The origin and the genetic regulation of the self-compatibility mechanism in clementine (Citrus clementina Hort. ex Tan.).

Self-incompatibility (SI) is a genetic mechanism common in flowering plants to prevent self-fertilization. Among citrus species, several pummelo, mandarin, and mandarin-like accessions show SI behavior. In these species, SI is coupled with a variable degree of parthenocarpy ensuring the production of seedless fruits, a trait that is highly appreciated by consumers. In Citrus, recent evidences have shown the presence of a gametophytic SI system based on S-ribonucleases (S-RNases) ability to impair self-pollen tube growth in the upper/middle part of the style. In the present study, we combined PCR analysis and next-generation sequencing technologies, to define the presence of S7- and S11-Rnases in the S-genotype of the Citrus clementina (Hort. ex Tan.), the self-incompatible 'Comune' clementine and its self-compatible natural mutant 'Monreal'. The reference genome of 'Monreal' clementine is presented for the first time, providing more robust results on the genetic sequence of the newly discovered S7-RNase. SNP discovery analysis coupled with the annotation of the variants detected enabled the identification of 7,781 SNPs effecting 5,661 genes in 'Monreal' compared to the reference genome of C. clementina. Transcriptome analysis of unpollinated pistils at the mature stage from both clementine genotypes revealed the lack of expression of S7-RNase in 'Monreal' suggesting its involvement in the loss of the SI response. RNA-seq analysis followed by gene ontology studies enabled the identification of 2,680 differentially expressed genes (DEGs), a significant number of those is involved in oxidoreductase and transmembrane transport activity. Merging of DNA sequencing and RNA data led to the identification of 164 DEGs characterized by the presence of at least one SNP predicted to induce mutations with a high effect on their amino acid sequence. Among them, four candidate genes referring to two Agamous-like MADS-box proteins, to MYB111 and to MLO-like protein 12 were validated. Moreover, the transcription factor MYB111 appeared to contain a binding site for the 2.0-kb upstream sequences of the S7- and S11-RNase genes. These results provide useful information about the genetic bases of SI indicating that SNPs present in their sequence could be responsible for the differential expression and the regulation of S7-RNase and consequently of the SI mechanism.

Expression of Clementine Asp-Rich Proteins (CcASP-RICH) in Tobacco Plants Interferes with the Mechanism of Pollen Tube Growth.

Low-molecular-weight, aspartic-acid-rich proteins (ASP-RICH) have been assumed to be involved in the self-incompatibility process of clementine. The role of ASP-RICH is not known, but hypothetically they could sequester calcium ions (Ca2+) and affect Ca2+-dependent mechanisms. In this article, we analyzed the effects induced by clementine ASP-RICH proteins (CcASP-RICH) when expressed in the tobacco heterologous system, focusing on the male gametophyte. The aim was to gain insight into the mechanism of action of ASP-RICH in a well-known cellular system, i.e., the pollen tube. Pollen tubes of tobacco transgenic lines expressing CcASP-RICH were analyzed for Ca2+ distribution, ROS, proton gradient, as well as cytoskeleton and cell wall. CcASP-RICH modulated Ca2+ content and consequently affected cytoskeleton organization and the deposition of cell wall components. In turn, this affected the growth pattern of pollen tubes. Although the expression of CcASP-RICH did not exert a remarkable effect on the growth rate of pollen tubes, effects at the level of growth pattern suggest that the expression of ASP-RICH may exert a regulatory action on the mechanism of plant cell growth.

Biotechnological Approaches for Genetic Improvement of Lemon (Citrus limon (L.) Burm. f.) against Mal Secco Disease.

Among Citrus species, lemon is one of the most susceptible to mal secco disease, a tracheomycosis caused by the mitosporic fungus Plenodomus tracheiphilus, which induces chlorosis followed by leaf drop and progressive desiccation of twigs and branches. Severe infection can cause the death of the plant. Since no effective control strategies are available to efficiently control the pathogen spread, host tolerance is the most desirable goal in the struggle against mal secco disease. To date, both traditional breeding programs and biotechnological techniques were not efficient in developing novel varieties coupling tolerance to mal secco with optimal fruit quality. Furthermore, the genetic basis of host resistance has not been fully deciphered yet, hampering the set-up of marker-assisted selection (MAS) schemes. This paper provides an overview of the biotechnological approaches adopted so far for the selection of mal secco tolerant lemon varieties and emphasizes the promising contribution of marker-trait association analysis techniques for both unraveling the genetic determinism of the resistance to mal secco and detecting molecular markers that can be readily used for MAS. Such an approach has already proved its efficiency in several crops and could represent a valuable tool to select novel lemon varieties coupling superior fruit quality traits and resistance to mal secco.

