Osmotic Stress and Ion-Specific Effects on Xylem Abscisic Acid and the Relevance to Salinity Tolerance in Poplar

We designed two experiments to investigate the osmotic stress and ion-specific effects on xylem abscisic acid (ABA) and the relevance to salinity tolerance in one-year-old seedlings of Populus euphratica Oliv. (a salt-resistant genotype) and one-year-old rooted cuttings of P. 'popularis 35-44' (P. popularis) (a salt-sensitive genotype). Net photosynthetic rates (Pn) and unit transpiration rates (TRN) of the two genotypes were significantly decreased upon osmotic shock caused by PEG 6000 (osmotic potential = -0.24 MPa) or iso-NaCl (50 mM). Shoot xylem ABA concentrations in both genotypes increased rapidly after the onset of PEG stress, resulting from a decreased water flow. NaCl-treated trees of P. euphratica maintained considerably greater concentrations of ABA than PEG-treated plants in a longer term, whereas salinized P. popularis exhibited a transient accumulation of ABA in the shoot. TRN was greatly enhanced in both genotypes when pressure (0.24 MPa) was applied to counteract the osmotic suction of 50 mM NaCl. Pressurizing of root systems diluted solutes in the root xylem, but the dilution effect was more pronounced in P. popularis. Root xylem ABA concentrations in P. euphratica steadily increased with salt stress although pressurization lowered its levels. In contrast, there were no observed changes in ABA response to salinity in pressured P. popularis. Therefore, we concluded that the salt-tolerant P. euphratica had a greater capacity to synthesize ABA under saline conditions, which may partially result from specific salt effects. In addition, P. euphratica exhibited a higher capacity for salt (Na+ and Cl-) transport control under salt stress, compared with P. popularis. The possible association between ABA and salt transport limitation, and the relevance to salinity tolerance were discussed.     

Osmotic Stress and Ion-Specific Effects on Xylem Abscisic Acid and the Relevance to Salinity Tolerance in Poplar

We designed two experiments to investigate the osmotic stress and ion-specific effects on xylem abscisic acid (ABA) and the relevance to salinity tolerance in one-year-old seedlings of Populus euphratica Oliv. (a salt-resistant genotype) and one-year-old rooted cuttings of P. 'popularis 35-44' (P. popularis) (a salt-sensitive genotype). Net photosynthetic rates (Pn) and unit transpiration rates (TRN) of the two genotypes were significantly decreased upon osmotic shock caused by PEG 6000 (osmotic potential = -0.24 MPa) or iso-NaCl (50 mM). Shoot xylem ABA concentrations in both genotypes increased rapidly after the onset of PEG stress, resulting from a decreased water flow. NaCl-treated trees of P. euphratica maintained considerably greater concentrations of ABA than PEG-treated plants in a longer term, whereas salinized P. popularis exhibited a transient accumulation of ABA in the shoot. TRN was greatly enhanced in both genotypes when pressure (0.24 MPa) was applied to counteract the osmotic suction of 50 mM NaCl. Pressurizing of root systems diluted solutes in the root xylem, but the dilution effect was more pronounced in P. popularis. Root xylem ABA concentrations in P. euphratica steadily increased with salt stress although pressurization lowered its levels. In contrast, there were no observed changes in ABA response to salinity in pressured P. popularis. Therefore, we concluded that the salt-tolerant P. euphratica had a greater capacity to synthesize ABA under saline conditions, which may partially result from specific salt effects. In addition, P. euphratica exhibited a higher capacity for salt (Na+ and Cl-) transport control under salt stress, compared with P. popularis. The possible association between ABA and salt transport limitation, and the relevance to salinity tolerance were discussed.     

