Increase In Abscisic Acid Levels Research Articles

A Rice NAC Transcription Factor Promotes Leaf Senescence via ABA Biosynthesis.

It is well known that abscisic acid (ABA)-induced leaf senescence and premature leaf senescence negatively affect the yield of rice (Oryza sativa). However, the molecular mechanism underlying this relationship, especially the upstream transcriptional network that modulates ABA level during leaf senescence, remains largely unknown. Here, we demonstrate a rice NAC transcription factor, OsNAC2, that participates in ABA-induced leaf senescence. Overexpression of OsNAC2 dramatically accelerated leaf senescence, whereas its knockdown lines showed a delay in leaf senescence. Chromatin immunoprecipitation-quantitative PCR, dual-luciferase, and yeast one-hybrid assays demonstrated that OsNAC2 directly activates expression of chlorophyll degradation genes, OsSGR and OsNYC3 Moreover, ectopic expression of OsNAC2 leads to an increase in ABA levels via directly up-regulating expression of ABA biosynthetic genes (OsNCED3 and OsZEP1) as well as down-regulating the ABA catabolic gene (OsABA8ox1). Interestingly, OsNAC2 is upregulated by a lower level of ABA but downregulated by a higher level of ABA, indicating a feedback repression of OsNAC2 by ABA. Additionally, reduced OsNAC2 expression leads to about 10% increase in the grain yield of RNAi lines. The novel ABA-NAC-SAGs regulatory module might provide a new insight into the molecular action of ABA to enhance leaf senescence and elucidates the transcriptional network of ABA production during leaf senescence in rice.

Leaf dehydration is needed to induce abscisic acid accumulation in roots of citrus plants

The root system is the main organ involved in the water uptake and it is assumed that it perceives soil dehydration, triggering water stress responses at the entire plant level, including abscisic acid (ABA) accumulation. However, several lines of evidence demonstrate that leaves directly adjust their water status (and induce increases in ABA levels) in response to changes in the evaporative demand. To evaluate the influence of leaf water status on ABA accumulation in roots, the canopy of citrus plants was kept at high humidity while roots were dehydrated. Leaves and roots of completely dehydrated plants increased levels of ABA whereas plants with turgid leaves did not accumulate ABA either in leaves or in roots. Furthermore, in dehydrated roots, the expression of several key genes involved in carotenoid and ABA biosynthetic pathways such as CsβCHX1 and CsNCED1 was strongly affected by the water status of leaves whereas water availability in roots showed a marginal influence. Overall, results confirm that leaves are a relevant source of ABA for roots during dehydration and also demonstrate that leaf-dehydration is required to trigger ABA-induced responses in roots.

Involvement of WRKY Transcription Factors in Abscisic-Acid-Induced Cold Tolerance of Banana Fruit

Phytohormone abscisic acid (ABA) and plant-specific WRKY transcription factors (TFs) have been implicated to play important roles in various stress responses. The involvement of WRKY TFs in ABA-mediated cold tolerance of economical fruits, such as banana fruit, however remains largely unknown. Here, we reported that ABA application could induce expressions of ABA biosynthesis-related genes MaNCED1 and MaNCED2, increase endogenous ABA contents, and thereby enhance cold tolerance in banana fruit. Four banana fruit WRKY TFs, designated as MaWRKY31, MaWRKY33, MaWRKY60, and MaWRKY71, were identified and characterized. All four of these MaWRKYs were nuclear-localized and displayed transactivation activities. Their expressions were induced by ABA treatment during cold storage. More importantly, the gel mobility shift assay and transient expression analysis revealed that MaWRKY31, MaWRKY33, MaWRKY60, and MaWRKY71 directly bound to the W-box elements in MaNCED1 and MaNCED2 promoters and activated their expressions. Taken together, our findings demonstrate that banana fruit WRKY TFs are involved in ABA-induced cold tolerance by, at least in part, increasing ABA levels via directly activating NECD expressions.

