plants and play a central role in seed maturation and dispersal. Molecular dissection of fruit ripening has received considerable interest because of the biological and dietary significance of fruit. To better understand the regulatory mechanisms underlying fruit ripening, we report here the first comprehensive analysis of the nuclear proteome in tomato fruits.

RESULTS

Nuclear proteins were isolated from tomatoes in different stages of ripening, and subjected to iTRAQ (isobaric tags for relative and absolute quantification) analysis. We show that the proteins whose abundances change during ripening stages are involved in various cellular processes. We additionally evaluate changes in the nuclear proteome in the ripening-deficient mutant, ripening-inhibitor (rin), carrying a mutation in the transcription factor RIN. A set of proteins were identified and particular attention was paid to SlUBC32 and PSMD2, the components of ubiquitin-proteasome pathway. Through chromatin immunoprecipitation and gel mobility shift assays, we provide evidence that RIN directly binds to the promoters of SlUBC32 and PSMD2. Moreover, loss of RIN function affects protein ubiquitination in nuclei. SlUBC32 encodes an E2 ubiquitin-conjugating enzyme and a genome-wide survey of the E2 gene family in tomatoes identified five more E2s as direct targets of RIN. Virus-induced gene silencing assays show that two E2s are involved in the regulation of fruit ripening.

CONCLUSIONS

Our results uncover a novel function of protein ubiquitination, identifying specific E2s as regulators of fruit ripening. These findings contribute to the unraveling of the gene regulatory networks that control fruit ripening.

Tomato nuclear proteome reveals the involvement of specific E2 ubiquitin-conjugating enzymes in fruit ripening

BACKGROUND

Fruits are unique to flowering plants and play a central role in seed maturation and dispersal. Molecular dissection of fruit ripening has received considerable interest because of the biological and dietary significance of fruit. To better understand the regulatory mechanisms underlying fruit ripening, we report here the first comprehensive analysis of the nuclear proteome in tomato fruits.

RESULTS

Nuclear proteins were isolated from tomatoes in different stages of ripening, and subjected to iTRAQ (isobaric tags for relative and absolute quantification) analysis. We show that the proteins whose abundances change during ripening stages are involved in various cellular processes. We additionally evaluate changes in the nuclear proteome in the ripening-deficient mutant, ripening-inhibitor (rin), carrying a mutation in the transcription factor RIN. A set of proteins were identified and particular attention was paid to SlUBC32 and PSMD2, the components of ubiquitin-proteasome pathway. Through chromatin immunoprecipitation and gel mobility shift assays, we provide evidence that RIN directly binds to the promoters of SlUBC32 and PSMD2. Moreover, loss of RIN function affects protein ubiquitination in nuclei. SlUBC32 encodes an E2 ubiquitin-conjugating enzyme and a genome-wide survey of the E2 gene family in tomatoes identified five more E2s as direct targets of RIN. Virus-induced gene silencing assays show that two E2s are involved in the regulation of fruit ripening.

CONCLUSIONS

Our results uncover a novel function of protein ubiquitination, identifying specific E2s as regulators of fruit ripening. These findings contribute to the unraveling of the gene regulatory networks that control fruit ripening.

Conserved changes in the dynamics of metabolic processes during fruit development and ripening across species

Computational analyses of molecular phenotypes traditionally aim at identifying biochemical components that exhibit differential expression under various scenarios (e.g. environmental and internal perturbations) in a single species. High-throughput metabolomics technologies allow the quantification of (relative) metabolite levels across developmental stages in different tissues, organs, and species. Novel methods for analyzing the resulting multiple data tables could reveal preserved dynamics of metabolic processes across species. The problem we address in this study is 2-fold. (1) We derive a single data table, referred to as a compromise, which captures information common to the investigated set of multiple tables containing data on different fruit development and ripening stages in three climacteric (i.e. peach [Prunus persica] and two tomato [Solanum lycopersicum] cultivars, Ailsa Craig and M82) and two nonclimacteric (i.e. strawberry [Fragaria × ananassa] and pepper [Capsicum chilense]) fruits; in addition, we demonstrate the power of the method to discern similarities and differences between multiple tables by analyzing publicly available metabolomics data from three tomato ripening mutants together with two tomato cultivars. (2) We identify the conserved dynamics of metabolic processes, reflected in the data profiles of the corresponding metabolites that contribute most to the determined compromise. Our analysis is based on an extension to principal component analysis, called STATIS, in combination with pathway overenrichment analysis. Based on publicly available metabolic profiles for the investigated species, we demonstrate that STATIS can be used to identify the metabolic processes whose behavior is similarly affected during fruit development and ripening. These findings ultimately provide insights into the pathways that are essential during fruit development and ripening across species.

