Molecular cloning and characterization of six cDNAs expressed during glucose starvation in excised maize (Zea mays L.) root tips

Water Stress and Black Cutworm Feeding Modulate Plant Response in Maize Colonized by Metarhizium robertsii

Plants face many environmental challenges and have evolved different strategies to defend against stress. One strategy is the establishment of mutualistic associations with endophytic microorganisms which contribute to plant defense and promote plant growth. The fungal entomopathogen Metarhizium robertsii is also an endophyte that can provide plant-protective and growth-promoting benefits to the host plant. We conducted a greenhouse experiment in which we imposed stress from deficit and excess soil moisture and feeding by larval black cutworm (BCW), Agrotis ipsilon, to maize plants that were either inoculated or not inoculated with M. robertsii (Mr). We evaluated plant growth and defense indicators to determine the effects of the interaction between Mr, maize, BCW feeding, and water stress. There was a significant effect of water treatment, but no effect of Mr treatment, on plant chlorophyl, height, and dry biomass. There was no effect of water or Mr treatment on damage caused by BCW feeding. There was a significant effect of water treatment, but not Mr treatment, on the expression of bx7 and rip2 genes and on foliar content of abscisic acid (ABA), 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA), and gibberellin 19 (GA19), whereas GA53 was modulated by Mr treatment. Foliar content of GA19 and cis-Zeatin (cZ) was modulated by BCW feeding. In a redundancy analysis, plant phenology, plant nutrient content, and foliar DIMBOA and ABA content were most closely associated with water treatments. This study contributes toward understanding the sophisticated stress response signaling and endophytic mutualisms in crops.

Comparative Transcriptome Profiling in a Segregating Peach Population with Contrasting Juiciness Phenotypes

Cold storage of fruit is one of the methods most commonly employed to extend the postharvest lifespan of peaches ( Prunus persica (L.) Batsch). However, fruit quality in this species is affected negatively by mealiness, a physiological disorder triggered by chilling injury after long periods of exposure to low temperatures during storage and manifested mainly as a lack of juiciness, which ultimately modifies the organoleptic properties of peach fruit. The aim of this study was to identify molecular components and metabolic processes underlying mealiness in susceptible and nonsusceptible segregants. Transcriptome and qRT-PCR profiling were applied to individuals with contrasting juiciness phenotypes in a segregating F₂ population. Our results suggest that mealiness is a multiscale phenomenon, because juicy and mealy fruit display distinctive reprogramming processes affecting translational machinery and lipid, sugar, and oxidative metabolism. The candidate genes identified may be useful tools for further crop improvement.

The Novel Gene VpPR4-1 from Vitis pseudoreticulata Increases Powdery Mildew Resistance in Transgenic Vitis vinifera L

Pathogenesis-related proteins (PRs) can lead to increased resistance of the whole plant to pathogen attack. Here, we isolate and characterize a PR-4 protein (VpPR4-1) from a wild Chinese grape Vitis pseudoreticulata which shows greatly elevated transcription following powdery mildew infection. Its expression profiles under a number of abiotic stresses were also investigated. Powdery mildew, salicylic acid, and jasmonic acid methyl ester significantly increased the VpPR4-1 induction while NaCl and heat treatments just slightly induced VpPR4-1 expression. Abscisic acid and cold treatment slightly affected the expression level of VpPR4-1. The VpPR4-1 gene was overexpressed in 30 regenerated V. vinifera cv. Red Globe via Agrobacterium tumefaciens-mediated transformation and verified by the Western blot. The 26 transgenic grapevines exhibited higher expression levels of PR-4 protein content than wild-type vines and six of them were inoculated with powdery mildew which showed that the growth of powdery mildew was repressed. The powdery mildew-resistance of Red Globe transformed with VpPR4-1 was enhanced inoculated with powdery mildew. Moreover, other powdery mildew resistant genes were associated with feedback regulation since VpPR4-1 is in abundance. This study demonstrates that PR-4 protein in grapes plays a vital role in defense against powdery mildew invasion.

High-resolution structure of a papaya plant-defense barwin-like protein solved by in-house sulfur-SAD phasing

The first crystal structure of a barwin-like protein, named carwin, has been determined at high resolution by single-wavelength anomalous diffraction (SAD) phasing using the six intrinsic S atoms present in the protein. The barwin-like protein was purified from Carica papaya latex and crystallized in the orthorhombic space group P212121. Using in-house Cu Kα X-ray radiation, 16 cumulative diffraction data sets were acquired to increase the signal-to-noise level and thereby the anomalous scattering signal. A sequence-database search on the papaya genome identified two carwin isoforms of 122 residues in length, both containing six S atoms that yield an estimated Bijvoet ratio of 0.93% at 1.54 Å wavelength. A systematic analysis of data quality and redundancy was performed to assess the capacity to locate the S atoms and to phase the data. It was observed that the crystal decay was low during data collection and that successful S-SAD phasing could be obtained with a relatively low data multiplicity of about 7. Using a synchrotron source, high-resolution data (1 Å) were collected from two different crystal forms of the papaya latex carwin. The refined structures showed a central β-barrel of six strands surrounded by several α-helices and loops. The β-barrel of carwin appears to be a common structural module that is shared within several other unrelated proteins. Finally, the possible biological function of the protein is discussed.

Characterization of chilling-shock responses in four genotypes of Miscanthus reveals the superior tolerance of M. x giganteus compared with M. sinensis and M. sacchariflorus

BACKGROUND AND AIMS

The bioenergy grass Miscanthus is native to eastern Asia. As Miscanthus uses C4 photosynthesis, the cooler temperatures experienced in much of northern Europe are expected to limit productivity. Identification of genetic diversity in chilling tolerance will enable breeders to generate more productive varieties for these cooler regions. Characterizing the temporal relationships between photosynthesis, carbohydrate and molecular expression of relevant genes is key to understanding genotypic differences in tolerance or sensitivity.

