Mechanism of the chilling-induced decrease in proton pumping across the tonoplast of rice cells

Review of recent advances in post-harvest techniques for tropical cut flowers and future prospects: Heliconia as a case-study

Aesthetic attributes and easy-to-grow nature of tropical cut flowers (TCFs) have contributedto their potential for increased production. The dearth of information regarding agronomic practices and lack of planting materials are the key hindrances against their fast expansion. Unconventional high-temperature storage requirements and the anatomy of the peduncle contribute topoor vase life performance, while troublesome packaging and transport due to unusual size and structureprimarily cause post-harvest quality deterioration. Nonetheless, the exotic floral structuresconsequently increase market demand, particularly in temperate countries. This boosts studies aimed at overcoming post-harvest hindrances. While a few TCFs (Anthurium, Strelitzia, Alpinia, and a few orchids) are under the spotlight, many others remain behind the veil. Heliconia, an emerging specialty TCF (False Bird-of-Paradise, family Heliconiaceae), is one of them. The structural uniquenessand dazzling hues of Heliconia genotypes facilitate shifting its position from the back to the forefrontof the world floriculture trade. The unsatisfactory state-of-the-art of Heliconia research and the absence of any review exclusively on it are the key impetus for structuring this review. In addition to the aforementioned setbacks, impaired water uptake capacity after harvest, high chilling sensitivity, and the proneness of xylem ducts to microbial occlusion may be counted as a few additional factors that hinder its commercialization. This review demonstrates the state-of-the-art of post-harvest research while also conceptualizing the implementation of advanced biotechnological aid to alleviate the challenges, primarily focusing on Heliconia (the model crop here) along with some relevant literature on its other allied members. Standard harvesting indices, grading, and packaging are also part of the entire post-harvest operational chain, but since these phases are barely considered in Heliconia and the majority of tropical ornamentals except a few, a comprehensive account of these aspects has also been given. The hypothesized cues to nip chilling injury, resorting to different bio-chemical treatments, nano-based technology, and advanced packaging techniques, may help overcome preservation difficulties and propel its transition from niche to the commercial flower market. In a nutshell, readers will gain a comprehensive overview of how optimum post-harvest handling practices can rewardingly characterize this unique group of TCFs as the most remunerative component.

Deciphering the Molecular Mechanisms of Chilling Tolerance in Lsi1-Overexpressing Rice

Improving tolerance to low-temperature stress during the rice seedling stage is of great significance in agricultural science. In this study, using the low silicon gene 1 (Lsi1)-overexpressing (Dular-OE) and wild-type rice (Dular-WT), we showed that Lsi1 overexpression enhances chilling tolerance in Dular-OE. The overexpression of the Lsi1 increases silicon absorption, but it was not the main reason for chilling tolerance in Dular-OE. Instead, our data suggest that the overexpression of a Lsi1-encoding NIP and its interaction with key proteins lead to chilling tolerance in Dular-OE. Additionally, we show that the high-mobility group protein (HMG1) binds to the promoter of Lsi1, positively regulating its expression. Moreover, Nod26-like major intrinsic protein (NIP)’s interaction with α and β subunits of ATP synthase and the 14-3-3f protein was validated by co-immunoprecipitation (Co-IP), bimolecular fluorescent complementary (BiFC), and GST-pulldown assays. Western blotting revealed that the overexpression of NIP positively regulates the ATP-synthase subunits that subsequently upregulate calcineurin B-like interacting protein kinases (CIPK) negatively regulating 14-3-3f. Overall, these NIP-mediated changes trigger corresponding pathways in an orderly manner, enhancing chilling tolerance in Dular-OE.

