Mechanisms and regulation of organic acid accumulation in plant vacuoles

Exploration of a European-centered strawberry diversity panel provides markers and candidate genes for the control of fruit quality traits

Fruit quality traits are major breeding targets in cultivated strawberry (Fragaria × ananassa). Taking into account the requirements of both growers and consumers when selecting high-quality cultivars is a real challenge. Here, we used a diversity panel enriched with unique European accessions and the 50 K FanaSNP array to highlight the evolution of strawberry diversity over the past 160 years, investigate the molecular basis of 12 major fruit quality traits by genome-wide association studies (GWAS), and provide genetic markers for breeding. Results show that considerable improvements of key breeding targets including fruit weight, firmness, composition, and appearance occurred simultaneously in European and American cultivars. Despite the high genetic diversity of our panel, we observed a drop in nucleotide diversity in certain chromosomal regions, revealing the impact of selection. GWAS identified 71 associations with 11 quality traits and, while validating known associations (firmness, sugar), highlighted the predominance of new quantitative trait locus (QTL), demonstrating the value of using untapped genetic resources. Three of the six selective sweeps detected are related to glossiness or skin resistance, two little-studied traits important for fruit attractiveness and, potentially, postharvest shelf life. Moreover, major QTL for firmness, glossiness, skin resistance, and susceptibility to bruising are found within a low diversity region of chromosome 3D. Stringent search for candidate genes underlying QTL uncovered strong candidates for fruit color, firmness, sugar and acid composition, glossiness, and skin resistance. Overall, our study provides a potential avenue for extending shelf life without compromising flavor and color as well as the genetic markers needed to achieve this goal.

Alternative electron sinks in chloroplasts and mitochondria of halophytes as a safety valve for controlling ROS production during salinity

Electron flow through the electron transport chain (ETC) is essential for oxidative phosphorylation in mitochondria and photosynthesis in chloroplasts. Electron fluxes depend on environmental parameters, e.g., ionic and osmotic conditions and endogenous factors, and this may cause severe imbalances. Plants have evolved alternative sinks to balance the reductive load on the electron transport chains in order to avoid overreduction, generation of reactive oxygen species (ROS), and to cope with environmental stresses. These sinks act primarily as valves for electron drainage and secondarily as regulators of tolerance-related metabolism, utilizing the excess reductive energy. High salinity is an environmental stressor that stimulates the generation of ROS and oxidative stress, which affects growth and development by disrupting the redox homeostasis of plants. While glycophytic plants are sensitive to high salinity, halophytic plants tolerate, grow, and reproduce at high salinity. Various studies have examined the ETC systems of glycophytic plants, however, information about the state and regulation of ETCs in halophytes under non-saline and saline conditions is scarce. This review focuses on alternative electron sinks in chloroplasts and mitochondria of halophytic plants. In cases where information on halophytes is lacking, we examined the available knowledge on the relationship between alternative sinks and gradual salinity resilience of glycophytes. To this end, transcriptional responses of involved components of photosynthetic and respiratory ETCs were compared between the glycophyte Arabidopsis thaliana and the halophyte Schrenkiella parvula, and the time-courses of these transcripts were examined in A. thaliana. The observed regulatory patterns are discussed in the context of reactive molecular species formation in halophytes and glycophytes.

High night temperature stress on rice (Oryza sativa) - insights from phenomics to physiology. A review

Rice (Oryza sativa ) faces challenges to yield and quality due to urbanisation, deforestation and climate change, which has exacerbated high night temperature (HNT). This review explores the impacts of HNT on the physiological, molecular and agronomic aspects of rice growth. Rise in minimum temperature threatens a potential 41% reduction in rice yield by 2100. HNT disrupts rice growth stages, causing reduced seed germination, biomass, spikelet sterility and poor grain development. Recent findings indicate a 4.4% yield decline for every 1°C increase beyond 27°C, with japonica ecotypes exhibiting higher sensitivity than indica. We examine the relationships between elevated CO2 , nitrogen regimes and HNT, showing that the complexity of balancing positive CO2 effects on biomass with HNT challenges. Nitrogen enrichment proves crucial during the vegetative stage but causes disruption to reproductive stages, affecting grain yield and starch synthesis. Additionally, we elucidate the impact of HNT on plant respiration, emphasising mitochondrial respiration, photorespiration and antioxidant responses. Genomic techniques, including CRISPR-Cas9, offer potential for manipulating genes for HNT tolerance. Plant hormones and carbohydrate enzymatic activities are explored, revealing their intricate roles in spikelet fertility, grain size and starch metabolism under HNT. Gaps in understanding genetic factors influencing heat tolerance and potential trade-offs associated with hormone applications remain. The importance of interdisciplinary collaboration is needed to provide a holistic approach. Research priorities include the study of regulatory mechanisms, post-anthesis effects, cumulative HNT exposure and the interaction between climate variability and HNT impact to provide a research direction to enhance rice resilience in a changing climate.

