Low Abscisic Acid Research Articles

ABSCISIC ACID INSENSITIVE 3 promotes auxin signalling by regulating SHY2 expression to control primary root growth in response to dehydration stress.

Plants combat dehydration stress through different strategies including root architectural changes. Here we show that when exposed to varying levels of dehydration stress, primary root growth in Arabidopsis is modulated by regulating root meristem activity. Abscisic acid (ABA) in concert with auxin signalling adjust primary root growth according to stress levels. ABSCISIC ACID INSENSITIVE 3 (ABI3), an ABA-responsive transcription factor, stands at the intersection of ABA and auxin signalling and fine-tunes primary root growth in response to dehydration stress. Under low ABA or dehydration stress, induction of ABI3 expression promotes auxin signalling by decreasing expression of SHY2, a negative regulator of auxin response. This further enhances the expression of auxin transporter gene PIN1 and cell cycle gene CYCB1;1, resulting in an increase in primary root meristem size and root length. Higher levels of dehydration stress or ABA repress ABI3 expression and promote ABSCISIC ACID INSENSITIVE 5 (ABI5) expression. This elevates SHY2 expression, thereby impairing primary root meristem activity and retarding root growth. Notably, ABI5 can promote SHY2 expression only in the absence of ABI3. Such ABA concentration-dependent expression of ABI3 therefore functions as a regulatory sensor of dehydration stress levels and orchestrates primary root growth by coordinating its downstream regulation.

Melatonin delayed senescence by modulating the contents of plant signalling molecules in postharvest okras.

Okra has been widely cultivated worldwide. Consumers appreciate its nutritional value and delicious taste. However, okra is very perishable after harvest because of rapid senescence and high susceptibility to mechanical injuries, which limits its storage life and reduces consumer acceptance. This study examined the influence of melatonin treatment on senescence process and endogenous plant signalling molecules in postharvest okras. The results indicated that melatonin treatment delayed senescence by increasing the endogenous melatonin content through upregulation of its biosynthetic genes. In addition, the treatment increased the contents of indole-3-acetic acid (IAA) and gibberellin (GA) due to the positive modulation of their metabolic and signalling genes. Furthermore, treated okras exhibited higher levels of γ-aminobutyric acid (GABA) but lower abscisic acid (ABA) content, contributing to the delayed senescence process compared to control. Overall, the findings suggested that melatonin postponed senescence in okras fruit by positively regulating endogenous signalling molecules such as melatonin, IAA, GABA, GA, and ABA.

Elevated CO2-conferred sodium homeostasis is linked to abscisic acid-mediated stomatal closure and signalling pathway in tomato plants

Soil salinity hampers plant performance. Elevated atmospheric CO2 (e[CO2]) could alleviate the detrimental effect of salinity on plants but whether abscisic acid (ABA) is involved in this process is unclear. To address this issue, three tomato (Solanum lycopersicum) genotypes with varying endogenous ABA concentrations (wild-type AC, ABA-deficient mutant flacca and ABA-overproduction line SP5) were grown in pots under ambient (400 μmol · mol–1) or elevated (800 μmol · mol–1) CO2 with or without the addition of 100 mmol · L−1sodium chloride (NaCl). The results showed that e[CO2] favored ion homeostasis by decreasing root-to-shoot delivery of Na+, which was mainly attributed to lowered transpiration rate rather than altered xylem-sap Na+ concentration. In AC and SP5, the low transpiration rate of e[CO2]-plants under salinity was accompanied by enhanced endogenous ABA levels, which might play a role in upregulating the abundance of specific transcripts related to Na+ homeostasis (i.e., SALT OVERLY SENSITIVE) under salt stress. In flacca, e[CO2]-induced Na+ homeostasis was abolished, which could be ascribed to the low and unaltered ABA levels, albeit the ethylene biosynthesis was enhanced in flacca under salt stress, indicating an antagonistic relationship between ABA and ethylene. Furthermore, e[CO2] inhibited ethylene biosynthesis under salt stress in all three genotypes. The results enrich our comprehension of the fundamental processes of e[CO2]-conferred salt tolerance in tomato.

