Transcription of ethylene perception and biosynthesis genes is altered by putrescine, spermidine and aminoethoxyvinylglycine (AVG) during ripening in peach fruit (Prunus persica)

Chinese cherry CpMYB44-CpSPDS2 module regulates spermidine content and florescence in tobacco

The flower bud differentiation plays a crucial role in cherry yield and quality. In a preliminary study, we revealed the promotion of spermidine (Spd) in bud differentiation and quality. However, the molecular mechanism underlying Spd regulating cherry bud differentiation remains unclear. To address this research gap, we cloned CpSPDS2, a gene that encodes Spd synthase and is highly expressed in whole flowers and pistils of the Chinese cherry (cv. 'Manaohong'). Furthermore, an overexpression vector with this gene was constructed to transform tobacco plants. The findings demonstrated that transgenic lines exhibited higher Spd content, an earlier flowering time by 6 d, and more lateral buds and flowers than wild-type lines. Additionally, yeast one-hybrid assays and two-luciferase experiments confirmed that the R2R3-MYB transcription factor (CpMYB44) directly binds to and activates the CpSPDS2 promoter transcription. It is indicated that CpMYB44 promotes Spd accumulation via regulating CpSPDS2 expression, thus accelerating the flower growth. This research provides a basis for resolving the molecular mechanism of CpSPDS2 involved in cherry bud differentiation.

Regulation of ethylene metabolism in tomato under salinity stress involving linkages with important physiological signaling pathways

The tomato is well-known for its anti-oxidative and anti-cancer properties, and with a wide range of health benefits is an important cash crop for human well-being. However, environmental stresses (especially abiotic) are having a deleterious effect on plant growth and productivity, including tomato. In this review, authors describe how salinity stress imposes risk consequences on growth and developmental processes of tomato through toxicity by ethylene (ET) and cyanide (HCN), and ionic, oxidative, and osmotic stresses. Recent research has clarified how salinity stress induced-ACS and - β-CAS expressions stimulate the accumulation of ET and HCN, wherein the action of salicylic acid (SA),compatible solutes (CSs), polyamines (PAs) and ET inhibitors (ETIs) regulate ET and HCN metabolism. Here we emphasize how ET, SA and PA cooperates with mitochondrial alternating oxidase (AOX), salt overly sensitive (SOS) pathways and the antioxidants (ANTOX) system to better understand the salinity stress resistance mechanism. The current literature evaluated in this paper provides an overview of salinity stress resistance mechanism involving synchronized routes of ET metabolism by SA and PAs, connecting regulated network of central physiological processes governing through the action of AOX, β-CAS, SOS and ANTOX pathways, which might be crucial for the development of tomato.

Interactions of Polyamines and Phytohormones in Plant Response to Abiotic Stress

Numerous environmental conditions negatively affect plant production. Abiotic stresses, such as salinity, drought, temperature, and heavy metals, cause damage at the physiological, biochemical, and molecular level, and limit plant growth, development, and survival. Studies have indicated that small amine compounds, polyamines (PAs), play a key role in plant tolerance to various abiotic stresses. Pharmacological and molecular studies, as well as research using genetic and transgenic approaches, have revealed the favorable effects of PAs on growth, ion homeostasis, water maintenance, photosynthesis, reactive oxygen species (ROS) accumulation, and antioxidant systems in many plant species under abiotic stress. PAs display a multitrack action: regulating the expression of stress response genes and the activity of ion channels; improving the stability of membranes, DNA, and other biomolecules; and interacting with signaling molecules and plant hormones. In recent years the number of reports indicating crosstalk between PAs and phytohormones in plant response to abiotic stresses has increased. Interestingly, some plant hormones, previously known as plant growth regulators, can also participate in plant response to abiotic stresses. Therefore, the main goal of this review is to summarize the most significant results that represent the interactions between PAs and plant hormones, such as abscisic acid, brassinosteroids, ethylene, jasmonates, and gibberellins, in plants under abiotic stress. The future perspectives for research focusing on the crosstalk between PAs and plant hormones were also discussed.