Evaluation of morphological traits, biochemical parameters and seeding availability pattern among Citrus limon 'Assam lemon' accessions across Assam

The Assam lemon is a highly valued Citrus cultivar known for its unique aroma, flavor, and appearance. This study aimed to investigate the morphological, seeding pattern and biochemical variations within 132 populations of Assam lemon from across 22 districts of Assam along with the control samples, with the objective to offer comprehensive understanding that could facilitate the improvement of breeding programs and further improvement of this important cultivar. Clustering based on UPGMA algorithm for morphological and seeding pattern data were analysed at population level, revealed two major clusters, where all the populations of Upper Assam districts were in the same cluster with the original stock (control population). The populations from Tinsukia and Dhemaji districts displayed more close similarities with the control population in comparison to populations of Upper Assam districts. Another interesting observation was regarding flowering patterns, while populations from Upper Assam districts excluding Golaghat district displayed both bisexual and unisexual flowers with less concentration of unisexual flowers, other remaining districts had bisexual and unisexual flowers of almost equal concentration. Unisexual flowers contained only the male reproductive organs with 40 anthers, while bisexual flowers had 36 anthers. Seeding patterns were examined across the districts, and it was found that populations from Tinsukia, Dhemaji, Lakhimpur, Dibrugarh, Jorhat, and the control population exhibited seedless characteristic while populations from other selected districts displayed a combination of seedless and seeded traits. Interestingly, Golaghat district appears as the linking district and showed availability of both seeded and seedless Assam lemon fruit, connecting the regions of Barak valley, Central, Lower, North and Upper Assam. Biochemical analysis showed significant variations across districts, however, the populations from Dhemaji, Tinsukia, Lakhimpur, Dibrugarh, and Jorhat districts displayed similarity with the control population. The study also investigated variability in soil nutrient content revealing substantial variation among the populations studied. This comprehensive investigation provides valuable insights into the morphological, seeding pattern, and biochemical diversity within the Assam lemon cultivar. These findings can be instrumental in breeding programs to enhance the cultivar, particularly in producing high-quality seedless fruits to meet consumer demands.

Assessing the Role of Gaseous Chlorine Dioxide in Modulating the Postharvest Ripening of Keitt Mangoes through the Induction of Ethylene Biosynthesis

Consumer acceptance of Keitt mangoes (Mangifera indica L.) is significantly affected by their slow postharvest ripening. This work used gaseous chlorine dioxide (ClO2(g)) to prepare the ready-to-eat Keitt mango and explored the potential mechanisms for the mango ripening. Harvested mangoes were treated with 20 mg·L-1 of ClO2(g) or ethephon for 3 h (25 °C) and left in a climatic chamber with a temperature of 25 ± 1 °C and a relative humidity of 85 ± 5% for 4 d. The results showed that ClO2(g) treatment significantly promoted the orange coloration of mango flesh compared to the untreated control group. Moreover, ClO2(g) treatment significantly elevated the total soluble solids, total soluble sugar, and total carotenoids content of mangoes, whereas the firmness and titratable acidity were reduced. ClO2(g)-treated mangoes reached the edible window on day 2, as did mangoes treated with ethephon at the same concentration, except that the sweetness was prominent. The residual ClO2 level of the mangoes was <0.3 mg/kg during the whole storage time, which is a safe level for fruit. In addition, ClO2(g) significantly advanced the onset of ethylene peaks by 0.5 days and increased its production between days 0.5 and 2 compared to the control group. Consistently, the genes involved in ethylene biosynthesis including miACS6, miACO1, and miACO were upregulated. In sum, ClO2(g) can be a potential technique to reduce the time for harvested mango to reach the edible window, and it functions in modulating postharvest ripening by inducing ethylene biosynthesis.

Optimization of apple fruit flavor by MdVHP1-2 via modulation of soluble sugar and organic acid accumulation

For fleshy fruits, the content and ratio of organic acids and soluble sugars are key factors for their flavor. Therefore, a better understanding of soluble sugar and organic acid accumulation in vacuoles is essential to the improvement of fruit quality. Vacuolar-type inorganic pyrophosphatase (V-PPase) has been found in various plants with crucial functions based on the hydrolysis of PPi. However, the effects of V-PPase on the soluble sugar and organic acid accumulation in apple fruit remain unclear. In this study, MdVHP1-2, a V-PPase protein in the vacuolar membrane, was identified. The results showed a positive correlation between the expression of MdVHP1-2 and the sugar/acid ratio during ripening of apple fruits. A series of transgenic analyses showed that overexpression of MdVHP1-2 significantly elevated the contents of soluble sugars and organic acids as well as the sugar/acid ratio in apple fruits and calli. Additionally, transient interference induced by MdVHP1-2 expression inhibited the accumulation of soluble sugars and organic acids in apple fruits. In summary, this study provides insight into the mechanisms by which MdVHP1-2 modulates fruit flavor through mediation of soluble sugar and organic acid accumulation, thereby facilitating improvement of the overall quality of apple and other fruits.