盐胁迫条件下杨树盐分与甜菜碱及糖类物质变化

以抗旱耐盐性强的胡杨（Populus euphratica）和非抗盐的群众杨（P. popularis‘35-44’）为实验材料,研究了盐胁迫条件下盐分与甜菜碱、还原糖、蔗糖以及水溶性糖等细胞相容溶质的动态变化。两种杨树在盐处理期间表现出明显差异：群众杨下部叶片首先表现出盐害症状,处理后两周苗木上部叶片也出现盐害并脱落。而胡杨在试验期间仅下部叶片发黄脱落,盐处理15天后落叶量仅为16%。群众杨盐害症状的出现主要是由叶片中盐离子的大量累积所致。与之比较,胡杨拒吸Na+的能力及控制Cl-转运的能力均优于群众杨。另外,胡杨的耐盐性强于群众杨也与其有机溶质的变化有关。受到盐胁迫后胡杨根叶中甜 菜碱浓度显著提高,在处理后15天达到最高值,特别是叶片中甜菜碱的浓度提高了243倍,达到1*!899.8 μmol/L,根中甜菜碱含量也增加了9倍。此外,盐处理后胡杨叶和根中的还原糖、水溶性糖和蔗糖含量均呈明显上升趋势,分别在第4天和第15天达到峰值。与胡杨相反,耐盐性弱的群众杨在盐胁迫期间,叶中甜菜碱和糖含量并无显著提高,根中糖分水平还明显降低。由此可以得出结论,胡杨渗透调节能力高于群众杨,是其耐盐性强的重要生理基础之一。     

Uniconazole-induced tolerance of soybean to water deficit stress in relation to changes in photosynthesis, hormones and antioxidant system

This study investigated whether uniconazole confers drought tolerance to soybean and if such tolerance is correlated with changes in photosynthesis, hormones and antioxidant system of leaves. Soybean plants were foliar treated with uniconazole at 50 mg L-1 at the beginning of bloom and then exposed to water deficit stress at pod initiation for 7 d. Uniconazole promoted biomass accumulation and seed yield under both water conditions. Plants treated with uniconazole showed higher leaf water potential only in water-stressed condition. Water stress decreased the chlorophyll content and photosynthetic rate, but those of uniconazole-treated plants were higher than the stressed control. Uniconazole increased the maximum quantum yield of photosystemand ribulose-1,5-bisphosphate carboxylase/oxygenase activity of water-stressed plants. Water stress decreased partitioning of assimilated 14C from labeled leaf to the other parts of the plant. In contrast, uniconazole enhanced translocation of assimilated 14C from labeled leaves to the other parts, except stems, regardless of water treatment. Uniconazole-treated plants contained less GA3, GA4 and ABA under well-watered condition than untreated plants, while the IAA and zeatin levels were increased substantially under both water conditions, and ABA concentration was also increased under water stressed condition. Under water-stressed conditions, uniconazole increased the content of proline and soluble sugars, and the activities of superoxide dismutase and peroxidase in soybean leaves but not the malondialdehyde content or electrical conductivity. These results suggest that uniconazole-induced tolerance to water deficit stress in soybean was related to the changes of photosynthesis, hormones and antioxidant system of leaves.     

Growth, Gas Exchange, Abscisic Acid, and Calmodulin Response to Salt Stress in Three Poplars

In the present study, we Investigated the effects of Increaslng sallnlty on growth, gas exchange, absclslc acld （ABA）, calmodulln （CAM）, and the relevance to salt tolerance In seedllngs of Populus euphratlca Ollv. and cutUngs of P. ＂pupularls 35-44＂ （P. popularls） and P. x euramerlcana cv. 1-214 （P. cv. Itallca）. The relatlve growth rates of shoot helght （RGR,） for P. cv. Itallca and P. popularls were severely reduced by Increaslng salt stress, whereas the growth reductlon was relatlvely less in P. euphratica. Slmllarly, P. euphratlca malntalned hlgher net photosynthetlc rates （Pn） and unlt transplration rotes （TRN） than P. cv. Itallca and P. popularls under condltlons of hlgher sallnlty. Sallnity caused a slgnlficant increase In leaf ABA and CaM In the three genotypes after the onset of stress, but NaCl-induced ABA and CaM accumulatlon was more pronounced In P. euphratlca, suggeeUng that P. euphratlca plants are more sensitlve in sensing soil salinlty than the other two poplars. Furthermore, P. euphratica maintained relatively higher ABA and CaM concentrations under conditions of high salinity. The higher capacity to synthesize stress signals, namely ABA and CaM, In P. euphratica and the contrlbuUon of this to the salt resistance of P. euphratica are discussed.     