METHODS

To characterize chilling responses in four Miscanthus genotypes, plants were exposed to a sudden reduction in temperature. The genotypes studied comprised of two M. sinensis, one M. sacchariflorus and one inter-species hybrid, M. × giganteus. Changes in photosynthesis (Asat), carbohydrate composition and the expression of target transcripts were observed following chilling-shock. After 4 d the decline in leaf elongation rate (LER) in the different genotypes was measured.

RESULTS

Following chilling-shock the greatest decline in Asat was observed in M. sacchariflorus and one M. sinensis genotype. Carbohydrate concentrations increased in all genotypes following chilling but to a lesser extent in M. sacchariflorus. Two stress inducible genes were most highly expressed in the genotypes that experienced the greatest declines in Asat and LER. Miscanthus × giganteus retained the highest Asat and was unique in exhibiting no decline in LER following transfer to 12 °C.

CONCLUSIONS

Miscanthus × giganteus exhibits a superior tolerance to chilling shock than other genotypes of Miscanthus. The absence of sucrose accumulation in M. sacchariflorus during chilling-shock suggests an impairment in enzyme function. A candidate transcription factor, MsCBF3, is most highly expressed in the most sensitive genotypes and may be a suitable molecular marker for predicting chilling sensitivity.

Isolation and expression analysis of cDNAs that are associated with alternate bearing in Olea europaea L. cv. Ayvalık

Background

Olive cDNA libraries to isolate candidate genes that can help enlightening the molecular mechanism of periodicity and / or fruit production were constructed and analyzed. For this purpose, cDNA libraries from the leaves of trees in “on year” and in “off year” in July (when fruits start to appear) and in November (harvest time) were constructed. Randomly selected 100 positive clones from each library were analyzed with respect to sequence and size. A fruit-flesh cDNA library was also constructed and characterized to confirm the reliability of each library’s temporal and spatial properties.

Results

Quantitative real-time RT-PCR (qRT-PCR) analyses of the cDNA libraries confirmed cDNA molecules that are associated with different developmental stages (e. g. “on year” leaves in July, “off year” leaves in July, leaves in November) and fruits. Hence, a number of candidate cDNAs associated with “on year” and “off year” were isolated. Comparison of the detected cDNAs to the current EST database of GenBank along with other non - redundant databases of NCBI revealed homologs of previously described genes along with several unknown cDNAs. Of around 500 screened cDNAs, 48 cDNA elements were obtained after eliminating ribosomal RNA sequences. These independent transcripts were analyzed using BLAST searches (cutoff E-value of 1.0E-5) against the KEGG and GenBank nucleotide databases and 37 putative transcripts corresponding to known gene functions were annotated with gene names and Gene Ontology (GO) terms. Transcripts in the biological process were found to be related with metabolic process (27%), cellular process (23%), response to stimulus (17%), localization process (8.5%), multicellular organismal process (6.25%), developmental process (6.25%) and reproduction (4.2%).

Conclusions

A putative P450 monooxigenase expressed fivefold more in the “on year” than that of “off year” leaves in July. Two putative dehydrins expressed significantly more in “on year” leaves than that of “off year” leaves in November. Homologs of UDP – glucose epimerase, acyl - CoA binding protein, triose phosphate isomerase and a putative nuclear core anchor protein were significant in fruits only, while a homolog of an embryo binding protein / small GTPase regulator was detected in “on year” leaves only. One of the two unknown cDNAs was specific to leaves in July while the other was detected in all of the libraries except fruits. KEGG pathway analyses for the obtained sequences correlated with essential metabolisms such as galactose metabolism, amino sugar and nucleotide sugar metabolisms and photosynthesis. Detailed analysis of the results presents candidate cDNAs that can be used to dissect further the genetic basis of fruit production and / or alternate bearing which causes significant economical loss for olive growers.

Root proteases: reinforced links between nitrogen uptake and mobilization and drought tolerance

Integral subcellular and cellular functions ranging from gene expression, protein targeting and nutrient supply to cell differentiation and cell death require proteases. Plants have unique organelles such as chloroplasts composed of unique proteins that carry out the unique process of photosynthesis. Hence, along with proteases common across kingdoms, plants contain unique proteases. Improved knowledge on proteases can lead to a better understanding of plant development, differentiation and death. Because of their importance in multiple processes, plant proteases are actively studied. However, root proteases specifically are not as well studied. The associated rhizosphere, organic matter and/or inorganic matter make roots a difficult system. Yet recent research conclusively demonstrated the occurrence of endocytosis of proteins, peptides and even microbes by root cells, which, hitherto known for specialized pathogenesis or symbiosis, was unsuspected for nutrient uptake. These results reinforced the importance of root proteases in endocytosis or root exudate-mediated nutrient uptake. Rhizoplane, rhizosphere or in planta protease action on proteins, peptides and microbes generates sources of nitrogen, especially during abiotic stresses such as drought. This article highlights the recent research on root proteases for nitrogen uptake and the connection of the two to drought-tolerance mechanisms. Drought-induced proteases in rice roots, as known from rice expression databases, are discussed for future research on certain M50, Deg, FtsH, AMSH and deubiquitination proteases. The recent emphasis on linking drought and plant hydraulics to nutrient metabolism is illustrated and connected to the value of a systematic study of root proteases in crop improvement.

