Characterization of Cd translocation and identification of the Cd form in xylem sap of the Cd-hyperaccumulator Arabidopsis halleri

Arabidopsis halleri is a Cd hyperaccumulator; however, the mechanismsinvolved in the root to shoot translocation of Cd are not wellunderstood. In this study, we characterized Cd transfer fromthe root medium to xylem in this species. Arabidopsis halleriaccumulated 1,500 mg kg^–1 Cd in the shoot without growthinhibition. A time-course experiment showed that the releaseof Cd into the xylem was very rapid; by 2 h exposure to Cd,Cd concentration in the xylem sap was 5-fold higher than thatin the external solution. The concentration of Cd in the xylemsap increased linearly with increasing Cd concentration in theexternal solution. Cd transfer to the xylem was completely inhibitedby the metabolic inhibitor carbonyl cyanide 3-chlorophenylhydrazone(CCCP). Cd concentration in the xylem sap was decreased by increasingthe concentration of external Zn, but enhanced by Fe deficiencytreatment. Analysis with ¹¹³Cd-nuclear magnetic resonance (NMR)showed that the chemical shift of ¹¹³Cd in the xylem sap wasthe same as that of Cd(NO₃)₂. Metal speciation with Geochem-PCalso showed that Cd occurred mainly in the free ionic form inthe xylem sap. These results suggest that Cd transfer from theroot medium to the xylem in A. halleri is an energy-dependentprocess that is partly shared with Zn and/or Fe transport. Furthermore,Cd is translocated from roots to shoots in inorganic forms.     

The mechanism selecting the guide strand from small RNA duplexes is different among argonaute proteins

Double-stranded RNA induces RNA silencing and is cleaved into21–24 nt small RNA duplexes by Dicer enzyme. A strandof Dicer-generated small RNA duplex (called the guide strand)is then selected by a thermodynamic mechanism to associate withArgonaute (AGO) protein. This AGO–small RNA complex functionsto cleave mRNA, repress translation or modify chromatin structurein a sequence-specific manner. Although a model plant, Arabidopsisthaliana, contains 10 AGO genes, their roles and molecular mechanismsremain obscure. In this study, we analyzed the roles of ArabidopsisAGO2 and AGO5. Interestingly, the 5' nucleotide of small RNAsthat associated with AGO2 was mainly adenine (85.7%) and thatwith AGO5 was mainly cytosine (83.5%). Small RNAs that wereabundantly cloned from the AGO2 immunoprecipitation fraction(miR163-LL, which is derived from the Lower Left of mature miR163in pre-miR163, and miR390) and from the AGO5 immunoprecipitationfraction (miR163-UL, which is derived from the Upper Left ofmature miR163 in pre-miR163, and miR390^*) are derived from thesingle small RNA duplexes, miR163-LL/miR163-UL and miR390/miR390^*.Each strand of the miR163-LL/miR163-UL duplex is selectivelysorted to associate with AGO2 or AGO5 in a 5' nucleotide-dependentmanner rather than in a thermodynamic stability-dependent manner.Furthermore, we showed that both AGO2 and AGO5 have the abilityto bind cucumber mosaic virus-derived small RNAs. These resultsclearly indicate that the mechanism selecting the guide strandis different among AGO proteins and that multiple AGO genesare involved in anti-virus defense in plants.     

52Fe Translocation in Barley as Monitored by a Positron-Emitting Tracer Imaging System (PETIS): Evidence for the Direct Translocation of Fe from Roots to Young Leaves via Phloem

The real-time translocation of iron (Fe) in barley (Hordeumvulgare L. cv. Ehimehadaka no. 1) was visualized using the positron-emittingtracer ⁵²Fe and a positron-emitting tracer imaging system (PETIS).PETIS allowed us to monitor Fe translocation in barley non-destructivelyunder various conditions. In all cases, ⁵²Fe first accumulatedat the basal part of the shoot, suggesting that this regionmay play an important role in Fe distribution in graminaceousplants. Fe-deficient barley showed greater translocation of⁵²Fe from roots to shoots than did Fe-sufficient barley, demonstratingthat Fe deficiency causes enhanced ⁵²Fe uptake and translocationto shoots. In the dark, translocation of ⁵²Fe to the youngestleaf was equivalent to or higher than that under the light condition,while the translocation of ⁵²Fe to the older leaves was decreased,in both Fe-deficient and Fe-sufficient barley. This suggeststhe possibility that the mechanism and/or pathway of Fe translocationto the youngest leaf may be different from that to the olderleaves. When phloem transport in the leaf was blocked by steamtreatment, ⁵²Fe translocation from the roots to older leaveswas not affected, while ⁵²Fe translocation to the youngest leafwas reduced, indicating that Fe is translocated to the youngestleaf via phloem in addition to xylem. We propose a novel modelin which root-absorbed Fe is translocated from the basal partof the shoots and/or roots to the youngest leaf via phloem ingraminaceous plants.     