The APETALA2/ethylene-responsive factor transcription factor OsDERF2 negatively modulates drought stress in rice by repressing abscisic acid responsive genes

SUMMARYAPETALA2/ethylene-responsive factor (AP2/ERF) family transcription factors play a vital role in plant growth and in response to hormones and abiotic stresses. In the current research, it is reported that OsDERF2, one of the drought-responsive ERF, is a member of the DREB sub-family. OsDERF2 is a nuclear-localized protein and has transcriptional activity in yeast. Expression of OsDERF2 was induced by drought and inhibited by abscisic acid (ABA). However, OsDERF2 RNA interference (RNAi) knock-down transgenic lines enhanced tolerance to drought stress at seedling stage and were much more sensitive to ABA treatment, which may result from the increased ABA level in vivo. The basic leucine zipper (bZIP) transcription factor family plays an important role in the ABA signalling pathway of abiotic stress. Quantitative real-time polymerase chain reaction analysis revealed that the bZIP family gene OsbZIP20 and ABA-response gene OsABA45 were up-regulated 25 times and 120 times, respectively, in OsDERF2 RNAi knock-down lines under drought stress, which were up-regulated five and seven times in wild type under drought stress. The current data reveal that OsDERF2 negatively modulates drought stress response in an ABA-mediated pathway through regulating gene expression of other ABA-response transcription factors.

Homologous Recombination Defective Arabidopsis Mutants Exhibit Enhanced Sensitivity to Abscisic Acid.

Abscisic acid (ABA) acts as an important plant hormone in regulating various aspects of plant growth and developmental processes particularly under abiotic stress conditions. An increased ABA level in plant cells inhibits DNA replication and cell division, causing plant growth retardation. In this study, we have investigated the effects of ABA on the growth responses of some major loss-of-function mutants of DNA double-stand break (DSB) repair genes in Arabidopsis during seed germination and early stages of seedling growth for understanding the role of ABA in the induction of genome instability in plants. A comparative analysis of ABA sensitivity of wild-type Arabidopsis and the knockout mutant lines related to DSB sensors, including atatm, atatr, the non-homologous end joining (NHEJ) pathway genes, and mutants related to homologous recombination (HR) pathway genes showed relatively enhanced sensitivity of atatr and HR-related mutants to ABA treatment. The expression levels of HR-related genes were increased in wild-type Arabidopsis (Col-0) during seed germination and early stages of seedling growth. Immunoblotting experiments detected phosphorylation of histone H2AX in wild-type (Col-0) and DSB repair gene mutants after ABA treatment, indicating the activation of DNA damage response due to ABA treatment. Analyses of DSB repair kinetics using comet assay under neutral condition have revealed comparatively slower DSB repair activity in HR mutants. Overall, our results have provided comprehensive information on the possible effect of ABA on DNA repair machinery in plants and also indicated potential functional involvement of HR pathway in repairing ABA induced DNA damage in Arabidopsis.

Significant contribution from foliage-derived ABA in regulating gas exchange in Pinus radiata.

The complex regulatory system controlling stomata involves physical and chemical signals that affect guard cell turgor to bring about changes in stomatal conductance (gs). Abscisic acid (ABA) closes stomata, yet the mechanisms controlling foliar ABA status in tree species remain unclear. The importance of foliage-derived ABA in regulating gas exchange was evaluated under treatments that affected phloem export through girdling and reduced water availability in the tree species, Pinus radiata (D. Don). Branch- and whole-plant girdling increased foliar ABA levels leading to declines in gs, despite no change in plant water status. Changes in gs were largely independent of the more transient increases in foliar non-structural carbohydrates (NSC), suggesting that gradual accumulation of foliar ABA was the primary mechanism for reductions in gs and assimilation. Whole-plant girdling eventually reduced root NSC, hindering root water uptake and decreasing foliar water potential, causing a dramatic increase in ABA level in leaves and concentrations in the xylem sap of shoots (4032 ng ml-1), while root xylem sap concentrations remained low (43 ng ml-1). Contrastingly, the drought treatment caused similar increases in xylem sap ABA in both roots and shoots, suggesting that declines in water potential result in relatively consistent changes in ABA along the hydraulic pathway. ABA levels in plant canopies can be regulated independently of changes in root water status triggered by changes by both phloem export and foliar water status.

