Water-stress-induced heat tolerance in geranium leaf tissues: A possible linkage through stress proteins?

Evidence is accumulating in favor of a linkage at the cellular level between various abiotic stresses. We conducted a study to evaluate the effect of water stress on the heat tolerance of zonal geraniums, Pelargonium × hortorum cv. Evening Glow. Water stress was imposed by withholding irrigation until pots reached 30% (by weight) of well‐watered controls, and by maintaining the pot weight by additions of water for another 7 days. Leaf xylem water potential (XWP, MPa), relative water content (RWC. %), and heat‐stress tolerance (HST; LT₅₀, defined as the temperature causing half‐maximal % injury based on electrolyte leakage) were measured in control, stressed, and recovered plants. Proteins were extracted from the leaves following the above treatments, and SDS‐PAGE and immunoblotting were performed by using standard procedures. Immunoblots were probed with antibodies to dehydrin and 70‐kDa heat shock cognate (HSC70) proteins. Data indicate that XWP and RWC, respectively, were −0.378 MPa and 92.3% for control plants and −0.804 MPa and 78.6% for stressed plants. Water‐stressed plants exhibited a significant increase in HST compared to control (LT₅₀ of 55°C vs 51°C). Water‐stress‐induced HST was not due to heat acclimation (leaf warming in stressed plants). Data also indicate that water‐stress treatment did not increase freezing tolerance of geranium leaves. Increased HST was associated with the accumulation of several heat‐stable, dehydrin proteins (25–60 kDa), and both cytosolic and ER luminal (BiP) HSC70 proteins. Leaf XWP, RWC, and HST reversed to control levels concomitant with the disappearance/reduction of dehydrins and HSC70 proteins in water‐stress‐relieved plants. The possibility of a cellular linkage between water stress and heat‐stress tolerance is discussed.     

Induction of thaumatin‐like proteins (TLPs) in Rhizoctonia solani‐infected rice and characterization of two new cDNA clones

Thaumatin‐like proteins (TLPs) were shown to be induced in rice plants (cv. IR58) that were infected with the sheath blight fungus, Rhizoctonia solani . Western blot analysis revealed the presence of two TLPs with sizes of 25 and 24 kDa which are different from a previously reported TLP with a size of 15.6 kDa from rice plants infiltrated with the non‐pathogenic bacterium, Pseudomonas syringae pv. syringae . By probing a cDNA expression library prepared from RNA isolated from R. solani ‐infected rice plants with a TLP antibody, several putative TLP cDNA clones were isolated and sequenced. The cDNA clones appeared to be derived from two different genes which shared only 77% sequence identity with each other and a lower percentage of sequence identity with the previously reported TLP cDNA clone. Southern blot analysis with the two TLP cDNAs revealed different rice genomic DNA fragments. Northern blot analysis also confirmed that a 1.1‐kb RNA detectable by the TLP cDNA inserts was induced by fungal infection. Thus rice TLPs are encoded by a family of at least three genes which are differentially expressed in responses to bacterial or fungal pathogens.     

A Nicotiana plumbaginifolia protein labeled with an azido cytokinin agonist is a glutathione S‐transferase

The mechanisms of reception/transduction of cytokinins still remain largely unknown. We used 1‐(2‐azido‐6‐chloropyrid‐4‐yl)‐3‐(4‐[³H])phenylurea ([³H]azido‐CPPU), a new photoaffinity probe to search for cytokinin‐binding proteins. A soluble protein that binds phenylurea‐type cytokinins has been specifically photolabeled in Nicotiana plumbaginifolia (cv. Viviani line pbH1D) leaf extracts. The protein was purified to homogeneity by affinity chromatography. Its N‐terminal amino acid sequence, as well as four internal peptidic sequences are highly homologous with the theta class of the glutathione S‐transferase superfamily (GST, EC 2.5.1.18) including Hyoscyamus muticus and Arabidopsis GSTs identified as auxin‐binding proteins. The purified N. plumbaginifolia protein also possesses GST enzymatic activity. To test the possible involvement of this GST in the mechanism of action of cytokinin, we studied the binding of tritiated‐CPPU to the purified GST in the presence of various compounds, cytokinin agonists, cytokinin antagonists, or inactive molecules. Thidiazuron is a poor competitor, and neither zeatin nor the active optical isomer R‐MeBA is able to inhibit the binding of CPPU. There is no correlation between the cytokinin activity and the binding properties of the molecules tested. Our results confirmed that plant GSTs bind different compounds, especially plant hormones but probably have no specific role in the mode of action of cytokinins.     