Cold-Tolerant Crop Species Have Greater Temperature Homeostasis of Leaf Respiration and Photosynthesis Than Cold-Sensitive Species

Some plant species show constant rates of respiration and photosynthesismeasured at their respective growth temperatures (temperaturehomeostasis), whereas others do not. However, it is unclearwhat species show such temperature homeostasis and what factorsaffect the temperature homeostasis. To analyze the inherentability of plants to acclimate respiration and photosynthesisto different growth temperatures, we examined 11 herbace-ouscrops with different cold tolerance. Leaf respiration (R_area)and photosynthetic rate (P_area) under high light at 360 µll^–1 CO₂ concentrations were measured in plants grown at15 and 30°C. Cold-tolerant species showed a greater extentof temperature homeostasis of both R_area and P_area than cold-sensitivespecies. The underlying mechanisms which caused differencesin the extent of temperature homeostasis were examined. Theextent of temperature homeostasis of P_area was not determinedby differences in leaf mass and nitrogen content per leaf area,but by differences in photosynthetic nitrogen use efficiency(PNUE). Moreover, differences in PNUE were due to differencesin the maximum catalytic rate of Rubisco, Rubisco contents andamounts of nitrogen invested in Rubisco. These findings indicatedthat the temperature homeostasis of photosynthesis was regulatedby various parameters. On the other hand, the extent of temperaturehomeostasis of R_area was unrelated to the maximum activity ofthe respiratory enzyme (NAD-malic enzyme). The R_area/P_area ratiowas maintained irrespective of the growth temperatures in allthe species, suggesting that the extent of temperature homeostasisof R_area interacted with the photosynthetic rate and/or thehomeostasis of photosynthesis.     

Measurement of Changes in Cytosolic Ca2+ in Arabidopsis Guard Cells and Mesophyll Cells in Response to Blue Light

Phototropins (phot1 and phot2) are blue light (BL) receptorsthat mediate responses including phototropism, chloroplast movementand stomatal opening, and increased cytosolic Ca²⁺. BL absorbedby phototropins activates plasma membrane H⁺-ATPase in guardcells, resulting in membrane hyperpolarization, and drives K⁺uptake and stomatal opening. However, it is unclear whetherthe phototropin-mediated Ca²⁺ increase activates the H⁺-ATPase.Here, we determined cytosolic Ca²⁺ concentrations in guard cellprotoplasts (GCPs) from Arabidopsis transformed with aequorin.Cytosolic Ca²⁺ increased rapidly in response to BL in GCPs fromboth the wild type and phot1 phot2 double mutants, but was mostlysuppressed by an inhibitor of photosynthetic electron flow (DCMU).With depleted external K⁺, we observed another slower Ca²⁺ increase,which was phototropin- dependent. Fusicoccin, a H⁺-ATPase activator,mimicked the effect of BL. The slow Ca²⁺ increase thus appearsto result from membrane hyperpolarization. The slow Ca²⁺ increasewas suppressed by external K⁺ and was restored by blockers ofinward-rectifying K⁺ channels, CsCl and tetraethylammonium,suggesting the preferential uptake of K⁺ over Ca²⁺. Such efficientK⁺ uptake in response to BL was not found in mesophyll cells.Both the fast and the slow Ca²⁺ increases were inhibited byCa²⁺ channel blockers (CoCl₂ and LaCl₃) and a chelating agent(EGTA). These results indicate that the phototropin-mediatedCa²⁺ increase was not observed prior to H⁺-ATPase activationin guard cells and that Ca²⁺ entered guard cells via Ca²⁺ channelsthrough photosynthesis and phototropin-mediated membrane hyperpolarization.     