Implications of leaf ontogeny on drought-induced gradients of CAM expression and ABA levels in rosettes of the epiphytic tank bromeliad Guzmania monostachia

Guzmania monostachia is an epiphytic heteroblastic bromeliad that exhibits rosette leaves forming water-holding tanks at maturity. Different portions along its leaf blades can display variable degrees of crassulacean acid metabolism (CAM) up-regulation under drought. Since abscisic acid (ABA) can act as an important long-distance signal, we conducted a joint investigation of ontogenetic and drought impacts on CAM intensity and ABA levels in different leaf groups within the G. monostachia rosette. For this, three groups of leaves were analysed according to their position within the mature-tank rosette (i.e., younger, intermediate, and older leaves) to characterize the general growth patterns and magnitude of drought-modulated CAM expression. CAM activity was evaluated by analysing key molecules in the biochemical machinery of this photosynthetic pathway, while endogenous ABA content was comparatively measured in different portions of each leaf group after seven days under well-watered (control) or drought treatment. The results revealed that G. monostachia shows more uniform morphological characteristics along the leaves when in the atmospheric stage. The drought treatment of mature-tank rosettes generally induced in older leaves a more severe water loss, followed by the lowest CAM activity and a higher increase in ABA levels, while younger leaves showed an opposite response. Therefore, leaf groups at distinct ontogenetic stages within the tank rosette of G. monostachia responded to drought with variable degrees of water loss and CAM expression. ABA seems to participate in this tissue-compartmented response as a long-distance signalling molecule, transmitting the drought-induced signals originated in older leaves towards the younger ones.

Moderate wetting and drying increases rice yield and reduces water use, grain arsenic level, and methane emission

To meet the major challenge of increasing rice production to feed a growing population under increasing water scarcity, many water-saving regimes have been introduced in irrigated rice, such as an aerobic rice system, non-flooded mulching cultivation, and alternate wetting and drying (AWD). These regimes could substantially enhance water use efficiency (WUE) by reducing irrigation water. However, such enhancements greatly compromise grain yield. Recent work has shown that moderate AWD, in which photosynthesis is not severely inhibited and plants can rehydrate overnight during the soil drying period, or plants are rewatered at a soil water potential of −10 to −15kPa, or midday leaf potential is approximately −0.60 to −0.80MPa, or the water table is maintained at 10 to 15cm below the soil surface, could increase not only WUE but also grain yield. Increases in grain yield WUE under moderate AWD are due mainly to reduced redundant vegetative growth; improved canopy structure and root growth; elevated hormonal levels, in particular increases in abscisic acid levels during soil drying and cytokinin levels during rewatering; and enhanced carbon remobilization from vegetative tissues to grain. Moderate AWD could also improve rice quality, including reductions in grain arsenic accumulation, and reduce methane emissions from paddies. Adoption of moderate AWD with an appropriate nitrogen application rate may exert a synergistic effect on grain yield and result in higher WUE and nitrogen use efficiency. Further research is needed to understand root–soil interaction and evaluate the long-term effects of moderate AWD on sustainable agriculture.

Implication of Abscisic Acid on Ripening and Quality in Sweet Cherries: Differential Effects during Pre- and Post-harvest.

Sweet cherry, a non-climacteric fruit, is usually cold-stored during post-harvest to prevent over-ripening. The aim of the study was to evaluate the role of abscisic acid (ABA) on fruit growth and ripening of this fruit, considering as well its putative implication in over-ripening and effects on quality. We measured the endogenous concentrations of ABA during the ripening of sweet cherries (Prunus avium L. var. Prime Giant) collected from orchard trees and in cherries exposed to 4°C and 23°C during 10 days of post-harvest. Furthermore, we examined to what extent endogenous ABA concentrations were related to quality parameters, such as fruit biomass, anthocyanin accumulation and levels of vitamins C and E. Endogenous concentrations of ABA in fruits increased progressively during fruit growth and ripening on the tree, to decrease later during post-harvest at 23°C. Cold treatment, however, increased ABA levels and led to an inhibition of over-ripening. Furthermore, ABA levels positively correlated with anthocyanin and vitamin E levels during pre-harvest, but not during post-harvest. We conclude that ABA plays a major role in sweet cherry development, stimulating its ripening process and positively influencing quality parameters during pre-harvest. The possible influence of ABA preventing over-ripening in cold-stored sweet cherries is also discussed.