Low sucrose availability reduces basal spikelet fertility by inducing abscisic acid and jasmonic acid synthesis in wheat.

Within a spike of wheat, the central spikelets usually generate three to four fertile florets, while the basal spikelets generate zero to one fertile floret. The physiological and transcriptional mechanism behind the difference in fertility between the basal and central spikelets is unclear. This study reports a high temporal resolution investigation of transcriptomes, number and morphology of floret primordia, and physiological traits. The W6.5-W7.5 stage was regarded as the boundary to distinguish between fertile and abortive floret primordia; those floret primordia reaching the W6.5-W7.5 stage during the differentiation phase (3-9 d after terminal spikelet stage) usually developed into fertile florets in the next dimorphism phase (12-27 d after terminal spikelet stage), whereas the others aborted. The central spikelets had a greater number of fertile florets than the basal spikelets, which was associated with more floret primordia reaching the W6.5-W7.5 stage. Physiological and transcriptional results demonstrated that the central spikelets had a higher sucrose content and lower abscisic acid (ABA) and jasmonic acid (JA) accumulation than the basal spikelets due to down-regulation of genes involved in ABA and JA synthesis. Collectively, we propose a model in which ABA and JA accumulation is induced under limiting sucrose availability (basal spikelet) through the up-regulation of genes involved in ABA and JA synthesis; this leads to floret primordia in the basal spikelets failing to reach their fertile potential (W6.5-W7.5 stage) during the differentiation phase and then aborting. This fertility repression model may also regulate spikelet fertility in other cereal crops and potentially provides genetic resources to improve spikelet fertility.

Involvement of brassinosteroids and abscisic acid in spikelet degeneration in rice under soil drying during meiosis.

Spikelet degeneration in rice (Oryza sativa L.) is a serious physiological defect, and can be regulated by soil moisture status and phytohormones. This study investigated the possibility that brassinosteroids (BRs) in collaboration with abscisic acid (ABA) are involved in mediating the effect of soil drying during meiosis on spikelet degeneration in rice. Three rice cultivars were field grown and three irrigation regimes including well watered (WW), moderate soil drying (MD), and severe soil drying (SD) were imposed during meiosis. MD significantly decreased spikelet degeneration in comparison with WW, due mainly to the alleviation in oxidative damage via enhancing ascorbate-glutathione (AsA-GSH) cycle activity in young panicles, and SD exhibited the opposite effects. Enhanced AsA-GSH cycle strength, decreased oxidative stress, and spikelet degeneration rate were closely associated with the synergistically elevated BR and ABA levels in young panicles in MD. In contrast, low BR and excessive ABA levels led to an increase in spikelet degeneration in SD. The three cultivars exhibited the same tendencies. The intrinsic link among AsA-GSH cycle, oxidative stress, spikelet degeneration rate, and BR and ABA levels was further verified by using transgenic rice lines and chemical regulators. BRs or ABA play a unique role in regulating spikelet degeneration. Synergistically increased BR and ABA levels in MD could work together to strengthen AsA-GSH cycle activity, leading to a reduction in oxidative damage and spikelet degeneration. On the other hand, a severe imbalance between low BR and excessive ABA levels may have contributed to the opposite effects in SD.

