Zooming in on a quantitative trait for tomato yield using interspecific introgressions

To explore natural biodiversity we developed and examined introgression lines (ILs) containing chromosome segments of wild species (Solanum pennellii) in the background of the cultivated tomato (S. lycopersicum). We identified Brix9-2-5, which is a S. pennellii quantitative trait locus (QTL) that increases sugar yield of tomatoes and was mapped within a flower- and fruit-specific invertase (LIN5). QTL analysis representing five different tomato species delimited the functional polymorphism of Brix9-2-5 to an amino acid near the catalytic site of the invertase crystal, affecting enzyme kinetics and fruit sink strength. These results underline the power of diverse ILs for high-resolution perspectives on complex phenotypes.

An effector-targeted protease contributes to defense against Phytophthora infestans and is under diversifying selection in natural hosts

Since the leaf apoplast is a primary habitat for many plant pathogens, apoplastic proteins are potent, ancient targets for apoplastic effectors secreted by plant pathogens. So far, however, only a few apoplastic effector targets have been identified and characterized. Here, we discovered that the papain-like cysteine protease C14 is a new common target of EPIC1 and EPIC2B, two apoplastic, cystatin-like proteins secreted by the potato (Solanum tuberosum) late blight pathogen Phytophthora infestans. C14 is a secreted protease of tomato (Solanum lycopersicum) and potato typified by a carboxyl-terminal granulin domain. The EPIC-C14 interaction occurs at a wide pH range and is stronger than the previously described interactions of EPICs with tomato defense proteases PIP1 and RCR3. The selectivity of the EPICs is also different when compared with the AVR2 effector of the fungal tomato pathogen Cladosporium fulvum, which targets PIP1 and RCR3, and only at apoplastic pH. Importantly, silencing of C14 increased susceptibility to P. infestans, demonstrating that this protease plays a role in pathogen defense. Although C14 is under conservative selection in tomato, it is under diversifying selection in wild potato species (Solanum demissum, Solanum verrucosum, and Solanum stoliniferum) that are the natural hosts of P. infestans. These data reveal a novel effector target in the apoplast that contributes to immunity and is under diversifying selection, but only in the natural host of the pathogen.

Host protein BSL1 associates with Phytophthora infestans RXLR effector AVR2 and the Solanum demissum Immune receptor R2 to mediate disease resistance

Plant pathogens secrete effector proteins to modulate plant immunity and promote host colonization. Plant nucleotide binding leucine-rich repeat (NB-LRR) immunoreceptors recognize specific pathogen effectors directly or indirectly. Little is known about how NB-LRR proteins recognize effectors of filamentous plant pathogens, such as Phytophthora infestans. AVR2 belongs to a family of 13 sequence-divergent P. infestans RXLR effectors that are differentially recognized by members of the R2 NB-LRR family in Solanum demissum. We report that the putative plant phosphatase BSU-LIKE PROTEIN1 (BSL1) is required for R2-mediated perception of AVR2 and resistance to P. infestans. AVR2 associates with BSL1 and mediates the interaction of BSL1 with R2 in planta, possibly through the formation of a ternary complex. Strains of P. infestans that are virulent on R2 potatoes express an unrecognized form, Avr2-like (referred to as A2l). A2L can still interact with BSL1 but does not promote the association of BSL1 with R2. Our findings show that recognition of the P. infestans AVR2 effector by the NB-LRR protein R2 requires the putative phosphatase BSL1. This reveals that, similar to effectors of phytopathogenic bacteria, recognition of filamentous pathogen effectors can be mediated via a host protein that interacts with both the effector and the NB-LRR immunoreceptor.

Functionally divergent alleles and duplicated Loci encoding an acyltransferase contribute to acylsugar metabolite diversity in Solanum trichomes

Glandular trichomes from tomato (Solanum lycopersicum) and other species in the Solanaceae produce and secrete a mixture of O-acylsugars (aliphatic esters of sucrose and glucose) that contribute to insect defense. Despite their phylogenetic distribution and diversity, relatively little is known about how these specialized metabolites are synthesized. Mass spectrometric profiling of acylsugars in the S. lycopersicum x Solanum pennellii introgression lines identified a chromosome 11 locus containing a cluster of BAHD acyltransferases with one gene (named Sl-ASAT3) expressed in tip cells of type I trichomes where acylsugars are made. Sl-ASAT3 was shown to encode an acyl-CoA-dependent acyltransferase that catalyzes the transfer of short (four to five carbons) branched acyl chains to the furanose ring of di-acylsucrose acceptors to produce tri-acylsucroses, which can be further acetylated by Sl-ASAT4 (previously Sl-AT2). Among the wild tomatoes, diversity in furanose ring acyl chains on acylsucroses was most striking in Solanum habrochaites. S. habrochaites accessions from Ecuador and northern Peru produced acylsucroses with short (≤C5) or no acyl chains on the furanose ring. Accessions from central and southern Peru had the ability to add short or long (up to C12) acyl chains to the furanose ring. Multiple ASAT3-like sequences were found in most accessions, and their in vitro activities correlated with observed geographical diversity in acylsugar profiles.

