Plant physiology: spreading a drought warning

Communication between the root and the shoot forms a central part of the coordinated response of plants to drought. Recent evidence suggests that a K+ channel expressed in the stelar tissue of the root may have a major role in this process.

Expression studies of the zeaxanthin epoxidase gene in nicotiana plumbaginifolia

Abscisic acid (ABA) is a plant hormone involved in the control of a wide range of physiological processes, including adaptation to environmental stress and seed development. In higher plants ABA is a breakdown product of xanthophyll carotenoids (C40) via the C15 intermediate xanthoxin. The ABA2 gene of Nicotiana plumbaginifolia encodes zeaxanthin epoxidase, which catalyzes the conversion of zeaxanthin to violaxanthin. In this study we analyzed steady-state levels of ABA2 mRNA in N. plumbaginifolia. The ABA2 mRNA accumulated in all plant organs, but transcript levels were found to be higher in aerial parts (stems and leaves) than in roots and seeds. In leaves ABA2 mRNA accumulation displayed a day/night cycle; however, the ABA2 protein level remained constant. In roots no diurnal fluctuation in mRNA levels was observed. In seeds the ABA2 mRNA level peaked around the middle of development, when ABA content has been shown to increase in many species. In conditions of drought stress, ABA levels increased in both leaves and roots. A concomitant accumulation of ABA2 mRNA was observed in roots but not in leaves. These results are discussed in relation to the role of zeaxanthin epoxidase both in the xanthophyll cycle and in the synthesis of ABA precursors.

Effects of xylem pH on transpiration from wild-type and flacca tomato leaves. A vital role for abscisic acid in preventing excessive water loss even from well-watered plants

The pH of xylem sap from tomato (Lycopersicon esculentum) plants increased from pH 5.0 to 8.0 as the soil dried. Detached wild-type but not flacca leaves exhibited reduced transpiration rates when the artificial xylem sap (AS) pH was increased. When a well-watered concentration of abscisic acid (0.03 &mgr;m) was provided in the AS, the wild-type transpirational response to pH was restored to flacca leaves. Transpiration from flacca but not from wild-type leaves actually increased in some cases when the pH of the AS was increased from 6.75 to 7.75, demonstrating an absolute requirement for abscisic acid in preventing stomatal opening and excessive water loss from plants growing in many different environments.

Effect of water stress on cell division and cell-division-cycle 2-like cell-cycle kinase activity in wheat leaves

In wheat (Triticum aestivum) seedlings subjected to a mild water stress (water potential of -0.3 MPa), the leaf-elongation rate was reduced by one-half and the mitotic activity of mesophyll cells was reduced to 42% of well-watered controls within 1 d. There was also a reduction in the length of the zone of mesophyll cell division to within 4 mm from the base compared with 8 mm in control leaves. However, the period of division continued longer in the stressed than in the control leaves, and the final cell number in the stressed leaves reached 85% of controls. Cyclin-dependent protein kinase enzymes that are required in vivo for DNA replication and mitosis were recovered from the meristematic zone of leaves by affinity for p13(suc1). Water stress caused a reduction in H1 histone kinase activity to one-half of the control level, although amounts of the enzyme were unaffected. Reduced activity was correlated with an increased proportion of the 34-kD Cdc2-like kinase (an enzyme sharing with the Cdc2 protein of other eukaryotes the same size, antigenic sites, affinity for p13(suc1), and H1 histone kinase catalytic activity) deactivated by tyrosine phosphorylation. Deactivation to 50% occurred within 3 h of stress imposition in cells at the base of the meristematic zone and was therefore too fast to be explained by a reduction in the rate at which cells reached mitosis because of slowing of growth; rather, stress must have acted more immediately on the enzyme. The operation of controls slowing the exit from the G1 and G2 phases is discussed. We suggest that a water-stress signal acts on Cdc2 kinase by increasing phosphorylation of tyrosine, causing a shift to the inhibited form and slowing cell production.