Genomic analysis of the polyamine biosynthesis pathway in duckweed Spirodela polyrhiza L.: presence of the arginine decarboxylase pathway, absence of the ornithine decarboxylase pathway, and response to abiotic stresses

Identification of the polyamine biosynthetic pathway genes in duckweed S. polyrhiza reveals presence of prokaryotic as well as land plant-type ADC pathway but absence of ODC encoding genes. Their differential gene expression and transcript abundance is shown modulated by exogenous methyl jasmonate, salinity, and acidic pH. Genetic components encoding for polyamine (PA) biosynthetic pathway are known in several land plant species; however, little is known about them in aquatic plants. We utilized recently sequenced three duckweed (Spirodela polyrhiza) genome assemblies to map PA biosynthetic pathway genes in S. polyrhiza. PA biosynthesis in most higher plants except for Arabidopsis involves two pathways, via arginine decarboxylase (ADC) and ornithine decarboxylase (ODC). ADC-mediated PA biosynthetic pathway genes, namely, one arginase (SpARG1), two arginine decarboxylases (SpADC1, SpADC2), one agmatine iminohydrolase/deiminase (SpAIH), one N-carbamoyl putrescine amidase (SpCPA), three S-adenosylmethionine decarboxylases (SpSAMDc1, 2, 3), one spermidine synthase (SpSPDS1) and one spermine synthase (SpSPMS1) in S. polyrhiza genome were identified here. However, no locus was found for ODC pathway genes in this duckweed. Hidden Markov Model protein domain analysis established that SpADC1 is a prokaryotic/biodegradative type ADC and its molecular phylogenic classification fell in a separate prokaryotic origin ADC clade with SpADC2 as a biosynthetic type of arginine decarboxylase. However, thermospermine synthase (t-SPMS)/Aculis5 genes were not found present. Instead, one of the annotated SPDS may also function as SPMS, since it was found associated with the SPMS phylogenetic clade along with known SPMS genes. Moreover, we demonstrate that S. polyrhiza PA biosynthetic gene transcripts are differentially expressed in response to unfavorable conditions, such as exogenously added salt, methyl jasmonate, or acidic pH environment as well as in extreme temperature regimes. Thus, S. polyrhiza genome encodes for complete polyamine biosynthesis pathway and the genes are transcriptionally active in response to changing environmental conditions suggesting an important role of polyamines in this aquatic plant.

Transcriptomics of Improved Fruit Retention by Hexanal in 'Honeycrisp' Reveals Hormonal Crosstalk and Reduced Cell Wall Degradation in the Fruit Abscission Zone

Apples (Malus domestica Borkh) are prone to preharvest fruit drop, which is more pronounced in 'Honeycrisp'. Hexanal is known to improve fruit retention in several economically important crops. The effects of hexanal on the fruit retention of 'Honeycrisp' apples were assessed using physiological, biochemical, and transcriptomic approaches. Fruit retention and fruit firmness were significantly improved by hexanal, while sugars and fresh weight did not show a significant change in response to hexanal treatment. At commercial maturity, abscisic acid and melatonin levels were significantly lower in the treated fruit abscission zone (FAZ) compared to control. At this stage, a total of 726 differentially expressed genes (DEGs) were identified between treated and control FAZ. Functional classification of the DEGs showed that hexanal downregulated ethylene biosynthesis genes, such as S-adenosylmethionine synthase (SAM2) and 1-aminocyclopropane-1-carboxylic acid oxidases (ACO3, ACO4, and ACO4-like), while it upregulated the receptor genes ETR2 and ERS1. Genes related to ABA biosynthesis (FDPS and CLE25) were also downregulated. On the contrary, key genes involved in gibberellic acid biosynthesis (GA20OX-like and KO) were upregulated. Further, hexanal downregulated the expression of genes related to cell wall degrading enzymes, such as polygalacturonase (PG1), glucanases (endo-β-1,4-glucanase), and expansins (EXPA1-like, EXPA6, EXPA8, EXPA10-like, EXPA16-like). Our findings reveal that hexanal reduced the sensitivity of FAZ cells to ethylene and ABA. Simultaneously, hexanal maintained the cell wall integrity of FAZ cells by regulating genes involved in cell wall modifications. Thus, delayed fruit abscission by hexanal is most likely achieved by minimizing ABA through an ethylene-dependent mechanism.