Transcriptional Analysis of Carotenoids Accumulation and Metabolism in a Pink-Fleshed Lemon Mutant.

Pink lemon is a spontaneous bud mutation of lemon (Citrus limon, L. Burm. f) characterized by the production of pink-fleshed fruits due to an unusual accumulation of lycopene. To elucidate the genetic determinism of the altered pigmentation, comparative carotenoid profiling and transcriptional analysis of both the genes involved in carotenoid precursors and metabolism, and the proteins related to carotenoid-sequestering structures were performed in pink-fleshed lemon and its wild-type. The carotenoid profile of pink lemon pulp is characterized by an increased accumulation of linear carotenoids, such as lycopene, phytoene and phytofluene, from the early stages of development, reaching their maximum in mature green fruits. The distinctive phenotype of pink lemon is associated with an up-regulation and down-regulation of the genes upstream and downstream the lycopene cyclase, respectively. In particular, 9-cis epoxycarotenoid dioxygenase genes were overexpressed in pink lemon compared with the wild-type, suggesting an altered regulation of abscisic acid biosynthesis. Similarly, during early development of the fruits, genes of the carotenoid-associated proteins heat shock protein 21, fibrillin 1 and 2 and orange gene were overexpressed in the pulp of the pink-fleshed lemon compared to the wild-type, indicating its increased capacity for sequestration, stabilization or accumulation of carotenes. Altogether, the results highlighted significant differences at the transcriptomic level between the pink-fleshed lemon and its wild-type, in terms of carotenoid metabolism and the capacity of stabilization in storage structures between the two accessions. Such changes may be either responsible for the altered carotenoid accumulation or in contrast, a metabolic consequence.

Temperature-Dependent Compatible and Incompatible Pollen-Style Interactions in Citrus clementina Hort. ex Tan. Show Different Transglutaminase Features and Polyamine Pattern.

In clementine, failure of fertilization can result in parthenocarpic fruit development, which has several advantages, such as seedless fruit, longer shelf-life, and greater consumer appeal. Recently, S-RNases have been identified in Citrus grandis, thus revealing that the self-incompatibility (SI) reaction relies on the S-RNase gametophytic mechanism. The fundamental role of environmental factors, mostly temperature, in determining the numbers of pollen tubes reaching the ovary is also well established in Citrus. In the present work, temperature-dependent pollen-pistil interactions in C. clementina were analyzed, focusing on several morphological aspects, as well as on polyamine (PA) content and the activity and distribution of transglutaminase (TGase), both reported to be involved in the SI response in pear and in pummelo. Results clearly indicate that temperature contributed to a different activation of the SI response, which occurs at optimal temperature of 25°C but was by-passed at 15°C. TGase activity was stimulated during the SI response, and it localized differently in the compatible and incompatible interaction: in compatible pollinated styles, TGase localized inside the style canal, while it was detected all around it in incompatible crosses. TGase localization and activity were congruent with the levels of soluble and insoluble conjugated PAs and with morphological evidences, which highlighted cell wall modification occurring as a result of SI.

Target-Genes Reveal Species and Genotypic Specificity of Anthocyanin Pigmentation in Citrus and Related Genera.

BACKGROUND: Anthocyanin pigmentation characterizes a number of tissues of Citrus and its relatives. The gain and loss of pigmentation is intriguing and is inherited variously among species. METHODS: Citrus germplasm was used to investigate the anthocyanin pigmentation of tissues never before considered, including stamen, style and stigma, and of young leaves, petals, rind and flesh of 28 genotypes belonging to 14 species. Citrus genotypes encompassed citron, lemon, sweet orange, lime, and Citrus relatives included Microcitrus, Murraya, and Severinia. A relative qRT-PCR analysis was carried out on the structural and regulatory genes: phenylalanine ammonia-lyase (PAL), chalcone synthase (CHS), chalcone isomerase (CHI), flavanone 3'-hydroxylase (F3H), dihydroflavonol 4-reductase (DFR), anthocyanidin synthase (ANS), uridine diphosphate glucose flavonoid glucosyl-transferase (UFGT), glutathione S-transferase (GST), Ruby and Noemi. Image analysis and a genomic approach were employed to evaluate how the red pigmentation is inherited among tissues and species. RESULTS: Pigmentation of young leaves and petals is specific to citron and its hybrids. Ruby controls the pigmentation of petals, but not of leaves. The red color of the rind and flesh is a trait that particularly characterizes a diversity of sweet oranges, citron hybrids and Citrus relatives. Color expression depends on external factors and also on developmental stage. The coloration of stamen and style is citron-specific, while a red stigma is exclusive to Moro orange and its hybrids. CONCLUSION: It is hypothesized that there is a relationship among Citrus species and genes controlling anthocyanin pigmentation.