Characterization of a pathogenesis-related protein 4 (PR-4) induced in Capsicum chinense L3 plants with dual RNase and DNase activities

Resistance conferred by the L(3) gene is active against most of the tobamoviruses, including the Spanish strain (PMMoV-S), a P(1),(2) pathotype, but not against certain strains of pepper mild mottle virus (PMMoV), termed as P(1),(2),(3) pathotype, such as the Italian strain (PMMoV-I). PMMoV-S induces a hypersensitive reaction (HR) in C. chinense PI159236 plant leaves with the formation of necrotic local lesions and restriction of the virus at the primary infection sites. In this paper, a C. chinense PR-4 protein induced during both the compatible and the incompatible interactions has been identified. It was strongly associated with HR induction and to a lesser extent with the compatible interaction, but only in the later stages of infection. Moreover, it was found to accumulate during the necrogenic reaction induced by Potato virus X. The C. chinense PR-4 protein belongs to the PR-4 protein subgroup II, based on the absence of a hevein domain. Furthermore, it is shown that the purified protein does not have chitinase activity, as previously proposed for PR-4 proteins. Instead, it has both RNase and DNase activity, although its contribution to the bulk activity of nucleases in infected plants is very low.

Identification and expression analysis of genes in response to high-salinity and drought stresses in Brassica napus

High salinity and drought are the major abiotic stresses that adversely affect plant growth and agricultural productivity. To investigate genes that are involved in response to abiotic stresses in Brassica napus, a comprehensive survey of genes induced by high-salinity and drought stresses was done by macroarray analysis. In total, 536 clones were identified to be putative high-salinity-or drought-responsive genes. Among them, 172 and 288 clones are detected to be putative high-salinity- and drought-inducible genes, whereas 141 and 189 are candidates for high-salinity- and drought-suppressed genes, respectively. The functional classification of these genes are indicated that belonged to gene families encoding metabolic enzymes, regulatory factors, components of signal transduction, hormone responses, some abiotic stresses-related proteins, and other processes related to growth and development of B. napus. From the upregulated candidate genes, some interested genes were further demonstrated to be high-salinity- or/and drought-induced expression by real-time quantitative RT-PCR analysis. The experimental results also revealed that some genes may function in abscisic acid-dependent signaling pathway related to drought or salinity stress. Collectively, the data presented in this study will facilitate the understanding of molecular mechanism of B. napus in response to high-salinity and drought stresses, and also provide us the basis of effective genetic engineering strategies for improving stress tolerance of B. napus.

Genome-wide identification, expression and chromosomal location of the genes encoding chitinolytic enzymes in Zea mays

Chitinolytic enzymes are important pathogenesis and stress related proteins. We identified 27 putative genes encoding endochitinases in the maize genome via in silico techniques and four exochitinases. Only seven of the endochitinases and segments of the exochitinases were heretofore known. The endochitinases included members of family 19 chitinases (classes I-IV of PR3, II of PR4) and members of family 18 chitinases (class III of PR8). Some similar enzymes were detected on adjacent regions of the same chromosome, and seem to result from duplication events. Most of the genes expressed were identified from EST libraries from plants exposed to biotic or abiotic stresses but also from libraries from tissues not exposed to stresses. We isolated proteins from seedlings of maize in the presence or absence of the symbiotic root colonizing fungus Trichoderma harzianum strain T22, and analyzed the activity of chitinolytic enzymes using an in-gel activity assay. The activity bands were identified by LC/MS/MS using the database from our in silico study. The identities of the enzymes changed depending on whether or not T22 was present. One activity band of about 95 kDa appeared to be a heterodimer between an exochitinase and any of several different endochitinases. The identity of the endochitinase component appeared to be dependent upon treatment.

A metabolic study of the regulation of proteolysis by sugars in maize root tips: effects of glycerol and dihydroxyacetone

Sugars, the main growth substrates of plants, act as physiological signals in the complex regulatory network of sugar metabolism. To investigate the function of different glycolytic steps in sugar sensing and signaling we compared the effects of carbon starvation with those of glucose, glycerol and dihydroxyacetone on carbon metabolism, proteolysis, and protease expression in excised maize (Zea mays L.) root tips. Respiration, soluble proteins, protein turnover and proteolytic activities were monitored as a function of time, along with in vitro and in vivo analysis of a variety of metabolites (sugars, amino and organic acids, phosphoesters, adenine nucleotides...) using (13)C, (31)P and (1)H NMR spectroscopy. Our results indicate that, in maize root tips, endopeptidase activities and protease expression are induced in response to a decrease in carbon supply to the upper part of the glycolytic pathway, i.e. at the hexokinase step. Proteolysis would be controlled downstream glycolysis, probably at the level of the respiratory substrate supply to mitochondria.

Senescence is accelerated, and several proteases are induced by carbon "feast" conditions in barley (Hordeum vulgare L.) leaves

Leaf senescence is characterized by nitrogen remobilization to developing seeds of annual plants, or surviving organs of perennial species. It has been demonstrated that high carbohydrate levels (carbon "feast") are associated with the onset of the senescence process. Therefore, the development of model systems allowing the manipulation of leaf carbohydrates constitutes a logical first step in the investigation of processes important during early phases of senescence, such as plastidial protein degradation. In this study, sugar accumulation was induced either by the incubation of excised, mature barley (Hordeum vulgare L.) leaves under relatively strong light, or by the interruption of sieve tubes at the base of the leaf lamina by "steam-girdling". Accelerated chlorophyll degradation and net proteolysis confirmed successful senescence induction in both model systems, but suggested that girdled leaves are more useful than excised leaves to study proteolysis. Activities or transcript levels of several proteolytic enzymes, including plastidial (aminopeptidases, Clp protease), cytosolic (proteasome) and vacuolar (thiol proteases, an aspartic protease and a serine carboxypeptidase) proteases were clearly induced under these conditions; some of these genes also reacted to other stimuli such as leaf excision. The most interesting finding was the specific induction of a carboxypeptidase gene (cp-mIII) in girdled leaves accumulating high carbohydrate levels. As a previous study from our laboratory, using a genetic approach, has indicated that one or several carboxypeptidases are involved in leaf N remobilization, the detailed characterization of cp-mIII (and, possibly, closely related genes) may considerably improve our understanding of whole-plant N recycling.