Evolution of TPS20-related terpene synthases influences chemical diversity in the glandular trichomes of the wild tomato relative Solanum habrochaites

A systematic screen of volatile terpene production in the glandular trichomes of 79 accessions of Solanum habrochaites was conducted and revealed the presence of 21 mono- and sesquiterpenes that exhibit a range of qualitative and quantitative variation. Hierarchical clustering identified distinct terpene phenotypic modules with shared patterns of terpene accumulation across accessions. Several terpene modules could be assigned to previously identified terpene synthase (TPS) activities that included members of the TPS-e/f subfamily that utilize the unusual cis-prenyl diphosphate substrates neryl diphosphate and 2z,6z-farnesyl diphosphate. DNA sequencing and in vitro enzyme activity analysis of TPS-e/f members from S. habrochaites identified three previously unassigned enzyme activities that utilize these cisoid substrates. These produce either the monoterpenes α-pinene and limonene, or the sesquiterpene 7-epizingiberene, with the in vitro analyses that recapitulated the trichome chemistry found in planta. Comparison of the distribution of S. habrochaites accessions with terpene content revealed a strong preference for the presence of particular TPS20 alleles at distinct geographic locations. This study reveals that the unusually high intra-specific variation of volatile terpene synthesis in glandular trichomes of S. habrochaites is due at least in part to evolution at the TPS20 locus.

Divergence in the enzymatic activities of a tomato and Solanum pennellii alcohol acyltransferase impacts fruit volatile ester composition

Tomato fruits accumulate a diverse set of volatiles including multiple esters. The content of ester volatiles is relatively low in tomato fruits (Solanum lycopersicum) and far more abundant in the closely related species Solanum pennellii. There are also qualitative variations in ester content between the two species. We have previously shown that high expression of a non-specific esterase is critical for the low overall ester content of S. lycopersicum fruit relative to S. pennellii fruit. Here, we show that qualitative differences in ester composition are the consequence of divergence in enzymatic activity of a ripening-related alcohol acyltransferase (AAT1). The S. pennellii AAT1 is more efficient than the tomato AAT1 for all the alcohols tested. The two enzymes have differences in their substrate preferences that explain the variations observed in the volatiles. The results illustrate how two related species have evolved to precisely adjust their volatile content by modulating the balance of the synthesis and degradation of esters.

Striking natural diversity in glandular trichome acylsugar composition is shaped by variation at the Acyltransferase2 locus in the wild tomato Solanum habrochaites

Acylsugars are polyesters of short- to medium-length acyl chains on sucrose or glucose backbones that are produced in secretory glandular trichomes of many solanaceous plants, including cultivated tomato (Solanum lycopersicum). Despite their roles in biotic stress adaptation and their wide taxonomic distribution, there is relatively little information about the diversity of these compounds and the genes responsible for their biosynthesis. In this study, acylsugar diversity was assessed for 80 accessions of the wild tomato species Solanum habrochaites from throughout the Andes Mountains. Trichome metabolites were analyzed by liquid chromatography-time of flight-mass spectrometry, revealing the presence of at least 34 structurally diverse acylsucroses and two acylglucoses. Distinct phenotypic classes were discovered that varied based on the presence of glucose or sucrose, the numbers and lengths of acyl chains, and the relative total amounts of acylsugars. The presence or absence of an acetyl chain on the acylsucrose hexose ring caused clustering of the accessions into two main groups. Analysis of the Acyltransferase2 gene (the apparent ortholog of Solyc01g105580) revealed differences in enzyme activity and gene expression correlated with polymorphism in S. habrochaites accessions that varied in acylsucrose acetylation. These results are consistent with the hypothesis that glandular trichome acylsugar acetylation is under selective pressure in some populations of S. habrochaites and that the gene mutates to inactivity in the absence of selection.

Salt tolerance in Solanum pennellii: antioxidant response and related QTL

BACKGROUND

Excessive soil salinity is an important problem for agriculture, however, salt tolerance is a complex trait that is not easily bred into plants. Exposure of cultivated tomato to salt stress has been reported to result in increased antioxidant content and activity. Salt tolerance of the related wild species, Solanum pennellii, has also been associated with similar changes in antioxidants. In this work, S. lycopersicum M82, S. pennellii LA716 and a S. pennellii introgression line (IL) population were evaluated for growth and their levels of antioxidant activity (total water-soluble antioxidant activity), major antioxidant compounds (phenolic and flavonoid contents) and antioxidant enzyme activities (superoxide dismutase, catalase, ascorbate peroxidase and peroxidase) under both control and salt stress (150 mM NaCl) conditions. These data were then used to identify quantitative trait loci (QTL) responsible for controlling the antioxidant parameters under both stress and nonstress conditions.