Recent Advances of In Vitro Culture for the Application of New Breeding Techniques in Citrus.

Citrus is one of the most important fruit crops in the world. This review will discuss the recent findings related to citrus transformation and regeneration protocols of juvenile and adult explants. Despite the many advances that have been made in the last years (including the use of inducible promoters and site-specific recombination systems), transformation efficiency, and regeneration potential still represent a bottleneck in the application of the new breeding techniques in commercial citrus varieties. The influence of genotype, explant type, and other factors affecting the regeneration and transformation of the most used citrus varieties will be described, as well as some examples of how these processes can be applied to improve fruit quality and resistance to various pathogens and pests, including the potential of using genome editing in citrus. The availability of efficient regeneration and transformation protocols, together with the availability of the source of resistance, is made even more important in light of the fast diffusion of emerging diseases, such as Huanglongbing (HLB), which is seriously challenging citriculture worldwide.

Substantial Equivalence of a Transgenic Lemon Fruit Showing Postharvest Fungal Pathogens Resistance.

The development of genetically modified (GM) crops speeds up the obtainment of novel varieties with improved agronomic characteristics. However, the risk evaluation of the use of GMs is mandatory before their release in the market. In this paper, an untargeted and comprehensive nuclear magnetic resonance-based metabolomic study was carried out on the peel and flesh of a transgenic lemon clone (E23) expressing the chit42 gene and exhibiting an increased tolerance to some pathogenic fungi and on its wild type. Results highlighted a substantial equivalence of the metabolomics profile of the transgenic clone compared to the wild type. In addition, an enhanced response of the E23 clone toward fungal pathogens affecting the postharvest management in lemon was evidenced. These results confirm the potential of genetic engineering for the punctual modification of specific agronomic traits without altering the whole pattern of metabolites and open new perspectives for a more sustainable and effective management of specific postharvest diseases in citrus.

Elucidating the contribution of wild related species on autochthonous pear germplasm: A case study from Mount Etna.

The pear (genus Pyrus) is one of the most ancient and widely cultivated tree fruit crops in temperate climates. The Mount Etna area claims a large number of pear varieties differentiated due to a long history of cultivation and environmental variability, making this area particularly suitable for genetic studies. Ninety-five pear individuals were genotyped using the simple sequence repeat (SSR) methodology interrogating both the nuclear (nDNA) and chloroplast DNA (cpDNA) to combine an investigation of maternal inheritance of chloroplast SSRs (cpSSRs) with the high informativity of nuclear SSRs (nSSRs). The germplasm was selected ad hoc to include wild genotypes, local varieties, and national and international cultivated varieties. The objectives of this study were as follows: (i) estimate the level of differentiation within local varieties; (ii) elucidate the phylogenetic relationships between the cultivated genotypes and wild accessions; and (iii) estimate the potential genetic flow and the relationship among the germplasms in our analysis. Eight nSSRs detected a total of 136 alleles with an average minor allelic frequency and observed heterozygosity of 0.29 and 0.65, respectively, whereas cpSSRs allowed identification of eight haplotypes (S4 Table). These results shed light on the genetic relatedness between Italian varieties and wild genotypes. Among the wild species, compared with P. amygdaliformis, few P. pyraster genotypes exhibited higher genetic similarity to local pear varieties. Our analysis revealed the presence of genetic stratification with a 'wild' subpopulation characterizing the genetic makeup of wild species and the international cultivated varieties exhibiting the predominance of the 'cultivated' subpopulation.

Altered sensitivity to ethylene in 'Tardivo', a late-ripening mutant of Clementine mandarin.