Identification of genes responsive to brown planthopper Nilaparvata lugens Stal (Homoptera: Delphacidae) feeding in rice

The brown planthopper (Nilaparvata lugens Stal) is a rice (Oryza sativa L.)-specific herbivore that ingests assimilates from the phloem, which leads to stunted growth and therefore poor yield of rice. In this study, 27 planthopper-responsive genes in rice plants were isolated by suppression subtractive hybridization (SSH), and confirmed by filter cDNA array hybridization and northern blot analysis. Among these transcripts, 25 were induced and 2 were suppressed by N. lugens feeding. The responsive genes are involved in photosynthesis, macromolecule degradation, signal transduction and defenses against stress and pathogens. Overall, genes involved in macromolecule degradation and plant defenses were up-regulated, whereas those involved in photosynthesis and cell growth were down-regulated. Our data suggest that leaf senescence is most likely activated in the susceptible rice plants damaged by N. lugens insects.

Inhibition of phosphoinositide-specific phospholipase C results in the induction of pathogenesis-related genes in soybean

The inositol 1,4,5-trisphosphate (IP3) content is decreased in soybean cells following infection with Pseudomonas syringae pv. glycinea (Psg). In this investigation, a differential display approach was applied to isolate soybean genes that are transcriptionally up-regulated by the inhibition of phosphoinositide-specific phospholipase C (PI-PLC) activity and to study if the transcription of those genes is altered following Psg infection. Four genes, transcriptionally activated following treatment with the PI-PLC-specific inhibitor U-73122, were cloned. Three of the four genes were induced following infection with Psg. The transcripts of a hydrolase homologue (GmHy) were induced in the incompatible but not compatible soybean-Psg interaction. The transcripts of a putative ascorbate oxidase gene (GmAO) were induced in both compatible and incompatible interactions. GmHy and GmAO may represent new classes of pathogenesis-related genes. In addition to these two novel genes, homologues of PR-10 and polygalacturonase inhibitor protein (GmPR10 and GmPGIP, respectively) were identified. These two genes have previously been reported as pathogenesis-related. Transcripts of GmPR-10, but not GmPGIP, were induced in both compatible and incompatible soybean-Psg interactions. Induction of these genes, except for GmPGIP, following inhibition of PI-PLC by either the U-73122 treatment or bacterial infection suggests that PI-PLC may negatively regulate the expression of defence genes.

Plant and mycorrhizal regulation of rhizodeposition

The loss of carbon from roots (rhizodeposition) and the consequent proliferation of microorganisms in the surrounding soil, coupled with the physical presence of a root and processes associated with nutrient uptake, gives rise to a unique zone of soil called the rhizosphere. In this review, we bring together evidence to show that roots can directly regulate most aspects of rhizosphere C flow either by regulating the exudation process itself or by directly regulating the recapture of exudates from soil. Root exudates have been hypothesized to be involved in the enhanced mobilization and acquisition of many nutrients from soil or the external detoxification of metals. With few exceptions, there is little mechanistic evidence from soil-based systems to support these propositions. We conclude that much more integrated work in realistic systems is required to quantify the functional significance of these processes in the field. We need to further unravel the complexities of the rhizosphere in order to fully engage with key scientific ideas such as the development of sustainable agricultural systems and the response of ecosystems to climate change. Contents I. Introduction 460 II. What is rhizodeposition? 460 III. Regulation of rhizodeposition 460 IV. How large is the root exudation C flux? 463 V. How responsive is the root exudation C flux? 463 VI. How responsive is the microbial community to root exudation? 464 VII. The role of root exudates in nutrient acquisition 464 VIII. Mycorrhizal fungi and rhizodeposition 471 IX. Future thoughts 474 Acknowledgements 474 References 474.

Transcriptional regulation of sorghum defense determinants against a phloem-feeding aphid

When attacked by a phloem-feeding greenbug aphid (Schizaphis graminum), sorghum (Sorghum bicolor) activates jasmonic acid (JA)- and salicylic acid (SA)-regulated genes, as well as genes outside known wounding and SA signaling pathways. A collection of 672 cDNAs was obtained by differential subtraction with cDNAs prepared from sorghum seedlings infested by greenbug aphids and those from uninfested seedlings. Subsequent expression profiling using DNA microarray and northern-blot analyses identified 82 transcript types from this collection responsive to greenbug feeding, methyl jasmonate (MeJA), or SA application. DNA sequencing analyses indicated that these encoded proteins functioning in direct defense, defense signaling, oxidative burst, secondary metabolism, abiotic stress, cell maintenance, and photosynthesis, as well as proteins of unknown function. In response to insect feeding, sorghum increased transcript abundance of numerous defense genes, with some SA-dependent pathogenesis-related genes responding to greenbug more strongly than to SA. In contrast, only weak induction of MeJA-regulated defense genes was observed after greenbug treatment. However, infestation tests confirmed that JA-regulated pathways were effective in plant defense against greenbugs. Activation of certain transcripts exclusively by greenbug infestation was observed, and may represent unique signal transduction events independent of JA- and SA-regulated pathways. Results indicate that plants coordinately regulate defense gene expression when attacked by phloem-feeding aphids, but also suggest that aphids are able to avoid triggering activation of some otherwise potentially effective plant defensive machinery, possibly through their particular mode of feeding.

