Regulation by biotic stress of tannins biosynthesis in Quercus ilex: Crosstalk between defoliation and Phytophthora cinnamomi infection

Genome-wide identification of TCP transcription factors and their potential roles in hydrolyzable tannin production in Quercus variabilis cupule

Hydrolyzable tannins (HTs) have garnered significant attention due to their proven beneficial effects in the clinical treatment of various diseases. The cupule of Chinese cork oak (Quercus variabilis Blume) has been used as raw material of traditional medicine for centuries for its high content of HTs. Previous studies have identified UGT84A13 as a key enzyme in the HT biosynthesis pathway in Q. variabilis, but the transcriptional regulation network of UGT84A13 remains obscure. Here, we performed a comprehensive genome-wide identification of the TCP transcription factors in Q. variabilis, elucidating their molecular evolution and gene structure. Gene expression analysis showed that TCP3 from the CIN subfamily and TCP6 from the PCF subfamily were co-expressed with UGT84A13 in cupule. Further functional characterization using dual-luciferase assays confirmed that TCP3, rather than TCP6, played a role in the transcriptional regulation of UGT84A13, thus promoting HT biosynthesis in the cupule of Q. variabilis. Our work identified TCP family members in Q. variabilis for the first time, and provided novel insights into the transcriptional regulatory network of UGT84A13 and HT biosynthesis in Q. variabilis, explaining the reason why the cupule enriches HTs that could be used for traditional medicine.

Genomic basis of the distinct biosynthesis of β-glucogallin, a biochemical marker for hydrolyzable tannin production, in three oak species

Hydrolyzable tannins (HTs), predominant polyphenols in oaks, are widely used in grape wine aging, feed additives, and human healthcare. However, the limited availability of a high-quality reference genome of oaks greatly hampered the recognition of the mechanism of HT biosynthesis. Here, high-quality reference genomes of three Asian oak species (Quercus variabilis, Quercus aliena, and Quercus dentata) that have different HT contents were generated. Multi-omics studies were carried out to identify key genes regulating HT biosynthesis. In vitro enzyme activity assay was also conducted. Dual-luciferase and yeast one-hybrid assays were used to reveal the transcriptional regulation. Our results revealed that β-glucogallin was a biochemical marker for HT production in the cupules of the three Asian oaks. UGT84A13 was confirmed as the key enzyme for β-glucogallin biosynthesis. The differential expression of UGT84A13, rather than enzyme activity, was the main reason for different β-glucogallin and HT accumulation. Notably, sequence variations in UGT84A13 promoters led to different trans-activating activities of WRKY32/59, explaining the different expression patterns of UGT84A13 among the three species. Our findings provide three high-quality new reference genomes for oak trees and give new insights into different transcriptional regulation for understanding β-glucogallin and HT biosynthesis in closely related oak species.

God save the queen! How and why the dominant evergreen species of the Mediterranean Basin is declining?

Quercus ilex may be considered the queen tree of the Mediterranean Basin, dominating coastal forest areas up to 2000 m above sea level at some sites. However, an increase in holm oak decline has been observed in the last decade. In this review, we analysed the current literature to answer the following questions: what are the traits that allow holm oak to thrive in the Mediterranean environment, and what are the main factors that are currently weakening this species? In this framework, we attempt to answer these questions by proposing a triangle as a graphical summary. The first vertex focuses on the main morpho-anatomical, biochemical and physiological traits that allow holm oak to dominate Mediterranean forests. The other two vertices consider abiotic and biotic stressors that are closely related to holm oak decline. Here, we discuss the current evidence of holm oak responses to abiotic and biotic stresses and propose a possible solution to its decline through adequate forest management choices, thus allowing the species to maintain its ecological domain.