Regulation of a vacuolar proton-pumping P-ATPase MdPH5 by MdMYB73 and its role in malate accumulation and vacuolar acidification

As the main organic acid in fruits, malate is produced in the cytoplasm and is then transported into the vacuole. It accumulates by vacuolar proton pumps, transporters, and channels, affecting the taste and flavor of fruits. Among the three types of proton pumps (V-ATPases, V-PPases, and P-ATPases), the P-ATPases play an important role in the transport of malate into vacuoles. In this study, the transcriptome data, collected at different stages after blooming and during storage, were analyzed and the results demonstrated that the expression of MdPH5, a vacuolar proton-pumping P-ATPase, was associated with both pre- and post-harvest malate contents. Moreover, MdPH5 is localized at the tonoplast and regulates malate accumulation and vacuolar pH. In addition, MdMYB73, an upstream MYB transcription factor of MdPH5, directly binds to its promoter, thereby transcriptionally activating its expression and enhancing its activity. In this way, MdMYB73 can also affect malate accumulation and vacuolar pH. Overall, this study clarifies how MdMYB73 and MdPH5 act to regulate vacuolar malate transport systems, thereby affecting malate accumulation and vacuolar pH.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42994-023-00115-7.

Non-Targeted Spectranomics for the Early Detection of Xylella fastidiosa Infection in Asymptomatic Olive Trees, cv. Cellina di Nardò

Olive quick decline syndrome (OQDS) is a disease that has been seriously affecting olive trees in southern Italy since around 2009. During the disease, caused by Xylella fastidiosa subsp. pauca sequence type ST53 (Xf), the flow of water and nutrients within the trees is significantly compromised. Initially, infected trees may not show any symptoms, making early detection challenging. In this study, young artificially infected plants of the susceptible cultivar Cellina di Nardò were grown in a controlled environment and co-inoculated with additional xylem-inhabiting fungi. Asymptomatic leaves of olive plants at an early stage of infection were collected and analyzed using nuclear magnetic resonance (NMR), hyperspectral reflectance (HSR), and chemometrics. The application of a spectranomic approach contributed to shedding light on the relationship between the presence of specific hydrosoluble metabolites and the optical properties of both asymptomatic Xf-infected and non-infected olive leaves. Significant correlations between wavebands located in the range of 530-560 nm and 1380-1470 nm, and the following metabolites were found to be indicative of Xf infection: malic acid, fructose, sucrose, oleuropein derivatives, and formic acid. This information is the key to the development of HSR-based sensors capable of early detection of Xf infections in olive trees.

Breeding of Modern Rose Cultivars Decreases the Content of Important Biochemical Compounds in Rose Hips

This study aimed to determine the content and composition of bioactive compounds in autochthonous rose hips (R. pendulina, R. spinosissima, and R. gallica) and to compare them with the content of bioactive compounds in some cultivars ('Harstad', 'Bourgogne', 'Mount Everest', 'Poppius', 'Fruhlingsduft', 'Single Cherry', 'Fruhlingsmorgen', 'Violacea', and 'Splendens') derived from these main species. Due to insufficient information on how bioactive compound content changes when crossing roses, this study also sought to ascertain whether modern rose hip cultivars are still a sufficiently rich source of bioactive compounds and could, therefore, be potentially used as a functional food. All material was collected in the Arboretum Volčji Potok (Slovenia). The ascorbic acid content was highest in the 'Harstad' cultivar (12.79 g/kg FW), and the total organic acid content varied from 1.57 g/kg FW (R. spinosissima) to 34.39 g/kg FW ('Harstad'). Of all the carotenoids analyzed, only lycopene and β-carotene were present in all the samples. The total carotenoid content was highest in the 'Fruhlingsmorgen' cultivar (100.84 mg/kg FW), derived from R. spinosissima, and lowest in the main species, R. spinosissima (9.26 mg/kg FW). It can be concluded, therefore, that the content of bioactive compounds in rose hips of modern cultivars is generally lower than in rose hips of old cultivars and original species included in this study. The research results confirm that modern breeding strategies are mainly focused on goals such as abundant flowering and resistance to diseases and pests and not so much on the content of bioactive compounds.

Dynamics of Organic Acids during the Droplet-Vitrification Cryopreservation Procedure Can Be a Signature of Oxidative Stress in Pogostemon yatabeanus