Mechanisms to avoid photoinhibition in a desiccation-tolerant cyanobacterium, Nostoc commune

A desiccation-tolerant cyanobacterium, Nostoc commune, showsunique responses to dehydration. These responses are: (i) lossof PSII activity in parallel with the loss of photosynthesis;(ii) loss of PSI activity; and (iii) dissipation of light energyabsorbed by pigment–protein complexes. In this study,the deactivation of PSII is shown to be important in avoidingphotoinhibition when the Calvin–Benson cycle is repressedby dehydration. Furthermore, our evidence suggests that dissipationof light energy absorbed by PSII blocks photoinhibition understrong light in dehydrated states.     

Electrophysiological characterization of the node in Chara corallina: functional differentiation for wounding response

Electrical characteristics of the node were analyzed in comparisonwith those of the flank of the internodal cell in Chara corallina.The dependence of the membrane potential of the node on pH andK⁺ concentration was almost the same as that of the flank. Inthe flank, the increase in the Ca²⁺ concentration stopped thedepolarization in the presence of 100 mM KCl. In the node, however,Ca²⁺ could not stop the depolarization induced by 100 mM KCl.It has been reported that the node has a function to tranducethe signal of osmotic shock into a transient depolarization.In combination with osmotic shock, 10 mM K⁺ could induce a long-lastingdepolarization of the node. These electrical characteristicsof the node were suggested to be responsible for the electricalresponse to wounding in Characeae.     

Involvement of Polypyrimidine Tract-Binding Protein (PTB)-Related Proteins in Pollen Germination in Arabidopsis

The pollen grains of most angiosperms contain stores of RNAsand their translation products required for pollen germinationand subsequent early elongation of pollen tubes. Polypyrimidinetract-binding protein (PTB), which is involved in the regulationof pre-mRNA alternative splicing, internal ribosomal entry site(IRES)-mediated translation and mRNA localization/sorting, isknown to act as a bridging molecule between RNAs and a varietyof cellular factors to fulfill cellular functions in both thenucleus and cytoplasm. Moreover, it has been reported that PTBplays roles in the differentiation and development of animalcells and tissues. In the Arabidopsis genome, there are twoPTB-related genes, tentatively termed AtPTB1 and AtPTB2. Inthe present study, the physiological functions of AtPTBs wereinvestigated using genetic and cytological approaches. The AtPTBpromoter was highly active in vegetative cells of mature pollengrains, and AtPTB was localized in the nucleus and cytoplasmof these vegetative cells. Mutations in the AtPTB genes resultedin decreased germination efficiency, and this effect was rescuedby introduction of the AtPTB2 promoter::AtPTB2–GFP. Takentogether, these findings suggest that AtPTB is involved in pollengermination through possible RNA metabolism processes in late-maturingand mature pollen grains.     

AtAMT1;4, a Pollen-Specific High-Affinity Ammonium Transporter of the Plasma Membrane in Arabidopsis

Pollen represents an important nitrogen sink in flowers to ensurepollen viability. Since pollen cells are symplasmically isolatedduring maturation and germination, membrane transporters arerequired for nitrogen import across the pollen plasma membrane.This study describes the characterization of the ammonium transporterAtAMT1;4, a so far uncharacterized member of the ArabidopsisAMT1 family, which is suggested to be involved in transportingammonium into pollen. The AtAMT1;4 gene encodes a functionalammonium transporter when heterologously expressed in yeastor when overexpressed in Arabidopsis roots. Concentration-dependentanalysis of ¹⁵N-labeled ammonium influx into roots of AtAMT1;4-transformedplants allowed characterization of AtAMT1;4 as a high-affinitytransporter with a K_m of 17 µM. RNA and protein gel blotanalysis showed expression of AtAMT1;4 in flowers, and promoter–genefusions to the green fluorescent protein (GFP) further definedits exclusive expression in pollen grains and pollen tubes.The AtAMT1;4 protein appeared to be localized to the plasmamembrane as indicated by protein gel blot analysis of plasmamembrane-enriched membrane fractions and by visualization ofGFP-tagged AtAMT1;4 protein in pollen grains and pollen tubes.However, no phenotype related to pollen function could be observedin a transposon-tagged line, in which AtAMT1;4 expression isdisrupted. These results suggest that AtAMT1;4 mediates ammoniumuptake across the plasma membrane of pollen to contribute tonitrogen nutrition of pollen via ammonium uptake or retrieval.     