Comparative transcriptome analysis of leaves during early stages of chilling stress in two different chilling-tolerant brown-fiber cotton cultivars

Chilling stress generates significant inhibition of normal growth and development of cotton plants and lead to severe reduction of fiber quality and yield. Currently, little is known for the molecular mechanism of brown-fiber cotton (BFC) to respond to chilling stress. Herein, RNA-sequencing (RNA-seq)-based comparative analysis of leaves under 4°C treatment in two different-tolerant BFC cultivars, chilling-sensitive (CS) XC20 and chilling-tolerant (CT) Z1612, was performed to investigate the response mechanism. A total of 72650 unigenes were identified with eight commonly used databases. Venn diagram analysis identified 1194 differentially expressed genes (DEGs) with significant up-regulation in all comparison groups. Furthermore, enrichment analyses of COG and KEGG, as well as qRT-PCR validation, indicated that 279 genes were discovered as up-regulated DEGs (UDEGs) with constant significant increased expression in CT cultivar Z1612 groups at the dimensions of both each comparison group and treatment time, locating in the enriched pathways of signal transduction, protein and carbohydrate metabolism, and cell component. Moreover, the comprehensive analyses of gene expression, physiological index and intracellular metabolite detections, and ascorbate antioxidative metabolism measurement validated the functional contributions of these identified candidate genes and pathways to chilling stress. Together, this study for the first time report the candidate key genes and metabolic pathways responding to chilling stress in BFC, and provide the effective reference for understanding the regulatory mechanism of low temperature adaptation in cotton.

Temperature modulation of thermal tolerance of a CAM-tank bromeliad and the relationship with acid accumulation in different leaf regions

Physiological changes that increase plant performance during exposure to high temperatures may play an inverse role during exposure to low temperatures. The objective of this study was to test variations in photosystem II response to heat and cold stress in the leaves of a bromeliad with crassulacean acid metabolism submitted to high or low temperatures. Leaves were maintained under constant temperatures of 10 and 35°C and used to examine possible relationships among physiological responses to high and low temperatures and organic acid accumulation. We also tested if distinct parts of bromeliad leaves show differences in photosynthetic thermotolerance. The samples from leaves maintained at 35°C showed greater heat tolerance values, while those from leaves maintained at 10°C showed lower cold tolerance values. Our results identified a strong negative relationship between the organic acid accumulation and thermal tolerance of bromeliad leaves that largely explained the differences in thermal tolerance among groups. One of these differences occurred among regions of a single leaf, with the base showing critical heat values of up to 8°C higher than the top region, suggesting a possible partitioning of leaf response among its regions. Differences in thermal tolerance were also observed between sampling times, with higher values observed in the morning.

Abscisic acid induced freezing tolerance in chilling-sensitive suspension cultures and seedlings of rice

BACKGROUND

The role of abscisic acid (ABA) as a possible activator of cold acclimation process was postulated since endogenous levels of ABA increase temporarily or constitutively during cold-hardening. Exogenous application of ABA has been known to induce freezing tolerance at ambient temperatures in in vitro systems derived from cold hardy plants. Yet, some cell cultures acquired much greater freezing tolerance by ABA than by cold whilst maintaining active growth. This raises questions about the relationships among ABA, cold acclimation and growth cessation. To address this question, we attempted to 1) determine whether exogenous ABA can confer freezing tolerance in chilling-sensitive rice suspension cells and seedlings, which obviously lack the mechanisms to acquire freezing tolerance in response to cold; 2) characterize this phenomenon by optimizing the conditions and compare with the case of cold hardy bromegrass cells.

RESULTS

Non-embryogenic suspension cells of rice suffered serious chilling injury when exposed to 4°C. When incubated with ABA at the optimal conditions (0.5-1 g cell inoculum, 75 μM ABA, 25-30°C, 7-10 days), they survived slow freezing (2°C/h) to -9.0 ~ -9.3°C (LT50: 50% killing temperature) while control cells were mostly injured at -3°C (LT50: -0.5 ~ -1.5°C). Ice-inoculation of the cell suspension at -3°C and survival determination by regrowth confirmed that ABA-treated rice cells survived extracellular freezing at -9°C. ABA-induced freezing tolerance did not require any exposure to cold and was best achieved at 25-30°C where the rice cells maintained high growth even in the presence of ABA. ABA treatment also increased tolerance to heat (43°C) as determined by regrowth. ABA-treated cells tended to have more augmented cytoplasm and/or reduced vacuole sizes compared to control cultures with a concomitant increase in osmolarity and a decrease in water content. ABA-treated (2-7 days) in vitro grown seedlings and their leaves survived slow freezing to -3°C with only marginal injury (LT50: -4°C) whereas untreated seedlings were killed at -3°C (LT50: -2°C).