Secondary metabolites related to the resistance of Psidium spp. against the nematode Meloidogyneenterolobii

The guava tree (Psidium guajava) is a tropical species native to South America and is recognized as the 11th most economically important fruit tree in Brazil. However, the presence of the nematode Meloidogyne enterolobii and the fungus Fusarium solani in the roots of guava plants leads to the development of root galls, causing significant damage. In contrast, the species P. guineense and P. cattleianum have been identified as resistant and immune to the nematode, respectively. In this study, the researchers aimed to compare the metabolomic profiles of infected and uninfected roots of P. guajava, P. cattleianum, and P. guineense using mass spectrometry coupled with liquid chromatography (LC-MS). The goal was to identify secondary metabolites that could potentially be utilized as biochemical resources for nematode control. The findings of the study demonstrated that the plant metabolism of all three species undergoes alterations in response to the phytopathogen inoculation. By employing molecular networks, the researchers identified that the secondary metabolites affected by the infection, whether produced or suppressed, are primarily of a polar chemical nature. Further analysis of the database confirmed the polar nature of the regulated substances after infection, specifically hydrolysable tannins and lignans in P. guineense and P. cattleianum. Interestingly, a group of non-polar substances belonging to the terpene class was also identified in the resistant and immune species. This suggests that these terpenes may act as inhibitors of M. enterolobii, working as repellents or as molecules that can reduce oxidative stress during the infection process, thus enhancing the guava resistance to the nematode. Overall, this study provides valuable insights into the metabolic alterations occurring in different Psidium spp. in response to M. enterolobii infection. The identification of specific secondary metabolites, particularly terpenes, opens up new possibilities for developing effective strategies to control the nematode and enhance guava resistance.

Traits variation of acorns and cupules during maturation process in Quercus variabilis and Quercus aliena

Quercus variabilis and Quercus aliena are two native tree species in China, which have similar habitats, and their regeneration mainly depends on acorn dispersal. This study analyzed the contents of water, soluble sugar, starch, soluble protein, and total phenolics in acorns and cupules during the whole development process to explore the difference between species. Thereinto, starch and total phenol occupied the dominant roles as their high contents. The acorn starch contents increased sharply during development in both species, but the contents in Q. variabilis were almost twice those of Q. aliena when mature. Similarly, high expression levels of starch synthase, soluble starch synthase 2 (SSS2) were also found in the acorns of Q. variabilis. The total phenol contents in Q. variabilis acorns were high at the early stages, and decreased sharply to similar contents in Q. aliena when mature. Additionally, the cupules in Q. variabilis had high contents of total phenols during the whole development period. Similar trends were also found in the expression patterns of UGT84A13 and SDH. The high total phenols in acorns and cupules of Q. variabilis probably protect the acorns from Mechoris ursulus, as only Q. aliena suffered a severe pest infestation in the early development stages. This study not only clarifies the interspecific difference between storage and defense substances during the development process in acorns and cupules, but also deepens understanding the specialized mechanisms of plant-pest/animal interactions in Quercus.

The D165H Polymorphism of QiMYB-like-1 Is Linked to Interactions between Tannin Accumulation, Herbivory and Biogeographical Determinants of Quercus ilex

The accumulation in the leaves and young stems of phenolic compounds, such as hydrolyzable and condensed tannins, constitutes a defense mechanism of plants against herbivores. Among other stressing factors, chronic herbivory endangers Quercus ilex, a tree playing a central role in Mediterranean forests. This work addressed the connections between the chemical defenses of Q. ilex leaves and their susceptibility to herbivory, quantitative traits whose relationships are modulated by environmental and genetic factors that could be useful as molecular markers for the selection of plants with improved fitness. A search for natural variants detected the polymorphism D165H in the effector domain of QiMYB-like-1, a TT2-like transcription factor whose family includes members that control the late steps of condensed tannins biosynthesis in different plant species. QiMYB-like-1 D165H polymorphism was screened by PCR-RFLP in trees from six national parks in Spain where Q. ilex has a relevant presence, revealing that, unlike most regions that match the Hardy-Weinberg equilibrium, homozygous plants are over-represented in "Monfragüe" and "Cabañeros", among the best examples to represent the continental Mediterranean (cM) ecosystem. Accordingly, the averages of two stress-related quantitative traits measured in leaves, herbivory index and accumulation of condensed tannins, showed asymmetric distributions depending on the clustering of trees based on ecological and genetic factors. Thus, the impact of herbivory was greater in managed forests with a low density of trees from the cM region, among which QiMYB-like-1 D165 homozygotes stand out, whereas condensed tannins accumulation was higher in leaves of QiMYB-like-1 H165 homozygotes from low-density forests, mainly in the Pyrenean (Py) region. Besides, the correlation between the contents of condensed tannins and total tannins vanished after clustering by the same factors: the cM region singularity, forest tree density, and QiMYB-like-1 genotype, among which homozygous shared the lowest link. The biogeographical and genetic constraints that modulate the contribution of condensed tannins to chemical defenses also mediated their interactions with the herbivory index, which was found positively correlated with total phenolics or tannins, suggesting an induction signal by this biotic stress. In contrast, a negative correlation was observed with condensed tannins after tree clustering by genetics factors where associations between tannins were lost. Therefore, condensed tannins might protect Q. ilex from defoliation in parks belonging to the cM ecosystem and carrying genetic factor(s) linked to the QiMYB-like-1 D165H polymorphism.

