Browsing by Author "Farrant, J"

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    A comparison of mechanisms of desiccation tolerance among three angiosperm resurrection plant species.
    (Springer, 2000) Farrant, J
    The mechanisms of protection against mechanical and oxidative stress were identified and compared in the angiosperm resurrection plants Craterostigma wilmsii, Myrothamnus flabellifolius and Xerophyta humilis. Drying-induced ultrastructural changes within mesophyll cells were followed to gain an understanding of the mechanisms of mechanical stabilisation. In all three species, water filled vacuoles present in hydrated cells were replaced by several smaller vacuoles filled with non-aqueous substances. In X. humilis, these occupied a large proportion of the cytoplasm, preventing plasmalemma withdrawal and cell wall collapse. In C. wilmsii, vacuoles were small but extensive cell wall folding occurred to prevent plasmalemma withdrawal. In M. flabellifolius, some degree of vacuolation and wall folding occurred, but neither were sufficient to prevent plasmalemma withdrawal. This membrane was not ruptured, possibly due to membrane repair at plasmodesmata junctions where tearing might have occurred. In addition, the extra-cytoplasmic compartment appeared to contain material (possibly similar to that in vacuoles) which could facilitate stabilisation of dry cells.
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    Differences in rehydration of three desiccation-tolerant angiosperm species.
    (Oxford University Press, 1996) Sherwin, H; Farrant, J
    The rehydration characteristics of the desiccation-tolerant plantsCraterostigma wilmsii andMyrothamnus flabellifolia (homoiochlorophyllous) andXerophyta viscosa (poikilochlorophyllous) were studied to determine differences among them. A desiccation-sensitive plant (Pisum sativum) was used as a control. Recovery of water content, quantum efficiency (FV/FM), photosynthetic pigments and chloroplast ultrastructure as well as damage to the plasmamembrane were studied. P. sativum did not recover after desiccation and considerable damage occurred during rehydration. The desiccation-tolerant plants appeared to differ in their responses to dehydration and rehydration. The small herbaceousC. wilmsii generally showed little damage in the dry state and recovered faster than the other tolerant species.M. flabellifolia took longer to recover thanC. wilmsii probably due to the presence of a woody stem in which dehydration-induced xylem embolisms slowed the rate of recovery. The poikilochlorophyllous speciesX. viscosa took the longest to recover because it took longer to reconstitute the chloroplasts and the photosynthetic pigments. Quantum efficiency recovered in all species before water content and chlorophyll content recovered to control levels. The significance of these different responses to desiccation and recovery from desiccation is discussed.
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    The effect of drying rate on the survival of three desiccation-tolerant angiosperm species.
    (Oxford University Press, 1999) Farrant, J; Cooper, K; Kruger, L; Sherwin, H
    The effect of drying rate on the survival of three angiosperm resurrection plants, Craterostigma wilmsii (homoiochlorophyllous), Xerophyta humilis (poikilochlorophyllous) and Myrothamnus flabellifolius (homoiochlorophyllous) was examined. All species survived slow drying, but only C. wilmsii was able to survive rapid drying. C. wilmsii was rapidly able to induce protection mechanisms such as folding of cell walls to prevent mechanical stress and curling of leaves to minimize light stress, and thus survived fast drying. Rapid drying of X. humilis andM. flabellifolius appeared to allow insufficient time for complete induction of protection mechanisms. In X. humilis, there was incomplete replacement of water in vacuoles, the photosynthetic apparatus was not dismantled, plasma membrane disruption occurred and quantum efficiency of photosystem II (FV/FM) did not recover on rehydration. Rapidly dried leaves of M. flabellifolius did not fold tightly against the stem and FV/FMdid not recover. Ultrastructural studies showed that subcellular damage incurred during drying was exacerbated on rehydration. The three species co-occur in environments in which they experience high desiccation pressures. C. wilmsii has few features to retard water loss and thus the ability for rapid induction of subcellular protection is vital to survival. X. humilis and M. flabellifolius are able to retard water loss and protection is acquired relatively slowly. Copyright 1999 Annals of Botany Company.
