Apparent absence of viruses in most symptomless field-grown sweet potato in Uganda

Heat tolerance of groundnut (Arachis hypogaea L.) genotypes was evaluated by solute leakage and chlorophyll fluorescence techniques in heat-hardened and non-hardened plants. To determine the appropriate hardening treatment, 1-month-old plants of two groundnut genotypes, ICGV 86707 and Chico were conditioned at five combinations of hardening (37°C) and non-hardening (30°C) air temperatures over a 5-day period. Heat injury, was assessed through measurements of electrolyte leakage after stressing leaf discs to 55°C for 15 min. The relative injury was significantly influenced by the conditioning temperatures and by the temperature during 24 h prior to measurement if those involved non-hardening conditions. Relative injury and chlorophyll fluorescence were measured after stressing leaves of six genotypes at a range of temperatures between 49°C and 55°C. Significant genotype × hardening treatment interactions were observed in relative injury and chlorophyll fluorescence. Chico was susceptible to heat stress, the relative injury test identified ICGV 86707 as tolerant, and the chlorophyll fluorescence test identified ICGV 86707 as tolerant under hardened conditions and ICGV 87358 as tolerant when non-hardened. When expressed as percentage of control values, the relative injury and chlorophyll fluorescence measurements over the 49–53°C stress temperature range were strongly correlated. Chlorophyll concentrations were increased by hardening in all genotypes except Chico. In Chico, chl_b concentration was decreased and the chl_a/b ratio increased by hardening, and chlorophyll concentrations were correlated with chlorophyll fluorescence parameters. Chlorophyll concentration may therefore provide an alternative means of screening for heat tolerance.     

Improving low-temperature tolerance in sugarcane by expressing the ipt gene under a cold inducible promoter

Sugarcane is cultivated in tropical and subtropical regions where cold stress is not very common, but lower yields and reduced industrial quality of the plants are observed when it occurs. In our efforts to enhance cold tolerance in sugarcane, the gene encoding the enzyme isopentenyltransferase (ipt) under control of the cold inducible gene promoter AtCOR15a was transferred via biolistic transformation into sugarcane (Saccharum spp.) cv. RB855536. Semiquantitative RT-PCR using GAPDH encoding glyceraldehyde-3-phosphate dehydrogenase as the normalizer gene showed the increased expression of the ipt gene under cold stress. The detached leaves of genetically modified plants subjected to low temperatures showed visible reduction of leaf senescence in comparison to non-transgenic control plants. Induced overexpression of ipt gene also enhanced cold tolerance of non-acclimated whole plants. After being subjected to freezing temperature, leaf total chlorophyll contents of transgenic plants were up to 31 % higher than in wild type plants. Also, lower malondialdehyde content and electrolyte leakage indicated less damage induced by cold in transgenic plants. Thus, the expression of ipt driven by the stress inducible COR15a promoter did not affect plant growth while providing a greater tolerance to cold stress.     

Physiological and morphological characteristics of chickpea accessions under low temperature stress

Economically important crop chickpea (Cicer arietinum L.) is sensitive to chilling stress, and breeding for chilling tolerance is the economic option even in countries with a high risk for drought and heat stresses. In this study, we have analyzed chilling-induced responses of ten chickpea accessions under field and growth-chamber conditions in order to screen, using phenotypic and physiological methods, for chilling tolerance. The field data analysis revealed that there were significant differences between accessions in their cold tolerance. The percent survival and cold-tolerance scores were the most important indices describing genotype tolerance to low temperature under field conditions; they can be used to assess chickpea cold tolerance. During environmentally controlled testing, the effects of low temperature regimes (−10°C for 15 and 30 min) were studied and cold tolerance was measured by electrolyte leakage from damaged leaves. The analysis of field data and cold treatments showed that two accessions, Sel 95Th1716 and Sel 96Th11439, grouped in one cluster, are good cold-tolerant genotypes (showing low scores for cold tolerance and electrolyte leakage). In comparison with ILC 8262, released as cold-tolerant accession, these genotypes showed more tolerance. Flip 00-6C, ILC 533, and Jam were less tolerant to cold stress. Thus, we have shown that as well as field studies, short-term cold treatment and electrolyte leakage assay can be used to evaluate low temperature tolerance of chickpea profitably in a short time.     