Effect of Methyl Jasmonate in Gene Expression, and in Hormonal and Phenolic Profiles of Holm Oak Embryogenic Lines Before and After Infection With Phytophthora cinnamomi

The dieback syndrome affecting Quercus ilex and other oak species impels the search for tolerant plant genotypes, as well as methods of plant immunization against such infections. Elicitation treatments can be an effective strategy to activate plant defense response and embryogenic lines represent a promising tool to generate new tolerant genotypes and also to study early markers involved in defense response. The aim of the presented work was to investigate changes in gene expression, and in hormonal and phenolic profiles induced in three holm oak embryogenic lines (ELs) elicited with methyl jasmonate (MeJA) before and after infection with the oomycete Phytophthora cinnamomi, which is the main biotic agent involved in this pathogenic process. The three ELs, derived from three genotypes, showed different basal profiles in all tested parameters, noting that the VA5 naïve genotype from a scape tree was characterized by a basal higher expression in NADPH-dependent cinnamyl alcohol dehydrogenase (CAD) and chalcone synthase (CHS) genes and also by higher caffeic acid content. Our work also identifies changes triggered by MeJA elicitation in holm oak embryogenic lines, such as increases in ABA and JA contents, as well as in levels of most of the determined phenolic compounds, especially in caffeic acid in Q8 and E00 ELs, but not in their biosynthesis genes. Irrespective of the EL, the response to oomycete infection in holm oak elicited plant material was characterized by a further increase in JA. Since JA and phenols have been described as a part of the Q. ilex defense response against P. cinnamomi, we propose that MeJA may act as an induced resistance (IR) stimulus and that in our embryogenic material induced both direct (detected prior to any challenge) and primed (detected after subsequent challenge) defense responses.

Variations in Acorn Traits in Two Oak Species: Quercus mongolica Fisch. ex Ledeb. and Quercus variabilis Blume

Quercus mongolica Fisch. ex Ledeb. and Q. variabilis Blume are two main oak species in China, producing large amounts of acorns every year. However, the trait variations in acorns, as a promising energy crop material, are not fully understood, hence we compared the traits of acorns from the different populations with the altered geographic distribution in this study. Thirteen acorn traits, including phenotype, proximate compositions and functional compounds, were analyzed in both Quercus L. species collected from 44 populations across China. The results showed that, except large differences found among accessions in acorn sizes, the starch varied from 140.96–297.09 mg/g in Q. mongolica and 130.99–306.28 mg/g in Q. variabilis, indicating its substantial differences among populations. The total polyphenols, total flavonoids and soluble tannins varied from 41.76–158.92, 23.43–91.94, and 15.11–17.81 mg/g, respectively, in Q. mongolica, 89.36–188.37, 50.59–116.07, 15.24–17.33 mg/g, respectively, in Q. variabilis, demonstrating their large variations in the levels of polyphenols among populations. Moreover, the acorns of Q. mongolica in North China and Q. variabilis in Southwest China had higher levels of starch and polyphenols. As the geographical location approached in the distribution of two Quercus species, the difference in acorn sizes gradually increased, while that in polyphenols were opposite. Principal component analysis and cluster analysis further revealed that the acorn sizes became larger and polyphenols became less with the increasing latitudes in both species. In North China, the acorns of Q. mongolica had small sizes and high polyphenols, which was contrary to those in Q. variabilis. These findings indicated that acorn traits were closely associated with the geographical distribution. Thus, our results will provide references for the selection breeding of acorn with the high starch, high or low polyphenols in the different regions.