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    Effects of the metal pollutants cadmium and nickel on soybean seed development
    (Cambridge University Press, 1998) Malan, H; Farrant, J
    The chloride salts of Cd or Ni were added to the nutrient solution in which soybean (Glycine max) plants were grown and the response of the plants to these pollutants examined. Both metals markedly reduced plant biomass and seed production. Accumulation was mostly in the roots. Nickel was more mobile than Cd, reaching higher levels in all plant parts, especially seeds. Within the tissues of mature seeds, the highest concentrations of Ni were found in the axis and testa. The highest concentrations of Cd were in the testa and cotyledon, and the lowest in the axis. When expressed on a per seed basis, metal contents of these organs increased with developmental age. Nickel amounts were lower in the pods than the seeds for all growth stages, however there was no significant difference for Cd. Cadmium reduced mature seed mass. This effect was mostly due to decreased yields of lipids, protein and carbohydrates. Although the number of seeds per pod declined as a response to Ni, seed mass was unaffected and there was no apparent effect on storage reserves.
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    Open Access
    Inclusion of polyvinylpyrrolidone in the polymerase chain reaction reverses the inhibitory effects of polyphenolic contamination of RNA.
    (Oxford University Press, 1999) Koonjul, P; Brandt, W; Farrant, J; Lindsey, G
    Polysaccharides, secondary metabolites and poly-phenolics are known to co-isolate with nucleic acids from plant tissues resulting in inhibition of molecular manipulations. RNA isolated from the polyphenolic-rich resurrection plant, Myrothamnus flabellifolius, was demonstrated to inhibit a standard polymerase chain reaction used as an assay despite the inclusion of the polyphenolic-binding compound poly(1-vinylpyrrolidone-2) (PVP) into the RNA isolation medium. This inhibition was, however, reversed by the addition of PVP into the PCR mixture itself. Confirmation of the inhibitory effect of polyphenolics on PCR was obtained by addition of green tea polyphenolics to the standard PCR assay. This inhibition was also reversed by the simultaneous inclusion of PVP.
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    Isolation and characterisation of chloroplasts from Myrothamnus flabellifolius Welw.
    (Elsevier, 2000) Koonjul, P; Brandt, W; Lindsey, G; Farrant, J
    Chloroplasts isolated from the resurrection plant Myrothamnus flabellifolius using trehalose gradients had a higher buoyant density than chloroplasts isolated from another resurrection plant Craterostigma wilmsii. The latter had the same buoyant density as those isolated from the desiccation-sensitive (DS) plant Pisum sativum. The increased buoyant density in M. flabellifolius was ascribed to the unusual ultrastructure of the thylakoid membranes. Standard chloroplast isolation protocols resulted in membrane damage in both resurrection plants. Trehalose rather than sucrose gradients were required for isolation of intact chloroplasts. Immunological studies showed that epitopes related to the desiccation stress protein dsp 21 from Craterostigma plantagineum were present in M. flabellifolius. Several small (10–15 kDa) unique stromal proteins were also present. Polyphenolics, including anthocyanins, accumulated in leaves and chloroplasts of M. flabellifolius during drying. Envisaged functions for these are maintenance of membrane integrity, chlorophyll masking and antioxidant protection. Metal (Ca2+, Cu2+, Fe2+, Mg2+ and Mn2+) concentrations declined and thylakoid membranes separated upon drying. We propose that this might be a mechanism to put a stasis on photosynthesis and minimise photo-oxidation damage under water stress conditions.
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    Leaf tensile properties of resurrection plants differ among species in their response to drying.
    (Elsevier, 2009) Hedderson, N; Balsamo, R; Farrant, J; Cooper, K
    Previous studies report that leaf tensile strength (TS) of the desiccation tolerant (resurrection) grass Eragrostis nindensis does not change on drying, but increases in dried desiccation sensitive Eragrostis species. In this paper we tested whether unchanging TS on dehydration is a common feature among 4 resurrection species, Craterostigma wilmsii, Sporobolus stapfianus, Xerophyta humilis and Xerophyta schlecteri, and how this might relate to leaf structure and mechanisms of protection against mechanical stress of drying. Desiccation sensitive controls were Zea mays and Arabidopsis thaliana. Light and transmission electron microscopy of leaves was performed to determine lignification and the nature of subcellular mechanical stabilization. There was a positive correlation between % lignin/unit cross-sectional area and TS of hydrated leaves. Only the grass, S. stapfianus, did not change TS when naturally dried. All others increased in TS when naturally dried, but there was variation among them when flash dried. In S. stapfianus, mechanical stabilization was by both wall folding (mesophyll) and vacuole packaging (bundle sheath) as reported for E. nindensis. This combination may account, in part, for unchanging TS during drying and may be a feature of resurrection grasses. We conclude that leaf tensile properties differ among resurrection plants and are not necessarily affected by protection mechanisms associated with mechanical stress.