Genetic regulation and structural changes during tomato fruit development and ripening

Fruits are an important evolutionary acquisition of angiosperms, which afford protection for seeds and ensure their optimal dispersal in the environment. Fruits can be divided into dry or fleshy. Dry fruits are the more ancient and provide for mechanical seed dispersal. In contrast, fleshy fruits develop soft tissues in which flavor compounds and pigments accumulate during the ripening process. These serve to attract animals that eat them and disseminate the indigestible seeds. Fruit maturation is accompanied by several striking cytological modifications. In particular, plastids undergo significant structural alterations, including the dedifferentiation of chloroplasts into chromoplasts. Chloroplast biogenesis, their remodeling in response to environmental constraints and their conversion into alternative plastid types are known to require communication between plastids and the nucleus in order to coordinate the expression of their respective genomes. In this review, we discuss the role of plastid modifications in the context of fruit maturation and ripening, and consider the possible involvement of organelle-nucleus crosstalk via retrograde (plastid to nucleus) and anterograde (nucleus to plastid) signaling in the process.

Proteomic analysis during capsicum ripening reveals differential expression of ACC oxidase isoform 4 and other candidates

Capsicum (Capsicum annuum L.) is categorised as a non-climacteric fruit that exhibits limited ethylene production during ripening and the molecular mechanisms associated with this process are poorly understood. A proteomic approach was used to identify the differentially expressed proteins during various ripening stages (Green (G), Breaker Red 1 (BR1) and Light Red (LR)) and the genes associated with their synthesis. From 2D gel electrophoresis (2DGE), seven protein spots were identified as selectively present either in G or BR1 and are involved in carbon metabolism, colour and fruit development, protein synthesis and chaperones or biosynthesis of amino acids and polyamines. One candidate of interest, 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase (ACO) is known to be involved in ethylene biosynthesis and was only present in BR1 and is related to the tomato ACO isoform 4 (LeACO4) and hence named CaACO4. CaACO4 RNA expression as well as total ACO protein expression in multiple stages of ripening (G, Breaker (B), BR1, Breaker Red 2 (BR2), LR and Deep Red (DR)) corresponded to the 2DGE protein spot abundance in breaker stages. Our findings highlight the involvement of the ethylene pathway in non-climacteric fruit ripening.

Transcriptomic analysis of genes involved in the biosynthesis, recycling and degradation of L-ascorbic acid in pepper fruits (Capsicum annuum L.)

Sweet pepper (Capsicum annuum L.) is widely recognized among the vegetables with high content of ascorbic acid (AsA). However, the metabolic pathways involved in the biosynthesis, recycling and degradation of AsA and their relative contribution to the concentration of AsA have not been established yet. In the present work, the expression levels of selected genes involved in the AsA biosynthesis, degradation and recycling pathways were analyzed during development and ripening of pepper fruit cv. Palermo and in mature fruit of four cultivars (Lipari, C-116, Surrentino and Italverde) with different AsA concentrations. An inverse correlation was found between the expression of the biosynthetic genes and AsA concentrations, which could indicate that a feedback mechanism regulates AsA homeostasis in pepper fruits. Interestingly, analysis of mRNA levels of ascorbate oxidase, involved in the degradation of AsA, suggests that this enzyme plays a critical role in the regulation of the AsA pool during fruit development and ripening.

Nutrition metabolism plays an important role in the alternate bearing of the olive tree (Olea europaea L.)

The olive tree (Olea europaea L.) is widely known for its strong tendency for alternate bearing, which severely affects the fruit yield from year to year. Microarray based gene expression analysis using RNA from olive samples (on-off years leaves and ripe-unripe fruits) are particularly useful to understand the molecular mechanisms influencing the periodicity in the olive tree. Thus, we carried out genome wide transcriptome analyses involving different organs and temporal stages of the olive tree using the NimbleGen Array containing 136,628 oligonucleotide probe sets. Cluster analyses of the genes showed that cDNAs originated from different organs could be sorted into separate groups. The nutritional control had a particularly remarkable impact on the alternate bearing of olive, as shown by the differential expression of transcripts under different temporal phases and organs. Additionally, hormonal control and flowering processes also played important roles in this phenomenon. Our analyses provide further insights into the transcript changes between "on year" and "off year" leaves along with the changes from unrpipe to ripe fruits, which shed light on the molecular mechanisms underlying the olive tree alternate bearing. These findings have important implications for the breeding and agriculture of the olive tree and other crops showing periodicity. To our knowledge, this is the first study reporting the development and use of an olive array to document the gene expression profiling associated with the alternate bearing in olive tree.

Analysis of ripening-related gene expression in papaya using an Arabidopsis-based microarray

BACKGROUND

Papaya (Carica papaya L.) is a commercially important crop that produces climacteric fruits with a soft and sweet pulp that contain a wide range of health promoting phytochemicals. Despite its importance, little is known about transcriptional modifications during papaya fruit ripening and their control. In this study we report the analysis of ripe papaya transcriptome by using a cross-species (XSpecies) microarray technique based on the phylogenetic proximity between papaya and Arabidopsis thaliana.