Physiological, biochemical and molecular analysis of sugar-starvation responses in tomato roots

Two-month-old tomato plants were submitted to day/night cycles and to prolonged darkness in order to investigate the physiological and biochemical response to sugar starvation in sink organs. Roots appeared particularly sensitive to the cessation of photosynthesis, as revealed by the reduction of the growth rate and the decline of the carbohydrate and protein content. Therefore, excised tomato roots were used as a model to deepen the characterization of sugar starvation symptoms. In excised roots, the endogenous sugars were rapidly exhausted and significant degradation of protein was observed. Glutamine and asparagine accounted for most of the nitrogen released by protein breakdown. Respiration declined and proliferation- and growth-associated genes were repressed soon after the beginning of the sugar depletion. Among the genes studied, only the gene encoding asparagine synthetase was strongly induced. All the starvation symptoms were reversible when the roots were resupplied with sugar. When the culture conditions deteriorated, the metabolic and molecular changes led to the triggering of apoptosis of the root cells.

Identification of dehydration-responsive cysteine proteases during post-harvest senescence of broccoli florets

Harvest-induced senescence of broccoli results in tissue wilting and sepal chlorosis. As senescence progresses, chlorophyll and protein levels in floret tissues decline and endo-protease activity (measured with azo-casein) increases. Protease activity increased from 24 h after harvest for tissues held in air at 20 degrees C. Activity was lower in floret tissues from branchlets that had been held in solutions of sucrose (2% w/v) or under high carbon dioxide, low oxygen (10% CO(2), 5% O(2)) conditions. Four protease-active protein bands were identified in senescing floret tissue by zymography, and the use of chemical inhibitors of protease action suggests that some 44% of protease activity in senescing floret tissue 72 h after harvest is due to the action of cysteine and serine proteases. Four putative cysteine protease cDNAs have been isolated from broccoli floret tissue (BoCP1, BoCP2, BoCP3, BoCP4). The cDNAs are most similar (73-89% at the amino acid level) to dehydration-responsive cysteine proteases previously isolated from Arabidopsis thaliana (RD19, RD21). The mRNAs encoded by the broccoli cDNAs are expressed in floret tissue during harvest-induced senescence with mRNA accumulating within 6 h of harvest for BoCP1, 12 h of harvest for BoCP4 and within 24 h of harvest for BoCP2 and BoCP3. Induction of the cDNAs is differentially delayed when broccoli branchlets are held in solutions of water or sucrose. In addition, the expression of BoCP1 and BoCP3 is inhibited in tissue held in atmospheres of high carbon dioxide/low oxygen (10% CO(2), 5% O(2)). The putative cysteine protease mRNAs are expressed before measurable increases in endo-protease activity, loss of protein, chlorophyll or tissue chlorosis.

In plants, 3-o-methylglucose is phosphorylated by hexokinase but not perceived as a sugar

In plants, sugars are the main respiratory substrates and important signaling molecules in the regulation of carbon metabolism. Sugar signaling studies suggested that sugar sensing involves several key components, among them hexokinase (HXK). Although the sensing mechanism of HXK is unknown, several experiments support the hypothesis that hexose phosphorylation is a determining factor. Glucose (Glc) analogs transported into cells but not phosphorylated are frequently used to test this hypothesis, among them 3-O-methyl-Glc (3-OMG). The aim of the present work was to investigate the effects and fate of 3-OMG in heterotrophic plant cells. Measurements of respiration rates, protein and metabolite contents, and protease activities and amounts showed that 3-OMG is not a respiratory substrate and does not contribute to biosynthesis. Proteolysis and lipolysis are induced in 3-OMG-fed maize (Zea mays L. cv DEA) roots in the same way as in sugar-starved organs. However, contrary to the generally accepted idea, phosphorous and carbon nuclear magnetic resonance experiments and enzymatic assays prove that 3-OMG is phosphorylated to 3-OMG-6-phosphate, which accumulates in the cells. Insofar as plant HXK is involved in sugar sensing, these findings are discussed on the basis of the kinetic properties because the catalytic efficiency of HXK isolated from maize root tips is five orders of magnitude lower for 3-OMG than for Glc and Man.

Molecular physiology of aquaporins in plants

In plants, membrane channels of the major intrinsic protein (MIP) super-family exhibit a high diversity with, for instance, 35 homologues in the model species Arabidopsis thaliana. As has been found in other organisms, plant MIPs function as membrane channels permeable to water (aquaporins) and in some cases to small nonelectrolytes. The aim of the present article is to integrate into plant physiology what has been recently learned about the molecular and functional properties of aquaporins in plants. Exhaustive compilation of data in the literature shows that the numerous aquaporin isoforms of plants have specific expression patterns throughout plant development and in response to environmental stimuli. The diversity of aquaporin homologues in plants can also be explained in part by their presence in multiple subcellular compartments. In recent years, there have been numerous reports that describe the activity of water channels in purified membrane vesicles, in isolated organelles or protoplasts, and in intact plant cells or even tissues. Altogether, these data suggest that the transport of water and solutes across plant membranes concerns many facets of plant physiology. Because of the high degree of compartmentation of plant cells, aquaporins may play a critical role in cell osmoregulation. Water uptake in roots represents a typical process in which to investigate the role of aquaporins in transcellular water transport, and the mechanisms and regulations involved are discussed.