Domestication has altered the ABA and gibberellin profiles in developing pea seeds

Main conclusionWe showed that wild pea seeds contained a more diverse combination of bioactive GAs and had higher ABA content than domesticated peas.Although the role of abscisic acid (ABA) and gibberellins (GAs) interplay has been extensively studied in Arabidopsis and cereals models, comparatively little is known about the effect of domestication on the level of phytohormones in legume seeds. In legumes, as in other crops, seed dormancy has been largely or entirely removed during domestication. In this study, we have measured the endogenous levels of ABA and GAs comparatively between wild and domesticated pea seeds during their development. We have shown that wild seeds contained more ABA than domesticated ones, which could be important for preparing the seeds for the period of dormancy. ABA was catabolised particularly by an 8´-hydroxylation pathway, and dihydrophaseic acid was the main catabolite in seed coats as well as embryos. Besides, the seed coats of wild and pigmented cultivated genotypes were characterised by a broader spectrum of bioactive GAs compared to non-pigmented domesticated seeds. GAs in both seed coat and embryo were synthesized mainly by a 13-hydroxylation pathway, with GA29 being the most abundant in the seed coat and GA20 in the embryos. Measuring seed water content and water loss indicated domesticated pea seeds´ desiccation was slower than that of wild pea seeds. Altogether, we showed that pea domestication led to a change in bioactive GA composition and a lower ABA content during seed development.

Abscisic acid (ABA) alleviates cadmium toxicity by enhancing the adsorption of cadmium to root cell walls and inducing antioxidant defense system of Cosmos bipinnatus

Cadmium (Cd) pollution is a global problem affecting soil ecology and plant growth. Abscisic acid (ABA) acts as a growth and stress hormone, regulates cell wall synthesis, and plays an important role in plant responses to stress. There are few studies on the mechanisms behind abscisic acid alleviation of cadmium stress in Cosmos bipinnatus, especially in regards to regulation of the root cell wall. This study examined the effects of different concentrations of abscisic acid at different concentrations of cadmium stress. Through adding 5 μmol/L and 30 μmol/L cadmium, followed by spraying 10 μmol/L and 40 μmol/L ABA in a hydroponic experiment, it was found that under two concentrations of cadmium stress, low concentration of ABA improved root cell wall polysaccharide, Cd, and uronic acid content. Especially in pectin, after the application of low concentration ABA, the cadmium concentration was significantly increased by 1.5 times and 1.2 times compared with the Cd concentration under Cd5 and Cd30 treatment alone, respectively. Fourier-Transform Infrared spectroscopy (FTIR) demonstrated that cell wall functional groups such as -OH and -COOH were increased with exposure to ABA. Additionally, the exogenous ABA also increased expression of three kinds of antioxidant enzymes and plant antioxidants. The results of this study suggest that ABA could reduce Cd stress by increasing Cd accumulation, promoting Cd adsorption on the root cell wall, and activating protective mechanisms. This result could help promote application of C. bipinnatus for phytostabilization of cadmium-contaminated soil.

Characterization of ZmPMP3g function in drought tolerance of maize

The genes enconding proteins containing plasma membrane proteolipid 3 (PMP3) domain are responsive to abiotic stresses, but their functions in maize drought tolerance remain largely unknown. In this study, the transgenic maize lines overexpressing maize ZmPMP3g gene were featured by enhanced drought tolerance; increases in total root length, activities of superoxide dismutase and catalase, and leaf water content; and decreases in leaf water potential, levels of O2−·and H2O2, and malondialdehyde content under drought. Under treatments with foliar spraying with abscisic acid (ABA), drought tolerance of both transgenic line Y7-1 overexpressing ZmPMP3g and wild type Ye478 was enhanced, of which Y7-1 showed an increased endogenous ABA and decreased endogenous gibberellin (GA) 1 (significantly) and GA3 (very slightly but not significantly) and Ye478 had a relatively lower ABA and no changes in GA1 and GA3. ZmPMP3g overexpression in Y7-1 affected the expression of multiple key transcription factor genes in ABA-dependent and -independent drought signaling pathways. These results indicate that ZmPMP3g overexpression plays a role in maize drought tolerance by harmonizing ABA-GA1-GA3 homeostasis/balance, improving root growth, enhancing antioxidant capacity, maintaining membrane lipid integrity, and regulating intracellular osmotic pressure. A working model on ABA-GA-ZmPMP3g was proposed and discussed.

Changes and transcriptome regulation of endogenous hormones during somatic embryogenesis in Ormosia henryi Prain.