RESULTS

Papaya transcriptome analyses resulted in the identification of 414 ripening-related genes with some having their expression validated by qPCR. The transcription profile was compared with that from ripening tomato and grape. There were many similarities between papaya and tomato especially with respect to the expression of genes encoding proteins involved in primary metabolism, regulation of transcription, biotic and abiotic stress and cell wall metabolism. XSpecies microarray data indicated that transcription factors (TFs) of the MADS-box, NAC and AP2/ERF gene families were involved in the control of papaya ripening and revealed that cell wall-related gene expression in papaya had similarities to the expression profiles seen in Arabidopsis during hypocotyl development.

CONCLUSION

The cross-species array experiment identified a ripening-related set of genes in papaya allowing the comparison of transcription control between papaya and other fruit bearing taxa during the ripening process.

Two wheat (Triticum aestivum) pathogenesis-related 10 (PR-10) transcripts with distinct patterns of abundance in different organs

PR-10 genes encode small, acidic, intracellular proteins that respond to abiotic and biotic stimuli. Transgenic expression of PR-10 genes has been shown to enhance early seedling growth of dicots in saline environments. To identify candidate PR-10 genes in cereals for increasing stress tolerance, we conducted phylogenetic analyses and real-time polymerase chain reaction of representatives of the two major clades of putative PR-10 genes in wheat. We observed that the abundance of BQ752893 was generally greater than the abundance of CV778999, particularly when measured in roots across four wheat genotypes. However, CV778999 transcripts were more abundant than BQ752893 in flag leaves. These data suggest that the transcripts define two functionally divergent groups of PR-10 type genes in wheat, both of which may be suitable targets for biotechnological manipulation under different circumstances.

Integrative comparative analyses of transcript and metabolite profiles from pepper and tomato ripening and development stages uncovers species-specific patterns of network regulatory behavior

Integrative comparative analyses of transcript and metabolite levels from climacteric and nonclimacteric fruits can be employed to unravel the similarities and differences of the underlying regulatory processes. To this end, we conducted combined gas chromatography-mass spectrometry and heterologous microarray hybridization assays in tomato (Solanum lycopersicum; climacteric) and pepper (Capsicum chilense; nonclimacteric) fruits across development and ripening. Computational methods from multivariate and network-based analyses successfully revealed the difference between the covariance structures of the integrated data sets. Moreover, our results suggest that both fruits have similar ethylene-mediated signaling components; however, their regulation is different and may reflect altered ethylene sensitivity or regulators other than ethylene in pepper. Genes involved in ethylene biosynthesis were not induced in pepper fruits. Nevertheless, genes downstream of ethylene perception such as cell wall metabolism genes, carotenoid biosynthesis genes, and the never-ripe receptor were clearly induced in pepper as in tomato fruit. While signaling sensitivity or actual signals may differ between climacteric and nonclimacteric fruit, the evidence described here suggests that activation of a common set of ripening genes influences metabolic traits. Also, a coordinate regulation of transcripts and the accumulation of key organic acids, including malate, citrate, dehydroascorbate, and threonate, in pepper fruit were observed. Therefore, the integrated analysis allows us to uncover additional information for the comprehensive understanding of biological events relevant to metabolic regulation during climacteric and nonclimacteric fruit development.

Exploring pathogenic mechanisms of Botrytis cinerea secretome under different ambient pH based on comparative proteomic analysis

Botrytis cinerea causes gray mold rot on over 200 plant species worldwide, resulting in great economic loss every year. Cooperation of proteins secreted by B. cinerea plays an important role in its successful infection to host plants. The ambient pH, as one of the most important environmental parameters, can regulate expression of secreted proteins in various fungal pathogens. In the present study, we mainly investigated the effect of ambient pH on secretome of B. cinerea strain B05.10 with a comparative proteomic method based on 2-DE. Distinct differences in secretome of B. cinerea were found between pH 4 and 6 treatments, and 47 differential spots, corresponding to 21 unique proteins, were identified using MALDI-TOF/TOF. At pH 4, more proteins related to proteolysis were induced, whereas most of up-accumulated proteins were cell wall degrading enzymes at pH 6. Analysis of gene expression using quantitative real-time PCR suggests that production of most of these proteins was regulated at the level of transcription. These findings indicate that B. cinerea can adjust protein profile of secretome responding to different ambient pH values and provide evidence to deeply understand the complicated infecting mechanisms of B. cinerea on a wide range of plant hosts.