Changes in the expression and the enzymic properties of the 20S proteasome in sugar-starved maize roots. evidence for an in vivo oxidation of the proteasome

The 20S proteasome (multicatalytic proteinase) was purified from maize (Zea mays L. cv DEA 1992) roots through a five-step procedure. After biochemical characterization, it was shown to be similar to most eukaryotic proteasomes. We investigated the involvement of the 20S proteasome in the response to carbon starvation in excised maize root tips. Using polyclonal antibodies, we showed that the amount of proteasome increased in 24-h-carbon-starved root tips compared with freshly excised tips, whereas the mRNA levels of alpha 3 and beta 6 subunits of 20S proteasome decreased. Moreover, in carbon-starved tissues, chymotrypsin-like and caseinolytic activities of the 20S proteasome were found to increase, whereas trypsin-like activities decreased. The measurement of specific activities and kinetic parameters of 20S proteasome purified from 24-h-starved root tips suggested that it was subjected to posttranslational modifications. Using dinitrophenylhydrazine, a carbonyl-specific reagent, we observed an increase in carbonyl residues in 20S proteasome purified from starved root tips. This means that 20S proteasome was oxidized during starvation treatment. Moreover, an in vitro mild oxidative treatment of 20S proteasome from non-starved material resulted in the activation of chymotrypsin-like, peptidyl-glutamyl-peptide hydrolase and caseinolytic-specific activities and in the inhibition of trypsin-like specific activities, similar to that observed for proteasome from starved root tips. Our results provide the first evidence, to our knowledge, for an in vivo carbonylation of the 20S proteasome. They suggest that sugar deprivation induces an oxidative stress, and that oxidized 20S proteasome could be associated to the degradation of oxidatively damaged proteins in carbon starvation situations.

Regulation of protein degradation and protease expression by mannose in maize root tips. Pi sequestration by mannose may hinder the study of its signaling properties

The effects of mannose (Man) and glucose (Glc) on central metabolism, proteolysis, and expression of the root starvation-induced protease (RSIP; F. James, R. Brouquisse, C. Suire, A. Pradet, P. Raymond [1996] Biochem J 320: 283-292) were investigated in maize (Zea mays L. cv DEA) root tips. Changes in metabolite concentrations (sugars, ester-phosphates, adenine nucleotides, and amino acids) were monitored using in vivo and in vitro (13)C- and (31)P-NMR spectroscopy, in parallel with the changes in respiration rates, protein contents, proteolytic activities, and RSIP amounts. The inhibition of proteolysis, the decrease in proteolytic activities, and the repression of RSIP expression triggered by Man, at concentrations usually used to study sugar signaling (2 and 10 mM), were found to be related to a drop of energy metabolism, primarily due to a Man-induced Pi sequestration. However, when supplied at low concentration (2 mM) and with the adequate phosphate concentration (30 mM), energy metabolism was restored and Man repressed proteolysis similarly to Glc, when provided at the same concentration. These results indicate that Man should be used with caution as a Glc analog to study signalization by sugars in plants because possible signaling effects may be hindered by Pi sequestration.

Dark-inducible genes from Arabidopsis thaliana are associated with leaf senescence and repressed by sugars

We have isolated 5 cDNA clones (din2, din6, din9, din10 and din11) corresponding to genes, the transcripts of which accumulated in leaves of Arabidopsis thaliana kept in the dark. These cDNA clones encode proteins similar to beta-glucosidase (EC 3.2.1.21, din2), asparagine synthetase (EC 6.3.5.4, din6), phosphomannose isomerase (EC 5.3.1.8, din9), seed imbibition protein (din10) and 2-oxoacid-dependent dioxygenases (din11). Accumulation of the transcripts from din6 and din10 occurred within 3 h after plants were transferred to darkness. The transcripts from din2, din9 and din11 were only detected after 24 h of dark treatment. We also observed the accumulation of the din transcripts in senescing leaves. Application of a photosynthesis inhibitor, 3-(3,4-dichlorophenyl)-1-1-dimethyl-urea, induced the expression of the din genes under illumination. Application of sucrose to detached leaves suppressed the accumulation of the din transcripts in the dark. These results indicate that expression of these genes partly depends on cellular sugar level. The sugar-modulated expression of the din genes suggests that dark-induced expression of these genes might be related to sugar starvation occurring in leaf cells in the dark, when the photosynthesis is hindered.

Sugar coordinately and differentially regulates growth- and stress-related gene expression via a complex signal transduction network and multiple control mechanisms

In plants, sugars are required to sustain growth and regulate gene expression. A large set of genes are either up- or down-regulated by sugars; however, whether there is a common mechanism and signal transduction pathway for differential and coordinated sugar regulation remain unclear. In the present study, the rice (Oryza sativa cv Tainan 5) cell culture was used as a model system to address this question. Sucrose and glucose both played dual functions in gene regulation as exemplified by the up-regulation of growth-related genes and down-regulation of stress-related genes. Sugar coordinately but differentially activated or repressed gene expression, and nuclear run-on transcription and mRNA half-life analyses revealed regulation of both the transcription rate and mRNA stability. Although coordinately regulated by sugars, these growth- and stress-related genes were up-regulated or down-regulated through hexokinase-dependent and/or hexokinase-independent pathways. We also found that the sugar signal transduction pathway may overlap the glycolytic pathway for gene repression. alpha-Amylase and the stress-related genes identified in this study were coordinately expressed under sugar starvation, suggesting a convergence of the nutritional and environmental stress signal transduction pathways. Together, our studies provide a new insight into the complex signal transduction network and mechanisms of sugar regulation of growth and stress-related genes in plants.