CONCLUSIONS

The results indicate that exogenous ABA can induce some levels of freezing tolerance in chilling-sensitive rice cells and seedlings, probably by eliciting mechanisms different from low temperature-induced cold acclimation.

Proteome changes in tomato fruits prior to visible symptoms of chilling injury are linked to defensive mechanisms, uncoupling of photosynthetic processes and protein degradation machinery

A comparative proteomic analysis between tomato fruits stored at chilling and non-chilling temperatures was carried out just before the appearance of visible symptoms of chilling injury. At this stage of the stress period it was possible to discriminate between proteins involved in symptoms and proteins implicated in response. To investigate the changes in the tomato fruit proteome under this specific stressful condition, two-dimensional differential in-gel electrophoresis coupled with spot identification by mass spectrometry was applied. This proteomic approach allowed the identification of differentially expressed proteins which are involved in two main biological functions: (i) defensive mechanisms represented by small heat shock and late embryogenesis proteins; and (ii) reaction to the uncoupling of photosynthetic processes and the protein degradation machinery. One of the first changes observed in chilled fruits is the down-regulation of ATP synthase, 26S proteasome subunit RPN11 and aspartic proteinase, whereas the first responses in order to deal with the stress are mainly multifunctional proteins involved not only in metabolism but also in stress regulation such as glyceraldehyde phosphate dehydrogenase, 2-oxoglutarate dehydrogenase and invertase. In addition, our data seem to indicate a possible candidate to be used as a protein marker for further studies on cold stress: aldose-1-epimerase, which seems to have an important role in low temperature tolerance.

Carbon partitioning and the impact of starch deficiency on the initial response of Arabidopsis to chilling temperatures

Metabolites and stress related transcripts were measured in Arabidopsis thaliana in response to chilling temperatures. Rates of carbon assimilation increased 17% on average in response to cold treatment. Sucrose, glucose and fructose accumulation consumed 42% of the carbon from A but leaf starch only could synthesize ~10% of observed changes in soluble sugars. Carbohydrates were the only major class of metabolites that accumulated during the first 24 h of cold treatment. Except maltose and raffinose, carbohydrate accumulation was abolished when cold treatments were in darkness. Starch hydrolysis was correlated with maltose accumulation and increased expression of BAM3, which encodes a β-amylase necessary for starch mobilization. Hexose accumulation was delayed 6 h and raffinose accumulation was not observed in a starchless (pgm1) mutant. Changes of expression of five stress-induced transcripts in response to cold were similar in the wild type and in the pgm1 mutant. Three of five stress related transcripts had decreased expression when cold treatments were performed in the dark compared to the light. Therefore, starch hydrolysis may augment hexose and raffinose accumulations during the first 24 h after a cold shock and a partial cold stress response was observed in Arabidopsis during cold treatments in the dark.

Characterization of membrane protein reconstitution in LUVs of different lipid composition by fluorescence anisotropy

A major requirement to perform structural studies with membrane proteins is not only to define efficient reconstitution protocols, that assure a high incorporation degree in preformed liposomes, but also a protein directionality and topology that mimics its in vivo conditions. For this kind of studies, protein reconstitution in membranes systems via a detergent-mediated pathway is usually successfully adopted, since detergents are generally used in the initial isolation and purification of membrane proteins. In this study we report the reconstitution of OmpF in preformed DMPC and E. coli liposomes using two different techniques for detergent removal: (1) exclusion chromatography and (2) incubation with detergent-adsorbing beads. The incorporation degree was determined by bicinchoninic acid assay and fluorescence anisotropy was used to determine OmpF effect on the structural order of membrane lipids. These results show that protein insertion in membranes depends both on the technique used to remove detergent and on the lipids used to prepare the liposomes. Furthermore, it is possible to state that although the insertion is directly related to the size distributions of proteoliposomes, it could be efficiently recognized by steady-state fluorescence anisotropy. This technique, more popular among cell biologists, can be a very practical and straightforward alternative to DLS to confirm membrane protein insertion.