Cryopreservation in liquid nitrogen (LN, -196 °C) is a unique option for the long-term conservation of threatened plant species with non-orthodox or limitedly available seeds. In previous studies, a systematic approach was used to develop a droplet-vitrification (DV) cryopreservation protocol for Postemon yatabeanus shoot tips that includes preculture with 10% sucrose, osmoprotection with C4-35%, cryoprotection with A3-80% vitrification solution, and a three-step regrowth starting with the ammonium-free medium. The tricarboxylic acid (TCA) cycle is a crucial component of plant cell metabolism as it is involved in redox state regulation and energy provision. We hypothesized that organic acids (OAs) associated with the TCA and its side reactions indirectly indicate metabolism intensity and oxidative stress development in shoot tips under the cryopreservation procedure. In this study, the contents of 14 OAs were analyzed using gas chromatography-tandem mass spectrometry (GC-MS/MS) in P. yatabeanus shoot tips in a series of treatments including individual steps of the DV procedure, additional stress imposed by non-optimum protocol conditions (no preculture, no osmoprotection, various vitrification solution composition, using vials instead of aluminum foils, etc.) and regrowth on different media with or without ammonium or growth regulators. The possible relation of OA content with the total cryoprotectant (CPA) concentration and shoot tips regeneration percentage was also explored. Regeneration of cryopreserved shoot tips reduced in descending order as follows: standard protocol condition (91%) > non-optimum vitrification solution (ca. 68%) > non-optimum preculture (60-62%) > regrowth medium (40-64%) > no osmoprotection, cryopreservation in vials (28-30%). Five OAs (glycolic, malic, citric, malonic, and lactic) were the most abundant in the fresh (control) shoot tips. The dynamic pattern of OAs during the DV procedure highly correlated (r = 0.951) with the total CPA concentration employed: it gradually increased through the preculture, osmoprotection, and cryoprotection, peaked at cooling/rewarming (6.38-fold above control level), and returned to the fresh control level after 5 days of regrowth (0.89-fold). The contents of four OAs (2-hydroxybutyric, 3-hydroxypropionic, lactic, and glycolic) showed the most significant (10-209-fold) increase at the cooling/rewarming step. Lactic and glycolic acids were the major OAs at cooling/rewarming, accounting for 81% of the total OAs content. The OAs were categorized into three groups based on their dynamics during the cryopreservation protocol, and these groups were differently affected by protocol step modifications. However, there was no straightforward relationship between the dynamics of OAs and shoot tip regeneration. The results suggest that active modulation of OAs metabolism may help shoot tips to cope with osmotic stress and the chemical cytotoxicity\ of CPAs. Further intensive studies are needed to investigate the effect of cryopreservation on cell primarily metabolism and identify oxidative stress-related biomarkers in plant materials.

Recent Updates on ALMT Transporters' Physiology, Regulation, and Molecular Evolution in Plants

Aluminium toxicity and phosphorus deficiency in soils are the main interconnected problems of modern agriculture. The aluminium-activated malate transporters (ALMTs) comprise a membrane protein family that demonstrates various physiological functions in plants, such as tolerance to environmental Al3+ and the regulation of stomatal movement. Over the past few decades, the regulation of ALMT family proteins has been intensively studied. In this review, we summarise the current knowledge about this transporter family and assess their involvement in diverse physiological processes and comprehensive regulatory mechanisms. Furthermore, we have conducted a thorough bioinformatic analysis to decipher the functional importance of conserved residues, structural components, and domains. Our phylogenetic analysis has also provided new insights into the molecular evolution of ALMT family proteins, expanding their scope beyond the plant kingdom. Lastly, we have formulated several outstanding questions and research directions to further enhance our understanding of the fundamental role of ALMT proteins and to assess their physiological functions.

PbWRKY26 positively regulates malate accumulation in pear fruit by activating PbMDH3

Malate is the main organic acid that affects fruit acidity and flavor in pear (Pyrus spp.). However, the regulatory mechanism of malic acid accumulation in pear remains unclear. We identified PbWRKY26 as a candidate gene using mRNA-seq, and quantification analysis verified the expression level. The expression of PbWRKY26 was positively correlated with the malic acid content in two P. pyrifolia cultivars ('Cuiguan', 'Hongsucui') and two P. ussuriensis cultivars ('Qiuxiang', 'Hanhong'), with respective correlation coefficients of 0.748*, 0.871**, 0.889**, and 0.910** (*, P < 0.05; **, P < 0.01). The expression of PbWRKY26 enhanced the malate content in overexpression transgenic pear fruit and callus. In contrast, silencing PbWRKY26 decreased the pear fruit malic acid content. Analysis of the neighbor-joining phylogenetic tree indicated that PbWRKY26 was a PH3 homolog. The WRKY26 (PH3) has been identified to regulate a proton pump gene, PH5, in a lot of plant species, but the LUC and Y1H assays showed that PbWRKY26 could not bind to PbPH5 promoter in our study. Interestingly, a malate dehydrogenase gene, PbMDH3, was identified to be regulated by PbWRKY26. This study might be valuable to understand the metabolic regulatory network associated with malate accumulation.

A pear S1-bZIP transcription factor PpbZIP44 modulates carbohydrate metabolism, amino acid, and flavonoid accumulation in fruits

Fruit quality is defined by attributes that give value to a commodity. Flavor, texture, nutrition, and shelf life are key quality traits that ensure market value and consumer acceptance. In pear fruit, soluble sugars, organic acids, amino acids, and total flavonoids contribute to flavor and overall quality. Transcription factors (TFs) regulate the accumulation of these metabolites during development or in response to the environment. Here, we report a novel TF, PpbZIP44, as a positive regulator of primary and secondary metabolism in pear fruit. Analysis of the transient overexpression or RNAi-transformed pear fruits and stable transgenic tomato fruits under the control of the fruit-specific E8 promoter demonstrated that PpZIP44 substantially affected the contents of soluble sugar, organic acids, amino acids, and flavonoids. In E8::PpbZIP44 tomato fruit, genes involved in carbohydrate metabolism, amino acid, and flavonoids biosynthesis were significantly induced. Furthermore, in PpbZIP44 overexpression or antisense pear fruits, the expression of genes in the related pathways was significantly impacted. PpbZIP44 directly interacted with the promoter of PpSDH9 and PpProDH1 to induce their expression, thereby depleting sorbitol and proline, decreasing citrate and malate, and enhancing fructose contents. PpbZIP44 also directly bound to the PpADT and PpF3H promoters, which led to the carbon flux toward phenylalanine metabolites and enhanced phenylalanine and flavonoid contents. These findings demonstrate that PpbZIP44 mediates multimetabolism reprogramming by regulating the gene expression related to fruit quality compounds.