Putrescine Depletion Affects Arabidopsis Root Meristem Size by Modulating Auxin and Cytokinin Signaling and ROS Accumulation

Polyamines (PAs) dramatically affect root architecture and development, mainly by unknown mechanisms; however, accumulating evidence points to hormone signaling and reactive oxygen species (ROS) as candidate mechanisms. To test this hypothesis, PA levels were modified by progressively reducing ADC1/2 activity and Put levels, and then changes in root meristematic zone (MZ) size, ROS, and auxin and cytokinin (CK) signaling were investigated. Decreasing putrescine resulted in an interesting inverted-U-trend in primary root growth and a similar trend in MZ size, and differential changes in putrescine (Put), spermidine (Spd), and combined spermine (Spm) plus thermospermine (Tspm) levels. At low Put concentrations, ROS accumulation increased coincidently with decreasing MZ size, and treatment with ROS scavenger KI partially rescued this phenotype. Analysis of double AtrbohD/F loss-of-function mutants indicated that NADPH oxidases were not involved in H2O2 accumulation and that elevated ROS levels were due to changes in PA back-conversion, terminal catabolism, PA ROS scavenging, or another pathway. Decreasing Put resulted in a non-linear trend in auxin signaling, whereas CK signaling decreased, re-balancing auxin and CK signaling. Different levels of Put modulated the expression of PIN1 and PIN2 auxin transporters, indicating changes to auxin distribution. These data strongly suggest that PAs modulate MZ size through both hormone signaling and ROS accumulation in Arabidopsis.

Double-sided battle: The role of ethylene during Monilinia spp. infection in peach at different phenological stages

Controversy exists on whether ethylene is involved in determining fruit resistance or susceptibility against biotic stress. In this work, the hypothesis that ethylene biosynthesis in peaches at different phenological stages may be modulated by Monilinia spp. was tested. To achieve this, at 49 and 126 d after full bloom (DAFB), ethylene biosynthesis of healthy and infected 'Merryl O'Henry' peaches with three strains of Monilinia spp. (M. fructicola (CPMC6) and M. laxa (CPML11 and ML8L) that differ in terms of aggressiveness) was analysed at the biochemical and molecular level along the course of infection in fruit stored at 20 °C. At 49 DAFB, results evidenced that infected fruit showed inhibition of ethylene production in comparison with non-inoculated fruit, suggesting that the three Monilinia strains were somehow suppressing ethylene biosynthesis to modify fruit defences to successfully infect the host. On the contrary, at 126 DAFB ethylene production increased concomitantly with brown rot spread, and values for non-inoculated fruit were almost undetectable throughout storage at 20 °C. The expression of several target genes involved in the ethylene biosynthetic pathway confirmed that they were differentially expressed upon Monilinia infection, pointing to a strain-dependent regulation. Notably, Prunus persica 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) (PpACS) family was the most over-expressed over time, demonstrating a positive ethylene regulation, especially at 126 DAFB. At this phenological stage it was demonstrated the ability of Monilinia spp. to alter ethylene biosynthesis through PpACS1 and benefit from the consequences of an ethylene burst likely on cell wall softening. Overall, our results put forward that infection not only among different strains but also at each stage is achieved by different mechanisms, with ethylene being a key factor in determining peach resistance or susceptibility to brown rot.