Ormosia henryi is a rare and endangered plant growing in southern China. Somatic embryo culture is an effective measure for the rapid propagation of O. henryi. It has not been reported how regulatory genes induce somatic embryogenesis by regulating endogenous hormone changes during the process of somatic embryogenesis in O. henryi. In this study, we analysed the endogenous hormone levels and transcriptome data of nonembryogenic callus (NEC), embryogenic callus (EC), globular embryo (GE) and cotyledon embryo (CE) in O. henryi. The results showed that the indole-3-acetic acid (IAA) content was higher and the cytokinins (CKs) content was lower in EC than in NEC, and the gibberellins (GAs) and abscisic acid (ABA) contents were significantly higher in NEC than in EC. The contents of IAA, CKs, GAs and ABA increased significantly with EC development. The expression patterns of differentially expressed genes (DEGs) involved in the biosynthesis and signal transduction of auxin (AUX) (YUCCA and SAUR), CKs (B-ARR), GAs (GA3ox, GA20ox, GID1 and DELLA) and ABA (ZEP, ABA2, AAO3, CYP97A3, PYL and ABF) were consistent with the levels of endogenous hormones during somatic embryogenesis (SE). In this study, 316 different transcription factors (TFs) regulating phytohormones were detected during SE. AUX/IAA were downregulated in the process of EC formation and GE differentiation into CE, but other TFs were upregulated and downregulated. Therefore, we believe that relatively high IAA content and low CKs, GAs and ABA contents contribute to EC formation. The differential expression of AUX, CKs, GAs and ABA biosynthesis and signal transduction genes affected the endogenous hormone levels at different stages of SE in O. henryi. The downregulated expression of AUX/IAA inhibited NEC induction, promoted EC formation and GE differentiation into CE.

An analysis of hormones and mineral nutrients in fruit from terminal and lateral racemes of two blueberry cultivars

BACKGROUND: In general, fruit that develops from early-opening flowers ripens earlier than fruit from late-opening flowers. However, the opposite has been observed in the rabbiteye blueberry cultivar “Brightwell” (Vaccinium ashei). OBJECTIVE: From the differences of endogenous hormones and mineral nutrition, with attempt to analyze the reasons for fruit from late-opening flowers on lateral racemes of “Brightwell” ripened earlier than fruit from early-opening flowers on terminal racemes. RESULTS: Fruit from late-opening flowers on lateral racemes of “Brightwell” ripened earlier than fruit from early-opening flowers on terminal racemes. This may be associated with the higher levels of giberellin 1, abscisic acid (ABA), salicylic acid, jasmonic acid, phosphorus, iron and boron in the early developmental stages, and the higher ABA level and lower giberellin 9 level in the fruit ripening stages on lateral racemes, compared with on terminal racemes. However, the fruit borne on the terminal racemes of “O’Neal” grew faster than fruit borne on lateral racemes, this may be correlated with the higher IAA, iron, manganese, zinc and boron levels, and the lower ABA level, in the early fruit developmental stages on terminal racemes, compared with on lateral racemes. CONCLUSIONS: The growth and ripening differences in fruit set at the two positions of the two blueberry cultivars may be associated with hormonal and nutritional control during fruit development.

Differential roles of abscisic acid in maize roots in the adaptation to soil drought