Unraveling the regulatory network of the MADS box transcription factor RIN in fruit ripening

The MADS box transcription factor RIN is a global regulator of fruit ripening. However, the direct targets modulated by RIN and the mechanisms underlying the transcriptional regulation remain largely unknown. Here we identified 41 protein spots representing 35 individual genes as potential targets of RIN by comparative proteomic analysis of a rin mutant in tomato fruits. Gene expression analysis showed that the mRNA level of 26 genes correlated well with the protein level. After examining the promoter regions of the candidate genes, a variable number of RIN binding sites were found. Five genes (E8, TomloxC, PNAE, PGK and ADH2) were identified as novel direct targets of RIN by chromatin immunoprecipitation. The results of a gel mobility shift assay confirmed the direct binding of RIN to the promoters of these genes. Of the direct target genes, TomloxC and ADH2, which encode lipoxygenase (LOX) and alcohol dehydrogenase, respectively, are critical for the production of characteristic tomato aromas derived from LOX pathway. Further study indicated that RIN also directly regulates the expression of HPL, which encodes hydroperoxide lyase, another rate-limiting enzyme in the LOX pathway. Loss of function of RIN causes de-regulation of the LOX pathway, leading to a specific defect in the generation of aroma compounds derived from this pathway. These results indicate that RIN modulates aroma formation by direct and rigorous regulation of expression of genes in the LOX pathway. Taken together, our findings suggest that the regulatory effect of RIN on fruit ripening is achieved by targeting specific molecular pathways.

Comprehensive QTL mapping survey dissects the complex fruit texture physiology in apple (Malus x domestica Borkh.)

Fruit ripening is a complex physiological process in plants whereby cell wall programmed changes occur mainly to promote seed dispersal. Cell wall modification also directly regulates the textural properties, a fundamental aspect of fruit quality. In this study, two full-sib populations of apple, with 'Fuji' as the common maternal parent, crossed with 'Delearly' and 'Pink Lady', were used to understand the control of fruit texture by QTL mapping and in silico gene mining. Texture was dissected with a novel high resolution phenomics strategy, simultaneously profiling both mechanical and acoustic fruit texture components. In 'Fuji × Delearly' nine linkage groups were associated with QTLs accounting from 15.6% to 49% of the total variance, and a highly significant QTL cluster for both textural components was mapped on chromosome 10 and co-located with Md-PG1, a polygalacturonase gene that, in apple, is known to be involved in cell wall metabolism processes. In addition, other candidate genes related to Md-NOR and Md-RIN transcription factors, Md-Pel (pectate lyase), and Md-ACS1 were mapped within statistical intervals. In 'Fuji × Pink Lady', a smaller set of linkage groups associated with the QTLs identified for fruit texture (15.9-34.6% variance) was observed. The analysis of the phenotypic variance over a two-dimensional PCA plot highlighted a transgressive segregation for this progeny, revealing two QTL sets distinctively related to both mechanical and acoustic texture components. The mining of the apple genome allowed the discovery of the gene inventory underlying each QTL, and functional profile assessment unravelled specific gene expression patterns of these candidate genes.

Engineering virus resistance using a modified potato gene

Natural mutations in translation initiation factor eIF4E confer resistance to potyviruses in many plant species. Potato is a staple food crop plagued by several potyviruses, yet to date no known eIF4E-mediated resistance genes have been identified. In this study, we demonstrate that transgenic expression of the pvr1(2) gene from pepper confers resistance to Potato virus Y (PVY) in potato. We then use this information to convert the susceptible potato ortholog of this allele into a de novo allele for resistance to PVY using site-directed mutagenesis. Potato plants overexpressing the mutated potato allele are resistant to virus infection. Resistant lines expressed high levels of eIF4E mRNA and protein. The resistant plants showed growth similar to untransformed controls and produced phenotypically similar tubers. This technique disrupts a key step in the viral infection process and may potentially be used to engineer virus resistance in a number of economically important plant-viral pathosystems. Furthermore, the general public may be more amenable to the 'intragenic' nature of this approach because the transferred coding region is modified from a gene in the target crop rather than from a distant species.

A tiling microarray for global analysis of chloroplast genome expression in cucumber and other plants

Plastids are small organelles equipped with their own genomes (plastomes). Although these organelles are involved in numerous plant metabolic pathways, current knowledge about the transcriptional activity of plastomes is limited. To solve this problem, we constructed a plastid tiling microarray (PlasTi-microarray) consisting of 1629 oligonucleotide probes. The oligonucleotides were designed based on the cucumber chloroplast genomic sequence and targeted both strands of the plastome in a non-contiguous arrangement. Up to 4 specific probes were designed for each gene/exon, and the intergenic regions were covered regularly, with 70-nt intervals. We also developed a protocol for direct chemical labeling and hybridization of as little as 2 micrograms of chloroplast RNA. We used this protocol for profiling the expression of the cucumber chloroplast plastome on the PlasTi-microarray. Owing to the high sequence similarity of plant plastomes, the newly constructed microarray can be used to study plants other than cucumber. Comparative hybridization of chloroplast transcriptomes from cucumber, Arabidopsis, tomato and spinach showed that the PlasTi-microarray is highly versatile.