ABA and cytokinins: challenge and opportunity for plant stress research.

Accumulation of the stress hormone abscisic acid (ABA) induces many cellular mechanisms associated with drought resistance. Recent years have seen a rapid advance in our knowledge of how increased ABA levels are perceived by ABA receptors, particularly the PYL/RCAR receptors, but there has been relatively less new information about how ABA accumulation is controlled and matched to stress severity. ABA synthesis and catabolism, conjugation and deconjugation to glucose, and ABA transport all are involved in controlling ABA levels. This highly buffered system of ABA metabolism represents both a challenge and opportunity in developing a mechanistic understanding of how plants detect and respond to drought. Recent data have also shown that direct manipulation of cytokinin levels in transgenic plants has dramatic effect on drought phenotypes and prompted new interest in the role of cytokinins and cytokinin signaling in drought. Both ABA and cytokinins will continue to be major foci of drought research but likely with different trajectories both in terms of basic research and in translational research aimed at increasing plant performance during drought.

Environmental Nitrate Stimulates Abscisic Acid Accumulation in Arabidopsis Root Tips by Releasing It from Inactive Stores.

Abscisic acid (ABA) signaling plays a major role in root system development, regulating growth and root architecture. However, the precise localization of ABA remains undetermined. Here, we present a mechanism in which nitrate signaling stimulates the release of bioactive ABA from the inactive storage form, ABA-glucose ester (ABA-GE). We found that ABA accumulated in the endodermis and quiescent center of Arabidopsis thaliana root tips, mimicking the pattern of SCARECROW expression, and (to lower levels) in the vascular cylinder. Nitrate treatment increased ABA levels in root tips; this stimulation requires the activity of the endoplasmic reticulum-localized, ABA-GE-deconjugating enzyme b-GLUCOSIDASE1, but not de novo ABA biosynthesis. Immunogold labeling demonstrated that ABA is associated with cytoplasmic structures near, but not within, the endoplasmic reticulum. These findings demonstrate a mechanism for nitrate-regulated root growth via regulation of ABA accumulation in the root tip, providing insight into the environmental regulation of root growth.

Cytogenetic and epigenetic alterations by cobalt and nickel on Zea mays L

Changes in DNA methylation in Zea mays plantlets in response to cobalt and nickel (5, 10, 20, and 40 mmol L−1) evaluated by the coupled restriction enzyme digestion-random amplification technique revealed dose-related increases in hypermethylation. The effects on mitotic index and phytohormone levels were also determined. Co and Ni caused a decrease in mitotic index and an attenuation in the growth-promoting hormones gibberellic acid, zeatin, and indole acetic acid, while abscisic acid levels rose. These findings have shown that the increase in abscisic acid levels and DNA methylation depend on the concentrations of Co and Ni.

LTP3 contributes to disease susceptibility in Arabidopsis by enhancing abscisic acid (ABA) biosynthesis.

Several plant lipid transfer proteins (LTPs) act positively in plant disease resistance. Here, we show that LTP3 (At5g59320), a pathogen and abscisic acid (ABA)-induced gene, negatively regulates plant immunity in Arabidopsis. The overexpression of LTP3 (LTP3-OX) led to an enhanced susceptibility to virulent bacteria and compromised resistance to avirulent bacteria. On infection of LTP3-OX plants with Pseudomonas syringae pv. tomato, genes involved in ABA biosynthesis, NCED3 and AAO3, were highly induced, whereas salicylic acid (SA)-related genes, ICS1 and PR1, were down-regulated. Accordingly, in LTP3-OX plants, we observed increased ABA levels and decreased SA levels relative to the wild-type. We also showed that the LTP3 overexpression-mediated enhanced susceptibility was partially dependent on AAO3. Interestingly, loss of function of LTP3 (ltp3-1) did not affect ABA pathways, but resulted in PR1 gene induction and elevated SA levels, suggesting that LTP3 can negatively regulate SA in an ABA-independent manner. However, a double mutant consisting of ltp3-1 and silent LTP4 (ltp3/ltp4) showed reduced susceptibility to Pseudomonas and down-regulation of ABA biosynthesis genes, suggesting that LTP3 acts in a redundant manner with its closest homologue LTP4 by modulating the ABA pathway. Taken together, our data show that LTP3 is a novel negative regulator of plant immunity which acts through the manipulation of the ABA-SA balance.