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    Longevity of dry Myrothamnus flabellifolius in simulated field conditions.
    (Springer, 2001) Farrant, J; Kruger, L
    This study tested the length of time the desiccation-tolerantMyrothamnus flabellifolius could remain in the dry stateunder simulated field conditions, without losing viability. Dry plants werekeptat 50% relative humidity, with a 25/16 °Cday/night temperature regime and a daylight intensity of 1200μmol·m−2 s−1. At threemonthly intervals plants were rehydrated and the ability to resume respiration,photosynthesis, protein synthesis and transcription was assessed and changes insubcellular organisation and plant growth regulator content (zeatin, zeatinriboside and abscisic acid) monitored. Plants survived in the dry state foronly one year. The ability toresurrect metabolism in existing leaves was lost after nine months, after whichsurvival occurred due to regrowth of new leaves from meristems. There waslittleevidence of subcellular damage in leaf tissues of plants kept dry for up to sixmonths. These plants recovered respiration before the onset of translation (andthus repair) suggesting considerable subcellular protection of this metabolismagainst desiccation damage. Furthermore, full recovery of metabolism, includingphotosynthesis, occurred before the onset of transcription in these plants.Somesubcellular damage occurred in plants maintained dry for nine months. There wasincreased electrolyte leakage indicative of membrane damage which was repairedwith the onset of protein synthesis. Since this repair occurred before theonsetof transcription, it is likely that stored mRNA, present in the dry leaves, wasused. Recovery of photosynthesis in plants dried for 9 months was delayed untilthe onset of transcription. We propose that mRNA for recovery of thismetabolismis not stored during drying and thus damage to the photosynthetic apparatus canbe repaired only upon de novo transcription of the genome.In leaves that were able to resurrect, cytokinin content increasedtransiently just prior to onset of chlorophyll biosynthesis and translation,andabscisic acid content increased just prior to the onset of transcription. Theseplant growth regulator changes did not occur in leaves which did not resurrect,but we cannot distinguish whether it was the lack of signal, or the extent oftissue damage (or both) which prevented the repair and recovery of metabolisminthese tissues.
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    Open Access
    Physiological and molecular insights into drought tolerance.
    (Academic Journals, 2002) Mundree, S; Baker, B; Mowla, S; Peters, S; Marais, S; Vander Willigen, C; Govender, K; Maredza, A; Muyanga, S; Farrant, J; Thomson, J
    Water is a major limiting factor in world agriculture. In general, most crop plants are highly sensitive to even a mild dehydration stress. There are however, a few genera of plants unique to Southern Africa, called "resurrection plants" which can tolerate extreme water loss or desiccation. We have used Xerophyta viscosa, a representative of the monocotyledonous resurrection plants to isolate genes that are associated with osmotic stress tolerance. Several genes that are differentially expressed, and that confer functional sufficiency to osmotically-stressed Escherichia coli are being studied at the molecular and biochemical levels. In this review, we use this as a basis to discuss the physiological and molecular insights into drought tolerance.
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    Protection mechanisms against excess light in the resurrection plants Craterostigma wilmsii and Xerophyta viscosa.
    (Springer, 1998) Sherwin, H; Farrant, J
    Mechanisms of avoidance and protection against light damage were studied in the resurrection plants Craterostigma wilmsii and Xerophyta viscosa. In C. wilmsii, a combination of both physical and chemical changes appeared to afford protection against free radical damage. During dehydration leaves curled inwards, and the abaxial surface became exposed to light. The tissue became purple/brown in colour, this coinciding with a three-fold increase in anthocyanin content and a 30% decline in chlorophyll content. Thus light-chlorophyll interactions are progressively reduced as chlorophyll became masked by anthocyanins in abaxial layers and shaded in the adaxial layers. Ascorbate peroxidase (AP) activity increased during this process but declined when the leaf was desiccated (5% RWC). During rehydration leaves uncurled and the potential for normal light-chlorophyll interaction was possible before full hydration had occurred. Superoxide dismutase (SOD) and glutathione reductase (GR) activities increased markedly during this stage, possibly affording free radical protection until full hydration and metabolic recovery had occurred. In contrast, the leaves of X. viscosa did not curl, but light-chlorophyll interactions were minimised by the loss of chlorophyll and dismantling of thylakoid membranes. During dehydration, free radical protection was afforded by a four-fold increase in anthocyanin content and increased activities of AP, GR and SOD. These declined during rehydration. It is suggested that potential free radical damage may be avoided by the persistence of anthocyanins during the period of thylakoid membrane re-assembly and full chlorophyll restitution which only occurred once the leaves were fully rehydrated.