Regulatory control of high levels of carotenoid accumulation in potato tubers

Potato (Solanum tuberosum L.) tubers contain a wide range of carotenoid contents. To decipher the key factors controlling carotenoid levels in tubers, four potato lines (Atlantic, Désirée, 91E22 and POR03) were examined by a combination of biochemical, molecular and genomics approaches. These lines contained incremental levels of carotenoids, which were found to be associated with enhanced capacity of carotenoid biosynthesis as evident from norflurazon treatment. Microarray analysis of high and low carotenoid lines (POR03 versus Atlantic) revealed 381 genes that showed significantly differential expression. The carotenoid metabolic pathway genes β-carotene hydroxylase 2 (BCH2) and β-carotene hydroxylase 1 (BCH1), along with zeaxanthin epoxidase (ZEP), and carotenoid cleavage dioxygenase 1A (CCD1A) were among the most highly differentially expressed genes. The transcript levels of BCH2 and BCH1 were lowest in Atlantic and highest in POR03, whereas those of ZEP and CCD1A were high in low carotenoid lines and low in high carotenoid lines. The high expression of BCH2 in POR03 line was associated with enhanced response to sugars. Our results indicate that high levels of carotenoid accumulation in potato tubers were due to an increased metabolic flux into carotenoid biosynthetic pathway, as well as the differential expression of carotenoid metabolic genes.

Transcriptional responses of Italian ryegrass during interaction with Xanthomonas translucens pv. graminis reveal novel candidate genes for bacterial wilt resistance

Xanthomonas translucens pv. graminis (Xtg) causes bacterial wilt, a severe disease of forage grasses such as Italian ryegrass (Lolium multiflorum Lam.). In order to gain a more detailed understanding of the genetic control of resistance mechanisms and to provide prerequisites for marker assisted selection, the partial transcriptomes of two Italian ryegrass genotypes, one resistant and one susceptible to bacterial wilt were compared at four time points after Xtg infection. A cDNA microarray developed from a perennial ryegrass (Lolium perenne) expressed sequence tag set consisting of 9,990 unique genes was used for transcriptome analysis in Italian ryegrass. An average of 4,487 (45%) of the perennial ryegrass sequences spotted on the cDNA microarray were detected by cross-hybridisation to Italian ryegrass. Transcriptome analyses of the resistant versus the susceptible genotype revealed substantial gene expression differences (>1,200) indicating that great gene expression differences between different Italian ryegrass genotypes exist which potentially contribute to the observed phenotypic divergence in Xtg resistance between the two genotypes. In the resistant genotype, several genes differentially expressed after Xtg inoculation were identified which revealed similarities to transcriptional changes triggered by pathogen-associated molecular patterns in other plant-pathogen interactions. These genes represent candidate genes of particular interest for the development of tools for marker assisted resistance breeding.

A ß-D: -xylosidase and a PR-4B precursor identified as genes accounting for differences in peach cold storage tolerance

A transcriptome analysis was applied on two peach (Prunus persica L.) cultivars with different sensitivity to low temperature regimes to identify genes that might be involved in tolerance to extended low temperature storage. Peach fruit from 'Morettini No2' to 'Royal Glory', cultivars sensitive and tolerant to chilling injury (CI), respectively, were harvested at commercial maturity stage and allowed to ripen at room temperature (shelf-life, 25°C) or subjected to 4 and 6 weeks of cold storage (0°C, 95% R.H.) followed by ripening at room temperature. The use of μPEACH 1.0 microarray platform identified a number of genes that were differentially expressed in 'Morettini No2' and 'Royal Glory' fruit after the extended storage period. Based on their possible involvement in physiological processes related to cold storage and on their differential expression pattern, two heat shock proteins, a β-D-xylosidase, an expansin, a dehydrin and a pathogenesis-related (PR) protein were further selected for detailed analysis via RNA blot analysis. It is suggested that β-D: -xylosidase and PR-4B precursor genes could be related to the different tolerance to CI observed in the two peach cultivars since generally higher expression levels were observed in cv. 'Royal Glory', the tolerant one. These two genes could play a role in peach tolerance to chilling injury.

Use of homologous and heterologous gene expression profiling tools to characterize transcription dynamics during apple fruit maturation and ripening

BACKGROUND

Fruit development, maturation and ripening consists of a complex series of biochemical and physiological changes that in climacteric fruits, including apple and tomato, are coordinated by the gaseous hormone ethylene. These changes lead to final fruit quality and understanding of the functional machinery underlying these processes is of both biological and practical importance. To date many reports have been made on the analysis of gene expression in apple. In this study we focused our investigation on the role of ethylene during apple maturation, specifically comparing transcriptomics of normal ripening with changes resulting from application of the hormone receptor competitor 1-methylcyclopropene.

