Expression of Delta(12) fatty acid desaturase during the induced accumulation of the antifungal diene in avocado fruits

Chia (Salvia hispanica L.), a functional 'superfood': new insights into its botanical, genetic and nutraceutical characteristics

BACKGROUND

Chia (Salvia hispanica L.) seeds have become increasingly popular among health-conscious consumers owing to their high content of ω-3 fatty acids, which provide various health benefits. Comprehensive chemical analyses of the fatty acids and proteins in chia seeds have been conducted, revealing their functional properties. Recent studies have confirmed the high ω-3 content of chia seed oil and have hinted at additional functional characteristics.

SCOPE

This review article aims to provide an overview of the botanical, morphological and biochemical features of chia plants, seeds and seed mucilage. Additionally, we discuss the recent developments in genetic and molecular research on chia, including the latest transcriptomic and functional studies that examine the genes responsible for chia fatty acid biosynthesis. In recent years, research on chia seeds has shifted its focus from studying the physicochemical characteristics and chemical composition of seeds to understanding the metabolic pathways and molecular mechanisms that contribute to their nutritional benefits. This has led to a growing interest in various pharmaceutical, nutraceutical and agricultural applications of chia. In this context, we discuss the latest research on chia and the questions that remain unanswered, and we identify areas that require further exploration.

CONCLUSIONS

Nutraceutical compounds associated with significant health benefits, including ω-3 polyunsaturated fatty acids, proteins and phenolic compounds with antioxidant activity, have been measured in high quantities in chia seeds. However, comprehensive investigations through both in vitro experiments and in vivo animal and controlled human trials are expected to provide greater clarity on the medicinal, antimicrobial and antifungal effects of chia seeds. The recently published genome of chia and gene-editing technologies, such as CRISPR, facilitate functional studies deciphering molecular mechanisms of biosynthesis and metabolic pathways in this crop. This necessitates development of stable transformation protocols and creation of a publicly available lipid database, mutant collection and large-scale transcriptomic datasets for chia.

Peruvian Amaranth (kiwicha) Accumulates Higher Levels of the Unsaturated Linoleic Acid

Grain amaranth (Amaranthus spp.) is an emerging crop rich in proteins and other valuable nutrients. It was domesticated twice, in Mexico and Peru. Although global trade is dominated by Mexican species of amaranth, Peruvian amaranth (A. caudatus, kiwicha) has remained neglected, although it harbours valuable traits. In the current study, we investigate the accumulation of polyunsaturated fatty acids, comparing four genotypes of A. caudatus with K432, a commercial variety deriving from the Mexican species A. hypochondriacus under the temperate environment of Southwest Germany. We show that the A. caudatus genotypes flowered later (only in late autumn), such that they were taller as compared to the Mexican hybrid but yielded fewer grains. The oil of kiwicha showed a significantly higher content of unsaturated fatty acids, especially of linoleic acid and α-linolenic acid compared to early flowering genotype K432. To gain insight into the molecular mechanisms behind these differences, we sequenced the genomes of the A. hypochondriacus × hybridus variety K432 and the Peruvian kiwicha genotype 8300 and identified the homologues for genes involved in the ω3 fatty-acid pathway and concurrent oxylipin metabolism, as well as of key factors for jasmonate signalling and cold acclimation. We followed the expression of these transcripts over three stages of seed development in all five genotypes. We find that transcripts for Δ6 desaturases are elevated in kiwicha, whereas in the Mexican hybrid, the concurrent lipoxygenase is more active, which is followed by the activation of jasmonate biosynthesis and signalling. The early accumulation of transcripts involved in cold-stress signalling reports that the Mexican hybrid experiences cold stress already early in autumn, whereas the kiwicha genotypes do not display indications for cold stress, except for the very final phase, when there were already freezing temperatures. We interpret the higher content of unsaturated fatty acids in the context of the different climatic conditions shaping domestication (tropical conditions in the case of Mexican amaranth, sharp cold snaps in the case of kiwicha) and suggest that kiwicha oil has high potential as functional food which can be developed further by tailoring genetic backgrounds, agricultural practice, and processing.