Salicylic acid decreases the levels of dehydrin-like proteins in Tibetan hulless barley leaves under water stress

The effects of salicylic acid (SA) on the accumulation of dehydrins in leaves of Tibetan hulless barley seedlings under water stress were investigated. The results indicated that SA decreased the levels of the four dehydrin-like proteins induced by water stress. The concentrations of these dehydrin-like proteins increased under water stress. However, their levels in SA-pretreated seedlings were always lower than in those receiving only water stress. Our results also indicated that the levels of dehydrin-like proteins decreased as the SA concentration increased. In SA-pretreated seedlings, electrolyte leakage, MDA and H2O2 content were rather higher than in seedlings receiving only water stress. By these results, we suggest that lower levels of dehydrin-like proteins in seedlings with SA treatment may be due to the greater accumulation of H2O2 induced by SA, which causes more oxidative injury under water stress.     

Physiological response of riparian plants to watering in hyper-arid areas of Tarim River,China

The physiological responses and adaptive strategies of Populus euphratica Oliv.(arbor species),Tamarix ramosissima Ldb.(bush species),and Apocynum venetum L.(herb species)to variations in water and salinity stress were studied in the hyper-arid environment of the Tarim River in China.The groundwater table,the saline content of the groundwater,as well as the content of free proline,soluble sugars,plant endogenous hormones (abscisic acid (ABA),and cytokinins (CTK))of the leaves of the three species were monitored and analyzed at the lower reaches of the Tarim River in the study area where five transects were fixed at 100 m intervals along a vertical sampling line before and after water release.Saline stress dramatically increased soluble sugar concentration of the three species.Differences in sugar accumulation were determined among the species at different transects.The free proline concentration of the leaves of T.ramosissima and P.euphratica showed a proportional decrease with various degrees of elevation of the groundwater table after water release.There was a least correlation between the soluble sugars and proline stimulation in T.ramosissima.It was strongly suggested that T.ramosissima developed a different strategy to accumulate organic solutes to adapt to the stress environment.The soluble sugars and proline accumulation responded to the changes of groundwater table independently:the former occurred under salt stress,whereas the latter was more significant under drought stress.The concentration and the increase in concentration of ABA and CTK involved in stress resistance of the three species were also determined.This increase in the hormone concentration in P.euphratica was different from that of the other two species.Expressed as a function of increase of ABA concentration in leaves,A.venetum and T.ramosissima showed a different solute accumulation in response to groundwater table.There was a significant correlation between ABA accumulation and A [proline] in A.venetum as well as between ABA accumulation and △ [sugar] in T.ramosissima.     

Abscisic acid and cytokinins in the root exudates and leaves and their relationship to senescence and remobilization of carbon reserves in rice subjected to water stress during grain filling

The possible regulation of senescence-initiated remobilization of carbon reserves in rice (Oryza sativa L.) by abscisic acid (ABA) and cytokinins was studied using two rice cultivars with high lodging resistance and slow remobilization. The plants were grown in pots and either well-watered (WW, soil water potential = 0 MPa) or water-stressed (WS, soil water potential = -0.05 MPa) from 9 days after anthesis until they reached maturity. Leaf water potentials of both cultivars markedly decreased at midday as a result of water stress but completely recovered by early morning. Chlorophyll (Chl) and photosynthetic rate (Pr) of the flag leaves declined faster in WS plants than in WW plants, indicating that the water deficit enhanced senescence. Water stress accelerated starch remobilization in the stems, promoted the re-allocation of pre-fixed (14)C from the stems to grains, shortened the grain-filling period and increased the grain-filling rate. Sucrose phosphate synthase (SPS, EC 2.4.1.14) activity was enhanced by water stress and positively correlated with sucrose accumulation in both the stem and leaves. Water stress substantially increased ABA but reduced zeatin (Z) + zeatin riboside (ZR) concentrations in the root exudates and leaves. ABA significantly and negatively, while Z+ZR positively, correlated with Pr and Chl of the flag leaves. ABA, not Z+ZR, was positively and significantly correlated with SPS activity and remobilization of pre-stored carbon. Spraying ABA reduced Chl in the flag leaves, and enhanced SPS activity and remobilization of carbon reserves. Spraying kinetin had the opposite effect. The results suggest that both ABA and cytokinins are involved in controlling plant senescence, and an enhanced carbon remobilization is attributed to an elevated ABA level in rice plants subjected to water stress.     