RESULTS

To gain insight into the molecular process regulating ripening in apple, and to compare to tomato (model species for ripening studies), we utilized both homologous and heterologous (tomato) microarray to profile transcriptome dynamics of genes involved in fruit development and ripening, emphasizing those which are ethylene regulated.The use of both types of microarrays facilitated transcriptome comparison between apple and tomato (for the later using data previously published and available at the TED: tomato expression database) and highlighted genes conserved during ripening of both species, which in turn represent a foundation for further comparative genomic studies. The cross-species analysis had the secondary aim of examining the efficiency of heterologous (specifically tomato) microarray hybridization for candidate gene identification as related to the ripening process. The resulting transcriptomics data revealed coordinated gene expression during fruit ripening of a subset of ripening-related and ethylene responsive genes, further facilitating the analysis of ethylene response during fruit maturation and ripening.

CONCLUSION

Our combined strategy based on microarray hybridization enabled transcriptome characterization during normal climacteric apple ripening, as well as definition of ethylene-dependent transcriptome changes. Comparison with tomato fruit maturation and ethylene responsive transcriptome activity facilitated identification of putative conserved orthologous ripening-related genes, which serve as an initial set of candidates for assessing conservation of gene activity across genomes of fruit bearing plant species.

Identification, phylogeny, and transcript profiling of ERF family genes during development and abiotic stress treatments in tomato

Ethylene responsive transcription factors have been shown to be intimately connected to plant development, defense responses and stress signaling pathways and in order to use them for plant improvement, we need to have better understanding of these proteins. In this study, 85 ERF genes have been identified from tomato using raw EST data in various public repositories. Phylogenetic analysis with tomato ERF domains revealed their distribution in all the groups, previously identified in model systems. MEME motif analysis resulted in identification of conserved domains, characteristic to member of each clade, in addition to ERF domain. Expression analysis during vegetative and reproductive stages of development using QPCR and tomato GeneChip arrays, revealed their tissue-specific/preferential accumulation. In total, 57 genes were found to be differentially expressed during temporal stages of tomato fruit development. The expression analysis of 23 ERF family genes representing each clade in response to seven abiotic stress treatments revealed their differential expression in response to more than one abiotic stress treatments. Results suggest that ERF genes play diverse roles in plant's life and comprehensive data generated will be helpful in conducting functional genomics studies to understand their precise role during plant development and stress response.

Transformation of metabolism with age and lifestyle in Antarctic seals: a case study of systems biology approach to cross-species microarray experiment

Background

The metabolic transformation that changes Weddell seal pups born on land into aquatic animals is not only interesting for the study of general biology, but it also provides a model for the acquired and congenital muscle disorders which are associated with oxygen metabolism in skeletal muscle. However, the analysis of gene expression in seals is hampered by the lack of specific microarrays and the very limited annotation of known Weddell seal (Leptonychotes weddellii) genes.

Results

Muscle samples from newborn, juvenile, and adult Weddell seals were collected during an Antarctic expedition. Extracted RNA was hybridized on Affymetrix Human Expression chips. Preliminary studies showed a detectable signal from at least 7000 probe sets present in all samples and replicates. Relative expression levels for these genes was used for further analysis of the biological pathways implicated in the metabolism transformation which occurs in the transition from newborn, to juvenile, to adult seals. Cytoskeletal remodeling, WNT signaling, FAK signaling, hypoxia-induced HIF1 activation, and insulin regulation were identified as being among the most important biological pathways involved in transformation.

Conclusion

In spite of certain losses in specificity and sensitivity, the cross-species application of gene expression microarrays is capable of solving challenging puzzles in biology. A Systems Biology approach based on gene interaction patterns can compensate adequately for the lack of species-specific genomics information.

Transcript profiling of two alfalfa genotypes with contrasting cell wall composition in stems using a cross-species platform: optimizing analysis by masking biased probes

BACKGROUND

The GeneChip(R) Medicago Genome Array, developed for Medicago truncatula, is a suitable platform for transcript profiling in tetraploid alfalfa [Medicago sativa (L.) subsp. sativa]. However, previous research involving cross-species hybridization (CSH) has shown that sequence variation between two species can bias transcript profiling by decreasing sensitivity (number of expressed genes detected) and the accuracy of measuring fold-differences in gene expression.

RESULTS

Transcript profiling using the Medicago GeneChip(R) was conducted with elongating stem (ES) and post-elongation stem (PES) internodes from alfalfa genotypes 252 and 1283 that differ in stem cell wall concentrations of cellulose and lignin. A protocol was developed that masked probes targeting inter-species variable (ISV) regions of alfalfa transcripts. A probe signal intensity threshold was selected that optimized both sensitivity and accuracy. After masking for both ISV regions and previously identified single-feature polymorphisms (SFPs), the number of differentially expressed genes between the two genotypes in both ES and PES internodes was approximately 2-fold greater than the number detected prior to masking. Regulatory genes, including transcription factor and receptor kinase genes that may play a role in development of secondary xylem, were significantly over-represented among genes up-regulated in 252 PES internodes compared to 1283 PES internodes. Several cell wall-related genes were also up-regulated in genotype 252 PES internodes. Real-time quantitative RT-PCR of differentially expressed regulatory and cell wall-related genes demonstrated increased sensitivity and accuracy after masking for both ISV regions and SFPs. Over 1,000 genes that were differentially expressed in ES and PES internodes of genotypes 252 and 1283 were mapped onto putative orthologous loci on M. truncatula chromosomes. Clustering simulation analysis of the differentially expressed genes suggested co-expression of some neighbouring genes on Medicago chromosomes.