New insight into the structure and regulation of the plant vacuolar H+-ATPase

Plant cells are characterized by a highly active secretory system that includes the large central vacuole found in most differentiated tissues. The plant vacuolar H+-ATPase plays an essential role in maintaining the ionic and metabolic gradients across endomembranes, in activating transport processes and vesicle dynamics, and, hence, is indispensable for plant growth, development, and adaptation to changing environmental conditions. The review summarizes recent advances in elucidating the structure, subunit composition, localization, and regulation of plant V-ATPase. Emerging knowledge on subunit isogenes from Arabidopsis and rice genomic sequences as well as from Mesembryanthemum illustrates another level of complexity, the regulation of isogene expression and function of subunit isoforms. To this end, the review attempts to define directions of future research on plant V-ATPase.

Modulation of proton pumping across proteoliposome membranes reconstituted with tonoplast H(+)-ATPase from cultured rice (Oryza sativa L. var. Boro) cells by acyl steryl glucoside and steryl glucoside

Tonoplast H(+)-ATPase purified from cultured rice cells (Oryza sativa L. var. Boro) was reconstituted into asolectin liposomes containing steryl glucoside (SG) or acyl steryl glucoside (ASG), and the effects of SG and ASG on proton pumping, ATP-hydrolysis activity and proton permeability of the proteoliposome membranes were investigated. In the proteoliposomes containing 10 mol% SG, proton pumping and ATP-hydrolysis activity were increased to around 140% of those in SG-free proteoliposomes. In the proteoliposomes containing ASG, proton pumping and ATP-hydrolysis activity were decreased to one-tenth of those in ASG-free proteoliposomes at 15 mol% ASG; however, activity increased again slightly in the range between 20 and 40 mol% ASG. The change in proton pumping across the proteoliposome membrane is not due to a change of proteoliposome size nor to the location of the catalytic site of the tonoplast H(+)-ATPase in the proteoliposomes. SG and ASG also reduced the passive proton permeability of the proteoliposomes. These results show that SG and ASG modulate proton pumping across the tonoplast toward stimulation and depression, respectively, and they reduce the passive proton permeability of the tonoplast.

Characterization of an Arabidopsis cDNA encoding a subunit of serine palmitoyltransferase, the initial enzyme in sphingolipid biosynthesis

Serine palmitoyltransferase (SPT; EC 2.3.1.50) catalyzes the condensation of serine with palmitoyl-CoA to form 3-ketosphinganine in the first step of de novo sphingolipid biosynthesis. In this study, we describe the cloning and functional characterization of a cDNA from Arabidopsis thaliana encoding the LCB2 subunit of SPT. The Arabidopsis LCB2 (AtLCB2) cDNA contains an open reading frame of 1,467 nucleotides, encoding 489 amino acids. The predicted polypeptide contains three transmembrane helices and a highly conserved motif involved in pyridoxal phosphate binding. Expression of this open reading frame in the Saccharomyces cerevisiae mutant strains defective in SPT activity resulted in the expression of a significant level of sphinganine, suggesting that AtLCB2 cDNA encodes SPT. Southern blot analysis and inspection of the complete Arabidopsis genome sequence database suggest that there is a second LCB2-like gene in Arabidopsis. Expression of a green fluorescent protein (GFP) fusion product in suspension-cultured tobacco BY-2 cells showed that AtLCB2 is localized to the endoplasmic reticulum. AtLCB2 cDNA may be used to study how sphingolipid synthesis is regulated in higher plants.

Significance of the V-type ATPase for the adaptation to stressful growth conditions and its regulation on the molecular and biochemical level

Two electrogenic H(+)-pumps, the vacuolar type H(+)-ATPase (V-ATPase) and the vacuolar pyrophosphatase, coexist at membranes of the secretory pathway of plants. The V-ATPase is the dominant H(+)-pump at endomembranes of most plant cells, both in terms of protein amount and, frequently, also in activity. The V-ATPase is indispensable for plant growth under normal conditions due to its role in energizing secondary transport, maintenance of solute homeostasis and, possibly, in facilitating vesicle fusion. Under stress conditions such as salinity, drought, cold, acid stress, anoxia, and excess heavy metals in the soil, survival of the cells depends strongly on maintaining or adjusting the activity of the V-ATPase. Regulation of gene expression and activity are involved in adapting the V-ATPase on long- and short-term bases. The mechanisms known to regulate the V-ATPase are summarized in this paper with an emphasis on their implications for growth and development under stress.