AbstractMaize yield reduction occurs frequently due to soil drought. Abscisic acid (ABA) is an important hormonal signal indicating drought. However, it remains unclear about the specific roles of root ABA in the adaptation to soil drought in maize. This study applied five different soil water potentials (SWP), referring to maintaining SWP at −15, −30, −45, −60, and −75 kPa, respectively, and investigated the ABA content in roots and the activities and gene expression levels of key enzymes involved in the ABA biosynthesis, the leaf photosynthetic properties, root traits, and kernel yield. The results showed that maize root ABA content, the activities and gene expressions of key enzymes involved in the ABA biosynthesis were increased with the decrease of SWP. The leaf transpiration rate, and root dry weight, length, volume, surface area, and activity, and kernel yield of maize were increased first and then decreased with the severity of soil drying. The leaf photosynthetic rate was not significantly reduced with ABA accumulation at a low ABA content in roots. Root ABA content was significantly positively correlated with leaf transpiration efficiency and root activity when ABA content was relatively low (9.03–22.02 nmol g−1 DW), whereas root ABA content was negatively correlated with leaf photosynthetic rate, leaf transpiration efficiency, root volume, and root activity when ABA content was high (28.32–39.23 nmol g−1 DW). The results indicate that maize root ABA exhibit differential roles in the adaptation to soil drought, and can positively regulate the drought‐resistance of maize at an appropriate level.

Physiological analysis reveals the mechanism of accelerated growth recovery for rice seedlings by nitrogen application after low temperature stress.

Low temperature and overcast rain are harmful to directly seeding early rice, it can hinder rice growth and lower rice biomass during the seedling stage, which in turn lowers rice yield. Farmers usually use N to help rice recuperate after stress and minimize losses. However, the effect of N application on the growth recovery for rice seedlings after such low temperature stress and its associated physiological changes remain unclearly. Two temperature settings and four post-stress N application levels were used in a bucket experiment to compare B116 (strong growth recovery after stress) with B144 (weak growth recovery). The results showed that the stress (average daily temperature at 12°C for 4 days) inhibited the growth of rice seedlings. Compared to the zero N group, the N application group's seedling height, fresh weight and dry weight significantly increased after 12 days. In particular, the increases in all three growth indicators were relatively higher than that of N application at normal temperature, indicating the importance of N application to rice seedlings after low temperature stress. The antioxidant enzyme activity of rice seedlings increased significantly after N application, which reduced the damaging effect of ROS (reactive oxygen species) to rice seedlings. At the same time, the soluble protein content of seedlings showed a slow decrease, while the H2O2 and MDA (malondialdehyde) content decreased significantly. Nitrogen could also promote nitrogen uptake and utilization by increasing the expression of genes related to and uptake and transport, as well as improving the activity of NR (nitrate reductase) and GS (glutamine synthetase) in rice. N could affect GA3 (gibberellin A3) and ABA (abscisic acid) levels by regulating the anabolism of GA3 and ABA. The N application group maintained high ABA levels as well as low GA3 levels from day 0 to day 6, and high GA3 levels as well as low ABA levels from day 6 to day 12. The two rice varieties showed obvious characteristics of accelerated growth recovery and positive physiological changes by nitrogen application after stress, while B116 generally showed more obvious growth recovery and stronger growth-related physiological reaction than that of B144. The N application of 40kg hm-2 was more conducive to the rapid recovery of rice growth after stress. The above results indicated that appropriate N application promoted rice seedling growth recovery after low temperature stress mainly by increasing the activities of antioxidant enzymes and nitrogen metabolizing enzymes as well as regulating the levels of GA3 and ABA. The results of this study will provide a reference for the regulation of N on the recovery of rice seedling growth after low temperature and weak light stress.

Role of exogenous abscisic acid in freezing tolerance of mangrove Kandelia obovata under natural frost condition at near 32°N