CONCLUSIONS

The problems associated with transcript profiling in alfalfa stems using the Medicago GeneChip as a CSH platform were mitigated by masking probes targeting ISV regions and SFPs. Using this masking protocol resulted in the identification of numerous candidate genes that may contribute to differences in cell wall concentration and composition of stems of two alfalfa genotypes.

Transcript profiling of common bean (Phaseolus vulgaris L.) using the GeneChip Soybean Genome Array: optimizing analysis by masking biased probes

BACKGROUND

Common bean (Phaseolus vulgaris L.) and soybean (Glycine max) both belong to the Phaseoleae tribe and share significant coding sequence homology. This suggests that the GeneChip(R) Soybean Genome Array (soybean GeneChip) may be used for gene expression studies using common bean.

RESULTS

To evaluate the utility of the soybean GeneChip for transcript profiling of common bean, we hybridized cRNAs purified from nodule, leaf, and root of common bean and soybean in triplicate to the soybean GeneChip. Initial data analysis showed a decreased sensitivity and accuracy of measuring differential gene expression in common bean cross-species hybridization (CSH) GeneChip data compared to that of soybean. We employed a method that masked putative probes targeting inter-species variable (ISV) regions between common bean and soybean. A masking signal intensity threshold was selected that optimized both sensitivity and accuracy of measuring differential gene expression. After masking for ISV regions, the number of differentially-expressed genes identified in common bean was increased by 2.8-fold reflecting increased sensitivity. Quantitative RT-PCR (qRT-PCR) analysis of 20 randomly selected genes and purine-ureide pathway genes demonstrated an increased accuracy of measuring differential gene expression after masking for ISV regions. We also evaluated masked probe frequency per probe set to gain insight into the sequence divergence pattern between common bean and soybean. The sequence divergence pattern analysis suggested that the genes for basic cellular functions and metabolism were highly conserved between soybean and common bean. Additionally, our results show that some classes of genes, particularly those associated with environmental adaptation, are highly divergent.

CONCLUSIONS

The soybean GeneChip is a suitable cross-species platform for transcript profiling in common bean when used in combination with the masking protocol described. In addition to transcript profiling, CSH of the GeneChip in combination with masking probes in the ISV regions can be used for comparative ecological and/or evolutionary genomics studies.

DNA chip analysis in diverse organisms with unsequenced genomes

Whether for basic research or biotechnology, DNA microarrays have become indispensable tools for studying the transcriptome. Normally, analyses begin with a set of known cDNA sequences to prepare microarray chips specific for a target organism with an extensively sequenced and annotated genome. For many organisms, however, genome programs are not complete or have not been initiated. The present study demonstrates that, whether using homologous or heterologous arrays, the chances of seeing interesting differences are similar. When a specific DNA microarray is not available, the results indicate that a reverse approach based on a heterologous array can be used to probe for interesting differences in gene expression. This may be sufficient in many studies but, if necessary, the genes exhibiting the most significant changes subsequently could be identified by traditional molecular approaches. Such a reverse strategy can provide a convenient and inexpensive approach to probe for significant genetic changes in many diverse studies, to monitor or mine critical biological information for basic or applied research, long before complete sequence data are available.

General and species-specific transcriptional responses to downy mildew infection in a susceptible (Vitis vinifera) and a resistant (V. riparia) grapevine species

Background

Downy mildew is a destructive grapevine disease caused by Plasmopara viticola (Berk. and Curt.) Berl. and de Toni, which can only be controlled by intensive fungicide treatments. Natural sources of resistance from wild grapevine (Vitis) species are used in conventional breeding approaches, but the signals and effectors involved in resistance in this important crop species are not well understood.

Results

Early transcriptional changes associated with P. viticola infection in susceptible V. vinifera and resistant V. riparia plants were analyzed using the Combimatrix microarray platform. Transcript levels were measured 12 and 24 h post-inoculation, reflecting the time points immediately preceding the onset of resistance in V. riparia, as determined by microscopic analysis. Our data indicate that resistance in V. riparia is induced after infection, and is not based on differences in basal gene expression between the two species. The strong and rapid transcriptional reprogramming involves the induction of pathogenesis-related proteins and enzymes required for the synthesis of phenylpropanoid-derived compounds, many of which are also induced, albeit to a lesser extent, in V. vinifera. More interestingly, resistance in V. riparia also involves the specific modulation of numerous transcripts encoding components of signal transduction cascades, hypersensitive reaction markers and genes involved in jasmonate biosynthesis. The limited transcriptional modulation in V. vinifera represents a weak attempted defense response rather than the activation of compatibility-specific pathways.