Allelic variation of MdMYB123 controls malic acid content by regulating MdMa1 and MdMa11 expression in apple

Acidity is a key determinant of fruit organoleptic quality. Here, a candidate gene for fruit acidity, designated MdMYB123, was identified from a comparative transcriptome study of two Ma1Ma1 apple (Malus domestica) varieties, "Qinguan (QG)" and "Honeycrisp (HC)" with different malic acid content. Sequence analysis identified an A→T SNP, which was located in the last exon, resulting in a truncating mutation, designated mdmyb123. This SNP was significantly associated with fruit malic acid content, accounting for 9.5% of the observed phenotypic variation in apple germplasm. Differential MdMYB123- and mdmyb123-mediated regulation of malic acid accumulation was observed in transgenic apple calli, fruits, and plantlets. Two genes, MdMa1 and MdMa11, were up- and down-regulated in transgenic apple plantlets overexpressing MdMYB123 and mdmyb123, respectively. MdMYB123 could directly bind to the promoter of MdMa1 and MdMa11, and induce their expression. In contrast, mdmyb123 could directly bind to the promoters of MdMa1 and MdMa11, but with no transcriptional activation of both genes. In addition, gene expression analysis in 20 different apple genotypes based on SNP locus from "QG" × "HC" hybrid population confirmed a correlation between A/T SNP with expression levels of MdMa1 and MdMa11. Our finding provides valuable functional validation of MdMYB123 and its role in the transcriptional regulation of both MdMa1 and MdMa11, and apple fruit malic acid accumulation.

ABRE-BINDING FACTOR3-WRKY DNA-BINDING PROTEIN44 module promotes salinity-induced malate accumulation in pear

Malate impacts fruit acidity and plays a vital role in stress tolerance. Malate accumulation is induced by salinity in various plants as a metabolite in coping with this stress. However, the exact molecular mechanism responsible for salinity-induced malate accumulation remains unclear. Here, we determined that salinity treatment induces malate accumulation in pear (Pyrus spp.) fruit, calli, and plantlets compared to the control. Genetic and biochemical analyses established the key roles of PpWRKY44 and ABRE-BINDING FACTOR3 (PpABF3) transcription factors in promoting malate accumulation in response to salinity. We found that PpWRKY44 is involved in salinity-induced malate accumulation by directly binding to a W-box on the promoter of the malate-associated gene aluminum-activated malate transporter 9 (PpALMT9) to activate its expression. A series of in-vivo and in-vitro assays revealed that the G-box cis-element in the promoter of PpWRKY44 was targeted by PpABF3, which further enhanced salinity-induced malate accumulation. Taken together, these findings suggest that PpWRKY44 and PpABF3 play positive roles in salinity-induced malate accumulation in pears. This research provides insights into the molecular mechanism by which salinity affects malate accumulation and fruit quality.

Relationship among Sugars, Organic Acids, Mineral Composition, and Chilling Injury Sensitivity on Six Pomegranate Cultivars Stored at 2 °C

Pomegranate is a sensitive fruit to chilling injury (CI) during storage at temperatures below 7 °C. However, sensitivity of pomegranate to CI is dependent on cultivar and exposure times to low temperatures. In this work, the sensitivity to CI of six pomegranate cultivars (Punica granatum L.) 'Wonderful', 'Kingdom', 'Bigful', 'Acco', 'Purple Queen', and 'Mollar de Elche', was evaluated after 30 d at 2 °C plus 2 d at 20 °C. Among cultivars, there was a great variability in the sensitivity to the appearance of CI symptoms. 'Kingdom' cultivar was the most CI sensitive and 'Mollar de Elche' cultivar was the least sensitive cultivar. CI symptoms were greater in the internal part of the skin than in the external part, although no correlation was found between ion leakage (IL) and CI severity after cold storage. However, both, external and internal CI index were correlated with the IL at harvest, with Pearson correlation of 0.63 and 0.80, respectively. In addition, this variability to CI among cultivars could also be due to composition and tissue structures in arils and peel. The solute content of the arils (anthocyanins, sugars, and organic acids, in particular citric acid), showed high correlations with CI sensitivity, with Pearson correlations (r) of 0.56 for total soluble solids, 0.87 for total acidity, 0.94 for anthocyanins, -0.94 for oxalic acid, 0.87 for citric acid, 0.62 for tartaric acid, -0.91 for malic acid, 0.8 for sucrose, and 0.71 for glucose, which can leak to the inner surface of the peel causing browning reactions. In addition, the high peel Ca/K ratio could play an important role on increasing fruit tolerance to CI, since it was negatively correlated with the internal and external CI indexes.