BackgroundMangroves possess substantial ecological, social, and economic functions in tropical and subtropical coastal wetlands. Kandelia obovata is the most cold-resistance species among mangrove plants, with a widespread distribution in China that ranges from Sanya (18° 12′ N) to Wenzhou (28° 20′ N). Here, we explored the temporal variations in physiological status and transcriptome profiling of K. obovata under natural frost conditions at ~ 32oN, as well as the positive role of exogenous abscisic acid (ABA) in cold resistance.ResultsThe soluble sugar (SS) and proline (Pro) functioned under freezing stress, of which SS was more important for K. obovata. Consistently, up-regulated DEGs responding to low temperature were significantly annotated to glycometabolism, such as starch and sucrose metabolism and amino sugar and nucleotide sugar metabolism. Notably, the top 2 pathways of KEGG enrichment were phenylpropanoid biosynthesis and flavonoid biosynthesis. For the antioxidant system, POD in conjunction with CAT removed hydrogen peroxide, and CAT appeared to be more important. The up-regulated DEGs responding to low temperature and ABA were also found to be enriched in arginine and proline metabolism, starch and sucrose metabolism, and peroxisome. Moreover, ABA triggered the expression of P5CS and P5CR, but inhibited the ProDH expression, which might contribute to Pro accumulation. Interestingly, there was no significant change in malondialdehyde (MDA) content during the cold event (P > 0.05), suggesting foliar application of ABA effectively alleviated the adverse effects of freezing stress on K. obovata by activating the antioxidant enzyme activity and increasing osmolytes accumulation, such as Pro, and the outcome was proportional to ABA concentration.ConclusionsThis study deepened our understanding of the physiological characters and molecular mechanisms underlying the response of K. obovata to natural frost conditions and exogenous ABA at the field level, which could provide a sound theoretical foundation for expanding mangroves plantations in higher latitudes, as well as the development coastal landscape.

The Role of ABA in the Interaction between Citrus Fruit and Penicillium digitatum

Abscisic acid (ABA) protects citrus fruit against Penicillium digitatum infection. The global mechanisms involved in the role of ABA in the P. digitatum-citrus fruit interaction are unknown. Here, we determine the transcriptome differences between the Navelate (Citrus sinensis (L.) Osbeck) orange and its ABA-deficient mutant Pinalate, which is less resistant to infection. Low ABA levels may affect both the constitutive mechanisms that protect citrus fruit against P. digitatum and early responses to infection. The repression of terpenoid, phenylpropanoid and glutation metabolism; of oxidation-reduction processes; and of processes related to the defense response to fungus and plant hormone signal transduction may be one part of the constitutive defense reduced in the mutant against P. digitatum. Our results also provide potential targets for developing P. digitatum-citrus fruit-resistant varieties. Of those up-regulated by ABA, a thaumatin protein and a bifunctional inhibitor/LTP, which are relevant in plant immunity, were particularly remarkable. It is also worth highlighting chlorophyllase 1 (CLH1), induced by infection in Pinalate, and the OXS3 gene, which was down-regulated by ABA, because the absence of OXS3 activates ABA-responsive genes in plants.

ABA biosynthesis gene OsNCED3 contributes to preharvest sprouting resistance and grain development in rice.

Preharvest sprouting (PHS) is an unfavorable trait in cereal crops and causes serious yield loss. However, the molecular mechanism underlying PHS remains largely elusive. Here, we identified a member of 9-cis-epoxycarotenoid dioxygenase family, OsNCED3, which regulates PHS and grain development in rice (Oryza sativa L.). OsNCED3 encodes a chloroplast-localized abscisic acid(ABA) biosynthetic enzyme highly expressed in the embryo of developing seeds. Disruption of OsNCED3 by CRISPR/Cas9-mediated mutagenesis led to a lower ABA and higher gibberellic acid(GA) levels (thus a skewed ABA/GA ratio) in the embryo, promoting embryos growth and breaking seed dormancy before seed maturity and harvest, thus decreased seed dormancy and enhanced PHS in rice. However, the overexpression of OsNCED3 enhanced PHS resistance by regulating proper ABA/GA ratio in the embryo. Intriguingly, the overexpression of OsNCED3 resulted in increased grain size and weight, whereas the disruption of OsNCED3 function decreased grain size and weight. Nucleotide diversity analyses suggested that OsNCED3 may be selected during japonica populations adaptation of seed dormancy and germination. Taken together, we have identified a new OsNCED regulator involved rice PHS and grain development, and provide a potential target gene for improving PHS resistance and grain development in rice.

Knockdown of NtCPS2 promotes plant growth and reduces drought tolerance in Nicotiana tabacum.