Alterations in Detergent-Resistant Plasma Membrane Microdomains in Arabidopsis thaliana During Cold Acclimation

Microdomains in the plasma membrane (PM) have been proposedto be involved in many important cellular events in plant cells.To understand the role of PM microdomains in plant cold acclimation,we isolated the microdomains as detergent-resistant plasma membranefractions (DRMs) from Arabidopsis seedlings and compared lipidand protein compositions before and after cold acclimation.The DRM was enriched in sterols and glucocerebrosides, and theproportion of free sterols in the DRM increased after cold acclimation.The protein-to-lipid ratio in the DRM was greater than thatin the total PM fraction. The protein amount recovered in DRMsdecreased gradually during cold acclimation. Cold acclimationfurther resulted in quantitative changes in DRM protein profiles.Subsequent mass spectrometry and Western blot analyses revealedthat P-type H⁺-ATPases, aquaporins and endocytosis-related proteinsincreased and, conversely, tubulins, actins and V-type H⁺-ATPasesubunits decreased in DRMs during cold acclimation. Functionalcategorization of cold-responsive proteins in DRMs suggeststhat plant PM microdomains function as platforms of membranetransport, membrane trafficking and cytoskeleton interaction.These comprehensive changes in microdomains may be associatedwith cold acclimation of Arabidopsis.     

The signaling role of extracellular ATP and its dependence on Ca2+ flux in elicitation of Salvia miltiorrhiza hairy root cultures

The application of a polysaccharide elicitor from yeast extract,YE, to Salvia miltiorrhiza hairy root cultures induced transientrelease of ATP from the roots to the medium, leading to a dose-dependentincrease in the extracellular ATP (eATP) level. The eATP levelrose to a peak (about 6.5 nM with 100 mg l^–1 YE) at about10 h after YE treatment, but dropped to the control level 6h later. The elicitor-induced ATP release was dependent on membraneCa²⁺ influx, and abolished by the Ca²⁺ chelator EGTA or thechannel blocker La³⁺. The YE-induced H₂O₂ production was stronglyinhibited by reactive blue (RB), a specific inhibitor of membranepurinoceptors. On the other hand, the application of exogenousATP at 10–100 µM to the cultures also induced rapidand dose-dependent increases in H₂O₂ production and medium pH,both of which were effectively blocked by RB and EGTA. The non-hydrolyzableATP analog ATPS was as effective as ATP, but the hydrolyzedderivatives ADP or AMP were not so effective in inducing thepH and H₂O₂ increases. Our results suggest that ATP releaseis an early event and that eATP plays a signaling role in theelicitation of plant cell responses; Ca²⁺ is required for activationof the elicitor-induced ATP release and the eATP signal transduction.This is the first report on ATP release induced by a fungalelicitor and its involvement in the elicitor-induced responsesin plant cells.     

Generation of Hydroxyl Radical in Isolated Pea Root Cell Wall, and the Role of Cell Wall-Bound Peroxidase, Mn-SOD and Phenolics in Their Production

The hydroxyl radical produced in the apoplast has been demonstratedto facilitate cell wall loosening during cell elongation. Cellwall-bound peroxidases (PODs) have been implicated in hydroxylradical formation. For this mechanism, the apoplast or cellwalls should contain the electron donors for (i) H₂O₂ formationfrom dioxygen; and (ii) the POD-catalyzed reduction of H₂O₂to the hydroxyl radical. The aim of the work was to identifythe electron donors in these reactions. In this report, hydroxylradical (·OH) generation in the cell wall isolated frompea roots was detected in the absence of any exogenous reductants,suggesting that the plant cell wall possesses the capacity togenerate ·OH in situ. Distinct POD and Mn-superoxidedismutase (Mn-SOD) isoforms different from other cellular isoformswere shown by native gel electropho-resis to be preferably boundto the cell walls. Electron paramagnetic resonance (EPR) spectroscopyof cell wall isolates containing the spin-trapping reagent,5-diethoxyphosphoryl-5-methyl-1-pyrroline-N-oxide (DEPMPO),was used for detection of and differentiation between ·OHand the superoxide radical (O₂^–·). The data obtainedusing POD inhibitors confirmed that tightly bound cell wallPODs are involved in DEPMPO/OH adduct formation. A decreasein DEPMPO/OH adduct formation in the presence of H₂O₂ scavengersdemonstrated that this hydroxyl radical was derived from H₂O₂.During the generation of ·OH, the concentration of quinhydronestructures (as detected by EPR spectroscopy) increased, suggestingthat the H₂O₂ required for the formation of ·OH in isolatedcell walls is produced during the reduction of O₂ by hydroxycinnamicacids. Cell wall isolates in which the proteins have been denaturated(including the endogenous POD and SOD) did not produce ·OH.Addition of exogenous H₂O₂ again induced the production of ·OH,and these were shown to originate from the Fenton reaction withtightly bound metal ions. However, the appearance of the DEPMPO/OOHadduct could also be observed, due to the production of O₂^–·when endogenous SOD has been inactivated. Also, O₂^–·was converted to ·OH in an in vitro horseradish peroxidase(HRP)/H₂O₂ system to which exogenous SOD has been added. Takentogether with the discovery of the cell wall-bound Mn-SOD isoform,these results support the role of such a cell wall-bound SODin the formation of ·OH jointly with the cell wall-boundPOD. According to the above findings, it seems that the hydroxycinnamicacids from the cell wall, acting as reductants, contribute tothe formation of H₂O₂ in the presence of O₂ in an autocatalyticmanner, and that POD and Mn-SOD coupled together generate ·OHfrom such H₂O₂.     