RESULTS

Under control conditions, cultivated tomato had higher levels of all antioxidants (except superoxide dismutase) than S. pennellii. However, under salt stress, the wild species showed greater induction of all antioxidants except peroxidase. The ILs showed diverse responses to salinity and proved very useful for the identification of QTL. Thus, 125 loci for antioxidant content under control and salt conditions were detected. Eleven of the total antioxidant activity and phenolic content QTL matched loci identified in an independent study using the same population, thereby reinforcing the validity of the loci. In addition, the growth responses of the ILs were evaluated to identify lines with favorable growth and antioxidant profiles.

CONCLUSIONS

Plants have a complex antioxidant response when placed under salt stress. Some loci control antioxidant content under all conditions while others are responsible for antioxidant content only under saline or nonsaline conditions. The localization of QTL for these traits and the identification of lines with specific antioxidant and growth responses may be useful for breeding potentially salt tolerant tomato cultivars having higher antioxidant levels under nonstress and salt stress conditions.

Engineering Flax with the GT Family 1 Solanum sogarandinum Glycosyltransferase SsGT1 Confers Increased Resistance to Fusarium Infection

The aim of this study was to engineer a flax with increased resistance to pathogens. The approach was based on the recent analysis of the Solanum sogarandinum -derived glycosyltransferase (UGT) protein, designated SsGT1 (previously called 5UGT). On the basis of enzyme studies, the recombinant SsGT1 is a 7-O-glycosyltransferase, the natural substrates of which include both anthocyanidins and flavonols such as kaempferol and quercetin. Because flavonoids act as antioxidants and glycosylation increases the stability of flavonoids, it has been suggested that the accumulation of a higher quantity of flavonoid glycosides in transgenic plants might improve their resistance to pathogen infection. Flax overproducing SsGT1 showed higher resistance to Fusarium infection than wild-type plants, and this was correlated with a significant increase in the flavonoid glycoside content in the transgenic plants. Overproduction of glycosyltransferase in transgenic flax also resulted in proanthocyanin, lignan, phenolic acid, and unsaturated fatty acid accumulation in the seeds. The last is meaningful from a biotechnological point of view and might suggest the involvement of polyphenol glycosides in the protection of unsaturated fatty acids against oxidation and thus improve oil storage. It is thus suggested that introduction of SsGT1 is sufficient for engineering altered pathogen resistance in flax.

Location of chlorogenic acid biosynthesis pathway and polyphenol oxidase genes in a new interspecific anchored linkage map of eggplant

BACKGROUND

Eggplant is a powerful source of polyphenols which seems to play a key role in the prevention of several human diseases, such as cancer and diabetes. Chlorogenic acid is the polyphenol most present in eggplant, comprising between the 70% and 90% of the total polyphenol content. Introduction of the high chlorogenic acid content of wild relatives, such as S. incanum, into eggplant varieties will be of great interest. A potential side effect of the increased level polyphenols could be a decrease on apparent quality due to browning caused by the polyphenol oxidase enzymes mediated oxidation of polyphenols. We report the development of a new interspecific S. melongena × S. incanum linkage map based on a first backcross generation (BC1) towards the cultivated S. melongena as a tool for introgressing S. incanum alleles involved in the biosynthesis of chlorogenic acid in the genetic background of S. melongena.

RESULTS

The interspecific genetic linkage map of eggplant developed in this work anchor the most informative previously published genetic maps of eggplant using common markers. The 91 BC1 plants of the mapping population were genotyped with 42 COSII, 99 SSRs, 88 AFLPs, 9 CAPS, 4 SNPs and one morphological polymorphic markers. Segregation marker data resulted in a map encompassing 1085 cM distributed in 12 linkage groups. Based on the syntheny with tomato, the candidate genes involved in the core chlorogenic acid synthesis pathway in eggplant (PAL, C4H, 4CL, HCT, C3'H, HQT) as well as five polyphenol oxidase (PPO1, PPO2, PPO3, PPO4, PPO5) were mapped. Except for 4CL and HCT chlorogenic acid genes were not linked. On the contrary, all PPO genes clustered together. Candidate genes important in domestication such as fruit shape (OVATE, SISUN1) and prickliness were also located.

CONCLUSIONS

The achievements in location of candidate genes will allow the search of favorable alleles employing marker-assisted selection in order to develop new varieties with higher chlorogenic content alongside a lower polyphenol oxidase activity. This will result into an enhanced product showing a lower fruit flesh browning with improved human health properties.