Multiple signaling pathways in gene expression during sugar starvation. Pharmacological analysis of din gene expression in suspension-cultured cells of Arabidopsis

We have identified many dark-inducible (din) genes that are expressed in Arabidopsis leaves kept in the dark. In the present study we addressed the question of how plant cells sense the depletion of sugars, and how sugar starvation triggers din gene expression in suspension-cultured cells of Arabidopsis. Depletion of sucrose in the medium triggered marked accumulation of din transcripts. Suppression of din gene expression by 2-deoxy-Glc, and a non-suppressive effect exerted by 3-O-methyl-Glc, suggested that sugar-repressible expression of din genes is mediated through the phosphorylation of hexose by hexokinase, as exemplified in the repression of photosynthetic genes by sugars. We have further shown that the signaling triggered by sugar starvation involves protein phosphorylation and dephosphorylation events, and have provided the first evidence that multiple pathways of protein dephosphorylation exist in sugar starvation-induced gene expression. An inhibitor of serine/threonine protein kinase, K-252a, inhibited din gene expression in sugar-depleted cells. Okadaic acid, which may preferentially inhibit type 2A protein phosphatases over type 1, enhanced the transcript levels of all din genes, except din6 and din10, under sugar starvation. Conversely, a more potent inhibitor of type 1 and 2A protein phosphatases, calyculin A, increased transcripts from din2 and din9, but decreased those from other din genes, in sugar-depleted cells. On the other hand, calyculin A, but not okadaic acid, completely inhibited the gene expression of chlorophyll a/b-binding protein under sugar starvation. These results indicate that multiple signaling pathways, mediated by different types of protein phosphatases, regulate gene expression during sugar starvation.

Immunolocalization of a cysteine protease in vacuoles, vesicles, and symbiosomes of pea nodule cells

PsCYP15A is a cysteine protease from pea (Pisum sativum L.). It was first recognized as an up-regulated transcript in wilted shoots and subsequently in root nodules containing Rhizobium. Proteolytic activity of PsCYP15A in nodule extracts is now reported following immunopurification with polyclonal antiserum raised against recombinant antigen. Western-blot analysis indicated two forms of PsCYP15A, a pro-form (approximately 38 kD) and a mature form (approximately 30 kD). Both forms were present in most tissue samples, but only the mature form was isolated from cell-fractionated symbiosomes containing nitrogen-fixing bacteroids. Immunolabeling of nodule sections showed localization of PsCYP15A antigen in large vacuolar bodies, cytoplasmic vesicles, and the perisymbiont space. Immunolabeling of tissue sections from wilted shoots also indicated the presence of PsCYP15A in vacuoles and cytoplasmic vesicles. This protease may be involved in the adaptation to changes in cell turgor, both in wilted shoots and in nodule tissue. Additionally, the protease may be involved in protein turnover in the symbiosome compartment.

Plasma membrane intrinsic proteins from maize cluster in two sequence subgroups with differential aquaporin activity

The transport of water through membranes is regulated in part by aquaporins or water channel proteins. These proteins are members of the larger family of major intrinsic proteins (MIPs). Plant aquaporins are categorized as either tonoplast intrinsic proteins (TIPs) or plasma membrane intrinsic proteins (PIPs). Sequence analysis shows that PIPs form several subclasses. We report on the characterization of three maize (Zea mays) PIPs belonging to the PIP1 and PIP2 subfamilies (ZmPIP1a, ZmPIP1b, and ZmPIP2a). The ZmPIP2a clone has normal aquaporin activity in Xenopus laevis oocytes. ZmPIP1a and ZmPIP1b have no activity, and a review of the literature shows that most PIP1 proteins identified in other plants have no or very low activity in oocytes. Arabidopsis PIP1 proteins are the only exception. Control experiments show that this lack of activity of maize PIP1 proteins is not caused by their failure to arrive at the plasma membrane of the oocytes. ZmPIP1b also does not appear to facilitate the transport of any of the small solutes tried (glycerol, choline, ethanol, urea, and amino acids). These results are discussed in relationship to the function and regulation of the PIP family of aquaporins.

Differential screening indicates a dramatic change in mRNA profiles during grape berry ripening. Cloning and characterization of cDNAs encoding putative cell wall and stress response proteins

We used differential screening to isolate ripening-associated cDNAs from a Shiraz grape (Vitis vinifera L.) berry cDNA library. A rapid increase in the mRNA levels of a number of cDNAs not present in unripe fruit occurred in grape berries at the onset of ripening. The putative translation products of some of these clones had homologs in other species that are involved in cell wall structure. These included four proline-rich proteins, a small protein that is similar to the non-catalytic, N-terminal domain of some pectin methylesterases, and two other glutamate-rich proteins. The remainder of the clones encoded putative stress response proteins. These included two thaumatin-like proteins, a metallothionein, a transcription factor, a cytochrome P450 enzyme, and proteins induced by water, sugar, and/or cold stress in other species. Many of the homologs of the grape cDNAs thought to be involved in cell wall structure or stress-related responses also accumulate in a developmental manner in other plants. This may indicate that the grape mRNAs accumulate in response to stresses such as the storage of high concentrations of sugars and rapid cell expansion, or they may accumulate as part of the ripening developmental program.