Abscisic acid prevents pollen abortion under high‐temperature stress by mediating sugar metabolism in rice spikelets

Heat stress at the pollen mother cell (PMC) meiotic stage leads to pollen sterility in rice, in which the reactive oxygen species (ROS) and sugar homeostasis are always adversely affected. This damage is reversed by abscisic acid (ABA), but the mechanisms underlying the interactions among the ABA, sugar metabolism, ROS and heat shock proteins in rice spikelets under heat stress are unclear. Two rice genotypes, Zhefu802 (a recurrent parent) and fgl (its near‐isogenic line) were subjected to heat stress of 40°C after pre‐foliage sprayed with ABA and its biosynthetic inhibitor fluridone at the meiotic stage of PMC. The results revealed that exogenous application of ABA reduced pollen sterility caused by heat stress. This was achieved through various means, including: increased levels of soluble sugars, starch and non‐structural carbohydrates, markedly higher relative expression levels of heat shock proteins (HSP24.1 and HSP71.1) and genes related to sugar metabolism and transport, such as sucrose transporters (SUT) genes, sucrose synthase (SUS) genes and invertase (INV) genes as well as increased antioxidant activities and increased content of adenosine triphosphate and endogenous ABA in spikelets. In short, exogenous application of ABA prior to heat stress enhanced sucrose transport and accelerated sucrose metabolism to maintain the carbon balance and energy homeostasis, thus ABA contributed to heat tolerance in rice.     

Accumulation of theanderose in association with development of freezing tolerance in the moss Physcomitrella patens

Mosses are known to have the ability to develop high degrees of resistance to desiccation and freezing stress at cellular levels. However, underlying cellular mechanisms leading to the development of stress resistance in mosses are not understood. We previously showed that freezing tolerance in protonema cells of the moss Physcomitrella patens was rapidly increased by exogenous application of the stress hormone abscisic acid (ABA) [Minami, A., Nagao, M., Arakawa, K., Fujikawa, S., Takezawa, D., 2003a. Abscisic acid-induced freezing tolerance in the moss Physcomitrella patens is accompanied by increased expression of stress-related genes. J. Plant Physiol. 160, 475-483]. Herein it is shown that protonema cells with acquired freezing tolerance specifically accumulate low-molecular-weight soluble sugars. Analysis of the most abundant trisaccharide revealed that the cells accumulated theanderose (G6-alpha-glucosyl sucrose) in close association with enhancement of freezing tolerance by ABA treatment. The accumulation of theanderose was inhibited by cycloheximide, an inhibitor of nuclear-encoded protein synthesis, coinciding with a remarkable decrease in freezing tolerance. Furthermore, theanderose accumulation was promoted by cold acclimation and treatment with hyperosmotic solutes, both of which had been shown to enhance cellular freezing tolerance. These results reveal a novel role for theanderose, whose biological function has been obscure, in high freezing tolerance in moss cells.     