Drought stress is one of the primary environmental stress factors that gravely threaten crop growth, development, and yields. After drought stress, plants can regulate the content and proportion of various hormones to adjust their growth and development, and in some cases to minimize the adverse effects of drought stress. In our previous study, the tobacco cis-abienol synthesis gene (NtCPS2) was found to affect hormone synthesis in tobacco plants. Unfortunately, the role of NtCPS2 genes in the response to abiotic stress has not yet been investigated. Here, we present data supporting the role of NtCPS2 genes in drought stress and the possible underlying molecular mechanisms. NtCPS2 gene expression was induced by polyethylene glycol, high-temperature, and virus treatments. The results of subcellular localization showed that NtCPS2 was localized in the cell membrane. The NtCPS2-knockdown plants exhibited higher levels of gibberellin (GA) content and synthesis pathway genes expression but lower abscisic acid (ABA) content and synthesis pathway genes expression in response to drought stress. In addition, the transgenic tobacco lines showed higher leaf water loss and electrolyte loss, lower soluble protein and reactive oxygen species content (ROS), and lower antioxidant enzyme activity after drought treatment compared to wild type plants (WT). In summary, NtCPS2 positively regulates drought stress tolerance possibly by modulating the ratio of GA to ABA, which was confirmed by evidence of related phenotypic and physiological indicators. This study may provide evidence for the feedback regulation of hormone to abiotic and biotic stresses.

Pre-Sowing Treatment of Tilia cordata Mill. by Seed Material Fermentation.

The aim of the work was to verify whether the fermentation of seed material is a suitable pre-sowing treatment of small-leaved lime. Seed material of two origins was treated for a period of 7, 14 and 21 days using anaerobic fermentation and for 10, 20 and 30 days using aerobic fermentation, the control treatment being the seed after warm-cold stratification. The water content, germination capacity and abscisic acid level of the seed were evaluated, together with the emergence and morphological parameters of annual lime seedlings that had emerged from the tested seeds. It was found that fermented seed had a low abscisic acid level (up to 700 pmol/g), a higher germination capacity (approx. 90%) than mature seed without fermentation and the morphological parameters of the seedlings that had emerged from seed after fermentation reached higher values, compared to those of the seedlings that had emerged from seed after warm-cold stratification. The seed material had to be collected green (e.g., immature seeds) and moistened. A suitable anaerobic fermentation time is 7 days; the aerobic phase can last 10, 20 or 30 days. The fermentation of seed material can shorten the preparation of seed for sowing by 1 year.

A comprehensive analysis of carotenoids metabolism in two red-fleshed mutants of Navel and Valencia sweet oranges (Citrus sinensis).

Kirkwood Navel and Ruby Valencia are two spontaneous bud mutations of the respective parental lines of sweet orange (Citrus sinensis) Palmer Navel and Olinda Valencia, showing an atypical red pigmentation of the pulp. These red-fleshed varieties are commercially available and highly attractive for consumers but their carotenoid metabolism and the basis of the mutation have not been investigated. The red colour of Kirkwood and Ruby pulp was observed from the very early stages of fruit development until full maturity and associated with an altered carotenoid profiling. The red-fleshed varieties accumulated from 6- up to 1000-times more total carotenoids compared to the standard oranges. Specifically, the pulp of Kirkwood and Ruby accumulated large amounts of phytoene and phytofluene, and moderate contents of lycopene. Moreover, the red-fleshed oranges contained other unusual carotenes as δ-carotene, and lower concentrations of downstream products such as β,β-xanthophylls, abscisic acid (ABA) and ABA-glucosyl ester. This peculiar profile was associated with chromoplasts with lycopene crystalloid structures and round vesicles likely containing colourless carotenes. The flavedo and leaves of Kirkwood and Ruby showed minor changes in carotenoids, mainly limited to higher levels of phytoene. The carotenoid composition in Kirkwood and Ruby fruits was not explained by differences in the transcriptional profile of 26 genes related to carotenoid metabolism, covering the main steps of biosynthesis, catabolism and other processes related to carotenoid accumulation. Moreover, sequence analysis of the lycopene cyclase genes revealed no alterations in those of the red-fleshed oranges compared to the genes of the standard varieties. A striking event observed in Kirkwood and Ruby trees was the reddish coloration of the inner side of the bark tissue, with larger amounts of phytoene, accumulation of lycopene and lower ABA content. These observation lead to the conclusion that the mutation is not only manifested in fruit, affecting other carotenogenic tissues of the mutant plants, but with different consequences in the carotenoid profile. Overall, the carotenoid composition in the red-fleshed mutants suggests a partial blockage of the lycopene β-cyclization in the carotenoid pathway, rendering a high accumulation of carotenes upstream lycopene and a reduced flow to downstream xanthophylls and ABA.