ABI2, a second protein phosphatase 2C involved in abscisic acid signal transduction in Arabidopsis

The abi2-1 (abscisic acid insensitive) mutant of Arabidopsis thaliana shows abscisic acid (ABA) insensitivity with respect to seed germination and vegetative ABA responses. We identified the ABI2 gene by a combination of positional mapping and homology to ABI1. The ABI2 protein shows 80% amino acid sequence identity to ABI1, a protein phosphatase 2C (PP2C) involved in ABA signaling. The mutation that confers the abi2-1 phenotype is equivalent to the mutation previously identified in abi1-1 and the resulting Gly168Asp abi2 protein shows a reduced PP2C activity. Thus, a pair of highly homologous PP2Cs regulate ABA signaling.

Stomata and plant water relations: does air pollution create problems?

Small changes in the gaseous composition of the atmosphere have many different impacts on terrestrial plants. Some of the most important involve changes in stomatal control of leaf conductance. Evolution has provided highly complex mechanisms by which stomata respond to a wide range of environmental factors to balance the conflicting priorities of carbon gain for photosynthesis and water conservation. These mechanisms involve direct responses of the guard cells to aspects of the aerial environment, and hormonal communication within the plant enabling conductance to be adjusted according to soil moisture status. Various aspects of these delicately balanced mechanisms can be disturbed by air pollutants. Impairment of the regulation of plant water use by SO2 and O3 has been known for some years, but there are still many obstacles to our understanding of the variations in response between species, or even between genotypes of the same species. A surprising outcome of some recent studies is the suggestion that CO2 pollution may disrupt the control of water relations in some species because their stomata do not close sufficiently in CO2-enriched air. It has often been taken for granted that the elevation of atmospheric CO2 would lead to economies in water use by plant canopies, but the underlying assumptions are now being seriously questioned.

Molecular and physiological responses to water deficit in drought-tolerant and drought-sensitive lines of sunflower. Accumulation of dehydrin transcripts correlates with tolerance

To investigate correlations between phenotypic adaptation to water limitation and drought-induced gene expression, we have studied a model system consisting of a drought-tolerant line (R1) and a drought-sensitive line (S1) of sunflowers (Helianthus annuus L.) subjected to progressive drought. R1 tolerance is characterized by the maintenance of shoot cellular turgor. Drought-induced genes (HaElip1, HaDhn1, and HaDhn2) were previously identified in the tolerant line. The accumulation of the corresponding transcripts was compared as a function of soil and leaf water status in R1 and S1 plants during progressive drought. In leaves of R1 plants the accumulation of HaDhn1 and HaDhn2 transcripts, but not HaElip1 transcripts, was correlated with the drought-adaptive response. Drought-induced abscisic acid (ABA) concentration was not associated with the varietal difference in drought tolerance. Stomata of both lines displayed similar sensitivity to ABA. ABA-induced accumulation of HaDhn2 transcripts was higher in the tolerant than in the sensitive genotype. HaDhn1 transcripts were similarly accumulated in the tolerant and in the sensitive plants in response to ABA, suggesting that additional factors involved in drought regulation of HaDhn1 expression might exist in tolerant plants.

Response of the water status of soybean to changes in soil water potentials controlled by the water pressure in microporous tubes

Water transport through a microporous tube-soil-plant system was investigated by measuring the response of soil and plant water status to step change reductions in the water pressure within the tubes. Soybeans were germinated and grown in a porous ceramic 'soil' at a porous tube water pressure of -0.5 kpa for 28 d. During this time, the soil matric potential was nearly in equilibrium with tube water pressure. Water pressure in the porous tubes was then reduced to either -1.0, -1.5 or -2.0 kPa. Sap flow rates, leaf conductance and soil, root and leaf water potentials were measured before and after this change. A reduction in porous tube water pressure from -0.5 to -1.0 or -1.5 kPa did not result in any significant change in soil or plant water status. A reduction in porous tube water pressure to -2.0 kPa resulted in significant reductions in sap flow, leaf conductance, and soil, root and leaf water potentials. Hydraulic conductance, calculated as the transpiration rate/delta psi between two points in the water transport pathway, was used to analyse water transport through the tube-soil-plant continuum. At porous tube water pressures of -0.5 to-1.5 kPa soil moisture was readily available and hydraulic conductance of the plant limited water transport. At -2.0 kPa, hydraulic conductance of the bulk soil was the dominant factor in water movement.