Effects of Temperature and Salt Stress on the Expression of delta-12 Fatty Acid Desaturase Genes and Fatty Acid Compositions in Safflower

The regulation of microsomal (e.g., FAD2) and plastidial (e.g., FAD6) oleate desaturases by cold, heat and salt stress were investigated. Gene expression levels and fatty acid compositions were determined in the roots, stems and leaves of safflower following stress treatments. A safflower plastidial oleate desaturase gene, CtFAD6, was cloned, and oleic acid desaturation was confirmed in Synechococcus sp. strain PCC7942. The results showed that temperature regulated oleate desaturation at the transcriptional level, and this regulation pattern was tissue-specific. CtFAD2-1, CtFAD2-2 and CtFAD6 were significantly induced under cold and heat stress in young leaves, and CtFAD2-2 and CtFAD6 were slightly induced in young stems. In contrast, CtFAD2-1, CtFAD2-11 and CtFAD2-10 were sensitive to salt stress in all safflower tissues (roots, stem and leaves). CtFAD6 was insensitive to salt and was slightly induced in leaves only.

How Temperatures May Affect the Synthesis of Fatty Acids during Olive Fruit Ripening: Genes at Work in the Field

A major concern for olive cultivation in many extra-Mediterranean regions is the adaptation of recently introduced cultivars to environmental conditions different from those prevailing in the original area, such as the Mediterranean basin. Some of these cultivars can easily adapt their physiological and biochemical parameters in new agro-environments, whereas others show unbalanced values of oleic acid content. The objective of this study was to evaluate the effects of the thermal regime during oil synthesis on the expression of fatty acid desaturase genes and on the unsaturated fatty acid contents at the field level. Two cultivars (Arbequina and Coratina) were included in the analysis over a wide latitudinal gradient in Argentina. The results suggest that the thermal regime exerts a regulatory effect at the transcriptional level on both OeSAD2 and OeFAD2-2 genes and that this regulation is cultivar-dependent. It was also observed that the accumulated thermal time affects gene expression and the contents of oleic and linoleic acids in cv. Arbequina more than in Coratina. The fatty acid composition of cv. Arbequina is more influenced by the temperature regime than Coratina, suggesting its greater plasticity. Overall, findings from this study may drive future strategies for olive spreading towards areas with different or extreme thermal regimes serve as guidance for the evaluation olive varietal patrimony.

The Enigmatic Aliphatic Acetogenins and Their Correlations With Lipids During Seed Germination and Leaf Development of Avocado (Persea americana Mill.)

Lipids in avocados have been widely studied due to their nutritional value and several reported bioactivities. Aliphatic acetogenins are a relevant component of the avocado lipidome and have been tested for several potential food and pharma industries applications. This work followed the evolution of avocado fatty acids (FAs) and aliphatic acetogenins during seed germination and leaf growth. Oil extracts of embryonic axes, cotyledons, and leaves from seedlings and trees were divided to analyze free acetylated acetogenins (AcO-acetogenins), and free FAs. Embryonic axes from germinating seeds contained the highest amount of AcO-acetogenins and FAs; this tissue also accumulated the most diverse FA profile with up to 22 detected moieties. Leaves presented the highest variations in AcO-acetogenin profiles during development, although leaves from seedlings accumulated the simplest FA profile with only 10 different FAs. Remarkably, AcO-acetogenins represented half of the carbons allocated to lipids in grown leaves, while embryonic axes and cotyledons always contained more carbons within FAs during germination. Thus, we hypothesized the use of the AcO-acetogenin acyl chain for energy production toward β-oxidation. Also, α-linolenic and docosahexaenoic acids (DHAs) were proposed as close AcO-acetogenin intermediaries based on a correlation network generated using all these data. Another part of the oil extract was fractionated into different lipid classes before transesterification to profile FAs and acetogenins bound to lipids. Acetogenin backbones were identified for the first time in triglycerides from cotyledons and mainly in polar lipids (which include phospholipids) in all developing avocado tissues analyzed. Seed tissues accumulated preferentially polar lipids during germination, while triglycerides were consumed in cotyledons. Seedling leaves contained minute amounts of triglycerides, and polar lipids increased as they developed. Results from this work suggest acetogenins might be part of the energy and signaling metabolisms, and possibly of membrane structures, underlining the yet to establish role(s) of these unusual lipids in the avocado plant physiology.