Polyamine Metabolism in Climacteric and Non-Climacteric Fruit Ripening

Polyamines are small aliphatic amines that are found in both prokaryotic and eukaryotic organisms. These growth regulators have been implicated in abiotic and biotic stresses as well as plant development and morphogenesis. Several studies have also suggested a key role of polyamines during fruit set and early development. Polyamines have also been linked to fruit ripening and in the regulation of fruit quality-related traits.Recent studies indicate that during ripening of both climacteric and non-climacteric fruits, a decline in total polyamine contents is observed together with an increased catabolism of these growth regulators.In this review, we explore the current knowledge on polyamine biosynthesis and catabolism during fruit set and ripening. The study of the role of polyamine metabolism in fruit ripening indicates the possible application of these natural polycations to control ripening and postharvest decay as well as to improve fruit quality traits.

Over-expression of mouse ornithine decarboxylase gene under the control of fruit-specific promoter enhances fruit quality in tomato

Diamine putrescine (Put) and polyamines; spermidine (Spd) and spermine (Spm) are essential component of every cell because of their involvement in the regulation of cell division, growth and development. The aim of this study is to enhance the levels of Put during fruit development and see its implications in ripening and quality of tomato fruits. Transgenic tomato plants over-expressing mouse ornithine decarboxylase gene under the control of fruit-specific promoter (2A11) were developed. Transgenic fruits exhibited enhanced levels of Put, Spd and Spm, with a concomitant reduction in ethylene levels, rate of respiration and physiological loss of water. Consequently such fruits displayed significant delay of on-vine ripening and prolonged shelf life over untransformed fruits. The activation of Put biosynthetic pathway at the onset of ripening in transgenic fruits is also consistent with the improvement of qualitative traits such as total soluble solids, titratable acids and total sugars. Such changes were associated with alteration in expression pattern of ripening specific genes. Transgenic fruits were also fortified with important nutraceuticals like lycopene, ascorbate and antioxidants. Therefore, these transgenic tomatoes would be useful for the improvement of tomato cultivars through breeding approaches.

ABA may promote or delay peach fruit ripening through modulation of ripening- and hormone-related gene expression depending on the developmental stage

Peach (Prunus persica laevis L. Batsch) was chosen as a model to further clarify the physiological role of ABA during fruit ripening. To this aim, branches bearing one fruit at mid-S3, S3/S4 and S4 stages of fruit development and characterized by a different ripening index (I(AD)), as revealed by a non-destructive device called a DA-meter, were treated with ABA (0.02 mM) for 1 and 5 days. Exogenously applied ABA interfered with the progression of ripening leading to less ripe or riper fruit depending on the physiological stage. To better understand the molecular basis of ABA interference with ripening, the time-course changes in the expression of ethylene-, cell wall-, and auxin-related genes as well as other genes (NCED, PIP, LOX, AOS and SOT) was evaluated in the fruit mesocarp. Real-time PCR analyses revealed that in mid-S3 fruit transcript levels of ethylene biosynthesis and signaling (ACS1, ACO1, ETR2, ERF2), cell wall softening-related (PG, PMEI, EXP1, EXP2) and auxin biosynthesis, conjugation, transport and perception (TRPB, IGPS, Aux/IAA, GH3, PIN1 and TIR1) genes were substantially down-regulated on day 5 indicating a ripening delay. On the contrary, in more advanced stages (S3/S4 and S4) the same genes were early (day 1) up-regulated suggesting an acceleration of ripening. Transcript profiling of other ripening-related genes revealed changes that were in accord with a ripening delay (mid-S3) or acceleration (S3/S4 and S4). Thus, in peach fruit, ABA appears to modulate ripening through interference not only with ethylene and cell wall but also with auxin-related genes.