Mannitol inhibits growth of intact cucumber but not pea seedlings by mechanically collapsing the root pressure

The positive xylem pressure (Px) in cucumber hypocotyls is a direct extension of root pressure and therefore depends on the root environment. Solutions of the electrolyte KCl (0-10 osm) reduced the hypocotyl Px transiently (biphasic response), while the Px reduction by mannitol solutions was sustained. The amplitudes of the induced Px reduction depended directly, and the degree of Px restoration after stress release depended indirectly, on the size of the initial positive Px indicating that mannitol released the root pressure by a mechanical rather than osmotic mechanism. Mannitol treatment and other means of root pressure reduction revealed two separate growth responses in the affected cucumber hypocotyls. Only steep Px drops (following root excision or root pressure release in mannitol) directly cause a rapid, transient drop in growth rate (GR). Both rapid and slow (after root incubation in KCN or NEM) decreases in root pressure, however, led to a sustained growth inhibition of cucumber hypocotyls after about 30 min. This delay characterizes the growth response as an indirect consequence of the Px change. Pea seedlings, which lacked root pressure and had a negative Px throughout, showed extremely small changes in epicotyl Px and GR after root incubation in mannitol. It is apparent that the higher sensitivity of cucumber growth to mannitol depended on the presence and release of root pressure.

A group 3 LEA cDNA of rice, responsive to abscisic acid, but not to jasmonic acid, shows variety-specific differences in salt stress response

A cDNA clone oslea3, encoding a group 3 late-embryogenesis abundant (LEA) protein was isolated from roots of rice seedlings (Oryza sativa L.). The encoded OSLEA3 protein has previously been found to accumulate to higher levels in roots of two salt-tolerant compared to a salt-sensitive rice variety in response to abscisic acid (ABA) [Moons et al., 1995. Plant Physiol. 107, 177-186]. The OSLEA3 protein (Mr 20.5, pI 6.5) characteristically contains ten imperfect 11-mer amino acid repeats. Exogenous application of ABA and exposure to salt shock (150 mM NaCl) rapidly induces a de novo, abundant oslea3 transcript accumulation in seedling roots, whereas application of jasmonic acid (9 microM) does not induce oslea3 expression. The stress-induced oslea3 transcript gradually declined upon prolonged salt shock, as wilting-induced damage became irreversible. oslea3 expression was compared for the salt-tolerant variety Pokkali and the salt-sensitive cultivar Taichung N1. Higher maximal mRNA levels were found in roots of the tolerant variety, also declining less rapidly upon sustained salt shock, concomitant with a delayed drop in shoot water content. DNA blot analysis indicated the existence of a small oslea3 gene family in rice with an equal gene number in both ecotypes. The results suggest that a differential regulation of oslea3 expression is an aspect of the varietal differences in salt stress tolerance.

Microviscosity of cucumber (Cucumis sativus L.) fruit protoplast membranes is altered by triacontanol and abscisic acid

Cucumber (Cucumis sativus L.) fruit protoplast membranes were probed with diphenylhexatriene (DPH) and pyrene, and also with two different plant growth regulators, triacontanol (TRIA) and abscisic acid (ABA). Fluorescence anisotropies of DPH and pyrene were measured after incorporating them into the membranes. The fluorescence lifetime of membrane-bound pyrene was also measured by using neodymium-doped yttrium aluminium garnet (Nd:YAG) laser of 35 ps pulses. The microviscosities of the membranes were calculated using the values of fluorescence anisotropy and lifetime. In the presence of TRIA and ABA, there was a sharp decrease in the fluorescence lifetime of pyrene. Similarly, there was also a decrease in the microviscosities of the membranes and increase in the rate of rotation of membrane-bound fluorophore, induced by the plant growth regulators. Furthermore, TRIA or TRIA + ABA could reduce the fluorescence anisotropy of both the fluorophores whereas, ABA decreased the anisotropy of only pyrene. This property of ABA may be due to its confinement to a specific spacial facet in the membrane. Fatty acid analysis indicated that membrane microviscosity fluctuations were not due to altered fatty acid composition alone as it is known that change in lipid-protein interaction would also alter the physical status of the membrane.