Differential remodeling of the lipidome during cold acclimation in natural accessions of Arabidopsis thaliana

Freezing injury is a major factor limiting the geographical distribution of plant species and the growth and yield of crop plants. Plants from temperate climates are able to increase their freezing tolerance during exposure to low but non‐freezing temperatures in a process termed cold acclimation. Damage to cellular membranes is the major cause of freezing injury in plants, and membrane lipid composition is strongly modified during cold acclimation. Forward and reverse genetic approaches have been used to probe the role of specific lipid‐modifying enzymes in the freezing tolerance of plants. In the present paper we describe an alternative ecological genomics approach that relies on the natural genetic variation within a species. Arabidopsis thaliana has a wide geographical range throughout the Northern Hemisphere with significant natural variation in freezing tolerance that was used for a comparative analysis of the lipidomes of 15 Arabidopsis accessions using ultra‐performance liquid chromatography coupled to Fourier‐transform mass spectrometry, allowing the detection of 180 lipid species. After 14 days of cold acclimation at 4°C the plants from most accessions had accumulated massive amounts of storage lipids, with most of the changes in long‐chain unsaturated triacylglycerides, while the total amount of membrane lipids was only slightly changed. Nevertheless, major changes in the relative amounts of different membrane lipids were also evident. The relative abundance of several lipid species was highly correlated with the freezing tolerance of the accessions, allowing the identification of possible marker lipids for plant freezing tolerance.     

Imaging chlorophyll a fluorescence reveals specific spatial distributions under different stress conditions

Chlorophyll a fluorescence has been adopted as a fast, non-invasive, and cheap method to detect stress effects in plants. The majority of these chl-fluorescence measurements have been carried out with ‘clamping’ fluorometers recording punctual chlorophyll a fluorescence at isolated parts of the leaf. However, this method is inherently limited in providing information on the homogeneity of responses to stresses at the leaf or whole plant level. Therefore the purpose of this study was to measure imaging chlorophyll a fluorescence and to compare the temporal and spatial distribution of this emission under allelochemical (2-3H-benzoxazolinone and 3,4-dihydroxybenzaldehyde), thermal and salt, and heavy metal (cadmium, copper and zinc) treatment in the model plant Arabidopsis thaliana (L.) Heynh. The results suggested different spatial distributions for each condition: the two allelochemicals showed inhibition spots at the edges of the oldest leaves and both did not affect the photosynthetic activity of young leaves; treatment with the three heavy metals revealed highly homogenous effects over the whole plant with a quite uniform decrease of maximum PSII efficiency (also in youngest leaves). On the contrary, temperature (heat and cold) and salt stress showed an initial decrease of fluorescence in the tissues around the vascular bundles that lasted between 2 and 3 h depending on the treatment. These irregularities in chlorophyll fluorescence make it difficult to correlate punctual measures (typical for clamping fluorometers) with the effect on the whole plant, ignoring effects that are evident when imaging is used. Therefore these results show that monitoring chlorophyll a fluorescence by imaging improves the measurement of stress effects on treated plants, suggesting that punctual fluorescence measurements do not always reveal the heterogeneity of the stress-related effects in treated plants.     

Applications of chlorophyll fluorescence can improve crop production strategies: an examination of future possibilities

Chlorophyll fluorescence has been routinely used for many years to monitor the photosynthetic performance of plants non-invasively. The relationships between chlorophyll fluorescence parameters and leaf photosynthetic performance are reviewed in the context of applications of fluorescence measurements to screening programmes which seek to identify improved plant performance. The potential role of chlorophyll fluorescence imaging in increasing both the sensitivity and throughput of plant screening programmes is examined. Finally, consideration is given to possible specific applications of chlorophyll fluorescence for screening of plants for tolerance to environmental stresses and for improvements in glasshouse production and post-harvest handling of crops.     