Arbuscular Mycorrhizal Symbiosis Triggers Major Changes in Primary Metabolism Together With Modification of Defense Responses and Signaling in Both Roots and Leaves of Vitis vinifera

Grapevine (Vitis vinifera L.) is one of the most important crops worldwide but is subjected to multiple biotic and abiotic stresses, especially related to climate change. In this context, the grapevine culture could take advantage of symbiosis through association with arbuscular mycorrhizal fungi (AMF), which are able to establish symbiosis with most terrestrial plants. Indeed, it is well established that mycorrhization improves grapevine nutrition and resistance to stresses, especially water stress and resistance to root pathogens. Thus, it appears essential to understand the effect of mycorrhization on grapevine metabolism and defense responses. In this study, we combined a non-targeted metabolomic approach and a targeted transcriptomic study to analyze changes induced in both the roots and leaves of V. vinifera cv. Gewurztraminer by colonization with Rhizophagus irregularis (Ri). We showed that colonization of grapevine with AMF triggers major reprogramming of primary metabolism in the roots, especially sugar and fatty acid metabolism. On the other hand, mycorrhizal roots had decreased contents of most sugars and sugar acids. A significant increase in several fatty acids (C16:1, linoleic and linolenic acids and the C20 arachidonic and eicosapentaenoic acids) was also detected. However, a downregulation of the JA biosynthesis pathway was evidenced. We also found strong induction of the expression of PR proteins from the proteinase inhibitor (PR6) and subtilase (PR7) families in roots, suggesting that these proteins are involved in the mycorrhiza development but could also confer higher resistance to root pathogens. Metabolic changes induced by mycorrhization were less marked in leaves but involved higher levels of linoleic and linolenic acids and decreased sucrose, quinic, and shikimic acid contents. In addition, Ri colonization resulted in enhanced JA and SA levels in leaves. Overall, this study provides a detailed picture of metabolic changes induced by AMF colonization in a woody, economically important species. Moreover, stimulation of fatty acid biosynthesis and PR protein expression in roots and enhanced defense hormone contents in leaves establish first insight in favor of better resistance of grapevine to various pathogens provided by AMF colonization.

The expression of several pepper fatty acid desaturase genes is robustly activated in an incompatible pepper-tobamovirus interaction, but only weakly in a compatible interaction

The replication of positive strand RNA viruses in plant cells is markedly influenced by the desaturation status of fatty acid chains in lipids of intracellular plant membranes. At present, little is known about the role of lipid desaturation in the replication of tobamoviruses. Therefore, we investigated the expression of fatty acid desaturase (FAD) genes and the fatty acid composition of pepper leaves inoculated with two different tobamoviruses. Obuda pepper virus (ObPV) inoculation induced a hypersensitive reaction (incompatible interaction) while Pepper mild mottle virus (PMMoV) inoculation caused a systemic infection (compatible interaction). Changes in the expression of 16 FADs were monitored in pepper leaves following ObPV and PMMoV inoculations. ObPV inoculation rapidly and markedly upregulated seven Δ12-FADs that encode enzymes putatively located in the endoplasmic reticulum membrane. In contrast, PMMoV inoculation resulted in a weaker but rapid upregulation of two Δ12-FADs and a Δ15-FAD. The expression of genes encoding plastidial FADs was not influenced neither by ObPV nor by PMMoV. In accordance with gene expression results, a significant accumulation of linoleic acid was observed by gas chromatography-mass spectrometry in ObPV-, but not in PMMoV-inoculated leaves. ObPV inoculation led to a marked accumulation of H₂O₂ in the inoculated leaves. Therefore, the effect of H₂O₂ treatments on the expression of six tobamovirus-inducible FADs was also studied. The expression of these FADs was upregulated to different degrees by H₂O₂ that correlated with ObPV-inducibility of these FADs. These results underline the importance of further studies on the role of pepper FADs in pepper-tobamovirus interactions.