Role of Pathogen-Related Protein 10 (PR 10) under Abiotic and Biotic Stresses in Plants

Members of the Pathogenesis Related (PR) 10 protein family have been identified in a variety of plant species and a wide range of functions ranging from defense to growth and development has been attributed to them. PR10 protein possesses ribonuclease (RNase) activity, interacts with phytohormones, involved in hormone-mediated signalling, afforded protection against various phytopathogenic fungi, bacteria, and viruses particularly in response to biotic and abiotic stresses. The resistance mechanism of PR10 protein may include activation of defense signalling pathways through possible interacting proteins involved in mediating responses to pathogens, degradation of RNA of the invading pathogens. Moreover, several morphological changes have been shown to accompany the enhanced abiotic stress tolerance. In this review, the possible mechanism of action of PR10 protein against biotic and abiotic stress has been discussed. Furthermore, our findings also confirmed that the in vivo Nitric oxide (NO) is essential for most of environmental abiotic stresses and disease resistance against pathogen infection. The proper level of NO may be necessary and beneficial, not only in plant response to the environmental abiotic stress, but also to biotic stress. The updated information on this interesting group of proteins will be useful in future research to develop multiple stress tolerance in plants.     

Effect of solar radiation (UV and visible) at high altitude on CAM‐cycling and phenolic compound biosynthesis in Sedum album

The field experiment was carried out in order to compare the response of a CAM plant, Sedum album L., to solar radiation at a high altitude (2 100 m) with that at a low altitude location with respect to CAM and phenolic content. Treatment sites included (1) sun‐exposed, low altitude, (2) sun‐exposed, high altitude with different light treatments, including UV‐B and UV‐B + A screening, and (3) shade at high altitude. After a 70‐day treatment period, CAM‐cycling and phenolic compound content were analysed, and high altitude treatments were compared to the low altitude control. The sun‐exposed low altitude control was characterized by CAM‐cycling and a low phenolic compound content during the experiment. In plants transplanted to the high altitude, only the shaded group maintained a CAM‐cycling and a phenolic compound content similar to those of the sun‐exposed low altitude control. Samples under UV‐B and UV‐B + A filters showed similar responses, suggesting the absence of a specific UV‐A radiation effect. The screening of UV‐B or UV‐B + A radiation allowed plants to partially maintain a CAM‐cycling and induced a decrease in phenolic compound content. These responses under UV filters were, however, intermediate between those observed in sun‐exposed and shaded groups. These results demonstrate a specific effect of radiation from both visible (400–800 nm) and UV‐B (280–320 nm) bands on both CAM‐cycling and phenolic biosynthesis in S. album L. plants. These light‐dependent effects are discussed on a physiological basis and a possible interaction between CAM‐cycling and phenolic metabolism is suggested.     

Proteins functionally and immunogenically related to pathogenesis‐related proteins are induced during parsley leaf senescence

The advancement of leaf senescence is accompanied by a reduction in cellular protein content together with the induction of specific proteins which are probably involved in the process. In the present study, with parsley, we followed the changes in the levels of proteins functionally and immunogenically related to pathogenesis‐related proteins during both senescence of detached leaves and natural senescence of attached leaves. Both chitinase activity and protein level were found to be induced during senescence, as was the level of two other proteins immunologically related to β‐1,3‐glucanase and P4 pathogenesis‐related proteins of citrus and tomato, respectively. A high correlation between the advancement of senescence and the induction of these proteins was demonstrated. Treatments with CO₂ or gibberellic acid, which retard senescence, reduced both chitinase activity and the level of the pathogenesis‐related proteins, whereas enhancement of senescence with ethylene induced them further. The induction of pathogenesis‐related proteins during senescence suggests that these proteins may have a primary role in this process.     