Purification and characterization of macrodontain I, a cysteine peptidase from unripe fruits of Pseudananas macrodontes (Morr.) harms (Bromeliaceae)

A new papain-like cysteine peptidase isolated from fruits of Pseudananas macrodontes (Morr.) Harms, a species closely related to pineapple (Ananas comosus L.), has been purified and characterized. The enzyme, named macrodontain I, is the main proteolytic component present in fruit extracts and was purified by acetone fractionation followed by anion-exchange chromatography. Separation was improved by selecting both an adequate pH value and a narrow saline gradient. Optimum pH range (more than 90% of maximum activity with casein) was achieved at pH 6.1-8.5. Homogeneity of the enzyme was confirmed by bidimensional electrophoresis and mass spectroscopy (MS). Molecular mass of the enzyme was 23,459 (MS) and its isoelectric point was 6.1. The alanine, glutamine, and tyrosine derivatives were strongly preferred when the enzyme was assayed on N-alpha-CBZ-l-amino acid p-nitrophenyl esters. The N-terminal sequence of macrodontain (by comparison with the N-terminus of 30 plant proteases with more than 50% homology) showed a great deal of sequence similarity to the other pineapple-stem-derived cysteine endopeptidases, being 85.7, 85. 2, and 77.8% identical to comosain, stem bromelain, and ananain, respectively. It seems clear that the Bromeliaceae endopeptidases are more closely related to each other than to other members of the papain family, suggesting relatively recent divergence.

Biotic and abiotic stress can induce cystatin expression in chestnut

A cysteine proteinase inhibitor (cystatin) from chestnut (Castanea sativa) seeds, designated CsC, has been previously characterized. Its antifungal, acaricide and inhibitory activities have allowed to involve CsC in defence mechanisms. The CsC transcription levels decreased during seed maturation and increased throughout germination, an opposite behavior to that shown by most phytocystatins. No inhibition of endogenous proteinase activity by purified CsC was found during the seed maturation or germination processes. CsC message accumulation was induced in chestnut leaves after fungal infection, as well as by wounding and jasmonic acid treatment. Induction in roots was also observed by the last two treatments. Furthermore, CsC transcript levels strongly raised, both in roots and leaves, when chestnut plantlets were subjected to cold- and saline-shocks, and also in roots by heat stress. All together, these data suggest that chestnut cystatin is not only involved in defence responses to pests and pathogen invasion, but also in those related to abiotic stress.

Molecular and biochemical characterization of the involvement of cyclin-dependent kinase A during the early development of tomato fruit

Following fruit set, the early development of tomato (Lycopersicon esculentum Mill.) fruit comprises two distinct phases: a cell division phase and a consecutive phase of cell expansion until the onset of ripening. In this study, we analyzed cytological and molecular changes characterizing these early phases of tomato fruit development. First we investigated the spatial and temporal regulation of the mitotic activity during fruit development. The DNA content of isolated nuclei from the different fruit tissues was determined by flow cytometry analysis. The results confirm the data of mitotic activity measurements and show that cell differentiation, leading to expanded cells, is characterized by endoreduplication. Second, we isolated two cDNAs, named Lyces;CDKA1 (accession no. Y17225) and Lyces;CDKA2 (accession no. Y17226), encoding tomato homologs of the cyclin-dependent kinase (CDK) p34(cdc2). Tomato CDKA gene expression was followed at both the transcriptional and translational levels during fruit development. The transcripts for Lyces;CDKA1 and Lyces;CDKA2 and the corresponding CDKA proteins are predominantly accumulated during the phase of cell division between anthesis and 5 d post anthesis (DPA). In whole fruits, the maximum CDK activity was obtained between 5 and 10 DPA. The determination of the kinase activity using protein extracts from the different fruit tissues was in agreement with mitotic activity analysis. It showed the particular disappearance of the activity in the gel tissue as early as 15 DPA. The overall data of CDK activity measurements suggest a strong post-translational regulation of CDK at the temporal and spatial levels during early tomato fruit development.

Structure and function of metal chelators produced by plants: the case for organic acids, amino acids, phytin, and metallothioneins

Plants produce a range of ligands for cadmium (Cd), copper (Cu), nickel (Ni), and zinc (Zn). Cd- and Zn-citrate complexes are prevalent in leaves, even though malate is more abundant. In the xylem sap moving from roots to leaves, citrate and histidine are the principal ligands for Cu, Ni, and Zn. Phosphorus-rich globular bodies in young roots are probably Zn-phytate. Metallothioneins (MTs) are cysteine (Cys)-rich ligands. Plants produce class II MTs (MT-IIs) which differ from the archetypal mammalian MT-I in the location and number of Cys. The Ec protein from wheat embryos has Cys in three domains, binds Zn, and disappears with seedling development. The first 59 amino acids have been sequenced for the protein. Fifty-eight genes for MT-IIs, from a range of plants and tissues, predict proteins with Cys in two domains. Most of the predicted proteins have not been isolated, and their metal binding is poorly documented. Three protein bands, corresponding to six MT genes, have been isolated from Arabidopsis, and the amino acids sequenced for nine fragments. The MT-IIIs are atypical, nontranslationally synthesized polypeptides with variously repeating gamma-glutamylcysteine units. Of the five families known, those with carboxy-terminal glycine are the most widespread among plants, algae, and certain yeasts. A heterogeneous grouping of these molecules form Cd-binding complexes with tetrahedral coordination and a Cd-sulfur interatomic distance of 2.52 A. One complex is cytosolic, the dominant one is vacuolar. Together, they can bind a large proportion of cellular Cd; other ligands may also function. Little is known about the counterpart situation for Cu and Zn.

RNA expression patterns of a type 2 metallothionein-like gene from rice

A type 2 metallothionein-like gene from rice, OsMT-2 (Oryza sativa metallothionein-like gene-2), was isolated in its cDNA form and sequenced. By northern analyses OsMT-2 expression was shown to be induced under stress by sucrose starvation, heat shock and, to a lesser extent, abscisic acid, but not excess metals, including copper. Its response to sucrose starvation was transient and different from OsMT-1, a type 1 metallothionein-like gene of rice inducible by copper. These results suggest that while OsMT-2 is also involved in cellular response to stress, its function may be complementary to that of OsMT-1.