Development of a biosensor based on gilo peroxidase immobilized on chitosan chemically crosslinked with epichlorohydrin for determination of rutin

A new reagentless biosensor for the square-wave voltammetric determination of rutin in pharmaceutical formulations was developed by immobilization of gilo (Solanum gilo) crude extract in chitosan matrix. The gilo tissue acts as a source of peroxidase. The highest biosensor performance was obtained after immobilization of the peroxidase in chemically crosslinked chitosan with epichlorohydrin and glutaraldehyde that was incorporated in a carbon paste electrode. In the presence of hydrogen peroxide this enzyme catalyses the oxidation of rutin to quinone and the electrochemical reduction of the product was obtained at a fixed potential of +124 mV versus Ag/AgCl (3.0 M KCl). The performance and factors influencing the resulting biosensor were studied in detail. The bioelectrode exhibited a linear response for rutin concentrations from 3.4x10(-7) to 7.2x10(-6) M (r=0.9998) and the recovery of rutin from the samples ranged from 96.2 to 102.4%. The detection and quantification limits were 2.0x10(-8) and 6.3x10(-8) M, respectively. The relative standard deviation was less than 1.0% for solutions containing 3.4x10(-7) to 7.2x10(-6) M rutin in 0.1 M phosphate buffer solution at pH 7.0 (n=10). The lifetime of this biosensor was 8 months (at least 500 determinations).

Polymethylated myricetin in trichomes of the wild tomato species Solanum habrochaites and characterization of trichome-specific 3'/5'- and 7/4'-myricetin O-methyltransferases

Flavonoids are a class of metabolites found in many plant species. They have been reported to serve several physiological roles, such as in defense against herbivores and pathogens and in protection against harmful ultraviolet radiation. They also serve as precursors of pigment compounds found in flowers, leaves, and seeds. Highly methylated, nonglycosylated derivatives of the flavonoid myricetin flavonoid, have been previously reported from a variety of plants, but O-methyltransferases responsible for their synthesis have not yet been identified. Here, we show that secreting glandular trichomes (designated types 1 and 4) and storage glandular trichomes (type 6) on the leaf surface of wild tomato (Solanum habrochaites accession LA1777) plants contain 3,7,3'-trimethyl myricetin, 3,7,3',5'-tetramethyl myricetin, and 3,7,3',4',5'-pentamethyl myricetin, with gland types 1 and 4 containing severalfold more of these compounds than type 6 glands and with the tetramethylated compound predominating in all three gland types. We have also identified transcripts of two genes expressed in the glandular trichomes and showed that they encode enzymes capable of methylating myricetin at the 3' and 5' and the 7 and 4' positions, respectively. Both genes are preferentially expressed in secreting glandular trichome types 1 and 4 and to a lesser degree in storage trichome type 6, and the levels of the proteins they encode are correspondingly higher in types 1 and 4 glands compared with type 6 glands.

A novel glucosyltransferase involved in steroid saponin biosynthesis in Solanum aculeatissimum

Steroidal saponins are widely distributed in many plant species. Their diverse structures have resulted in a wide range of applications, including drug and medicine production. It has been suggested that the nature of the non-saccharide and oligosaccharide portions of the saponin molecule both contribute to the properties of individual saponins. Despite numerous studies on the occurrence, chemical structure, and varying pharmaceutical activities of steroidal saponins, their biosynthesis pathway is poorly understood. Glycosylation is thought to be the final step in steroidal saponin biosynthesis and it is thought to be involved in regulating the biological activities of saponins. Isolation of the glycosyltransferases that catalyze the transfer of sugar molecules to steroidal compounds will help to clarify the mechanisms that produce diverse saponins and control their activities in plants. In this study, we obtained three cDNAs encoding putative glycosyltransferases from Solanum aculeatissimum. One of the three, SaGT4A showed UDP-glucosyltransferase activity. This is the first cloned glucosyltransferase involved in steroidal saponin biosynthesis. SaGT4A catalyzes the 3-O-glucosylation of steroidal sapogenins, such as diosgenin, nuatigenin, and tigogenin. This enzyme also glucosylates steroidal alkaloids, such as solanidine, solasodine, and tomatidine. Gene expression analysis revealed that the accumulation of SaGT4A transcripts showed a unique response to wounding stress indicating the involvement of SaGT4A in plant defense system.

A Feedback-Insensitive Isopropylmalate Synthase Affects Acylsugar Composition in Cultivated and Wild Tomato

Acylsugars are insecticidal specialized metabolites produced in the glandular trichomes of plants in the Solanaceae family. In the tomato clade of the Solanum genus, acylsugars consist of aliphatic acids of different chain lengths esterified to sucrose, or less frequently to glucose. Through liquid chromatography-mass spectrometry screening of introgression lines, we previously identified a region of chromosome 8 in the Solanum pennellii LA0716 genome (IL8-1/8-1-1) that causes the cultivated tomato Solanum lycopersicum to shift from producing acylsucroses with abundant 3-methylbutanoic acid acyl chains derived from leucine metabolism to 2-methylpropanoic acid acyl chains derived from valine metabolism. We describe multiple lines of evidence implicating a trichome-expressed gene from this region as playing a role in this shift. S. lycopersicum M82 SlIPMS3 (Solyc08g014230) encodes a functional end product inhibition-insensitive version of the committing enzyme of leucine biosynthesis, isopropylmalate synthase, missing the carboxyl-terminal 160 amino acids. In contrast, the S. pennellii LA0716 IPMS3 allele found in IL8-1/8-1-1 encodes a nonfunctional truncated IPMS protein. M82 transformed with an SlIPMS3 RNA interference construct exhibited an acylsugar profile similar to that of IL8-1-1, whereas the expression of SlIPMS3 in IL8-1-1 partially restored the M82 acylsugar phenotype. These IPMS3 alleles are polymorphic in 14 S. pennellii accessions spread throughout the geographical range of occurrence for this species and are associated with acylsugars containing varying amounts of 2-methylpropanoic acid and 3-methylbutanoic acid acyl chains.