Identification of maize silicon influx transporters

Maize (Zea mays L.) shows a high accumulation of silicon (Si),but transporters involved in the uptake and distribution havenot been identified. In the present study, we isolated two genes(ZmLsi1 and ZmLsi6), which are homologous to rice influx Sitransporter OsLsi1. Heterologous expression in Xenopus laevisoocytes showed that both ZmLsi1 and ZmLsi6 are permeable tosilicic acid. ZmLsi1 was mainly expressed in the roots. By contrast,ZmLsi6 was expressed more in the leaf sheaths and blades. Differentfrom OsLsi1, the expression level of both ZmLsi1 and ZmLsi6was unaffected by Si supply. Immunostaining showed that ZmLsi1was localized on the plasma membrane of the distal side of rootepidermal and hypodermal cells in the seminal and crown roots,and also in cortex cells in lateral roots. In the shoots, ZmLsi6was found in the xylem parenchyma cells that are adjacent tothe vessels in both leaf sheaths and leaf blades. ZmLsi6 inthe leaf sheaths and blades also exhibited polar localizationon the side facing towards the vessel. Taken together, it canbe concluded that ZmLsi1 is an influx transporter of Si, whichis responsible for the transport of Si from the external solutionto the root cells and that ZmLsi6 mainly functions as a Si transporterfor xylem unloading.     

Normalizing Development of Cultured Triticum aestivum L. Embryos: I. LOW OXYGEN TENSIONS AND EXOGENOUS ABA

Wheat (Triticum aestivum L.) embryos form in dynamically-regulatedovular environments. Our objectives were to improve developmentof cultured immature wheat embryos by simulating, in vitro,abscisic acid (ABA) levels and O₂ tensions as found in wheatovules during zygotic embryogenesis. We characterized from intactwheat kernels embryo respiration, embryo morphology and embryoand endosperm + ABA levels at 13, 19 and 25 d post-anthesis(DPA). Young (13 DPA) embryos were then excised and culturedin vitro, where they were exposed to 0·2 or 2·Ommol m^–3 ±ABA and 2.·1, 2·5 or 7·4mol m^–3 (6, 7 and 21%, respectively) gaseous O₂. At 6and 12 d in culture, + ABA levels, embryo respiration and embryomorphology were characterized by treatment. Thirteen-day-oldembryos from two different plant populations differed by 17-foldin initial ABA content. However, this difference did not affectprecocious germination in vitro, nor did it affect the amountof exogenous ABA required to reduce precocious germination by40%. In this respect, embryos from both populations were equallysensitive to exogenous ABA. Cavity sap O₂ levels (2·1to 2·5 mol m^–3) were much more effective in preventingprecocious germination of cultured embryos than were cavitysap levels of ABA (0·2 to 2·0 mmol m^–3).The combination of physiological levels of both ABA and O₂ largelynormalized DW accumulation and embryo morphology without alteringendogenous + ABA levels. Residual respiration of cultured embryoswas higher than that of embryos grown in situ, and was not influencedby the exogenous O₂ and ABA treatments Key words: Abscisic acid, embryo development, oxygen tensions, respiration, wheat     

Four TFL1/CEN-Like Genes on Distinct Linkage Groups Show Different Expression Patterns to Regulate Vegetative and Reproductive Development in Apple (Malusxdomestica Borkh.)