'Tardivo' mandarin is a mutant of 'Comune' Clementine with a delay in peel degreening and coloration, allowing late harvesting. In this work, we have explored if the late-harvesting phenotype of 'Tardivo' mandarin is related to altered perception and sensitivity to ethylene. The peel degreening rate was examined after a single ethephon treatment or during a continuous ethylene application in fruits at two maturation stages. In general, ethylene-induced peel degreening was considerably delayed and reduced in fruits of 'Tardivo', as well as the concomitant reduction of chlorophyll (Chl) and chloroplastic carotenoids, and the accumulation of chromoplastic carotenoids. Analysis of the expression of genes involved in Chl degradation, carotenoids, ABA, phenylpropanoids and ethylene biosynthesis revealed an impairment in the stimulation of most genes by ethylene in the peel of 'Tardivo' fruits with respect to 'Comune', especially after 5 days of ethylene application. Moreover, ethylene-induced expression of two ethylene receptor genes, ETR1 and ETR2, was also reduced in mutant fruits. Expression levels of two ethylene-responsive factors, ERF1 and ERF2, which were repressed by ethylene, were also impaired to a different extent, in fruits of both genotypes. Collectively, results suggested an altered sensitivity of the peel of 'Tardivo' to ethylene-induced physiological and molecular responses, including fruit degreening and coloration processes, which may be time-dependent since an early moderated reduction in the responses was followed by the latter inability to sustain ethylene action. These results support the involvement of ethylene in the regulation of at least some aspects of peel maturation in the non-climacteric citrus fruit.

Highly ordered alignment of a vinyl polymer by host-guest cross-polymerization.

Chain alignment can significantly influence the macroscopic properties of a polymeric material, but no general and versatile methodology has yet been reported to obtain highly ordered crystalline packing of polymer chains, with high stability. Here, we disclose a strategy that relies on 'ordered crosslinks' to produce polymeric materials that exhibit a crystalline arrangement. Divinyl crosslinkers (2,5-divinyl-terephthalate) were first embedded, as substitutional ligands, into the structure of a porous coordination polymer (PCP), [Cu(terephthalate)triethylenediamine0.5]n. A representative vinyl monomer, styrene, was subsequently polymerized inside the channels of the host PCP. The polystyrene chains that form within the PCP channels also crosslink with the divinyl species. This bridges together the polymer chains of adjacent channels and ensures that, on selective removal of the PCP, the polymer chains remain aligned. Indeed, the resulting material exhibits long-range order and is stable to thermal and solvent treatments, as demonstrated by X-ray powder diffraction and transmission electron microscopy.

High resolution melting analysis is a more sensitive and effective alternative to gel-based platforms in analysis of SSR--an example in citrus.

High resolution melting curve analysis (HRM) has been used as an efficient, accurate and cost-effective tool to detect single nucleotide polymorphisms (SNPs) or insertions or deletions (INDELs). However, its efficiency, accuracy and applicability to discriminate microsatellite polymorphism have not been extensively assessed. The traditional protocols used for SSR genotyping include PCR amplification of the DNA fragment and the separation of the fragments on electrophoresis-based platform. However, post-PCR handling processes are laborious and costly. Furthermore, SNPs present in the sequences flanking repeat motif cannot be detected by polyacrylamide-gel-electrophoresis based methods. In the present study, we compared the discriminating power of HRM with the traditional electrophoresis-based methods and provided a panel of primers for HRM genotyping in Citrus. The results showed that sixteen SSR markers produced distinct polymorphic melting curves among the Citrus spp investigated through HRM analysis. Among those, 10 showed more genotypes by HRM analysis than capillary electrophoresis owing to the presence of SNPs in the amplicons. For the SSR markers without SNPs present in the flanking region, HRM also gave distinct melting curves which detected same genotypes as were shown in capillary electrophoresis (CE) analysis. Moreover, HRM analysis allowed the discrimination of most of the 15 citrus genotypes and the resulting genetic distance analysis clustered them into three main branches. In conclusion, it has been approved that HRM is not only an efficient and cost-effective alternative of electrophoresis-based method for SSR markers, but also a method to uncover more polymorphisms contributed by SNPs present in SSRs. It was therefore suggested that the panel of SSR markers could be used in a variety of applications in the citrus biodiversity and breeding programs using HRM analysis. Furthermore, we speculate that the HRM analysis can be employed to analyse SSR markers in a wide range of applications in all other species.

Comparative transcriptome analysis of stylar canal cells identifies novel candidate genes implicated in the self-incompatibility response of Citrus clementina.