Phylogenetic analyses in cornus substantiate ancestry of xylem supercooling freezing behavior and reveal lineage of desiccation related proteins

The response of woody plant tissues to freezing temperature has evolved into two distinct behaviors: an avoidance strategy, in which intracellular water supercools, and a freeze-tolerance strategy, where cells tolerate the loss of water to extracellular ice. Although both strategies involve extracellular ice formation, supercooling cells are thought to resist freeze-induced dehydration. Dehydrin proteins, which accumulate during cold acclimation in numerous herbaceous and woody plants, have been speculated to provide, among other things, protection from desiccative extracellular ice formation. Here we use Cornus as a model system to provide the first phylogenetic characterization of xylem freezing behavior and dehydrin-like proteins. Our data suggest that both freezing behavior and the accumulation of dehydrin-like proteins in Cornus are lineage related; supercooling and nonaccumulation of dehydrin-like proteins are ancestral within the genus. The nonsupercooling strategy evolved within the blue- or white-fruited subgroup where representative species exhibit high levels of freeze tolerance. Within the blue- or white-fruited lineage, a single origin of dehydrin-like proteins was documented and displayed a trend for size increase in molecular mass. Phylogenetic analyses revealed that an early divergent group of red-fruited supercooling dogwoods lack a similar protein. Dehydrin-like proteins were limited to neither nonsupercooling species nor to those that possess extreme freeze tolerance.     

Alteration of starch- sucrose transition in germinating wheat seed under sodium chloride salinity

Alterations in starch-sucrose transition during germination were studied in wheat seeds under saline conditions. NaCI significantly reduced the speed of germination and resultant seedling growth, but delayed the degradation of seed storage components. The endogenous level of ABA increased while osmotic potential decreased. NaCI also inhibited the expression of α-amylase. Increasing the concentration of NaCI induced the expression of sucrose phosphate synthase, and sugars, including sucrose, were accumulated in the seedlings. This accumulation of sugar closely correlated with an increase in ABA. However, sugar accumulation was reversible when the salt stress was removed. Overall our results strongly suggest that the germinating wheat seeds alter the starch-to-sucrose conversion to adapt for salt stress. This is probably mediated by the increase in ABA.     

Photoperiodic induction of dormancy and freezing tolerance in Betula pubescens. Involvement of ABA and dehydrins

In many woody plants photoperiod signals the initiation of dormancy and cold acclimation. The photoperiod-specific physiological and molecular mechanisms have remained uncharacterised. The role of abscisic acid (ABA) and dehydrins in photope-riod-induced dormancy and freezing tolerance was investigated in birch, Betula pubescens Ehrh. The experiments were designed to investigate if development of dormancy and freezing tolerance under long-day (LD) and short-day (SD) conditions could be affected by manipulation of the endogenous ABA content, and if accumulation of dehydrin-like proteins was correlated with SD and/or the water content of the buds. Experimentally, the internal ABA content was increased by ABA application and by water stress treatment under LD, and decreased by blocking the synthesis of ABA with fluridone under SD. Additionally, high humidity (95% RH) was applied to establish if accidental water stress was involved in SD. ABA content was monitored by gas chromatography-mass spectrometry with selective ion monitoring (SIM). Short days induced a transient increase in ABA content, which was absent in 95% RH, whereas fluridone treatment decreased ABA. Short days induced a typical pattern of bud desiccation and growth cessation regardless of the treatment, and improved freezing tolerance except in the fluridone treatment. ABA content of the buds was significantly increased after spraying ABA on leaves and after water stress, treatments that did not induce cessation of growth and dormancy, but improved freezing tolerance. In addition to several constitutively produced dehydrins, two SD-specific proteins of molecular masses 34 and 36 kDa were found. Photoperiod- and experimentally-induced alterations in ABA contents affected freezing tolerance but not cessation of growth and dormancy. Therefore, involvement of ABA in the photoperiodic control of cold acclimation is more direct than in growth cessation and dormancy. As the typical desiccation pattern of the buds was found in all SD plants, and was not directly related to ABA content or to freezing tolerance, this pattern characterises the onset of photo-period-induced growth cessation and dormancy. The results provide evidence for the existence of various constitutively and two photoperiod-induced dehydrins in buds of birch, and reveal characteristics of dormancy and freezing tolerance that may facilitate further investigations of photoperiodic control of growth in trees.     