Spermidine application to young developing peach fruits leads to a slowing down of ripening by impairing ripening-related ethylene and auxin metabolism and signaling

Peach (Prunus persica var. laevis Gray) was chosen to unravel the molecular basis underlying the ability of spermidine (Sd) to influence fruit development and ripening. Field applications of 1 mM Sd on peach fruit at an early developmental stage, 41 days after full bloom (dAFB), i.e. at late stage S1, led to a slowing down of fruit ripening. At commercial harvest (125 dAFB, S4II) Sd-treated fruits showed a reduced ethylene production and flesh softening. The endogenous concentration of free and insoluble conjugated polyamines (PAs) increased (0.3-2.6-fold) 1 day after treatment (short-term response) butsoon it declined to control levels; starting from S3/S4, when soluble conjugated forms increased (up to five-fold relative to controls at ripening), PA levels became more abundant in treated fruits, (long-term response). Real-time reverse transcription-polymerase chain reaction analyses revealed that peaks in transcript levels of fruit developmental marker genes were shifted ahead in accord with a developmental slowing down. At ripening (S4I-S4II) the upregulation of the ethylene biosynthetic genes ACO1 and ACS1 was dramatically counteracted by Sd and this led to a strong downregulation of genes responsible for fruit softening, such as PG and PMEI. Auxin-related gene expression was also altered both in the short term (TRPB) and in the long term (GH3, TIR1 and PIN1), indicating that auxin plays different roles during development and ripening processes. Messenger RNA amounts of other hormone-related ripening-regulated genes, such as NCED and GA2-OX, were strongly downregulated at maturity. Results suggest that Sd interferes with fruit development/ripening by interacting with multiple hormonal pathways.

Enhanced polyamine accumulation alters carotenoid metabolism at the transcriptional level in tomato fruit over-expressing spermidine synthase

Polyamines are involved in crucial plant physiological events, but their roles in fruit development remain unclear. We generated transgenic tomato plants that show a 1.5- to 2-fold increase in polyamine content by over-expressing the spermidine synthase gene, which encodes a key enzyme for polyamine biosynthesis. Pericarp-columella and placental tissue from transgenic tomato fruits were subjected to (1)H-nuclear magnetic resonance (NMR) for untargeted metabolic profiling and high-performance liquid chromatography-diode array detection for carotenoid profiling to determine the effects of high levels of polyamine accumulation on tomato fruit metabolism. A principal component analysis of the quantitative (1)H NMR data from immature green to red ripe fruit showed a clear discrimination between developmental stages, especially during ripening. Quantification of 37 metabolites in pericarp-columella and 41 metabolites in placenta tissues revealed distinct metabolic profiles between the wild type and transgenic lines, particularly at the late ripening stages. Notably, the transgenic tomato fruits also showed an increase in carotenoid accumulation, especially in lycopene (1.3- to 2.2-fold), and increased ethylene production (1.2- to 1.6-fold) compared to wild-type fruits. Genes responsible for lycopene biosynthesis, including phytoene synthase, phytoene desaturase, and deoxy-d-xylulose 5-phosphate synthase, were significantly up-regulated in ripe transgenic fruits, whereas genes involved in lycopene degradation, including lycopene-epsilon cyclase and lycopene beta cyclase, were down-regulated in the transgenic fruits compared to the wild type. These results suggest that a high level of accumulation of polyamines in the tomato regulates the steady-state level of transcription of genes responsible for the lycopene metabolic pathway, which results in a higher accumulation of lycopene in the fruit.

Jasmonate-induced ripening delay is associated with up-regulation of polyamine levels in peach fruit