Primary Metabolism in Avocado Fruit

Avocado (Persea americana Mill) is rich in a variety of essential nutrients and phytochemicals; thus, consumption has drastically increased in the last 10 years. Avocado unlike other fruit is characterized by oil accumulation during growth and development and presents a unique carbohydrate pattern. There are few previous and current studies related to primary metabolism. The fruit is also quite unique since it contains large amounts of C7 sugars (mannoheptulose and perseitol) acting as transportable and storage sugars and as potential regulators of fruit ripening. These C7 sugars play a central role during fruit growth and development, but still confirmation is needed regarding the biosynthetic routes and the physiological function during growth and development of avocado fruit. Relatively recent transcriptome studies on avocado mesocarp during development and ripening have revealed that most of the oil is synthesized during early stages of development and that oil synthesis is halted when the fruit is harvested (pre-climacteric stage). Most of the oil is accumulated in the form of triacylglycerol (TAG) representing 60-70% in dry basis of the mesocarp tissue. During early stages of fruit development, high expression of transcripts related to fatty acid and TAG biosynthesis has been reported and downregulation of same genes in more advanced stages but without cessation of the process until harvest. The increased expression of fatty acid key genes and regulators such as PaWRI1, PaACP4-2, and PapPK-β-1 has also been reported to be consistent with the total fatty acid increase and fatty acid composition during avocado fruit development. During postharvest, there is minimal change in the fatty acid composition of the fruit. Almost inexistent information regarding the role of organic acid and amino acid metabolism during growth, development, and ripening of avocado is available. Cell wall metabolism understanding in avocado, even though crucial in terms of fruit quality, still presents severe gaps regarding the interactions between cell wall remodeling, fruit development, and postharvest modifications.

Activation of the phenylpropanoid biosynthesis pathway reveals a novel action mechanism of the elicitor effect of chitosan on avocado fruit epicarp

Secondary metabolites play an important role in the avocado fruit defense system. Phenolic compounds are the main biosynthesized metabolites of this system response. Our objective in this investigation was to evaluate the induction of specific metabolic pathways using chitosan as an elicitor. Extracts obtained from avocado in intermediate and consumption maturity stages treated with chitosan exhibited an increase in antifungal activity, which caused inhibition of mycelial growth and a decrease in sporulation as well as spore germination of Colletotrichum gloeosporioides. Additionally, RNA from epicarp of the fruits treated and untreated with chitosan was obtained in order to evaluate the expression of genes related to phenylpropanoids and the antifungal compound 1-acetoxy-2-hydroxy-4-oxo-heneicosa-12,15-diene biosynthesis. An increased in gene expression of genes that participates in the phenylpropanoids route was observed during the stage of physiological fruit maturity, others genes such as Flavonol synthase (Fls), increased only in samples obtained from fruit treated with chitosan at consumption maturity. Our results reveal a new molecular mechanism where chitosan induces a specific accumulation of phenylpropanoids and antifungal diene; this partially explains avocado's resistance against fungal pathogens. Finally, we discuss the molecular connections between chitosan induction and gene expression to explain the biological events that orchestrate the resistance pathways in fruits.