Recent molecular analyses in several plant species revealedthat TERMINAL FLOWER1 (TFL1) and CENTRORADIALIS (CEN) homologsare involved in regulating the flowering time and/or maintainingthe inflorescence meristem. In apple (Malusxdomestica Borkh.),four TFL1/CEN-like genes, MdTFL1, MdTFL1a, MdCENa and MdCENb,were found and mapped by a similar position on putatively homoeologouslinkage groups. Apple TFL1/CEN-like genes functioned equivalentlyto TFL1 when expressed constitutively in transgenic Arabidopsisplants, suggesting that they have a potential to complementthe TFL1 function. Because MdTFL1 and MdTFL1a were expressedin the vegetative tissues in both the adult and juvenile phases,they could function redundantly as a flowering repressor anda regulator of vegetative meristem identity. On the other hand,MdCENa was mainly expressed in fruit receptacles, cultured tissuesand roots, suggesting that it is involved in the developmentof proliferating tissues but not in the control of the transitionfrom the juvenile to the adult phase. In contrast, MdCENb wassilenced in most organs probably due to gene duplication bythe polyploid origin of apple. The expression patterns of MdTFL1and MdCENa in apple were also supported by the heterologousexpression of β-glucuronidase fused with their promoterregions in transgenic Arabidopsis. Our results suggest thatfunctional divergence of the roles in the regulation of vegetativemeristem identity may have occurred among four TFL1/CEN-likegenes during evolution in apple.     

Carrier-Mediated ABA Uptake by Suspension-Cultured Phaseolus coccineus L. Cells: Stereospecificity and Inhibition by Ionones and ABA Esters

Astle, M. and Rubery, P. 1987. Carrier-mediated ABA uptake bysuspension-cultured Phaseolus coccineus L. cells: Stereospecificityand inhibition by ionones and ABA esters.—J. exp. Bot.38: 150–163. The substrate for the abscisic acid (ABA) carrier in Phaseoluscoccineus L. suspension-cultured cells is shown to be the (S)ABAenantiomer, K_m = 1?0 mmol m^–3. The methyl (MeABA) andphenyl (PheABA) esters of ABA inhibit carrier-mediated uptakeof ABA with half-maximal inhibition achieved at about 7?0 mmolm^–3 and 10 mmol m^–3 respectively: with (S)MeABAthis value is decreased to about 2?0 mmol m^–3. There isno demethylation of radioactive MeABA by the cells during 5min incubations. Although MeABA reversibly inhibits the ABAcarrier, it is not a transport substrate: association of radioactiveMeABA with living cells is unaffected by non-radioactive MeABAor ABA and, by comparison with frozen-and-thawed cells, it isshown that the radioactivity remains extracellular. It is proposedthat MeABA binds to the carrier to form an abortive complexthat is not translocated. The terpenoid ABA analogue LAB 144143also inhibits carrier-mediated ABA uptake. At concentrationsup to about 20 mmol m^–3 - and ß-ionone specificallyinhibit the ABA carrier with the half-maximal effect at about0?6 mmol m^–3 ß-ionone. However, at higher iononeconcentrations, the uptake of ABA, indol-3-yl acetic acid andof 5,5-dimethyloxazolidine-2,4-dione (DMO) are all stimulated:this may reflect general permeabilization of the membrane toweak acids by ionone. Key words: Uptake carrier, abscisic acid, methyl and phenyl esters of ABA, ionone, Phaseolus coccineus L. suspension culture     

Abscisic Acid Content and Effects During Dehydration of Detached Leaves of Desiccation Tolerant Plants

Borya nitida is an angiospcrm whose detached leaves developcomplete tolerance to dehydration when they are equilibratedto air of 96% r.h. This treatment causes leaves to yellow aschlorophyll is destroyed, and abscisic acid contents increaseseveral-fold. Exogenous ABA (at 0.038–0.38 mol m^–3)promoted desiccation tolerance (a) in leaves undergoing toleranceinduction at 96% r.h., (b) only slightly during rapid dryingat rates which are normally injurious, and (c) considerablyin turgid tissue treated with ABA 48 h before rapid drying. ABA content also increased with intense water stress in Myrothamnusflabellifolia, a desiccation tolerant angiosperm which, unlikeBorya, retains most of its chlorophyll when dehydrated. Preliminaryincubation in ABA of detached leaves of this ‘resurrectionplant’ also promoted survival during rapid drying. Theability of ABA to substitute for the normal induction periodsuggests that this hormone participates in the development ofdesiccation tolerance. Key words: Abscisic acid, ABA, Drought tolerance, Resurrection plant