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    Ultraviolet irradiation effects on serotinous shape Leucadendron laureolum seeds: altered seed physiology and ultrastructure, and seedling performance.
    (Springer, 1998) Musil, C; Newton, R; Farrant, J
    Dry seeds of Leucadendron laureolum (Lam.) Fourc. (Proteaceae) were exposed for different intervals (range: 7 to 84 days) to visible, UV-A and UV-B radiation of different biologically effective dose (range: 0 to 11.43 kJ m-2 d-1). Changes in seed germination, physiology and ultrastructure, and residual UV effects on seedling performance, were examined. Germination was depressed in seeds following short (7-day) exposures to UV radiation. This depression was intensified with increased UV exposure dose, and most pronounced at shorter UV-B wavelengths. Also glutathione reductase (GR) activities increased in seeds exposed to shorter UV-B wavelengths, but these were unaffected by irradiation dose level in the UV-B range. Electrolyte leakage rates from UV-irradiated seeds were unaltered, which indicated that germination depression did not result from intrinsic membrane damage. The reversal of germination depression (UV-induced dormancy) in UV-irradiated seeds by red light pointed to the possible involvement of phytochrome in this photo-response. Germination depression disappeared in seeds after 56-days irradiation, possibly due to photoreceptor damage by excess UV light. At this stage, all UV irradiated seeds, irrespective of treatment wavelength or dose level, exhibited increased electrolyte leakage rates, which indicated membrane perturbation. Also, increased GR activities were observed in irradiated seeds, but these were proportionately smaller in seeds exposed to shorter wavelength UV-B radiation (9.1 to 35.8% increase) than longer wavelength UV-A (73.4% increase) and visible (97.7% increase) radiation. This implied a metabolic limitation for scavenging of free radicals and peroxides in aging seeds exposed to UV-B radiation, which pointed to accelerated seed deterioration. It was indirectly supported by ultrastructural evidence of sub-cellular damage (lipid coagulation and plasmalemma withdrawal from cell walls) in embryonic tissues of seeds after 84 days UV-B exposure, and reflected in decreased leaf numbers, photochemical efficiencies, and foliar chlorophyll a and carotenoid levels in seedlings cultured from these seeds.
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    Use of metabolic inhibitors to elucidate mechanisms of recovery from desiccation stress in the resurrection plant Xerophyta humilis.
    (Springer, 1998) Dace, H; Sherwin, H; Illing, N; Farrant, J
    Xerophyta humilis (Bak.) Dur. and Schinz is a poikilochlorophyllous resurrection plant in that it is tolerant of considerable water loss (< 5% relative water content [RWC]) and thylakoid membranes are dismantled and chlorophyll is lost during dehydration. In this paper we examined the processes associated with recovery from desiccation upon rehydration. Dried leaf explants were rehydrated in water (control) or in solutions of actinomycin-D or cyclohexamide in order to determine to what extent initial recovery was dependant on de novo transcription and translation respectively. Our results suggest that considerable protection of subcellular organisation and components of metabolism occurs during drying such that the initial recovery of metabolism on rehydration is virtually independent of de novo transcription of nuclear genes. However recovery does require the synthesis of new proteins. The plasmalemma remains intact and macromolecular synthesis is not required for maintenance of its integrity. Messenger RNA's for chlorophyll biosynthesis appear to be stored in a stable form in the dried leaves and are translated on rehydration. Similarly most of the mRNA's necessary for recovery of electron transport in the chloroplast (as determined by measuring the quantum efficiency of photosystem II [FV/FM] using chlorophyll fluorescence) appear to be stabily present in the dried leaves. However, for total recovery of FV/FM new genomic transcription is necessary.