Organic Acid Accumulation and Associated Dynamic Changes in Enzyme Activity and Gene Expression during Fruit Development and Ripening of Common Loquat and Its Interspecific Hybrid

Loquats have gained increasing attention from consumers and growers for their essential nutrients and unusual phenology, which could help plug a gap period at market in early spring. Fruit acid is a critical contributor to fruit quality. The dynamic changes in organic acid (OA) during fruit development and ripening of common loquat (Dawuxing, DWX) and its interspecific hybrid (Chunhua, CH) were compared, as well as the corresponding enzyme activity and gene expression. At harvest, titratable acid was significantly lower (p ≤ 0.01) in CH (0.11%) than in DWX loquats (0.35%). As the predominant OA compound, malic acid accounted for 77.55% and 48.59% of the total acid of DWX and CH loquats at harvest, followed by succinic acid and tartaric acid, respectively. PEPC and NAD-MDH are key enzymes that participate in malic acid metabolism in loquat. The OA differences in DWX loquat and its interspecific hybrid could be attributed to the coordinated regulation of multiple genes and enzymes associated with OA biosynthesis, degradation, and transport. The data obtained in this work will serve as a fundamental and important basis for future loquat breeding programs and even for improvements in loquat cultural practices.

Arsenic inhibits citric acid accumulation via downregulating vacuolar proton pump gene expression in citrus fruits

Citric acid content is a critical quality determinant in citrus (Citrus spp.) fruits. Although arsenic (As) can effectively reduce citric acid content to improve citrus fruit quality, it can have adverse environmental effects. The discovery of nontoxic substitutes is hampered by the incomplete elucidation of the underlying mechanisms of As action in citrus fruits. Metabolic, transcriptomic, and physiological analyses were employed to investigate As action on citric acid accumulation to discover the mechanisms of As action in citrus. The enzyme activity related to citrate biosynthesis was not inhibited and the content of the involved metabolites was not reduced in As-treated fruits. However, the proton pump genes CitPH5 and CitPH1 control the vacuolar citric acid accumulation and transcription factor genes CitTT8 and CitMYB5, which regulate CitPH5 and CitPH1, were downregulated. The oxidative stress-response genes were upregulated in As-treated fruits. The reactive oxygen species (ROS) treatment also downregulated CitTT8 and CitMYB5 in juice cells. The mitochondrial ROS production rate increased in As-treated fruits. As^III was more potent in stimulating isolated mitochondria to overproduce ROS compared to As^V. Our results indicate that the As inhibition of citric acid accumulation may be primarily due to the transcriptional downregulation of CitPH5, CitPH1, CitTT8, and CitMYB5. As-induced oxidative stress signaling may operate upstream to downregulate these acid regulator genes. Mitochondrial thiol proteins may be the principal targets of As action in citrus fruits.

Exogenous salicylic acid regulates organic acids metabolism in postharvest blueberry fruit

Fruit acidity is an essential factor affecting blueberry organoleptic quality. The organic acid content in blueberry fruit mainly contributes to fruit acidity. This study aims to evaluate the effect of exogenous salicylic acid (SA), the principal metabolite of aspirin, on the organoleptic quality and organic acid metabolism in rabbiteye blueberry (Vaccinium virgatum Ait, 'Powderblue') during cold storage (4 °C). Results showed that SA-treated fruit reduced fruit decay and weight loss delayed fruit softening, and decline of total soluble solids (TSS). TA and total organic acid amounts stayed the same during the late storage period in SA-treated fruit. Four kinds of organic acid components, malic acid, quinic acid, citric acid, and succinic acid, were at higher levels in fruit treated by SA as compared to control. SA enhanced the activities of PEPC, NAD-MDH, and CS to promote the synthesis of malic acid and citric acid. Meanwhile, the activities of NADP-ME, ACL, and ACO, which participated in the degradation of malic acid and citric acid, were inhibited by SA. qPCR results also showed that the expression of VcPEPC, VcNAD-MDH, and VcCS genes were upregulated. In contrast, SA downregulated the expression of VcNADP-ME, VcACL, and VcACO genes. In conclusion, SA could regulate the key genes and enzymes that participated in organic acids metabolism to maintain the freshness of blueberry during cold storage, therefore minimizing the economic loss.