Over-expression of a gibberellin 2-oxidase gene from Phaseolus coccineus L. enhances gibberellin inactivation and induces dwarfism in Solanum species

Gibberellins (GAs) are endogenous hormones that play a predominant role in regulating plant stature by increasing cell division and elongation in stem internodes. The product of the GA 2-oxidase gene from Phaseolus coccineus (PcGA2ox1) inactivates C(19)-GAs, including the bioactive GAs GA(1 )and GA(4), by 2beta-hydroxylation, reducing the availability of these GAs in plants. The PcGA2ox1 gene was introduced into Solanum melanocerasum and S. nigrum (Solanaceae) by Agrobacterium-mediated transformation with the aim of decreasing the amounts of bioactive GA in these plants and thereby reducing their stature. The transgenic plants exhibited a range of dwarf phenotypes associated with a severe reduction in the concentrations of the biologically active GA(1) and GA(4). Flowering and fruit development were unaffected. The transgenic plants contained greater concentrations of chlorophyll b (by 88%) and total chlorophyll (11%), although chlorophyll a and carotenoid contents were reduced by 8 and 50%, respectively. This approach may provide an alternative to the application of chemical growth retardants for reducing the stature of plants, particularly ornamentals, in view of concerns over the potential environmental and health hazards of such compounds.

Expression of potato S-adenosyl-L-methionine synthase (SbSAMS) gene altered developmental characteristics and stress responses in transgenic Arabidopsis plants

S-adenosyl-L-methionine (SAM) synthase (SAMS) catalyze the biosynthesis of SAM, which is a precursor for ethylene and polyamines, and a methyl donor for a number of biomolecules. A full-length cDNA of SAMS from Solanum brevidens was expressed in Arabidopsis thaliana to study its physiological function. RT-PCR analysis showed that SbSAMS expression was enhanced significantly in S. brevidens leaves upon treatment with salt, mannitol, ethephon, IAA and ABA. The transgenic SbSAMS overexpression lines accumulated higher levels S-adenosyl homocysteine (SAHC) and ethylene concomitantly with increased SAM level. Expression levels of genes related to ethylene biosynthesis such as ACC synthase, but not polyamine biosynthesis genes were enhanced in SbSAMS overexpressing Arabidopsis lines. In addition, ABA responsive, wound and pathogen-inducible genes were upregulated in SbSAMS transgenic Arabidopsis plants. Transgenic Arabidopsis lines exhibited higher salt and drought stress tolerance compared to those of vector control. Based on these results we conclude that SbSAMS is expressed under abiotic stress to produce SAM as a broad-spectrum signal molecule to upregulate stress-related genes including ethylene and ABA biosynthetic pathway genes responsible for ABA, pathogen and wound responses.

Dubiumin, a chymotrypsin-like serine protease from the seeds of Solanum dubium Fresen

A serine protease was purified 6.7-fold and with 35% recovery from the seeds Solanum dubium Fresen by a simple purification procedure that combined ammonium sulfate fractionation, cation exchange and gel filtration chromatographies. The enzyme, named dubiumin, has a molecular mass of 66kDa as estimated by gel filtration and SDS-PAGE. Carbohydrate staining established the existence of a carbohydrate moiety attached to the enzyme. Inhibition of enzyme activity by serine protease inhibitors such as PMSF and chymostatin indicated that the enzyme belongs to the chymotrypsin-like serine protease class. Dubiumin is a basic protein with pI value of 9.3, acts optimally at pH 11.0, and is stable over a wide range of pH (3.0-12.0). The enzyme is also thermostable retaining complete activity at 60 degrees C after 1h and acts optimally at 70 degrees C for 30 min. Furthermore, it is highly stable in the presence of various denaturants (2.0% SDS, 7.0M urea and 3.0M guanidine hydrochloride) and organic solvents [CH(3)CN-H(2)O (1:1, v/v) and MeOH-H(2)O (1:1, v/v)] when incubated for 1h. The enzyme showed a high resistance to autodigestion even at low concentrations.