Increased expression of two cDNAs encoding metallothionein‐like proteins during growth of Cicer arietinum epicotyls

The present study was undertaken to identify and characterize clones whose expression increase during Cicer arietinum epicotyl growth. Two cDNAs encoding two different plant metallothionein (MT)‐like proteins have been isolated from a cDNA library from epicotyls of Cicer arietinum L. cv. Castellana. The CanMT‐1 deduced protein appears to have the typical structure of type 1 MT where all Cys residues are in Cys‐X‐Cys motifs, while the CanMT‐2 has the typical structure of type 2 MT having Cys‐Cys and Cys‐X‐X‐Cys motifs within the N‐terminal domain. Both chickpea CanMTs are up‐regulated during epicotyl growth, showing increased expression in mature tissues, mostly CanMT‐1, which is undetectable in young epicotyls. Accordingly, stem of chickpea plants displayed the highest level of CanMT‐1 expression in the basal internode, with reduced growth, decreasing towards the apex. Osmotic stress by PEG, which inhibited growth, and ABA treatment induced the expression of MT‐like genes, which points to a relationship between chickpea MTs and ABA‐mediated stress response. Unlike CanMT‐2, CanMT‐1 is induced in chickpea epicotyls by cadmium indicating a different function for both clones. We conclude that these MT‐like proteins, in particular CanMT‐1, are regulated by the developmental stage and may participate in cell maturation process.     

Role of sodium in the ABA‐mediated long‐term growth response of bean to salt stress

Long‐term salt effects on plant growth have often been related to direct ion toxicity due to the accumulation of high ion concentrations in plant tissue. This work examines the relative importance of endogenous ABA, as well as Na⁺ and Cl⁻ toxicity, in the inhibition of leaf growth and photosynthesis, in bean plants grown at 1, 25, 50 and 75 m M NaCl until the fruit‐bearing stage. All salt‐treated plants showed very high leaf Cl⁻ concentrations, with little difference between plants exposed to 50 or 75 m M NaCl. The 25 and 50 mM salt‐treated plants were able to successfully exclude Na⁺ from their leaves, and only suffered an initial decline in the rate of leaf growth. Plants exposed to 75 m M NaCl showed an increase in Na⁺ leaf concentrations with an accompanying decrease in growth and photosynthesis as salt exposure progressed. A high correlation was found between leaf Na⁺ and leaf growth. Leaf ABA significantly increased with salt supply, and was highly correlated with both leaf Na⁺ and leaf growth. Our results suggest that in bean plants under long‐term salt stress, leaf ABA may participate in the regulation of leaf growth, and leaf Na⁺ would be at least partly responsible for increased ABA levels.     

Accumulation of γ‐aminobutyric acid in nodulated soybean in response to drought stress

Nitrogen fixation and nodule permeability to O₂ diffusion are decreased by drought stress. Since γ‐aminobutyric acid (GABA) synthesis is rapidly stimulated by a variety of stress conditions including hypoxia, it was hypothesized that decreased O₂ availability in nodules stimulates glutamate decarboxylase (GAD) activity (EC 4.1.1.15), thereby resulting in GABA accumulation. First, the amino acid composition of xylem sap was determined in plants subjected to soil water deficits. While the xylem sap concentration of several amino acids increased when the plant was subjected to a water deficit, the greatest increase was in GABA. GABA accumulation was examined in response to stress induced by hypoxia or the addition of polyethylene glycol (PEG) to the nutrient solution. The exposure of soybean nodules to hypoxia for 6 h enhanced the GABA concentration by 6‐fold, but there was no change in GABA concentration in response to the PEG treatment. No major changes in the in vitro GAD activity were measured in nodule cytosol or bacteroids. The present data do not support the hypothesis that decreased nodule O₂ permeability and a resulting O₂ deprivation inside nodules may stimulate in vitro GAD activity and thus GABA accumulation. However, the data could indicate a possible effect of hypoxia and drought stress on the in vivo activity of GAD.     