CARBON ECONOMY AND MINERAL NUTRITION IN A SUSTAINABLE PEACH ORCHARD

Abscisic acid (ABA) is a phytohormone able to regulate the stomatal behaviour, transpiration and photosynthesis. ABA synthesis is usually associated to drought conditions. Based on the evidence that drip irrigation wets only 15-20% of soil surface and that ~80% is dry during summer, this study tests the hypothesis that in a well drip irrigated tree the ABA concentration in roots located in the inter-row (non-irrigated) is higher than that measured in roots under the emitter line (irrigated row). Abscisic acid was determined by a competitive enzyme-linked immunosorbent assay in roots collected from a well irrigated tree (x 3) in summer at row and inter-row positions in a nectarine orchard grown in Southern Italy. Results show that leaf water potential was optimal (~0.3 MPa at pre-dawn). However, ABA concentration was 3.74 pmol g-1 FW in roots from inter-row while it was 2.58 pmol g-1 FW in that growing along the row. Possible effects of the increased ABA levels at a part of the root-zone on leaf gas exchanges are discussed.

Ecophysiological response to seasonal variations in water availability in the arborescent, endemic plant Vellozia gigantea.

The physiological response of plants growing in their natural habitat is strongly determined by seasonal variations in environmental conditions and the interaction of abiotic and biotic stresses. Here, leaf water and nutrient contents, changes in cellular redox state and endogenous levels of stress-related phytohormones (abscisic acid (ABA), salicylic acid and jasmonates) were examined during the rainy and dry season in Vellozia gigantea, an endemic species growing at high elevations in the rupestrian fields of the Espinhaço Range in Brazil. Enhanced stomatal closure and increased ABA levels during the dry season were associated with an efficient control of leaf water content. Moreover, reductions in 12-oxo-phytodienoic acid (OPDA) levels during the dry season were observed, while levels of other jasmonates, such as jasmonic acid and jasmonoyl-isoleucine, were not affected. Changes in ABA and OPDA levels correlated with endogenous concentrations of iron and silicon, hydrogen peroxide, and vitamin E, thus indicating complex interactions between water and nutrient contents, changes in cellular redox state and endogenous hormone concentrations. Results also suggested crosstalk between activation of mechanisms for drought stress tolerance (as mediated by ABA) and biotic stress resistance (mediated by jasmonates), in which vitamin E levels may serve as a control point. It is concluded that, aside from a tight ABA-associated regulation of stomatal closure during the dry season, crosstalk between activation of abiotic and biotic defences, and nutrient accumulation in leaves may be important modulators of plant stress responses in plants growing in their natural habitat.

Transcriptional regulation of genes encoding ABA metabolism enzymes during the fruit development and dehydration stress of pear 'Gold Nijisseiki'

To investigate the contribution of abscisic acid (ABA) in pear 'Gold Nijisseiki' during fruit ripening and under dehydration stress, two cDNAs (PpNCED1 and PpNCED2) which encode 9-cis-epoxycarotenoid dioxygenase (NCED) (a key enzyme in ABA biosynthesis), two cDNAs (PpCYP707A1 and PpCYP707A2) which encode 8'-hydroxylase (a key enzyme in the oxidative catabolism of ABA), one cDNA (PpACS3) which encodes 1-aminocyclopropane-1-carboxylic acid (ACC), and one cDNA (PpACO1) which encodes ACC oxidase involved in ethylene biosynthesis were cloned from 'Gold Nijisseiki' fruit. In the pulp, peel and seed, expressions of PpNCED1 and PpNCED2 rose in two stages which corresponded with the increase of ABA levels. The expression of PpCYP707A1 dramatically declined after 60–90 days after full bloom (DAFB) in contrast to the changes of ABA levels during this period, while PpCYP707A2 stayed low during the whole development of fruit. Application of exogenous ABA at 100 DAFB increased the soluble sugar content and the ethylene release but significantly decreased the titratable acid and chlorophyll contents in fruits. When fruits harvested at 100 DAFB were stored in the laboratory (25 °C, 50% relative humidity), the ABA content and the expressions of PpNCED1/2 and PpCYP707A1 in the pulp, peel and seed increased significantly, while ethylene reached its highest value after the maximum peak of ABA accompanied with the expressions of PpACS3 and PpACO1. In sum the endogenous ABA may play an important role in the fruit ripening and dehydration of pear 'Gold Nijisseiki' and the ABA level was regulated mainly by the dynamics of PpNCED1, PpNCED2 and PpCYP707A1 at the transcriptional level.