Tolerances of wild potato species from different altitudes to cold and heat

The ability of wild potatoes (Solanum spp.) to adapt to potentially stressful environmental temperatures was investigated by measuring the cold and heat tolerances of plants grown near sea-level in Lima following collection of tubers from plants growing naturally at altitudes ranging from 450 to 4,200 m. Relative cold tolerance was measured in leaves stored at 0°C by the decrease in the induced rise of chlorophyll fluorescence. Similarly, changes in chlorophyll fluorescence were used to determine the relative heat tolerance of leaves heated at 41°C for 10 min. With increasing altitude, the cold tolerance of different species tended to increase and conversely, heat tolerance decreased. However, these two genotypic adaptations were not closely correlated and appear to vary independently of each other in response to climate.     

Genome-wide association study for electrolyte leakage in rapeseed/canola (<Emphasis Type="Italic">Brassica napus</Emphasis> L.)

Freezing temperature/frosts can cause significant damage to plants by rupturing plant cells. Rapeseed/canola (Brassica napus L.) is susceptible to freezing temperature at early seedling stage. The degree of cell rupture or seedling damage can be evaluated through the measurement of electrolyte leakage. Here, we measured the electrolyte leakage of a diversity panel of B. napus germplasm accessions under simulated freezing conditions. Preliminary data for electrolyte leakage measurement indicated that cold acclimation of two-week-old seedlings for 7 days at 4 °C followed by freezing treatment at ??12 °C for 2 h provided a reasonable diversity in response. With this protocol for electrolyte leakage, a genome-wide association study was conducted on 157 winter, semi-winter, and spring types of B. napus accessions that originated from 17 countries. A total of 37,454 single-nucleotide polymorphism (SNP) markers based upon genotyping-by-sequencing were used for the analysis. Ten QTL were identified as associated with electrolyte leakage of canola seedlings, which together explained 43% phenotypic variation. Five of the QTL were located on A-genome. We identified at least 33 orthologs of the functional candidate genes. Although no well-characterized cold regulatory genes were identified, there were some indications that genes involved in membrane structure, developmental processes, and extracellular transport may be involved in altering the electrolyte leakage following the short-term hard freeze and rapid defrosting suffered by the plants in our protocol.     

Melatonin-related mitochondrial respiration responses are associated with growth promotion and cold tolerance in plants

Melatonin has the ability to improve plant growth and strengthened plant tolerance to environmental stresses; however, the effects of melatonin on mitochondrial respiration in plants and the underlying biochemical and molecular mechanisms are still unclear. The objective of the study is to determine possible effects of melatonin on mitochondrial respiration and energy efficiency in maize leaves grown under optimum temperature and cold stress and to reveal the relationship between melatonin-induced possible alterations in mitochondrial respiration and cold tolerance. Melatonin and cold stress, alone and in combination, caused significant increases in activities and gene expressions of pyruvate dehydrogenase, citrate synthase, and malate dehydrogenase, indicating an acceleration in the rate of tricarboxylic acid cycle. Total mitochondrial respiration rate, cytochrome pathway rate, and alternative respiration rate were increased by the application of melatonin and/or cold stress. Similarly, gene expression and protein levels of cytochrome oxidase and alternative oxidase were also enhanced by melatonin and/or cold stress. The highest values for all these parameters were obtained from the seedlings treated with the combined application of melatonin and cold stress. The activity and gene expression of ATP synthase and ATP concentration were augmented by melatonin under control and cold stress. On the other hand, cold stress reduced markedly plant growth parameters, including root length, plant height, leaf surface area, and chlorophyll content and increased the content of reactive oxygen species (ROS), including superoxide anion and hydrogen peroxide and oxidative damage, including malondialdehyde content and electrolyte leakage level; however, melatonin significantly promoted the plant growth parameters and reduced ROS content and oxidative damage under control and cold stress. These data revealed that melatonin-induced growth promotion and cold tolerance in maize is associated with its modulating effect on mitochondrial respiration.     