Water relations and gas exchange in young coconut palm (Cocos nucifera L.) as influenced by water deficit

Drought is the main climatic limitation to coconut palm (Cocos nucifera L.) production. To identify early screening parameters for drought resistance, physiological responses to water deficit were investigated for the first time using containerized young plants grown outside. Three varieties were studied: 'West Coast Tall' (drought avoiding), 'Malayan Yellow Dwarf' (drought susceptible), and their progeny, the hybrid 'PB 121' (drought resistant). Leaf water status (relative water content, leaf water potential) and leaf gas exchange parameters (stomatal conductance to water vapor, net photosynthetic rate, and leaf internal CO2 concentration) were measured throughout a drying cycle induced by withholding watering. On fully hydrated plants, the sensitivity of stomata to atmospheric water vapor deficit was also investigated. In the three varieties, stomatal conductance to water vapor declined before leaf water status parameters were affected. The existence of a root-to-shoot communication system was proposed. Net photosynthetic rate was highly dependent on stomatal aperture, but nonstomatal factors also participated in the reduction of CO2 fixation. Since gas exchange rates were equally sensitive to drought in the three varieties, they could not be used as screening parameters for drought resistance. Under severe drought stress, both relative water content and leaf water potential differed significantly among populations. 'West Coast Tall' maintained a higher leaf water status than 'Malayan Yellow Dwarf'. Remarkably, the water status parameters of 'PB 121' were intermediate between those of the two parents. Similar ranking was obtained in experiments with excised leaflets. The high reproductibility of the results suggests that leaf water status parameters might be useful as early selection criteria for drought resistance in coconut palm. Key words: Cocos nucifera L., gas exchange, leaf water status parameters, water deficit, water relations.

Reciprocal effects of platinum and lead on the water household of poplar cuttings

In order to study the accumulation rates and effects of platinum as influenced by lead, experiments were performed with poplar cuttings in a growth chamber. The heavy metals were added at a final concentration of 34.8 ppb each to nutrient solutions as PtCl4 and Pb(NO3)2. The variants were 1) control; 2) permanent Pt treatment for 6 weeks; 3) pretreatment with Pt plus subsequent treatment with Pb (three weeks each), and 4) heavy metal application in inverse order to variant 3.The experiments revealed that platinum accumulates in the roots of poplar cuttings to a higher degree than lead. It is translocated from the roots to other plant parts to an extremely low degree. Lead is displaced from the roots by subsequent Pt treatment. Insoluble platinum was found to be associated especially on the cell walls of the rhizodermis and exodermis of the root tips.Accumulation of platinum in the roots leads to a gradual depletion of the plants' water supply. The disturbance of the water household causes a reduction of the transpirational surface, lowered transpiration rates and enhanced root growth. All these alterations are induced as a means of coping water stress.From the results of this experiment, the conclusion can be drawn that, under the chosen experimental conditions, platinum manifests a higher toxicity than lead in plant roots because of its higher accumulation rates.

Immunogold localization and quantification of cellular and subcellular abscisic acid, prior to and during drought stress

An immunogold labeling procedure and experimental data are presented, which demonstrate that antibodies produced against a bovine serum albumin-abscisic acid conjugate can be used both to characterize the cellular and subcellular localization of abscisic acid (ABA), and to permit quantitative comparisons of this hormone in the subcellular compartments prior to and at times of drought stress. At the control leaf water potential (approximately -0.45 MPa), a quantitatively similar positive labeling pattern was observed in the chloroplasts and apoplast. A twofold drought stress-induced increase in the apoplastic ABA concentration was observed in the drought stressed leaf tissue (i.e., at a leaf water potential of approximately -1.55 MPa), while the ABA concentration in the chloroplasts did not differ from that of the controls. Three histochemical controls and the physiological observations validated the specificity of the procedure. Based on the labeling patterns we observed and literature cited, the validity of the hypothesis that drought stress induces a release of chloroplastic ABA is questioned. We interpreted our results as providing indirect evidence for a drought stress-induced root source origin for the increased apoplastic ABA concentrations.