The FAD2 Gene in Plants: Occurrence, Regulation, and Role

Vegetable oils rich in oleic acid are more desirable than oils rich in polyunsaturated and saturated fatty acids. The biological switch of oleic acid to linoleic acid is facilitated by fatty acid desaturase 2 enzyme that is further classified into FAD2-1, FAD2-2, FAD2-3, and FAD2-4. The genes coding these enzymes have high sequence similarity, but differ mostly in their expression patterns. The seed-type FAD2 genes had evolved independently after segregation by duplication from constitutively expressed FAD2 genes. Temperature, light and wounding effectively regulate FAD2 expression in plants. FAD2 genes are expressed differently in different tissues of the plant, and the over-expression of FAD2 modifies physiological and vegetative characteristics. The activity of FAD2 leads to an increase in the content of dienoic fatty acids, and hence increases the resistance toward cold and salt stress. The thorough study of the FAD2 gene is important for understanding the expression, regulation and mechanism that will help in improving the quality of oil and stress resistance in plants.

Avocado fruit maturation and ripening: dynamics of aliphatic acetogenins and lipidomic profiles from mesocarp, idioblasts and seed

BACKGROUND

Avocado fruit contains aliphatic acetogenins (oft-acetylated, odd-chain fatty alcohols) with promising bioactivities for both medical and food industries. However, we have scarce knowledge about their metabolism. The present work aimed to study changes in acetogenin profiles from mesocarp, lipid-containing idioblasts, and seeds from 'Hass' cultivar during fruit development, germination, and three harvesting years. An untargeted LC-MS based lipidomic analysis was also conducted to profile the lipidome of avocado fruit in each tissue.

RESULTS

The targeted analysis showed that acetogenin profiles and contents remained unchanged in avocado mesocarp during maturation and postharvest ripening, germination, and different harvesting years. However, a shift in the acetogenin profile distribution, accompanied with a sharp increase in concentration, was observed in seed during early maturation. Untargeted lipidomics showed that this shift was accompanied with remodeling of glycerolipids: TAGs and DAGs decreased during fruit growing in seed. Remarkably, the majority of the lipidome in mature seed was composed by acetogenins; we suggest that this tissue is able to synthesize them independently from mesocarp. On the other hand, lipid-containing idioblasts accumulated almost the entire acetogenin pool measured in the whole mesocarp, while only having 4% of the total fatty acids. The lipidome of this cell type changed the most when the fruit was ripening after harvesting, TAGs decreased while odd-chain DAGs increased. Notably, idioblast lipidome was more diverse than that from mesocarp.

CONCLUSIONS

Evidence shown here suggests that idioblasts are the main site of acetogenin biosynthesis in avocado mesocarp. This work unveiled the prevalence of aliphatic acetogenins in the avocado fruit lipidome and evidenced TAGs as initial donors of the acetogenin backbones in its biosynthesis. It also sets evidence for acetogenins being included in future works aimed at characterizing the avocado seed, as they are a main component of their lipidome.

Timing of Apple Fruit Infection by Neofabraea perennans and Neofabraea kienholzii in Relation to Bull's-eye Rot Development in Stored Apple Fruit

Bull's-eye rot is a postharvest disease of pome fruit in the U.S. Pacific Northwest. The disease is caused by the fungi Neofabraea kienholzii, N. malicorticis, N. perennans, and N. vagabunda. Fruit infection by these pathogens is initiated in the orchard during the fruit-growing season but remains latent at harvest. For fruit held in postharvest cold storage, bull's-eye rot symptom development is slow to progress, requiring at least 3 months before symptoms are first visible. In order to determine the timing of preharvest fruit infection in relation to bull's-eye rot development in cold storage, 'Red Delicious' and 'Fuji' apple fruit were inoculated with a conidial suspension of N. perennans or N. kienholzii at different inoculation timings throughout the fruit growing seasons of 2012 to 2014. Fruit were harvested and stored at 0°C for up to 10 months, during which time disease incidence was recorded periodically. Results from this study demonstrate that apple fruit infection by either pathogen may occur at any point during the growing season. However, infections occurring over the 8-week period immediately prior to harvest yield a higher incidence of bull's-eye rot in stored fruit compared with infections initiated earlier in the growing season.