Solanum torvum responses to the root-knot nematode Meloidogyne incognita

BACKGROUND

Solanum torvum Sw is worldwide employed as rootstock for eggplant cultivation because of its vigour and resistance/tolerance to the most serious soil-borne diseases as bacterial, fungal wilts and root-knot nematodes. The little information on Solanum torvum (hereafter Torvum) resistance mechanisms, is mostly attributable to the lack of genomic tools (e.g. dedicated microarray) as well as to the paucity of database information limiting high-throughput expression studies in Torvum.

RESULTS

As a first step towards transcriptome profiling of Torvum inoculated with the nematode M. incognita, we built a Torvum 3' transcript catalogue. One-quarter of a 454 full run resulted in 205,591 quality-filtered reads. De novo assembly yielded 24,922 contigs and 11,875 singletons. Similarity searches of the S. torvum transcript tags catalogue produced 12,344 annotations. A 30,0000 features custom combimatrix chip was then designed and microarray hybridizations were conducted for both control and 14 dpi (day post inoculation) with Meloidogyne incognita-infected roots samples resulting in 390 differentially expressed genes (DEG). We also tested the chip with samples from the phylogenetically-related nematode-susceptible eggplant species Solanum melongena. An in-silico validation strategy was developed based on assessment of sequence similarity among Torvum probes and eggplant expressed sequences available in public repositories. GO term enrichment analyses with the 390 Torvum DEG revealed enhancement of several processes as chitin catabolism and sesquiterpenoids biosynthesis, while no GO term enrichment was found with eggplant DEG.The genes identified from S. torvum catalogue, bearing high similarity to known nematode resistance genes, were further investigated in view of their potential role in the nematode resistance mechanism.

CONCLUSIONS

By combining 454 pyrosequencing and microarray technology we were able to conduct a cost-effective global transcriptome profiling in a non-model species. In addition, the development of an in silico validation strategy allowed to further extend the use of the custom chip to a related species and to assess by comparison the expression of selected genes without major concerns of artifacts. The expression profiling of S. torvum responses to nematode infection points to sesquiterpenoids and chitinases as major effectors of nematode resistance. The availability of the long sequence tags in S. torvum catalogue will allow precise identification of active nematocide/nematostatic compounds and associated enzymes posing the basis for exploitation of these resistance mechanisms in other species.

The RING Finger E3 Ligase SpRing is a Positive Regulator of Salt Stress Signaling in Salt-Tolerant Wild Tomato Species

Protein ubiquitination in plants plays critical roles in many biological processes, including adaptation to abiotic stresses. Previously, RING finger E3 ligase has been characterized during salt stress response in several plant species, but little is known about its function in tomato. Here, we report that SpRing, a stress-inducible gene, is involved in salt stress signaling in wild tomato species Solanum pimpinellifolium 'PI365967'. In vitro ubiquitination assay revealed that SpRing is an E3 ubiquitin ligase and the RING finger conserved region is required for its activity. SpRing is expressed in all tissues of wild tomato and up-regulated by salt, drought and osmotic stresses, but repressed by low temperature. Green fluorescent protein (GFP) fusion analysis showed that SpRing is localized at the endoplasmic reticulum. Silencing of SpRing through a virus-induced gene silencing approach led to increased sensitivity to salt stress in wild tomato. Overexpression of SpRing in Arabidopsis thaliana resulted in enhanced salt tolerance during seed germination and early seedling development. The expression levels of certain key stress-related genes are altered both in SpRing-overexpressing Arabidopsis plants and virus-induced gene silenced tomato seedlings. Taken together, our results indicate that SpRing is involved in salt stress and functions as a positive regulator of salt tolerance.

Determination of residues responsible for substrate and product specificity of Solanum habrochaites short-chain cis-prenyltransferases

Isoprenoids are diverse compounds that have their biosynthetic origin in the initial condensation of isopentenyl diphosphate and dimethylallyl diphosphate to form C10 prenyl diphosphates that can be elongated by the addition of subsequent isopentenyl diphosphate units. These reactions are catalyzed by either cis-prenyltransferases (CPTs) or trans-prenyltransferases. The synthesis of volatile terpenes in plants typically proceeds through either geranyl diphosphate (C10) or trans-farnesyl diphosphate (C15), to yield monoterpenes and sesquiterpenes, respectively. However, terpene biosynthesis in glandular trichomes of tomato (Solanum lycopersicum) and related wild relatives also occurs via the cis-substrates neryl diphosphate (NPP) and 2Z,6Z-farnesyl diphosphate (Z,Z-FPP). NPP and Z,Z-FPP are synthesized by neryl diphosphate synthase1 (NDPS1) and Z,Z-farnesyl diphosphate synthase (zFPS), which are encoded by the orthologous CPT1 locus in tomato and Solanum habrochaites, respectively. In this study, comparative sequence analysis of NDPS1 and zFPS enzymes from S. habrochaites accessions that synthesize either monoterpenes or sesquiterpenes was performed to identify amino acid residues that correlate with the ability to synthesize NPP or Z,Z-FPP. Subsequent structural modeling, coupled with site-directed mutagenesis, highlighted the importance of four amino acids located within conserved domain II of CPT enzymes that form part of the second α-helix, for determining substrate and product specificity of these enzymes. In particular, the relative positioning of aromatic amino acid residues at positions 100 and 107 determines the ability of these enzymes to synthesize NPP or Z,Z-FPP. This study provides insight into the biochemical evolution of terpene biosynthesis in the glandular trichomes of Solanum species.