Relationships between the Pathogen Erysiphe alphitoides, the Phytophagous Mite Schizotetranychus garmani (Acari: Tetranychidae) and the Predatory Mite Euseius finlandicus (Acari: Phytoseiidae) in Oak

Simple Summary

Knowledge about the relationships between plant pathogens, arthropods, and their natural enemies is scarce. We studied the relationships between the plant fungal pathogen, Erysiphe alphitoides, the phytophagous mite Schizotetranychus garmani, and the predatory mite Euseius finlandicus in leaves of pedunculate oak. In June, July and August 2016, in 30 trees located in three forests near Belgrade, Serbia, the presence of E. alphitoides, S. garmani and E. finlandicus was assessed. The occurrence of E. alphitoides was high where the population of S. garmani was high. However, the presence of the leaf pathogen E. alphitoides was not related to the amount of the predatory mite E. finlandicus. The relationships between powdery mildew and the two mite species were stable across time and space, and the presence of one mite was not influenced by the presence of the other mite.

Abstract

Food webs on forest trees include plant pathogens, arthropods, and their natural enemies. To increase the understanding of the impact of a plant pathogen on herbivore-natural enemy interactions, we studied the powdery mildew fungus Erysiphe alphitoides, the phytophagous mite Schizotetranychus garmani, and the predatory and mycophagous mite Euseius finlandicus in pedunculate oak (Quercus robur) leaves. In June, July and August of 2016, we assessed the severity of powdery mildew, mite population density and adult female mite size in 30 trees in three forests near Belgrade, Serbia. In August, the infection severity of E. alphitoides related positively to the population density of S. garmani and negatively to the body size of S. garmani females. Throughout the vegetative season, the infection severity of E. alphitoides related positively to the population density of E. finlandicus but not to its body size. The effect of E. alphitoides on the population density and adult size of S. garmani was not mediated by the population density of E. finlandicus, and vice versa. Interactions were consistent in all forests and varied with the summer month. Our findings indicate that E. alphitoides can influence the average body size and population densities of prey and predatory mites studied, irrespective of predator-prey relationships.

Global Change and Forest Disturbances in the Mediterranean Basin: Breakthroughs, Knowledge Gaps, and Recommendations

Forest ecosystems in the Mediterranean Basin are mostly situated in the north of the Basin (mesic). In the most southern and dry areas, the forest can only exist where topography and/or altitude favor a sufficient availability of water to sustain forest biomass. We have conducted a thorough review of recent literature (2000–2021) that clearly indicates large direct and indirect impacts of increasing drought conditions on the forests of the Mediterranean Basin, their changes in surface and distribution areas, and the main impacts they have suffered. We have focused on the main trends that emerge from the current literature and have highlighted the main threatens and management solution for the maintenance of these forests. The results clearly indicate large direct and indirect impacts of increasing drought conditions on the forests of the Mediterranean Basin. These increasing drought conditions together with over-exploitation, pest expansion, fire and soil degradation, are synergistically driving to forest regression and dieback in several areas of this Mediterranean Basin. These environmental changes have triggered responses in tree morphology, physiology, growth, reproduction, and mortality. We identified at least seven causes of the changes in the last three decades that have led to the current situation and that can provide clues for projecting the future of these forests: (i) The direct effect of increased aridity due to more frequent and prolonged droughts, which has driven Mediterranean forest communities to the limit of their capacity to respond to drought and escape to wetter sites, (ii) the indirect effects of drought, mainly by the spread of pests and fires, (iii) the direct and indirect effects of anthropogenic activity associated with general environmental degradation, including soil degradation and the impacts of fire, species invasion and pollution, (iv) human pressure and intense management of water resources, (v) agricultural land abandonment in the northern Mediterranean Basin without adequate management of new forests, (vi) very high pressure on forested areas of northern Africa coupled with the demographic enhancement, the expansion of crops and higher livestock pressure, and the more intense and overexploitation of water resources uses on the remaining forested areas, and (vii) scarcity and inequality of human management and policies, depending on the national and/or regional governments and agencies, being unable to counteract the previous changes. We identified appropriate measures of management intervention, using the most adequate techniques and processes to counteract these impacts and thus to conserve the health, service capacity, and biodiversity of Mediterranean forests. Future policies should, moreover, promote research to improve our knowledge of the mechanisms of, and the effects on, nutrient and carbon plant-soil status concurrent with the impacts of aridity and leaching due to the effects of current changes. Finally, we acknowledge the difficulty to obtain an accurate quantification of the impacts of increasing aridity rise that warrants an urgent investment in more focused research to further develop future tools in order to counteract the negative effects of climate change on Mediterranean forests.