Reversible redox control of plant vacuolar H+-ATPase activity is related to disulfide bridge formation in subunit E as well as subunit A

The plant vacuolar proton pump can be subjected to reversible redox regulation in vitro. The redox-dependent activity change involves disulfide bridge formation not only in Vatp A, as reported for bovine V-ATPase, but also in the stalk subunit Vatp E. Microsomal membranes isolated from barley leaves were analysed for their activity of bafilomycin-sensitive ATP hydrolysis and proton pumping using quinacrine fluorescence quenching in vesicle preparations. ATP hydrolysis and proton pumping activity were inhibited by H2O2. H2O2-deactivated ATPase was reactivated by cysteine and glutathione. The glutathione concentration needed for half maximal reactivation was 1 mmol l-1. The activity loss was accompanied by shifts in electrophoretic mobility of Vatp A and E which were reversed upon reductive reactivation. The redox-dependent shift was also seen with recombinant Vatp E, and was absent following site-directed mutagenesis of either of the two cys residues conserved throughout all plant Vatp E sequences. V-ATPase was also inhibited by oxidized thioredoxin. These results support the hypothesis that tuning of vacuolar ATPase activity can be mediated by redox control depending on the metabolic requirements.

TONOPLAST TRANSPORTERS: Organization and Function

Regulation of the contents and volume of vacuoles in plant cells depends on the coordinated activities of transporters and channels located in the tonoplast (vacuolar membrane). The three major components of the tonoplast are two proton pumps, the vacuolar H+-ATPase (V-ATPase) and H+-pyrophosphatase (V-PPase), and aquaporins. The tertiary structure of the V-ATPase complex and properties of its subunits have been characterized by biochemical and genetic techniques. These studies and a comparison with the F-type ATPase have enabled estimation of the dynamics of V-ATPase activity during catalysis. V-PPase, a simple proton pump, has been identified and cloned from various plant species and other organisms, such as algae and phototrophic bacteria, and functional motifs of the enzyme have been determined. Aquaporin, serving as the water channel, is the most abundant protein in the tonoplast in most plants. A common molecular architecture of aquaporins in mammals and plants has been determined by two-dimensional crystallographic analysis. Furthermore, recent molecular biological studies have revealed several other types of tonoplast transporters, such as the Ca2+-ATPase, Ca2+/H+ antiporter and Na+/H+ antiporter. Many other transporters and channels in the tonoplast remain to be identified; their activities have already been detected. This review presents an overview of the field and discusses recent findings on the tonoplast protein components that have been identified and their physiological consequences.

Modulation in the activity of purified tonoplast H+-ATPase by tonoplast glycolipids prepared from cultured rice (Oryza sativa L. var. Boro) cells

Glycolipids, phospholipids, and neutral lipids were extracted from the tonoplast fraction of cultured rice cells (Oryza sativa L. var. Boro). Acyl steryl glucoside (ASG) and glucocerebroside (GlcCer) were also prepared from this fraction. We determined the effects of these tonoplast lipids on the activity of H+-ATPase which was delipidated and purified from the tonoplast fraction. Exogenously added tonoplast phospholipids stimulated the activity of purified tonoplast H+-ATPase, but tonoplast glycolipids did not. When tonoplast glycolipids or tonoplast ASG was added in the presence of tonoplast phospholipids, they decreased the phospholipid-induced activation of the tonoplast H+-ATPase; tonoplast GlcCer only caused a small decrease. Steryl glucoside (SG) did not cause any decrease in this activation. Phospholipids, ASG, and GlcCer made up 35 mol%, 20 mol% and 7 mol% of the total lipids of the tonoplast fraction of cultured rice cells, respectively, and these glycolipid levels were enough to depress the phospholipid-induced activation of the tonoplast H+-ATPASE: These results revealed that H+-ATPase activity in the tonoplast may be modulated toward activation and depression by tonoplast phospholipids and glycolipids, respectively. The acylation of SG would be responsible for the depression in the phospholipid-induced H+-ATPase activity.