Isolation and functional characterization of lycopene beta-cyclase (CYC-B) promoter from Solanum habrochaites

BACKGROUND

Carotenoids are a group of C40 isoprenoid molecules that play diverse biological and ecological roles in plants. Tomato is an important vegetable in human diet and provides the vitamin A precursor beta-carotene. Genes encoding enzymes involved in carotenoid biosynthetic pathway have been cloned. However, regulation of genes involved in carotenoid biosynthetic pathway and accumulation of specific carotenoid in chromoplasts are not well understood. One of the approaches to understand regulation of carotenoid metabolism is to characterize the promoters of genes encoding proteins involved in carotenoid metabolism. Lycopene beta-cyclase is one of the crucial enzymes in carotenoid biosynthesis pathway in plants. Its activity is required for synthesis of both alpha-and beta-carotenes that are further converted into other carotenoids such as lutein, zeaxanthin, etc. This study describes the isolation and characterization of chromoplast-specific Lycopene beta-cyclase (CYC-B) promoter from a green fruited S. habrochaites genotype EC520061.

RESULTS

A 908 bp region upstream to the initiation codon of the Lycopene beta-cyclase gene was cloned and identified as full-length promoter. To identify promoter region necessary for regulating developmental expression of the ShCYC-B gene, the full-length promoter and its three different 5' truncated fragments were cloned upstream to the initiation codon of GUS reporter cDNA in binary vectors. These four plant transformation vectors were separately transformed in to Agrobacterium. Agrobacterium-mediated transient and stable expression systems were used to study the GUS expression driven by the full-length promoter and its 5' deletion fragments in tomato. The full-length promoter showed a basal level activity in leaves, and its expression was upregulated > 5-fold in flowers and fruits in transgenic tomato plants. Deletion of -908 to -577 bp 5' to ATG decreases the ShCYC-B promoter strength, while deletion of -908 to -437 bp 5' to ATG led to significant increase in the activity of GUS in the transgenic plants. Promoter deletion analysis led to the identification of a short promoter region (-436 bp to ATG) that exhibited a higher promoter strength but similar developmental expression pattern as compared with the full-length ShCYC-B promoter.

CONCLUSION

Functional characterization of the full-length ShCYC-B promoter and its deletion fragments in transient expression system in fruto as well as in stable transgenic tomato revealed that the promoter is developmentally regulated and its expression is upregulated in chromoplast-rich flowers and fruits. Our study identified a short promoter region with functional activity and developmental expression pattern similar to that of the full-length ShCYC-B promoter. This 436 bp promoter region can be used in promoter::reporter fusion molecular genetic screens to identify mutants impaired in CYC-B expression, and thus can be a valuable tool in understanding carotenoid metabolism in tomato. Moreover, this short promoter region of ShCYC-B may be useful in genetic engineering of carotenoid content and other agronomic traits in tomato fruits.

The genetic architecture of branched-chain amino acid accumulation in tomato fruits

Previous studies of the genetic architecture of fruit metabolic composition have allowed us to identify four strongly conserved co-ordinate quantitative trait loci (QTL) for the branched-chain amino acids (BCAAs). This study has been extended here to encompass the other 23 enzymes described to be involved in the pathways of BCAA synthesis and degradation. On coarse mapping the chromosomal location of these enzymes, it was possible to define the map position of 24 genes. Of these genes eight co-localized, or mapped close to BCAA QTL including those encoding ketol-acid reductoisomerase (KARI), dihydroxy-acid dehydratase (DHAD), and isopropylmalate dehydratase (IPMD). Quantitative evaluation of the expression levels of these genes revealed that the S. pennellii allele of IPMD demonstrated changes in the expression level of this gene, whereas those of KARI and DHAD were invariant across the genotypes. Whilst the antisense inhibition of IPMD resulted in increased BCAA, the antisense inhibition of neither KARI nor DHAD produced a clear effect in fruit BCAA contents. The results are discussed both with respect to the roles of these specific enzymes within plant amino acid metabolism and within the context of current understanding of the regulation of plant branched-chain amino acid metabolism.