Molecular Research on Stress Responses in Quercus spp.: From Classical Biochemistry to Systems Biology through Omics Analysis

The genus Quercus (oak), family Fagaceae, comprises around 500 species, being one of the most important and dominant woody angiosperms in the Northern Hemisphere. Nowadays, it is threatened by environmental cues, which are either of biotic or abiotic origin. This causes tree decline, dieback, and deforestation, which can worsen in a climate change scenario. In the 21st century, biotechnology should take a pivotal role in facing this problem and proposing sustainable management and conservation strategies for forests. As a non-domesticated, long-lived species, the only plausible approach for tree breeding is exploiting the natural diversity present in this species and the selection of elite, more resilient genotypes, based on molecular markers. In this direction, it is important to investigate the molecular mechanisms of the tolerance or resistance to stresses, and the identification of genes, gene products, and metabolites related to this phenotype. This research is being performed by using classical biochemistry or the most recent omics (genomics, epigenomics, transcriptomics, proteomics, and metabolomics) approaches, which should be integrated with other physiological and morphological techniques in the Systems Biology direction. This review is focused on the current state-of-the-art of such approaches for describing and integrating the latest knowledge on biotic and abiotic stress responses in Quercus spp., with special reference to Quercus ilex, the system on which the authors have been working for the last 15 years. While biotic stress factors mainly include fungi and insects such as Phytophthora cinnamomi, Cerambyx welensii, and Operophtera brumata, abiotic stress factors include salinity, drought, waterlogging, soil pollutants, cold, heat, carbon dioxide, ozone, and ultraviolet radiation. The review is structured following the Central Dogma of Molecular Biology and the omic cascade, from DNA (genomics, epigenomics, and DNA-based markers) to metabolites (metabolomics), through mRNA (transcriptomics) and proteins (proteomics). An integrated view of the different approaches, challenges, and future directions is critically discussed.

Transgenerational Induction of Resistance to Phytophthora cinnamomi in Holm Oak

The maternal environment of a tree species can influence the development and resistance of its offspring. Transgenerational induction of resistance is well known in plants but its occurrence in forest tree species has been less reported. Quercus ilex L. (holm oak) is a widespread Mediterranean tree species threatened by the invasive Phytophthora cinnamomi Rands pathogen. The influence of P. cinnamomi on the offspring of infected Q. ilex mother trees has not been studied. This study compared the performance and tolerance to P. cinnamomi of seedlings from non-infected and P. cinnamomi-infected trees. Acorns from Q. ilex trees were collected from five forests. After isolations were conducted in the rhizosphere of several trees, in each forest, three trees were selected as non-infected and three were selected as P. cinnamomi-infected. Forty acorns per tree were weighed and sown under greenhouse conditions, and when plants were aged ~9 months they were challenged with P. cinnamomi. Plant mortality was higher in the offspring of non-infected trees than in the offspring of P. cinnamomi-infected trees (26.2% vs. 21.1%, respectively). Consistently, survival probabilities of seedlings from P. cinnamomi-infected trees were higher than those of seedlings from non-infected trees, particularly in seedlings with reduced growth. Although acorns from healthy Q. ilex trees were heavier than acorns from P. cinnamomi-infected trees, the time to death of inoculated seedlings was not influenced by seed weight. The time to death of seedlings was positively related to belowground mass, particularly to an increased proportion of fine secondary roots. We report transgenerational-induced resistance to P. cinnamomi in Q. ilex triggered by an unknown mechanism independent of acorn mass. Information about the persistence of transgenerational effects in Q. ilex offspring and the influence of these effects on plant fitness is crucial to improve the management and regeneration of this declining species.