The effects of water,nutrient availability and their interaction on the growth,morphology and physiology of two poplar species

We exposed cuttings of two poplar species, Populus cathayana Rehder and Populus przewalskii Maximowicz, from Sect. Tacamahaca Spach to two watering regimes (well-watered and water-stressed conditions) and to two nutrient regimes (with or without fertilization) in a greenhouse to determine how fertilization affects the growth, morphology and physiology of poplars under different water conditions. Under stress conditions, changes in early growth and dry matter allocation, and decrease in gas exchange and the related functions are usually observed. Moreover, the measurement of carbon isotope composition (δ¹³C) provides an integrated measurement of water use efficiency. And abscisic acid (ABA) is a phytohormone which plays a prominent role in various physiological and biochemical processes related to environmental stresses. So we determine these characteristics and related parameters, and our results showed the following: (1) Fertilization promoted the growth of poplars under well-watered conditions, while under water-stressed conditions its effect on growth was negative. (2) Fertilization increased δ¹³C, total N concentration, chlorophyll a/b and intrinsic efficiency of photosystem II (Fv/Fm) but decreased relative water content of leaves, stomatal conductance, transpiration rate and C/N ratio under both well-watered and water-stressed conditions. (3) Fertilization appeared to increase net photosynthesis rate and decrease ABA content under well-watered conditions, while it decreased net photosynthesis rate and increased ABA content under water-stressed conditions. Moreover, compared to P. cathayana, collected from a lower altitude region, P. przewalskii, collected from a high-altitude region, has a slower growth rate and stronger adaptability to drought stress, which perhaps resulted from its chronic adaptability to the low water availability of high-altitude region; but to the nutrient stress, there was no difference between the two species.     

Sugar accumulation in roots of two grape varieties with contrasting response to water stress

Root sugar accumulation was studied in two grapevine varieties contrasting in tolerance to water stress. During a 10‐day water withholding treatment, the drought‐tolerant variety, Grenache, sustained less negative predawn and midday leaf water potentials as well as root water potential compared with the sensitive variety, Semillon. Grenache vines also maintained lower stomatal conductance and transpiration than Semillon vines throughout the drying period. In both varieties there was accumulation of sucrose in the roots and concentrations were inversely correlated to leaf and root water status. In both Grenache and Semillon, elevated root osmolality was associated with decreased soil moisture indicating that sugar accumulation may play a role in osmotic protection. Petiole xylem sap abscisic acid (ABA) concentrations increased with water deficit in both varieties and were highest for vines with the most negative root and predawn leaf water potentials. Furthermore, root sucrose concentrations were positively correlated with leaf xylem sap ABA concentrations, indicative of integration between carbohydrate metabolism and the ABA signalling system. Similar root sugar accumulation patterns between the two varieties, however, demonstrate that other factors are likely influencing the ability of the drought‐tolerant variety to remain hydrated.     

Accumulation of dehydrin-like proteins in the mitochondria of cereals in response to cold, freezing, drought and ABA treatment

Background

Dehydrins are known as Group II late embryogenesis abundant proteins. Their high hydrophilicity and thermostability suggest that they may be structure stabilizers with detergent and chaperone-like properties. They are localised in the nucleus, cytoplasm, and plasma membrane. We have recently found putative dehydrins in the mitochondria of some cereals in response to cold. It is not known whether dehydrin-like proteins accumulate in plant mitochondria in response to stimuli other than cold stress.

Results

We have found five putative dehydrins in the mitochondria of winter wheat, rye and maize seedlings. Two of these polypeptides had the same molecular masses in all three species (63 and 52 kD) and were thermostable. Drought, freezing, cold, and exogenous ABA treatment led to higher accumulation of dehydrin-like protein (dlp) 63 kD in the rye and wheat mitochondria. Protein 52 kD was induced by cold adaptation and ABA. Some accumulation of these proteins in the maize mitochondria was found after cold exposition only. The other three proteins appeared to be heat-sensitive and were either slightly induced or not induced at all by all treatments used.

Conclusions

We have found that, not only cold, but also drought, freezing and exogenous ABA treatment result in accumulation of the thermostable dehydrins in plant mitochondria. Most cryotolerant species such as wheat and rye accumulate more heat-stable dehydrins than cryosensitive species such as maize. It has been supposed that their function is to stabilize proteins in the membrane or in the matrix. Heat-sensitive putative dehydrins probably are not involved in the stress reaction and adaptation of plants.     