Root system regulation of whole plant growth

New evidence confirms earlier postulates that root signals to shoots, including abscisic acid, nitrate flux, and cytokinins, modify whole plant growth processes including leaf expansion, stomatal behavior, and biosynthesis of photosynthetic enzymes. Root signals are thought to reflect soil water, nutrient, and mechanical attributes, as sensed by roots. Meristematic activities in root tips initiate changes in root architecture, modifying the soil volume subject to root uptake, and may provide multiple sensory and signaling capabilities. Knowledge of root signals regulating whole plant growth processes suggests new analytical and experimental tools for integrated analysis of plant phasic development, optimal growth, and ecological fitness.

Expression of the betaine aldehyde dehydrogenase gene in barley in response to osmotic stress and abscisic acid

When subjected to salt stress or drought, some vascular plants such as barley respond with an increased accumulation of the osmoprotectant glycine betaine (betaine), being the last step of betaine synthesis catalyzed by betaine aldehyde dehydrogenase (BADH). We report here cloning and characterization of BADH cDNA from barley, a monocot, and the expression pattern of a BADH transcript. An open reading frame of 1515 bp encoded a protein which showed high homology to BADH enzymes present in other plants (spinach and sugar-beet) and in Escherichia coli. Transgenic tobacco plants harboring the clone expressed high levels of both BADH protein and its enzymatic activity. Northern blot analyses indicated that BADH mRNA levels increased almost 8-fold and 2-fold, respectively, in leaves and roots of barley plants grown in high-salt conditions, and that these levels decreased upon release of the stress, whereas they did not decrease under continuous salt stress. BADH transcripts also accumulate in response to water stress or drought, indicating a common response of the plant to osmotic changes that affect its water status. The addition of abscisic acid (ABA) to plants during growth also increased the levels of BADH transcripts dramatically, although the response was delayed when compared to that found for salt-stressed plants. Removal of plant roots before transferring the plants to high-salt conditions reduced only slightly the accumulation of BADH transcripts in the leaves.

Cytokinins in the xylem sap of desert-grown almond (Prunus dulcis†) trees: Daily courses and their possible interactions with abscisic acid and leaf conductance

Xylem sap samples were obtained from one- to four-year-old almond trees [Prunus dulcis (Miller) D. A. Webb] that had been grown in lysimeters of different volumes with different amounts of available water and subjected to an annual drying cycle. The samples were analyzed for cytokinin patterns by ELISA during the growing season. Free bases, ribosides and nucleotides of the N⁶ -(Δ² -isopentenyl) adenine (iP) and the zeatin (Z) type could be identified and quantified. Z-type cytokinins were always predominant. In many cases the concentrations of the cytokinin fractions were rather constant during the day. In a limited number of days, however, Z-type cytokinins showed peak concentrations in the morning and a rapid decrease in the afternoon. A correlation between water status of the trees and the concentrations of cytokinins or their daily variations in the xylem sap could not be established. When the concentration of abscisic acid in the xylem sap was not limiting leaf conductance, daily-variation of a cytokinin fraction preceded daily variation in leaf conductance. We conclude that in almond trees, cytokinins may affect stomatal behaviour on a short-term basis. This is an outcome of changes in their xylem sap concentrations during the course of a day. Abscisic acid, on the other hand, acts as an opposing signal, the size of which reflects long-term water deficit.

Abscisic acid biosynthesis in roots : I. The identification of potential abscisic acid precursors, and other carotenoids