Genome-Wide Identification of Tannase Genes and Their Function of Wound Response and Astringent Substances Accumulation in Juglandaceae

Tannins are important polyphenol compounds with different component proportions in different plant species. The plants in the Juglandaceae are rich in tannins, including condensed tannins and hydrolyzable tannins. In this study, we identified seven tannase genes (TAs) responsible for the tannin metabolism from walnut, pecan, and Chinese hickory, and three nut tree species in the Juglandaceae, which were divided into two groups. The phylogenetic and sequence analysis showed that TA genes and neighboring clade genes (TA-like genes) had similar sequences compared with other carboxylesterase genes, which may be the origin of TA genes produced by tandem repeat. TA genes also indicated higher expressions in leaf than other tissues and were quickly up-regulated at 3 h after leaf injury. During the development of the seed coat, the expression of the synthesis-related gene GGTs and the hydrolase gene TAs was continuously decreased, resulting in the decrease of tannin content in the dry sample of the seed coat of Chinese hickory. However, due to the reduction in water content during the ripening process, the tannin content in fresh sample increased, so the astringent taste was obvious at the mature stage. In addition, the CcGGTs' expression was higher than CiGGTs in the initiation of development, but CcTAs continued to be down-regulated while CiTA2a and CiTA2b were up-regulated, which may bring about the significant differences in tannin content and astringent taste between Chinese hickory and pecan. These results suggested the crucial role of TAs in wound stress of leaves and astringent ingredient accumulation in seed coats of two nut tree species in the Juglandaceae.

Lythrum salicaria Ellagitannins Stimulate IPEC-J2 Cells Monolayer Formation and Inhibit Enteropathogenic Escherichia coli Growth and Adhesion

Lythrum salicaria herb (LSH) was applied in diarrhea therapy since ancient times. Despite empirically referenced therapeutic effects, the bioactivity mechanisms and chemical constituents responsible for pharmacological activity remain not fully resolved. Taking into consideration the historical use of LSH in treatment of diarrhea in humans and farm animals, the aim of the study was to examine in vitro the influence of LSH and its C-glycosylic ellagitannins on processes associated with maintaining intestinal epithelium integrity and enteropathogenic Escherichia coli (EPEC) growth and adhesion. LSH was not only inhibiting EPEC growth in a concentration dependent manner but also its adhesion to IPEC-J2 intestinal epithelial cell monolayers. Inhibitory activity toward EPEC growth was additionally confirmed ex vivo in distal colon samples of postweaning piglets. LSH and its dominating C-glycosylic ellagitannins, castalagin (1), vescalagin (2), and salicarinins A (3) and B (4) were stimulating IPEC-J2 monolayer formation by enhancing claudin 4 production. Parallelly tested gut microbiota metabolites of LSH ellagitannins, urolithin C (5), urolithin A (6), and its glucuronides (7) were inactive. The activities of LSH and the isolated ellagitannins support its purported antidiarrheal properties and indicate potential mechanisms responsible for its beneficial influence on the intestinal epithelium.

Growth Rates of Lymantria dispar Larvae and Quercus robur Seedlings at Elevated CO2 Concentration and Phytophthora plurivora Infection

Interactions between plants, insects and pathogens are complex and not sufficiently understood in the context of climate change. In this study, the impact of a root pathogen on a leaf-eating insect hosted by a tree species at elevated CO2 concentration is reported for the first time. The combined and isolated effects of CO2 and infection by the root pathogen Phytophthora plurivora on English oak (Quercus robur) seedlings were used to assess growth rates of plants and of gypsy moth (Lymantria dispar) larvae. For this purpose, two Q. robur provenances (Belgrade and Sombor) were used. At ambient CO2 concentration, the relative growth rates of larvae consuming leaves of plants infected by P. plurivora was higher than those consuming non-infected plants. However, at elevated CO2 concentration (1000 ppm) higher relative growth rates were detected in the larvae consuming the leaves of non-infected plants. At ambient CO2 concentration, lower growth rates were recorded in L. dispar larvae hosted in Q. robur from Belgrade in comparison to larvae hosted in Q. robur from Sombor. However, at elevated CO2 concentration, similar growth rates irrespective of the provenance were observed. Defoliation by the gypsy moth did not influence the growth of plants while P. plurivora infection significantly reduced tree height in seedlings from Belgrade. The results confirm that a rise of CO2 concentration in the atmosphere modifies the existing interactions between P. plurivora, Q. robur, and L. dispar. Moreover, the influence of the tree provenances on both herbivore and plant performance at elevated CO2 concentrations suggests a potential for increasing forest resilience through breeding.