Metabolomic and transcriptomic analyses reveal the mechanism of sweet-acidic taste formation during pineapple fruit development

Pineapple (Ananas comosus L.) is one of the most valuable subtropical fruit crop in the world. The sweet-acidic taste of the pineapple fruits is a major contributor to the characteristic of fruit quality, but its formation mechanism remains elusive. Here, targeted metabolomic and transcriptomic analyses were performed during the fruit developmental stages in two pineapple cultivars ("Comte de Paris" and "MD-2") to gain a global view of the metabolism and transport pathways involved in sugar and organic acid accumulation. Assessment of the levels of different sugar and acid components during fruit development revealed that the predominant sugar and organic acid in mature fruits of both cultivars was sucrose and citric acid, respectively. Weighted gene coexpression network analysis of metabolic phenotypes and gene expression profiling enabled the identification of 21 genes associated with sucrose accumulation and 19 genes associated with citric acid accumulation. The coordinated interaction of the 21 genes correlated with sucrose irreversible hydrolysis, resynthesis, and transport could be responsible for sucrose accumulation in pineapple fruit. In addition, citric acid accumulation might be controlled by the coordinated interaction of the pyruvate-to-acetyl-CoA-to-citrate pathway, gamma-aminobutyric acid pathway, and tonoplast proton pumps in pineapple. These results provide deep insights into the metabolic regulation of sweetness and acidity in pineapple.

Transcriptome analysis of the pulp of citrus fruitlets suggests that domestication enhanced growth processes and reduced chemical defenses increasing palatability

To identify key traits brought about by citrus domestication, we have analyzed the transcriptomes of the pulp of developing fruitlets of inedible wild Ichang papeda (Citrus ichangensis), acidic Sun Chu Sha Kat mandarin (C. reticulata) and three palatable segregants of a cross between commercial Clementine (C. x clementina) and W. Murcott (C. x reticulata) mandarins, two pummelo/mandarin admixtures of worldwide distribution. RNA-seq comparison between the wild citrus and the ancestral sour mandarin identified 7267 differentially expressed genes, out of which 2342 were mapped to 117 KEGG pathways. From the remaining genes, a set of 2832 genes was functionally annotated and grouped into 45 user-defined categories. The data suggest that domestication promoted fundamental growth processes to the detriment of the production of chemical defenses, namely, alkaloids, terpenoids, phenylpropanoids, flavonoids, glucosinolates and cyanogenic glucosides. In the papeda, the generation of energy to support a more active secondary metabolism appears to be dependent upon upregulation of glycolysis, fatty acid degradation, Calvin cycle, oxidative phosphorylation, and ATP-citrate lyase and GABA pathways. In the acidic mandarin, downregulation of cytosolic citrate degradation was concomitant with vacuolar citrate accumulation. These changes affected nitrogen and carbon allocation in both species leading to major differences in organoleptic properties since the reduction of unpleasant secondary metabolites increases palatability while acidity reduces acceptability. The comparison between the segregants and the acidic mandarin identified 357 transcripts characterized by the occurrence in the three segregants of additional downregulation of secondary metabolites and basic structural cell wall components. The segregants also showed upregulation of genes involved in the synthesis of methyl anthranilate and furaneol, key substances of pleasant fruity aroma and flavor, and of sugar transporters relevant for sugar accumulation. Transcriptome and qPCR analysis in developing and ripe fruit of a set of genes previously associated with citric acid accumulation, demonstrated that lower acidity is linked to downregulation of these regulatory genes in the segregants. The results suggest that the transition of inedible papeda to sour mandarin implicated drastic gene expression reprograming of pivotal pathways of the primary and secondary metabolism, while palatable mandarins evolved through progressive refining of palatability properties, especially acidity.

Overexpression of MsRCI2D and MsRCI2E Enhances Salt Tolerance in Alfalfa (Medicago sativa L.) by Stabilizing Antioxidant Activity and Regulating Ion Homeostasis

Rare cold-inducible 2 (RCI2) genes from alfalfa (Medicago sativa L.) are part of a multigene family whose members respond to a variety of abiotic stresses by regulating ion homeostasis and stabilizing membranes. In this study, salt, alkali, and ABA treatments were used to induce MsRCI2D and MsRCI2E expression in alfalfa, but the response time and the expression intensity of the MsRCI2D,-E genes were different under specific treatments. The expression intensity of the MsRCI2D gene was the highest in salt- and alkali-stressed leaves, while the MsRCI2E gene more rapidly responded to salt and ABA treatment. In addition to differences in gene expression, MsRCI2D and MsRCI2E differ in their subcellular localization. Akin to MtRCI2D from Medicago truncatula, MsRCI2D is also localized in the cell membrane, while MsRCI2E is different from MtRCI2E, localized in the cell membrane and the inner membrane. This difference might be related to an extra 20 amino acids in the C-terminal tail of MsRCI2E. We investigated the function of MsRCI2D and MsRCI2E proteins in alfalfa by generating transgenic alfalfa chimeras. Compared with the MsRCI2E-overexpressing chimera, under high-salinity stress (200 mmol·L⁻¹ NaCl), the MsRCI2D-overexpressing chimera exhibited a better phenotype, manifested as a higher chlorophyll content and a lower MDA content. After salt treatment, the enzyme activities of SOD, POD, CAT, and GR in MsRCI2D- and -E-overexpressing roots were significantly higher than those in the control. In addition, after salt stress, the Na⁺ content in MsRCI2D- and -E-transformed roots was lower than that in the control; K⁺ was higher than that in the control; and the Na⁺/K⁺ ratio was lower than that in the control. Correspondingly, H⁺-ATPase, SOS1, and NHX1 genes were significantly up-regulated, and the HKT gene was significantly down-regulated after 6 h of salt treatment. MsRCI2D was also found to regulate the expression of the MsRCI2B and MsRCI2E genes, and the MsRCI2E gene could alter the expression of the MsRCI2A, MsRCI2B, and MsRCI2D genes. MsRCI2D- and -E-overexpressing alfalfa was found to have higher salt tolerance, manifested as improved activity of antioxidant enzymes, reduced content of reactive oxygen species, and sustained Na⁺ and K⁺ ion balance by regulating the expression of the H⁺-ATPase, SOS1, NHX1, HKT, and MsRCI2 genes.