The pathway of water-stress-induced abscisic acid (ABA) biosynthesis in etiolated and light-grown leaves has been elucidated (see A.D. Parry and R. Horgan, 1991, Physiol. Plant. 82, 320-326). Roots also have the ability to synthesise ABA in response to stress and it was therefore of interest to examine root extracts for the presence of carotenoids, including those known to be ABA precursors in leaves. All-trans- and 9'-cis-neoxanthin, all-trans- and 9-cis-violaxanthin, antheraxanthin (all potential ABA precursors), lutein and β-carotene were identified on the basis of absorbance spectra, reactions with dilute acid, retention times upon high-performance liquid chromatography and by comparison with leaf carotenoids that had been analysed by mass spectrometry. The source of the extracted carotenoids was proved to be root tissue, and not contaminating compost or leaf material. The levels of total carotenoids in roots varied between 0.03-0.07% of the levels in light-grown leaves (Arabidopsis thaliana (L.) Heynh, Nicotiana plumbaginifolia Viv., Phaseolus vulgaris L. and Pisum sativum L.) up to 0.27% (Lycopersicon esculentum Mill.). The relative carotenoid composition was very different from that found in leaves, and varied much more between species. All-trans-neoxanthin and violaxanthin were the major carotenoids present (64-91 % of the total), but while Lycopersicon contained 67-80% all trans-neoxanthin, Phaseolus, Pisum and Zea mays L. contained 61-79% all-trans-violaxanthin. Carotenoid metabolism also varied between species, with most of the carotenoids in older roots of Phaseolus being esterified. Roots and leaves of the ABA-deficient aba mutant of Arabidopsis had reduced epoxy-xanthophyll levels compared to the wild-type.

Abscisic acid biosynthesis in roots : II. The effects of water-stress in wild-type and abscisic-acid-deficient mutant (notabilis) plants of Lycopersicon esculentum Mill

The ubiquity of the apo-carotenoid abscisic acid (ABA) biosynthetic pathway elucidated in water-stressed, etiolated leaves of Phaseolus vulgaris (see A.D. Parry and R. Horgan, 1991, Physiol. Plant. 82, 320-326), has been difficult to establish. Light-grown leaves contain very high carotenoid: ABA ratios, preventing correlative studies, and no etiolated leaves so far studied, other than those of Phaseolus, have been found capable of synthesising significant amounts of ABA in response to stress. Roots are known to synthesise ABA and contain low carotenoid levels; therefore ABA biosynthesis was investigated in soil- and hydroponically grown roots of Lycopersicon esculentum Mill. Hydroponically grown roots were stressed by immersion in 100 mM mannitol and soil-grown roots by withholding water. In both cases stress led to an increase in ABA levels and a decrease in the levels of specific xanthophylls, namely all-trans- and 9'-cis-neoxanthin and all-trans-violaxanthin. In hydroponically grown roots, and soil-grown roots stressed after removal of the shoot, ratios of xanthophyll cleaved:ABA synthesised of approx. 1∶1 were obtained. These findings are consistent with the operation of an apo-carotenoid pathway in roots, involving the conversion of all-trans-violaxanthin via all-trans-neoxanthin, to 9'-cis-neoxanthin, and the specific cleavage of 9'-cis-neoxanthin to yield the C15 ABA precursor xanthosin. Similar experiments with roots of the "leaky", ABA-deficient mutant of Lycopersicon, notabilis, indicate that the mutation does not affect the perception or transduction of stress, or the ability of the plant to cleave carotenoids. Rather, it appears that notabilis possesses an enzyme with reduced substrate specificity which cleaves more all-trans-than 9'-cis-neoxanthin.

Mechanisms of action of abscisic acid at the cellular level

Abscisic acid (ABA) has been implicated in the control of a diverse range of physiological processes in higher plants. In this review, we focus on the events which constitute the cellular responses to ABA. Current evidence suggests that it is possible to classify the responses to ABA on the basis of whether they are rapid, involving ion fluxes (typified by the stomatal response), or slower and requiring alterations to gene expression (for example the response of cereal embryos to ABA). In our consideration of ABA stimulus response coupling pathways, we have chosen to highlight the role of the calcium ion in the rapid responses, while we have concentrated on the contribution of as-acting elements and trans-acting factors in the regulation of ABA-responsive genes. We also draw attention to the possibility that interaction may exist between these pathways. Additionally, we discuss the controls of ABA concentrations during development and in response to environmental stimuli. Factors which contribute to the controls of ABA sensitivity are also reviewed. In our conclusions, we suggest that a general role for ABA may be to prepare tissue for entry into a new and different physiological state, perhaps by resetting the direction of cellular metabolism. CONTENTS Summary 9 I. Introduction 10 II. Stimulus response coupling 17 Synopsis 27 Acknowledgements 28 References 28.