BACKGROUND: Reproductive biology in citrus is still poorly understood. Although in recent years several efforts have been made to study pollen-pistil interaction and self-incompatibility, little information is available about the molecular mechanisms regulating these processes. Here we report the identification of candidate genes involved in pollen-pistil interaction and self-incompatibility in clementine (Citrus clementina Hort. ex Tan.). These genes have been identified comparing the transcriptomes of laser-microdissected stylar canal cells (SCC) isolated from two genotypes differing for self-incompatibility response ('Comune', a self-incompatible cultivar and 'Monreal', a self- compatible mutation of 'Comune'). RESULTS: The transcriptome profiling of SCC indicated that the differential regulation of few specific, mostly uncharacterized transcripts is associated with the breakdown of self-incompatibility in 'Monreal'. Among them, a novel F-box gene showed a drastic up-regulation both in laser microdissected stylar canal cells and in self-pollinated whole styles with stigmas of 'Comune' in concomitance with the arrest of pollen tube growth. Moreover, we identify a non-characterized gene family as closely associated to the self-incompatibility genetic program activated in 'Comune'. Three different aspartic-acid rich (Asp-rich) protein genes, located in tandem in the clementine genome, were over-represented in the transcriptome of 'Comune'. These genes are tightly linked to a DELLA gene, previously found to be up-regulated in the self-incompatible genotype during pollen-pistil interaction. CONCLUSION: The highly specific transcriptome survey of the stylar canal cells identified novel genes which have not been previously associated with self-pollen rejection in citrus and in other plant species. Bioinformatic and transcriptional analyses suggested that the mutation leading to self-compatibility in 'Monreal' affected the expression of non-homologous genes located in a restricted genome region. Also, we hypothesize that the Asp-rich protein genes may act as Ca2+ "entrapping" proteins, potentially regulating Ca2+ homeostasis during self-pollen recognition.

Polyamines and transglutaminase activity are involved in compatible and self-incompatible pollination of Citrus grandis.

Pollination of pummelo (Citrus grandis L. Osbeck) pistils has been studied in planta by adding compatible and self-incompatible (SI) pollen to the stigma surface. The pollen germination has been monitored inside the pistil by fluorescent microscopy showing SI altered morphologies with irregular depositions of callose in the tube walls, and heavy callose depositions in enlarged tips. The polyamine (PA) content as free, perchloric acid (PCA)-soluble and -insoluble fractions and transglutaminase (TGase) activity have been analyzed in order to deepen their possible involvement in the progamic phase of plant reproduction. The conjugated PAs in PCA-soluble fraction were definitely higher than the free and the PCA-insoluble forms, in both compatible and SI pollinated pistils. In pistils, pollination caused an early decrease of free PAs and increase of the bound forms. The SI pollination, showed highest values of PCA-soluble and -insoluble PAs with a maximum in concomitance with the pollen tube arrest. As TGase mediates some of the effects of PAs by covalently binding them to proteins, its activity, never checked before in Citrus, was examined with two different assays. In addition, the presence of glutamyl-PAs confirmed the enzyme assay data and excluded the possibility of a misinterpretation. The SI pollination caused an increase in TGase activity, whereas the compatible pollination caused its decrease. Similarly to bound PAs, the glutamyl-PAs and the enzyme activity peaked in the SI pollinated pistils in concomitance with the observed block of the pollen tube growth, suggesting an involvement of TGase in SI response.

New microsatellite loci for pomegranate, Punica granatum (Lythraceae).

PREMISE OF THE STUDY: A new set of pomegranate microsatellites was selected and characterized to assess the level of genetic diversity among cultivars and wild genotypes. • METHODS AND RESULTS: Nine Simple Sequence Repeat (SSR) markers were obtained using the Microsatellite-AFLP technique and were successfully amplified in 34 genotypes belonging to Italian, Spanish, and Turkish germplasm collections. The number of alleles per locus ranged from 1 to 5, and the total number of alleles was 22. • CONCLUSIONS: Because only a few codominant markers are available for this species, the newly identified SSRs will facilitate genetic diversity studies, fingerprinting, and mapping. In addition, the 9 loci successfully amplified in P. granatum var. nana. No cross transferability was observed for Cuphea micropetala and Lagerstroemia indica (Lythraceae).

Physiological and molecular analysis of the maturation process in fruits of Clementine Mandarin and one of its late-ripening mutants.