Water-Deficit Tolerance and Field Performance of Transgenic Alfalfa Overexpressing Superoxide Dismutase

Transgenic alfalfa (Medicago sativa) expressing Mn-superoxide dismutase cDNA tended to have reduced injury from water-deficit stress as determined by chlorophyll fluorescence, electrolyte leakage, and regrowth from crowns. A 3-year field trial indicated that yield and survival of transgenic plants were significantly improved, supporting the hypothesis that tolerance of oxidative stress is important in adaptation to field environments.     

Ectopic expression of a cold-inducible transcription factor, CBF1/DREB1b, in transgenic rice (Oryza sativa L.)

The gene encoding C-repeat/dehydration-responsive element binding factor 1 (CBF1/DREB1b) of Arabidopsis was introduced into rice (Oryza sativa L.) under the control of the maize ubiquitin promoter. Its incorporation and expression in transgenic rice plants were confirmed by DNA and RNA gel-blot analyses. Cold tolerance in the transgenics was not significantly different from that of the wild-type plants, as determined by ion leakage, chlorophyll fluorescence, and survival rates. However, the cold-responsive genes lip5, lip9, and OsDhn1 were up-regulated in the transgenic plants, suggesting that the cold signal transduction pathway involving CBF1 is partially conserved in this cold-labile plant.     

Structural and functional characteristics of photosynthetic apparatus of chlorophyll-containing grape vine tissue

Photosynthesis in tissues under periderm of woody stems and shoots of perennial plants occurs in environment that is very different from the internal environment of leaf chloroplasts. These tissues are characterized by high CO₂ and low O₂ concentrations, more acidic surroundings, besides that only light which have passed through periderm reaches photosynthetic antennas. In contrast to leaves of deciduous plants chlorenchyma tissues of wintering plant organs are exposed to temperature fluctuations during all seasons, that is why the photosynthetic apparatus of woody stems has to be able to adapt to a wide range of environmental temperatures. In order to reveal unique features, which enable photosynthetic apparatus of chlorenchyma cells in woody plant organs to implement biological functions under different light and temperature conditions, we studied photosynthetic tissues of stem cortex in grapevine (Vitis vinifera L.) under normal conditions and after exposure to suboptimal temperatures and high light intensity. Comparative analysis of photosynthetic pigment composition and low-temperature chlorophyll fluorescence emission spectrum of leaves, young shoots and chlorenchyma of lignified shoots revealed relatively high level of chlorophyll b and carotenoids, and high photosystem II (PSII) to photosystem I (PSI) ratio in woody shoots. Analysis of parameters of variable chlorophyll fluorescence revealed high PSII activity in grapevine shoot cortex and demonstrated improved freeze tolerance and higher sensitivity to light of photosynthetic apparatus in grape vine in comparison to leaves. It was shown for the first time that photosynthetic apparatus in chlorenchyma cells of vine undergoes so-called “state-transition”–fast rearrangements leading to redistribution of energy between photosystems. Analysis of fatty acid (FA) compositions of lipids in examined tissues showed that the FA unsaturation index in green tissue of vine is lower than in leaves. A distinct feature of FA compositions of lipids in vine cortex was relatively high level of linoleic acid.     

Overexpression of a western white pine PR10 protein enhances cold tolerance in transgenic Arabidopsis

The PmPR10-1.10 protein from western white pine is known to be associated with frost hardiness, and up-regulated by seasonal cold acclimation and biotic and abiotic stresses. To gain insight into the molecular basis of cold hardiness, we investigated the potential physiological role of PmPR10-1.10 by gene overexpression in transgenic Arabidopsis plants. A binary vector was constructed for PmPR10-1.10 synthesis in higher plants and transgenic Arabidopsis lines were generated by Agrobacterium-mediated transformation. Following Western protein blot analysis confirming target protein production, transgenic Arabidopsis lines were tested for cold tolerance by electrolyte leakage analysis post treatment of different freezing temperatures. Our results demonstrate that accumulation of PmPR10-1.10 protein resulted in significantly greater freezing tolerance in transgenic plants than in wild type plants. This indicates that the transfer and selection of cold acclimation proteins like PmPR10-1.10 may be a breeding strategy for the development of freezing tolerance in conifers.