Mild and severe salt stress responses are age-dependently regulated by abscisic acid in tomato.

Salt stress hampers plant growth and development through both osmotic and ionic imbalances. One of the key players in modulating physiological responses towards salinity is the plant hormone abscisic acid (ABA). How plants cope with salinity largely depends on the magnitude of the soil salt content (stress severity), but also on age-related developmental processes (ontogeny). Here we studied how ABA directs salt stress responses in tomato plants for both mild and severe salt stress in leaves of different ages. We used the ABA-deficient mutant notabilis, which contains a null-mutation in the gene of a rate-limiting ABA biosynthesis enzyme 9-cis-epoxycarotenoid dioxygenase (NCED1), leading to impaired stomatal closure. We showed that both old and young leaves of notabilis plants keep a steady-state transpiration and photosynthesis rate during salt stress, probably due to their dysfunctional stomatal closure. At the whole plant level, transpiration declined similar to the wild-type, impacting final growth. Notabilis leaves were able to produce osmolytes and accumulate ions in a similar way as wild-type plants, but accumulated more proline, indicating that osmotic responses were not impaired by the NCED1 mutation. Besides NCED1, also NCED2 and NCED6 are strongly upregulated under salt stress, which could explain why the notabilis mutant did not show a lower ABA content upon salt stress, except in young leaves. This might be indicative of a salt-mediated feedback mechanism on NCED2/6 in notabilis and might explain why notabilis plants seem to perform better under salt stress compared to wild-type plants with respect to biomass and water content accumulation.

A Comprehensive Analysis of Physiologic and Hormone Basis for the Difference in Room-Temperature Storability between 'Shixia' and 'Luosanmu' Longan Fruits.

Although the effects of phytohormones (mainly salicylic acid) on the storability of longan fruit have been reported, the relationship between postharvest hormone variation and signal transduction and storability remains unexplored. The basis of physiology, biochemistry, hormone content and signalling for the storability difference at room-temperature between ‘Shixia’ and ‘Luosanmu’ longan fruit were examined. ‘Luosanmu’ longan exhibited faster pericarp browning, aril breakdown and rotting during storage. ‘Luosanmu’ pericarp exhibited higher malondialdehyde but faster decreased total phenolics, flavonoid, glutathione, vitamin C, catalase activity and gene expression. Higher H2O2 and malondialdehyde but lower glutathione, glutathione-reductase and peroxidase activities, while higher activities and gene expressions of polygalacturonase, β-galactosidase and cellulose, lower covalent-soluble pectin, cellulose and hemicellulose but higher water-soluble pectin were observed in ‘Luosanmu’ aril. Lower abscisic acid and methyl jasmonate but higher expressions of LOX2, JAZ and NPR1 in pericarp, while higher abscisic acid, methyl jasmonate and salicylic acid together with higher expressions of ABF, JAZ, NPR1 and PR-1 in ‘Luosanmu’ aril were observed. In conclusion, the imbalance between the accumulation and scavenging of active oxygen in ‘Luosanmu’ longan might induce faster lipid peroxidation and senescence-related hormone signalling and further the polymerization of phenolics in pericarp and polysaccharide degradation in aril.