Elucidating the role of shikimate dehydrogenase in controlling the production of anthocyanins and hydrolysable tannins in the outer peels of pomegranate

BACKGROUND

The outer peels of pomegranate (Punica granatum L.) possess two groups of polyphenols that have health beneficial properties: anthocyanins (ATs, which also affect peel color); and hydrolysable tannins (HTs). Their biosynthesis intersects at 3-dehydroshikimate (3-DHS) in the shikimate pathway by the activity of shikimate dehydrogenase (SDH), which converts 3-DHS to shikimate (providing the precursor for AT biosynthesis) or to gallic acid (the precursor for HTs biosynthesis) using NADPH or NADP⁺ as a cofactor. The aim of this study is to gain more knowledge about the factors that regulate the levels of HTs and ATs, and the role of SDH.

RESULTS

The results have shown that the levels of ATs and HTs are negatively correlated in the outer fruit peels of 33 pomegranate accessions, in the outer peels of two fruits exposed to sunlight, and in those covered by paper bags. When calli obtained from the outer fruit peel were subjected to light/dark treatment and osmotic stresses (imposed by different sucrose concentrations), it was shown that light with high sucrose promotes the synthesis of ATs, while dark at the same sucrose concentration promotes the synthesis of HTs. To verify the role of SDH, six PgSDHs (PgSDH1, PgSDH3-1,2, PgSDH3a-1,2 and PgSDH4) were identified in pomegranate. The expression of PgSDH1, which presumably contributes to shikimate biosynthesis, was relatively constant at different sucrose concentrations. However, the transcript levels of PgSDH3s and PgSDH4 increased with the accumulation of gallic acid and HTs under osmotic stress, which apparently accumulates to protect the cells from the stress.

CONCLUSIONS

The results strongly suggest that the biosynthesis of HTs and ATs competes for the same substrate, 3-DHS, and that SDH activity is regulated not only by the NADPH/NADP⁺ ratio, but also by the expression of the PgSDHs. Since the outer peel affects the customer's decision regarding fruit consumption, such knowledge could be utilized for the development of new genetic markers for breeding pomegranates having higher levels of both ATs and HTs.

Hormone and secondary metabolite profiling in chestnut during susceptible and resistant interactions with Phytophthora cinnamomi

Phytophthora cinnamomi (Pc) is a dangerous pathogen that causes root rot (ink disease) and threatens the production of chestnuts worldwide. Despite all the advances recently reported at molecular and physiological level, there are still gaps of knowledge that would help to unveil the defence mechanisms behind plant-Pc interactions. Bearing this in mind we quantified constitutive and Pc-induced stress-related signals (hormones and metabolites) complemented with changes in photosynthetic related parameters by exploring susceptible and resistant Castanea spp.-Pc interactions. In a greenhouse experiment, five days before and nine days after inoculation with Pc, leaves and fine roots from susceptible C. sativa and resistant C. sativa × C. crenata clonal 2-year-old plantlets were sampled (clones Cs14 and 111-1, respectively). In the resistant clone, stomatal conductance (g_s) and net photosynthesis (A) decreased significantly and soluble sugars in leaves increased, while in the susceptible clone g_s and A remained unchanged and proline levels in leaves increased. In the resistant clone, higher constitutive content of root SA and foliar ABA, JA and JA-Ile as compared to the susceptible clone were observed. Total phenolics and condensed tannins were highest in roots of the susceptible clone. In response to infection, a dynamic hormonal response in the resistant clone was observed, consisting of accumulation of JA, JA-Ile and ABA in roots and depletion of total phenolics in leaves. However, in the susceptible clone only JA diminished in leaves and increased in roots. Constitutive and Pc-induced levels of JA-Ile were only detectable in the resistant clone. From the hormonal profiles obtained in leaves and roots before and after infection, it is concluded that the lack of effective hormonal changes in C. sativa explains the lack of defence responses to Pc of this susceptible species.