Regulation of sucrose and starch metabolism in potato tubers in response to short-term water deficit

To investigate the effect of water stress on carbon metabolism in growing potato tubers (Solanum tuberosum L.), freshly cut and washed discs were incubated in a range of mannitol concentrations corresponding to external water potential between 0 and −1.2 MPa. (i) Incorporation of [¹⁴C]glucose into starch was inhibited in water-stressed discs, and labeling of sucrose was increased. High glucose overrode the changes at low water stress (up to −0.5 MPa) but not at high water stress. (ii) Although [¹⁴C]sucrose uptake increased in water-stressed discs, less of the absorbed [¹⁴C]sucrose was metabolised. (iii) Analysis of the sucrose content of the discs confirmed that increasing water deficit leads to a switch, from net sucrose degradation to net sucrose synthesis. (iv) In parallel incubations containing identical concentrations of sugars but differing in which sugar was labeled, degradation of [¹⁴C]sucrose and labeling of sucrose from [¹⁴C]glucose and fructose was found at each mannitol concentration. This shows that there is a cycle of sucrose degradation and resynthesis in these tuber discs. Increasing the extent of water stress changed the relation between sucrose breakdown and sucrose synthesis, in favour of synthesis. (v) Analysis of metabolites showed a biphasic response to increasing water deficit. Moderate water stress (0–200 mM mannitol) led to a decrease of the phosphorylated intermediates, especially 3-phosphoglycerate (3PGA). The decrease of metabolites at moderate water stress was not seen when high concentrations of glucose were supplied to the discs. More extreme water stress (300–500 mM mannitol) was accompanied by an accumulation of metabolites at low and high glucose. (vi) Moderate water stress led to an activation of sucrose phosphate synthase (SPS) in discs, and in intact tubers. The stimulation involved a change in the kinetic properties of SPS, and was blocked␣by protein phosphatase inhibitors. (vii) The amount of ADP-glucose (ADPGlc) decreased when discs were incubated on 100 or 200 mM mannitol. There was a strong correlation between the in vivo levels of ADPGlc and 3PGA when discs were subjected to moderate water stress, and when the sugar supply was varied. (viii) The level of ADPGlc increased and starch synthesis was further inhibited when discs were incubated in 300–500 mM mannitol. (ix) It is proposed that moderate water stress leads to an activation of SPS and stimulates sucrose synthesis. The resulting decline of 3PGA leads to a partial inhibition of ADP-glucose pyrophosphorylase and starch synthesis. More-extreme water stress leads to a further alteration of partitioning, because it inhibits the activities of one or more of the enzymes involved in the terminal reactions of starch synthesis. Received: 26 August 1996 / Accepted: 5 November 1996     

Overexpression of a Harpin-encoding gene hrf1 in rice enhances drought tolerance

Harpin proteins are well known as eliciters that induce multiple responses in plants, such as systemic acquired resistance, hypersensitive response, enhancement of growth, resistance to the green peach aphid, and tolerance to drought. Overexpression of Harpin-encoding genes enhances plant resistance to diseases in tobacco, rice, rape, and cotton; however, it is not yet known whether the expression of Harpin-encoding genes in vivo improves plant tolerance to abiotic stresses. The results of this study showed that overexpression of a Harpin-encoding gene hrf1 in rice increased drought tolerance through abscisic acid (ABA) signalling. hrf1- overexpression induces an increase in ABA content and promotes stomatal closure in rice. The hrf1 transgenic rice lines exhibited a significant increase in water retention ability, levels of free proline and soluble sugars, tolerance to oxidative stress, reactive oxygen species-scavenging ability, and expression levels of four stress-related genes, OsLEA3-1, OsP5CS, Mn-SOD, and NM_001074345, under drought stress. The study confirmed that hrf1 conferred enhanced tolerance to drought stress on transgenic crops. These results suggest that Harpins may offer new opportunities for generating drought resistance in other crops.