Peel color is one of the main features affecting citrus quality. Clementine is a widespread citrus species with several mutants showing a delay in pigmentation and harvesting. This work characterizes the fruit development and ripening of two clementine clones, 'Comune', a widespread variety, and one of its natural mutations, 'Tardivo', which differ by a delayed color-break and extended harvest period. Morphological, chemical, and molecular analyses were carried out on fruits of both genotypes during the whole maturation process. Analysis showed that mutation did not affect ripening characteristics such as juice acidity and TSS. However, biochemical and molecular analysis revealed marked differences in the flavedo regarding carotenogenesis and chlorophyllase gene expression. Carotenoid showed quantitative differences at biochemical and molecular levels. Results demonstrated that the mutation in 'Tardivo' influenced the transcriptional activation of PSY, a key step in carotenoid biosynthesis. The differential PSY expression led to a significant quantitative difference in phytoene accumulation between the two genotypes. Also, 'Tardivo' showed delayed accumulation of carotenes, lutein, and beta,beta-xanthophylls. The differential expression of genes involved in ethylene biosynthesis and perception suggested differing responses to ethylene signaling between the two genotypes. Moreover, exogenous application of ethylene revealed a different sensitivity of the two varieties to this hormone. The analysis added new information to better understand the complex process of ripening in citrus.

Histological and molecular analysis of pollen-pistil interaction in clementine.

In contrast to model species, the self-incompatibility reaction in citrus has been poorly studied. It is assumed to be gametophytically determined and genetically controlled by the S-locus, which in other species encodes for glycoproteins (S-RNases) showing ribonuclease activity. To investigate pollen-pistil interaction, the pollen tube growth of two clementine varieties, 'Comune' (self-incompatible) and 'Monreal' (a 'Comune' self-compatible mutation) was analysed by histological assays in self- and cross-pollination conditions. Cross-pollination assays demonstrated that the mutation leading to self-compatibility in 'Monreal' occurred in the stylar tissues. Similar rates of pollen germination were observed in both genotypes. However, 'Comune' pollen tubes showed altered morphology and arrested growth in the upper style while in 'Monreal' they grew straight toward the ovary. Moreover, to identify genes putatively involved in pollen-pistil interaction and self-incompatibility, research based on the complementary DNA-amplified fragment length polymorphism technique was carried out to compare the transcript profiles of unpollinated and self-pollinated styles and stigmas of the two cultivars. This analysis identified 96 unigenes such as receptor-like kinases, stress-induced genes, transcripts involved in the phenylpropanoid pathway, transcription factors and genes related to calcium and hormone signalling. Surprisingly, a high percentage of active long terminal repeat (LTR) and non-LTR retrotransposons were identified among the unigenes, indicating their activation in response to pollination and their possible role in the regulation of self-incompatibility genes. The quantitative reverse trascription-polymerase chain reaction analysis of selected gene tags showed transcriptional differences between the two genotypes during pollen germination and pollen tube elongation.

Methane, carbon dioxide and hydrogen storage in nanoporous dipeptide-based materials.

Dipeptide crystals containing nanochannels of various sizes show remarkable and selective absorption of methane, carbon dioxide and hydrogen.

Defence-related gene expression in transgenic lemon plants producing an antimicrobial Trichoderma harzianum endochitinase during fungal infection.

Constitutive over-expression of antifungal genes from microorganisms involved in plant defence mechanisms represents a promising strategy for conferring genetic resistance against a broad range of plant pathogenic fungi. In the present work, two transgenic lemon clones with the chit42 gene from Trichoderma harzianum were tested for resistance to fungal disease and expression level of defence-related genes was evaluated. Different resistance-related processes, such as production of reactive oxygen species (ROS), systemic acquired resistance (SAR) and induced systemic resistance (ISR), were monitored in transgenic and wild type lemon clones inoculated with Botrytis cinerea, the causal agent of grey mould in citrus. Expression of genes that encode gluthatione peroxidase (GPX), a producer of ROS, chitinases, glucanases (SAR), PAL, HPL, and AOS (ISR) was measured by quantitative PCR during the first 24 h after leaf inoculation. Leaves of transgenic lemon plants inoculated with B. cinerea showed significantly less lesion development than wild type leaves. Tissues from detached leaves of different transgenic lemon clones showed a significant correlation between resistance and transgene expression. On the other hand, the over-expression of the transgenic fungal gene enhanced by two-three folds transcript levels of genes associated with enhanced ROS production and ISR establishment, while the expression of native chitinase and glucanase genes involved in SAR was down-regulated.