Mechanisms of Al‐induced iron chlorosis in wheat (Triticum aestivum). Al‐inhibited biosynthesis and secretion of phytosiderophore

Although Al‐induced iron chlorosis has been observed in many plants, the mechanisms responsible for this phenomenon are yet to be understood. We investigated the effect of Al on iron acquisition in a Strategy II plant, wheat ( Triticum aestivum L.) using both Al‐tolerant (Atlas 66) and ‐sensitive (Scout 66) cultivars. When iron was supplied as insoluble iron, ferric hydroxide, in the culture solution, both cultivars without Al treatment grew normally, while those with 100 µ M AlCl₃ developed chlorosis of the young leaves after 3 days of the treatment. A 21‐h treatment with 100 µ M AlCl₃ in 0.5 m M CaCl₂ solution (pH 4.5) decreased the amount of 2'‐deoxymugineic acid (DMA) secreted by Fe‐deficient Atlas 66 and Scout 66 plants by 85 and 90%, respectively. The amount of DMA secreted decreased with increasing external Al concentrations. Al treatment during the biosynthesis process caused the inhibition of that of DMA within 3 h. The secretion process was also found to be inhibited by Al, resulting in the biosynthesized DMA remaining in the roots. These results demonstrate the inhibition by Al of both biosynthesis and secretion of DMA attributed to Al‐induced iron chlorosis.     

Enhancement of pH‐resistant iron‐binding activity in supernatants of rainbow trout blood leucocytes by in vitro treatment with phorbol‐12‐myristate‐13‐acetate

Leucocyte lysates from rainbow trout Oncorhynchus mykiss showed an iron‐binding activity that was retained even if the samples were exposed to an acid pH (4·5). Iron‐binding activity of leucocyte supernatants was enhanced by the presence of 1 μg ml⁻¹ phorbol‐12‐myristate‐13‐acetate in the cell medium.     

Plant abiotic stress: deciphering remedial strategies for emerging problem

Growing in their natural environment, plants often encounter unfavorable environmental conditions that interrupt normal plant growth and productivity. Drought, high/low temperature and saline soils are the most common abiotic stresses that plants encounter in their natural environments. Molecular and genomic analyses have facilitated gene discovery and enabled genetic engineering using several functional or regulatory genes that are known to be involved in stress response and preliminary tolerance, to activate specific or broad pathways related to abiotic stress tolerance in plants. Through the use of transgenic technology, goals such as production of plants with desired traits that were unattainable with traditional selection programs are achieved. This review deals with recent advancement in understanding the role of various stress responsive genes and their critical importance for explaining the control mechanism of abiotic stress tolerance and engineering stress tolerant crops based on the expression of specific stress related genes.     

Reproductive biology of female big‐bellied seahorses

In this study, ovarian morphology, reproductive condition and sex steroid levels were investigated in the big‐bellied seahorse Hippocampus abdominalis , collected by snorkel and SCUBA diving in Wellington Harbour, New Zealand. Within the ovary, oocytes were contained between an outer muscular wall and an inner layer of luminal epithelium. Two germinal ridges ran along the entire length of the ovary. In cross‐section, oocytes were arranged in sequential order of development beginning at the germinal ridges and ending at the mature edge. Ovarian lamellae were absent. Vitellogenic and advanced cortical alveoli oocytes were elongated in shape, whereas maturing oocytes were distinctively pear‐shaped. Mature oocytes were large (2·6 – 4·4 mm in length) and aligned with the animal pole towards the muscular wall. Reproductively mature females were found throughout the year indicating a protracted reproductive season. The gonado‐somatic index was significantly different between all ovarian stages, but the hepato‐somatic index was not. Females with previtellogenic ovaries had significantly higher plasma concentrations of testosterone than females with vitellogenic or maturing ovaries. There was no significant difference in plasma concentrations of testosterone between females with vitellogenic or maturing ovaries, or in plasma concentrations of 17β‐oestradiol between females in all ovarian stages. This study contributes to the knowledge on the reproductive biology of female syngnathids.     