Lipid signaling in plants. Cloning and expression analysis of the obtusifoliol 14alpha-demethylase from Solanum chacoense Bitt., a pollination- and fertilization-induced gene with both obtusifoliol and lanosterol demethylase activity

The sterol 14alpha-demethylase (CYP51) is the most widely distributed cytochrome P450 gene family being found in all biological kingdoms. It catalyzes the first step following cyclization in sterol biosynthesis, leading to the formation of precursors of steroid hormones, including brassinosteroids, in plants. Most enzymes involved in the plant sterol biosynthesis pathway have been characterized biochemically and the corresponding genes cloned. Genes coding for enzymes promoting substrate modifications before 24-methylenelophenol lead to embryonic and seed defects when mutated, while mutants downstream the 24-methylenelophenol intermediate show phenotypes characteristic of brassinosteroid mutants. By a differential display approach, we have isolated a fertilization-induced gene, encoding a sterol 14alpha-demethylase enzyme, named CYP51G1-Sc. Functional characterization of CYP51G1-Sc expressed in yeast (Saccharomyces cerevisiae) showed that it could demethylate obtusifoliol, as well as nontypical plant sterol biosynthetic intermediates (lanosterol), in contrast with the strong substrate specificity of the previously characterized obtusifoliol 14alpha-demethylases found in other plant species. CYP51G1-Sc transcripts are mostly expressed in meristems and in female reproductive tissues, where they are induced following pollination. Treatment of the plant itself with obtusifoliol induced the expression of the CYP51G1-Sc mRNA, suggesting a possible role of this transient biosynthetic intermediate as a bioactive signaling lipid molecule. Furthermore, treatments of leaves with (14)C-labeled obtusifoliol demonstrated that this sterol could be transported in distal parts of the plant away from the sprayed leaves. Arabidopsis (Arabidopsis thaliana) CYP51 homozygous knockout mutants were also lethal, suggesting important roles for this enzymatic step and its substrate in plant development.

New evidence of similarity between human and plant steroid metabolism: 5alpha-reductase activity in Solanum malacoxylon

The physiological role of steroid hormones in humans is well known, and the metabolic pathway and mechanisms of action are almost completely elucidated. The role of plant steroid hormones, brassinosteroids, is less known, but an increasing amount of data on brassinosteroid biosynthesis is showing unexpected similarities between human and plant steroid metabolic pathways. Here we focus our attention on the enzyme 5alpha-reductase (5alphaR) for which a plant ortholog of the mammalian system, DET2, was recently described in Arabidopsis thaliana. We demonstrate that campestenone, the natural substrate of DET2, is reduced to 5alpha-campestanone by both human 5alphaR isozymes but with different affinities. Solanum malacoxylon, which is a calcinogenic plant very active in the biosynthesis of vitamin D-like molecules and sterols, was used to study 5alphaR activity. Leaves and calli were chosen as examples of differentiated and undifferentiated tissues, respectively. Two separate 5alphaR activities were found in calli and leaves of Solanum using campestenone as substrate. The use of progesterone allowed the detection of both activities in calli. Support for the existence of two 5alphaR isozymes in S. malacoxylon was provided by the differential actions of inhibitors of the human 5alphaR in calli and leaves. The evidence for the presence of two isozymes in different plant tissues extends the analogies between plant and mammalian steroid metabolic pathways.

Salicylic acid-induced ROS production by mitochondrial electron transport chain depends on the activity of mitochondrial hexokinases in tomato (Solanum lycopersicum L.)

The growth regulator, salicylic acid (SA) plays an important role in the induction of cell death in plants. Production of reactive oxygen species (ROS) by mitochondrial electron transport chain (mtETC), cytochrome c (cyt c) release from mitochondria and loss of mitochondrial integrity can be observed during cell death execution in plant tissues. The aim of this work was to study the putative role of hexokinases (HXKs) in the initiation of cell death using tomato (Solanum lycopersicum L.) leaves and mitochondria isolated from plants exposed to a sublethal, 0.1 mM and a cell death-inducing, 1 mM concentrations of SA. Both treatments enhanced ROS and nitric oxide (NO) production in the leaves, which contributed to a concentration-dependent loss of membrane integrity. Images prepared by transmission electron microscopy showed swelling and disorganisation of mitochondrial cristae and vacuolization of mitochondria after SA exposure. Using post-embedding immunohistochemistry, cyt c release from mitochondria was also detected after 1 mM SA treatment. Both SA treatments decreased the activity and transcript levels of HXKs in the leaves and the total mtHXK activity in the mitochondrial fraction. The role of mitochondrial hexokinases (mtHXKs) in ROS and NO production of isolated mitochondria was investigated by the addition of HXK substrate, glucose (Glc) and a specific HXK inhibitor, N-acetylglucosamine (NAG) to the mitochondrial suspension. Both SA treatments enhanced ROS production by mtETC in the presence of succinate and ADP, which was slightly inhibited by Glc and increased significantly by NAG in control and in 0.1 mM SA-treated mitochondria. These changes were not significant at 1 mM SA, which caused disorganisation of mitochondrial membranes. Thus the inhibition of mtHXK activity can contribute to the mitochondrial ROS production, but it is not involved in NO generation in SA-treated leaf mitochondria suggesting that SA can promote cell death by suppressing mtHXK transcription and activity.