AtRH57, a DEAD‐box RNA helicase, is involved in feedback inhibition of glucose‐mediated abscisic acid accumulation during seedling development and additively affects pre‐ribosomal RNA processing with high glucose

The Arabidopsis thalianaT-DNA insertion mutant rh57-1 exhibited hypersensitivity to glucose (Glc) and abscisic acid (ABA). The other two rh57 mutants also showed Glc hypersensitivity similar to rh57-1, strongly suggesting that the Glc-hypersensitive feature of these mutants results from mutation of AtRH57. rh57-1 and rh57-3 displayed severely impaired seedling growth when grown in Glc concentrations higher than 3%. The gene, AtRH57 (At3g09720), was expressed in all Arabidopsis organs and its transcript was significantly induced by ABA, high Glc and salt. The new AtRH57 belongs to class II DEAD-box RNA helicase gene family. Transient expression of AtRH57-EGFP (enhanced green fluorescent protein) in onion cells indicated that AtRH57 was localized in the nucleus and nucleolus. Purified AtRH57-His protein was shown to unwind double-stranded RNA independent of ATP in vitro. The ABA biosynthesis inhibitor fluridone profoundly redeemed seedling growth arrest mediated by sugar. rh57-1 showed increased ABA levels when exposed to high Glc. Quantitative real time polymerase chain reaction analysis showed that AtRH57 acts in a signaling network downstream of HXK1. A feedback inhibition of ABA accumulation mediated by AtRH57 exists within the sugar-mediated ABA signaling. AtRH57 mutation and high Glc conditions additively caused a severe defect in small ribosomal subunit formation. The accumulation of abnormal pre-rRNA and resistance to protein synthesis-related antibiotics were observed in rh57 mutants and in the wild-type Col-0 under high Glc conditions. These results suggested that AtRH57 plays an important role in rRNA biogenesis in Arabidopsis and participates in response to sugar involving Glc- and ABA signaling during germination and seedling growth.

The Role of ABA in the Maturation and Postharvest Life of a Nonclimacteric Sweet Cherry Fruit

The purpose of this study was to investigate the role of abscisic acid (ABA) in the pre- and postharvest maturation of the nonclimacteric sweet cherry. It was found that ABA content increased rapidly at the straw-colored stage and reached its highest level 4 days before commercial harvest time. The increase in ABA level was paralleled by an increase in the expression of a gene involved in ABA synthesis [9-cis-epoxycarotenoid dioxygenase (PacNCED1)]. Well before the ABA increase, the expression of a gene involved in ABA catabolism (PacCYP707A2) was downregulated. During the straw-colored stage, the application of exogenous ABA induced ABA accumulation, anthocyanin biosynthesis, and an increase in the maturity index (SSC/TA), thereby promoting fruit ripening. ABA treatment stimulated the expression of PacNCED1 and of genes involved in anthocyanin biosynthesis (PacCHS, PacF3H, PacCHI, PacDFR, and PacUFGT). The application of NiCl2 inhibited the release of ethylene and the expression of PacACO1 encoded 1-aminocyclopropane-1-carboxylic acid oxidase, of which the effect on fruit ripening was opposite that of ABA. Ethephon and nordihydroguaiaretic acid treatments, however, had no effect on fruit ripening. For postharvest fruit, the pedicel was a major route for water loss, and dehydration induced the transcription of PacNCED1 and the accumulation of ABA. Followed by ethylene release, the start of postharvest fruit senescence is triggered. In conclusion, endogenous ABA triggered the ripening of fruit via the modulation of ripening-related metabolism pathways such as anthocyanin accumulation. ABA also induced postharvest fruit senescence through stimulation of ethylene release in cherry fruit.