Methyl jasmonate (MJ, 0.20mM) and its synthetic analog n-propyl dihydrojasmonate (PDJ, 0.22mM) were applied to peach fruit (Prunus persica L. Batsch) at a late developmental stage under field conditions (in planta). On the basis of a previously demonstrated jasmonate (JA)-induced ripening delay in peach, the effects of JAs on the time course of the endogenous polyamine (PA) accumulation and expression of their biosynthetic genes arginine decarboxylase (ADC), ornithine decarboxylase (ODC), spermidine synthase (SPDS) and S-adenosylmethionine decarboxylase (SAMDC) were evaluated in control and JA-treated fruit during the 21-d trial period. In parallel, the main ripening-related parameters (ethylene production, flesh firmness and soluble solids contents) were measured, and transcription profiles of aminocyclopropane-1-carboxylic acid oxidase (PpACO1) and of two ethylene perception genes were evaluated. PDJ, but not MJ, reduced ethylene production and fruit softening, impaired PpACO1 transcription and altered the expression of PpERS1 (ethylene sensor 1), but not the expression of PpETR1 (ethylene receptor 1). In the epicarp and mesocarp, the pattern of PA accumulation was altered in a biphasic manner leading to a higher overall PA level in PDJ-treated fruit. Short and long term increases in putrescine, spermidine and/or spermine, the latter only in the epicarp, were observed in PDJ-treated fruit. MJ induced this behavior only with putrescine in the mesocarp. PpADC transcription was also enhanced soon after the PDJ treatment. Since PDJ-treated fruit were less ripe, their higher PA concentrations in treated fruit are discussed in light of the dual role of these molecules as stress/defense protective compounds and rejuvenating effectors.

Enhancement of spermidine content and antioxidant capacity in transgenic pear shoots overexpressing apple spermidine synthase in response to salinity and hyperosmosis

In our previous work, an apple spermidine synthase (SPDS)-overexpressing transgenic European pear (Pyrus communis L. 'Ballad'), line no. 32 (#32), demonstrated attenuated susceptibility to stress treatment. In the current paper, changes in enzymatic and non-enzymatic antioxidant capacity of the transgenic pear (line #32) were investigated in response to NaCl or mannitol stress. Under non-stressed conditions (before stress treatment), spermidine (Spd) contents and SPDS activity of line #32 were higher than those of the non-transformant (wild type). However, no significant differences were detected between line #32 and the wild type as regards contents of malondialdehyde (MDA) and H2O2, and activities of antioxidant enzymes like superoxide dismutase (SOD), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR) and glutathione reductase (GR). When exposed to NaCl or mannitol stress, both the wild type and line #32 exhibited accumulation of Spd with the latter accumulating more. The transgenic line contained higher antioxidant enzyme activities, less MDA and H2O2 than the wild, implying it suffered from less injury. These results suggested that increase of Spd content in the transgenic line could, at least in part, lead to enhancing enzymatic and non-enzymatic antioxidant capacity.

Jasmonate-induced transcriptional changes suggest a negative interference with the ripening syndrome in peach fruit

Peach (Prunus persica L. Batsch) was chosen as a model to shed light on the physiological role of jasmonates (JAs) during fruit ripening. To this aim, the effects of methyl jasmonate (MJ, 0.40 mM) and propyl dihydrojasmonate (PDJ, 0.22 mM), applied in planta at different fruit developmental stages, on the time-course of ethylene production and fruit quality traits were evaluated. MJ-induced changes in fruit transcriptome at harvest and the expression profiling of relevant JA-responsive genes were analysed in control and JA-treated fruit. Exogenously applied JAs affected the onset of ripening depending upon the fruit developmental stage, with PDJ being more active than MJ. Both compounds enhanced the transcription of allene oxide synthase (PpAOS1), the first specific enzyme in the biosynthesis of jasmonic acid, and altered the pattern of jasmonic acid accumulation. Microarray transcriptome profiling showed that MJ down-regulated some ripening-related genes, such as 1-aminocyclopropane-1-carboxylic acid oxidase (PpACO1) and polygalacturonase (PG), and the transcriptional modulator IAA7. MJ also altered the expression of cell wall-related genes, namely pectate lyase (PL) and expansins (EXPs), and up-regulated several stress-related genes, including some of those involved in JA biosynthesis. Time-course expression profiles of PpACO1, PL, PG, PpExp1, and the transcription factor LIM confirmed the array results. Thus, in peach fruit, exogenous JAs led to a ripening delay due to an interference with ripening- and stress/defence-related genes, as reflected in the transcriptome of treated fruit at harvest.