Conclusions

Several candidate resistance genes were identified that could be exploited in future biotechnological approaches to increase disease resistance in susceptible grapevine species. Measurements of jasmonic acid and methyl jasmonate in infected leaves suggest that this hormone may also be involved in V. riparia resistance to P. viticola.

Modulation of carotenoid accumulation in transgenic potato by inducing chromoplast formation with enhanced sink strength

An increasing interest in carotenoids as nutritional sources of provitamin A and health-promoting compounds has prompted a significant effort in metabolic engineering of carotenoid content and composition in food crops. The strategy commonly used in plants is to increase the biosynthetic capacity by altering the carotenogenic enzyme activities. The recent isolation of the Or gene from a cauliflower orange mutant has brought a new endeavor for carotenoid enhancement by increasing the sink strength to sequester and store the synthesized carotenoids. Potato as one of the major staple crops usually accumulates low levels of carotenoids. In this chapter, we describe a detailed protocol for metabolic engineering of carotenoids in potato plants with the Or gene and the analysis of the Or transformants.

Genome analysis and genetic enhancement of tomato

The Solanaceae is an important family of vegetable crops, ornamentals and medicinal plants. Tomato has served as a model member of this family largely because of its enriched cytogenetic, genetic, as well as physical, maps. Mapping has helped in cloning several genes of importance such as Pto, responsible for resistance against bacterial speck disease, Mi-1.2 for resistance against nematodes, and fw2.2 QTL for fruit weight. A high-throughput genome-sequencing program has been initiated by an international consortium of 10 countries. Since heterochromatin has been found to be concentrated near centromeres, the consortium is focusing on sequencing only the gene-rich euchromatic region. Genomes of the members of Solanaceae show a significant degree of synteny, suggesting that the tomato genome sequence would help in the cloning of genes for important traits from other Solanaceae members as well. ESTs from a large number of cDNA libraries have been sequenced, and microarray chips, in conjunction with wide array of ripening mutants, have contributed immensely to the understanding of the fruit-ripening phenomenon. Work on the analysis of the tomato proteome has also been initiated. Transgenic tomato plants with improved abiotic stress tolerance, disease resistance and insect resistance, have been developed. Attempts have also been made to develop tomato as a bioreactor for various pharmaceutical proteins. However, control of fruit quality and ripening remains an active and challenging area of research. Such efforts should pave the way to improve not only tomato, but also other solanaceous crops.

Heterologous oligonucleotide microarrays for transcriptomics in a non-model species; a proof-of-concept study of drought stress in Musa

BACKGROUND

'Systems-wide' approaches such as microarray RNA-profiling are ideally suited to the study of the complex overlapping responses of plants to biotic and abiotic stresses. However, commercial microarrays are only available for a limited number of plant species and development costs are so substantial as to be prohibitive for most research groups. Here we evaluate the use of cross-hybridisation to Affymetrix oligonucleotide GeneChip(R) microarrays to profile the response of the banana (Musa spp.) leaf transcriptome to drought stress using a genomic DNA (gDNA)-based probe-selection strategy to improve the efficiency of detection of differentially expressed Musa transcripts.

RESULTS

Following cross-hybridisation of Musa gDNA to the Rice GeneChip(R) Genome Array, ~33,700 gene-specific probe-sets had a sufficiently high degree of homology to be retained for transcriptomic analyses. In a proof-of-concept approach, pooled RNA representing a single biological replicate of control and drought stressed leaves of the Musa cultivar 'Cachaco' were hybridised to the Affymetrix Rice Genome Array. A total of 2,910 Musa gene homologues with a >2-fold difference in expression levels were subsequently identified. These drought-responsive transcripts included many functional classes associated with plant biotic and abiotic stress responses, as well as a range of regulatory genes known to be involved in coordinating abiotic stress responses. This latter group included members of the ERF, DREB, MYB, bZIP and bHLH transcription factor families. Fifty-two of these drought-sensitive Musa transcripts were homologous to genes underlying QTLs for drought and cold tolerance in rice, including in 2 instances QTLs associated with a single underlying gene. The list of drought-responsive transcripts also included genes identified in publicly-available comparative transcriptomics experiments.

CONCLUSION

Our results demonstrate that despite the general paucity of nucleotide sequence data in Musa and only distant phylogenetic relations to rice, gDNA probe-based cross-hybridisation to the Rice GeneChip(R) is a highly promising strategy to study complex biological responses and illustrates the potential of such strategies for gene discovery in non-model species.