A study on ABA involvement in the response of tomato to suboptimal root temperature using reciprocal grafts with notabilis, a null mutant in the ABA-biosynthesis gene LeNCED1

To elucidate the role of abscisic acid (ABA) in tomato (Solanum lycopersicum L.) responses to suboptimal root temperature (T), a near-isogenic line carrying the notabilis null mutation in the ABA biosynthesis gene LeNCED1 was reciprocally grafted with its parental cultivar Ailsa Craig. Exposure of tomato to suboptimal root T (15°C) decreased leaf area expansion, shoot elongation and plant biomass in comparison with optimal root T (25°C). Both suboptimal root T and null mutation of the LeNCED1 gene in root and shoot reduced leaf area and total plant biomass, but these two factors did not interact. Transpiration rates and stomatal conductances decreased, while net CO2 assimilation was not influenced by root exposure to suboptimal T. However, notabilis scions exhibited higher net assimilation rates, stomatal conductances and transpiration rates than Ailsa Craig scions. Moreover, notabilis plants invested much more biomass into the root than Ailsa Craig self-grafts, thereby improving their water uptake capacity. Lipid peroxidation as well as polyamines and guaiacol-peroxidase (G-POD), which are considered to possess antioxidant properties, increased in the leaves of all grafting combinations of tomato when exposed to suboptimal root T. All grafting combinations showed an increase in shoot ABA levels when exposed to suboptimal root T. However, the levels of ABA in the shoot of notabilis did not differ significantly from those found in Ailsa Craig, indicating that inactivation of LeNCED1 did not impair ABA accumulation in the leaves at low root-zone T. The genes LeNCED2 and LeNCED6 were not up-regulated at low root-zone T and therefore not responsible for ABA biosynthesis in notabilis. Increased ABA levels in tomato exposed to suboptimal root T were associated with elevated levels of the polyamine putrescine which contributes to ABA biosynthesis. In conclusion, the introgression in Ailsa Craig that contains the mutation resulting from inactivation of the LeNCED1 gene includes further genetic differences that influence responses to suboptimal root-zone T, such as plant growth restriction and ROS scavenging modification.

Modelling the widespread effects of TOC1 signalling on the plant circadian clock and its outputs

Background24-hour biological clocks are intimately connected to the cellular signalling network, which complicates the analysis of clock mechanisms. The transcriptional regulator TOC1 (TIMING OF CAB EXPRESSION 1) is a founding component of the gene circuit in the plant circadian clock. Recent results show that TOC1 suppresses transcription of multiple target genes within the clock circuit, far beyond its previously-described regulation of the morning transcription factors LHY (LATE ELONGATED HYPOCOTYL) and CCA1 (CIRCADIAN CLOCK ASSOCIATED 1). It is unclear how this pervasive effect of TOC1 affects the dynamics of the clock and its outputs. TOC1 also appears to function in a nested feedback loop that includes signalling by the plant hormone Abscisic Acid (ABA), which is upregulated by abiotic stresses, such as drought. ABA treatments both alter TOC1 levels and affect the clock’s timing behaviour. Conversely, the clock rhythmically modulates physiological processes induced by ABA, such as the closing of stomata in the leaf epidermis. In order to understand the dynamics of the clock and its outputs under changing environmental conditions, the reciprocal interactions between the clock and other signalling pathways must be integrated.ResultsWe extended the mathematical model of the plant clock gene circuit by incorporating the repression of multiple clock genes by TOC1, observed experimentally. The revised model more accurately matches the data on the clock’s molecular profiles and timing behaviour, explaining the clock’s responses in TOC1 over-expression and toc1 mutant plants. A simplified representation of ABA signalling allowed us to investigate the interactions of ABA and circadian pathways. Increased ABA levels lengthen the free-running period of the clock, consistent with the experimental data. Adding stomatal closure to the model, as a key ABA- and clock-regulated downstream process allowed to describe TOC1 effects on the rhythmic gating of stomatal closure.ConclusionsThe integrated model of the circadian clock circuit and ABA-regulated environmental sensing allowed us to explain multiple experimental observations on the timing and stomatal responses to genetic and environmental perturbations. These results crystallise a new role of TOC1 as an environmental sensor, which both affects the pace of the central oscillator and modulates the kinetics of downstream processes.