A comparative study on the protective role of trehalose and LEA proteins against abiotic stresses in transgenic Chinese cabbage (<Emphasis Type="Italic">Brassica campestris</Emphasis>) overexpressing<Emphasis Type="Italic">CaLEA</Emphasis> or<Emphasis Type="Italic">otsA</Emphasis>

Trehalose and LEA proteins, representative low MW chemicals that are synthesized under dehydration, are known to protect plants from drought stress. To compare their effectiveness on enhancing tolerance against various abiotic stresses, we generated transgenic Chinese cabbage plants overexpressingE. ctdi trehalose-6-phosphate synthase gene (otsA) or hot pepper (Capsicum annuum) LEA protein gene(CaLEA). Both transgenic plants exhibited altered phenotype including stunted growth and aberrant root development When subjected to drought, salt or heat stress, these plants showed remarkably improved tolerance against those stresses compared with nontransformants. After dehydration treatment, leaf turgidity and fresh weight was better maintained in both transgenic plants. GaUEA-plants performed somewhat better under dehydrated condition. When treated with 250 mM NaCI, both otsA-plants and CaLEA-plants remained equally healthier than nontransformants in maintaining leaf turgidity and delaying necrosis. Furthermore, leaf Chi content and Fv/Fm was maintained considerably higher in both transgenic plants than nontransformants. After heat-treatment at 45°C, both transgenic plants appeared much less damaged in external shape and PSII function, but LEA proteins were more protective. Our results indicate that although both trehalose and LEA proteins are effective in protecting plants against various abiotic stresses, LEA proteins seem to be more promising in generating stress-tolerant transgenic plants.     

Expression of apoplastically secreted tobacco osmotin in cotton confers drought tolerance

Osmotin or osmotin-like proteins have been shown to be induced in several plant species in response to various types of biotic and abiotic challenges. The protein is generally believed to be involved in protecting the plant against these stresses. Although some understanding of the possible mechanism underlying the defense function of osmotin against biotic stresses is beginning to emerge, its role in abiotic stress response is far from clear. We have transformed cotton plants with a tobacco-osmotin gene, lacking the sequence encoding its 20 amino acid-long, C-terminal vacuolar-sorting motif, under the control of CaMV 35S promoter. Apoplastic secretion of the recombinant protein was confirmed and the plants were evaluated for their ability to tolerate drought conditions. Under polyethylene glycol-mediated water stress, the osmotin-expressing seedlings showed better growth performance. The transformants showed a slower rate of wilting during drought and faster recovery following the termination of dry conditions in a greenhouse setting. During drought, the leaves from transgenic plants had higher relative water content and proline levels, while showing reduced H₂O₂ levels, lipid peroxidation and electrolyte leakage. Importantly, following a series of dry periods, the osmotin transformants performed better in terms of most growth and developmental parameters tested. Most relevant, the fiber yield of transgenic plants did not suffer as much as that of their non-transgenic counterparts under drought conditions. The results provide direct support for a protective role of osmotin in cotton plants experiencing water stress and suggest a possible way to achieve tolerance to drought conditions by means of genetic engineering. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The effect of salt stress on lipid peroxidation and antioxidants in the leaf of the cultivated tomato and its wild salt‐tolerant relative Lycopersicon pennellii

The possible involvement of the antioxidative system in the tolerance to salt stress was studied in the cultivated tomato Lycopersicon esculentum Mill. cv. M82 (M82) and its wild salt‐tolerant relative L. pennellii (Corn) D'Arcy accession Atico (Lpa). All analyses, except that of monodehydroascorbate reductase (MDHAR), were performed of the youngest fully‐expanded leaf of control and salt (100 m M NaCl) stressed plants, 4, 7, 10, 14, 18 and 22 days after completing the stress treatment. In Lpa, constitutive level of lipid peroxidation and activities of catalase (CAT) and glutathione reductase (GR) were lower while the activities of superoxide dismutase (SOD), ascorbate peroxidase (APX) and dehydroascorbate reductase (DHAR) were inherently higher than in M82. Relative to M82, lipid peroxidation was much lower and the activities of SOD, CAT and APX were higher in Lpa at 100 m M NaCl. The activity of DHAR decreased more in Lpa than in M82 under salt stress, and the activity of MDHAR, which was lower in Lpa than in M82 under control conditions, increased much more and to a higher level in salt‐treated Lpa plants. GR activity decreased similarly in the two species under salt stress. The results of these analyses suggest that the wild salt‐tolerant Lpa plants are better protected against active oxygen species (AOS), inherently and under salt stress, than the relatively sensitive plants of the cultivated species.