Visualizing the spatial distribution of functional metabolites in Forsythia suspensa at different harvest stages by MALDI mass spectrometry imaging

As a traditional Chinese medicine, Forsythia suspensa (F. suspensa) has attracted much attention due to its significant pharmacological activity. Revealing the spatial distribution of metabolites during F. suspensa development is important for understanding its biosynthesis rules and improving the quality of medicinal materials. However, there is currently a lack of information on the spatial distribution of F. suspensa metabolites. In this work, the spatial distribution and growth metabolism patterns of important metabolites of F. suspensa were studied for the first time using matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI). Using 2,5-dimethylnaphthalene (DAN) as the matrix and detecting in negative ion mode, the spatial distribution and growth patterns of 11 metabolites obtained from longitudinal sections of F. suspensa included pinoresinol, phillygenin, forsythoside A, forsythoside E, rutin, caffeic acid, malic acid, citric acid, stearic acid, oleic acid, and linoleic acid. These results showed the mesocarp and endosperm tissues of F. suspensa were important for storing important functional metabolites. Changes in mesocarp and endosperm growth and development tissues caused large changes in the content of important functional metabolites in F. suspensa. These results provide a basis for understanding the spatial distribution of metabolites in F. suspensa tissues and the significant changes that occur during growth and development, exploring the mechanism of important synthesis of metabolites, regulating the harvest of F. suspensa, and improving the quality of medicinal herbs.

Changes and response mechanism of sugar and organic acids in fruits under water deficit stress

The content and the ratio of soluble sugars and organic acids in fruits are significant indicators for fruit quality. They are affected by multiple environmental factors, in which water-deficient is the most concern. Previous studies found that the content of soluble sugars and organic acids in fruit displayed great differences under varied water stress. It is important to clarify the mechanism of such difference and to provide researchers with systematic knowledge about the response to drought stress and the mechanism of sugar and acid changes in fruits, so that they can better carry out the study of fruit quality under drought stress. Therefore, the researchers studied dozens of research articles about the content of soluble sugar and organic acid, the activity of related metabolic enzymes, and the expression of related metabolic genes in fruits under water stress, and the stress response of plants to water stress. We found that after plants perceived and transmitted the signal of water deficit, the expression of genes related to the metabolism of soluble sugars and organic acids changed. It was then affected the synthesis of metabolic enzymes and changed their metabolic rate, ultimately leading to changes in soluble sugar and organic acid content. Based on the literature review, we described the pathway diagrams of sugar metabolism, organic acid metabolism, mainly malic acid, tartaric acid, and citric acid metabolism, and of the response to drought stress. From many aspects including plants' perception of water stress signal, signal conversion and transmission, induced gene expression, the changes in soluble sugar and the enzyme activities of organic acids, as well as the final sugar and acid content in fruits, this thesis summarized previous studies on the influence of water stress on soluble sugars and the metabolism of organic acids in fruits.

The Interaction Between CitMYB52 and CitbHLH2 Negatively Regulates Citrate Accumulation by Activating CitALMT in Citrus Fruit

Citric acid plays significant roles in numerous physiological processes in plants, including carbon metabolism, signal transduction, and tolerance to environmental stress. For fruits, it has a major effect on fruit organoleptic quality by directly influencing consumer taste. Citric acid in citrus is mainly regulated by the balance between synthesis, degradation, and vacuolar storage. The genetic and molecular regulations of citric acid synthesis and degradation have been comprehensively elucidated. However, the transporters for citric acid in fruits are less well understood. Here, an aluminum-activated malate transporter, CitALMT, was characterized. Transient overexpression and stable transformation of CitALMT significantly reduced citrate concentration in citrus fruits and transgenic callus. Correspondingly, transient RNA interference-induced silencing of CitALMT and increased citrate significantly, indicating that CitALMT plays an important role in regulating citrate concentration in citrus fruits. In addition, dual-luciferase assays indicated that CitMYB52 and CitbHLH2 could trans-activate the promoter of CitALMT. EMSA analysis showed that CitbHLH2 could physically interact with the E-box motif in the CitALMT promoter. Bimolecular fluorescence complementation assays, yeast two-hybrid, coimmunoprecipitation and transient overexpression, and RNAi assay indicated that the interaction between CitMYB52 and CitbHLH2 could synergistically trans-activate CitALMT to negatively regulate citrate accumulation.