Virus-induced alterations in primary metabolism modulate susceptibility to Tobacco rattle virus in Arabidopsis

During compatible virus infections, plants respond by reprogramming gene expression and metabolite content. While gene expression studies are profuse, our knowledge of the metabolic changes that occur in the presence of the virus is limited. Here, we combine gene expression and metabolite profiling in Arabidopsis (Arabidopsis thaliana) infected with Tobacco rattle virus (TRV) in order to investigate the influence of primary metabolism on virus infection. Our results revealed that primary metabolism is reconfigured in many ways during TRV infection, as reflected by significant changes in the levels of sugars and amino acids. Multivariate data analysis revealed that these alterations were particularly conspicuous at the time points of maximal accumulation of TRV, although infection time was the dominant source of variance during the process. Furthermore, TRV caused changes in lipid and fatty acid composition in infected leaves. We found that several Arabidopsis mutants deficient in branched-chain amino acid catabolism or fatty acid metabolism possessed altered susceptibility to TRV. Finally, we showed that increments in the putrescine content in TRV-infected plants correlated with enhanced tolerance to freezing stress in TRV-infected plants and that impairment of putrescine biosynthesis promoted virus multiplication. Our results thus provide an interesting overview for a better understanding of the relationship between primary metabolism and virus infection.

Anti-Trypanosoma cruzi activity of 10 medicinal plants used in northeast Mexico

The aim of this study was to screen the trypanocidal activity of plants used in traditional Mexican medicine for the treatment of various diseases related to parasitic infections. Cultured Trypanosoma cruzi epimastigotes were incubated for 96h with different concentrations of methanolic extracts obtained from Artemisia mexicana, Castela texana, Cymbopogon citratus, Eryngium heterophyllum, Haematoxylum brasiletto, Lippia graveolens, Marrubium vulgare, Persea americana, Ruta chalepensis and Schinus molle. The inhibitory concentration (IC50) was determined for each extract via a colorimetric method. Among the evaluated species, the methanolic extracts of E. heterophyllum, H. brasiletto, M. vulgare and S. molle exhibited the highest trypanocidal activity, showing percentages of growth inhibition between 88 and 100% at a concentration of 150μg/ml. These medicinal plants may represent a valuable source of new bioactive compounds for the therapeutic treatment of trypanosomiasis.

Activity-guided identification of acetogenins as novel lipophilic antioxidants present in avocado pulp (Persea americana)

Avocado fruit is a rich source of health-related lipophilic phytochemicals such as monounsaturated fatty acids, tocopherols, carotenes, acetogenins and sterols. However, limited information is available on the contribution of specific phytochemicals to the overall antioxidant capacity (AOC) of the fruit. Centrifugal partition chromatography was used as fractionation tool, guided by an in vitro chemical assay of oxygen radical absorbance capacity (ORAC). Subsequent experiments focused on isolation and characterization of the chemical nature of the main contributors to lipophilic AOC of avocado pulp. ORAC values obtained for acetogenins were contrasted with results from an isolated kidney mitochondria membrane lipid peroxidation bioassay. The present study established that lipophilic AOC of the pulp was significantly higher than its hydrophilic AOC. Our results confirmed the presence of acetogenins in the fractions with highest lipophilic AOC, and for the first time linked them as contributors to lipophilic-ORAC values. Further HPLC-PDA/MS-TOF analysis led to structural elucidation of two novel acetogenins, not previously reported as present in avocado pulp, along with five already known related-compounds. Antioxidant properties observed for avocado pulp acetogenins by the ORAC assay suggested that, in the presence of an emulsifying agent, acetogenins could serve as novel lipophilic antioxidants in a food matrix. Results from isolated mitochondria lipid peroxidation bioassay, indicated that L-ORAC values which may have relevance for food matrix applications, should not be interpreted to have a direct relevance in health-related claims, compounds need to be evaluated considering the complexity of biological systems.

Identification and functional analysis of a Δ6-desaturase gene and the effects of temperature and wounding stresses on its expression in Microula sikkimensis leaves

A Δ6-desaturase gene was isolated from Microula sikkimensis. Sequence analysis indicated that the gene, designated MsD6DES, had an open reading frame of 1,357 bp and encoded 448 amino acids. Heterologous expression in tobacco indicated that MsD6DES can use endogenous substrates to synthesize γ-linolenic acid (GLA, 18:3(Δ 6,9,12)) and octadecatetraenoic acid (OTA, 18:4(Δ 6,9,12,15)). MsD6DES transcripts were distributed in all tested tissues, with high expression levels in seeds and young leaves. The effects of temperature and wounding stresses on MsD6DES expression were analyzed. The results indicated that temperature regulates MsD6DES at the transcriptional level. MsD6DES expression increased first, reaching a maximum 4 h after low-temperature treatment. A slight increase in MsD6DES transcript levels was also observed under high temperature. We found that the response of MsD6DES to temperature stress was different from those of fungi and algae. In addition, MsD6DES was found to be wound-inducible.

Activation of quiescent infections by postharvest pathogens during transition from the biotrophic to the necrotrophic stage

Insidious fungal infections of postharvest pathogens remain quiescent, as biotrophs, during fruit growth and harvest, but activate their development and transform to necrotrophs, which elicit decay symptoms, during ripening and senescence. Exposure of unripe hosts to pathogens quickly initiates defensive signal-transduction cascades that limit fungal growth and development, but exposure to the same pathogens during ripening and storage activates a substantially different signaling cascade that facilitates fungal colonization. The first step in the activation of quiescent infections may involve the fungal capability to cope with plant defense responses by detoxification and efflux transport of antifungals, or by overcoming the suppression of pathogenicity factors. The second step toward the activation of quiescent infections is actively modulated by the pathogen in response to a host signal(s), and includes alkalization or ammonification of the host tissue, which sensitizes the host and activates the transcription and secretion of fungal-degradative enzymes that promote maceration of the host tissue. Feedback signals involving, for example, nitrogen and sugar further enhance pH changes, synthesis of hydrolytic enzymes and saprophytic development in the macerated tissue. This review describes the coordinated series of mechanisms that regulate the activation of quiescent infections in various fruit/vegetable-pathogen interactions.

Metabolism of the Flavonoid Epicatechin by Laccase of Colletotrichum gloeosporioides and Its Effect on Pathogenicity on Avocado Fruits

ABSTRACT During avocado fruit ripening, decreasing levels of the flavonoid epicatechin have been reported to modulate the metabolism of preformed antifungal compounds and the activation of quiescent Colletotrichum gloeosporioides infections. Epicatechin levels decreased as well when C. gloeosporioides was grown in the presence of epicatechin in culture. Extracts of laccase enzyme obtained from decayed tissue and culture media fully metabolized the epicatechin substrate within 4 and 20 h, respectively. Purified laccase protein from C. gloeosporioides showed an apparent MW of 60,000, an isoelectric point at pH 3.9, and maximal epicatechin degradation at pH 5.6. Inhibitors of fungal laccase such as EDTA and thioglycolic acid reduced C. gloeosporioides symptom development when applied to ripening susceptible fruits. Isolates of C. gloeosporioides with reduced laccase activity and no capability to metabolize epicatechin showed reduced pathogenicity on ripening fruits. On the contrary, Mexican isolates with increasing capabilities to metabolize epicatechin showed early symptoms of disease in unripe fruits. Transcript levels of cglac1, encoding C. gloeosporioides laccase, were enhanced during fungal development in the presence of epicatechin at pH 6.0, where avocado fruits are susceptible to fungal attack. But transcript increase was not detected at pH 5.0, where the fruit is resistant to fungal attack. The present results suggest that biotransformation of epicatechin by C. gloeosporioides in ripening fruits is followed by the decline of the preformed antifungal diene compound, resulting in the activation of quiescent infections.