Involvement of Abscisic Acid in Regulating Water Status in Phaseolus vulgaris L. during Chilling

During the first hours of chilling, bean (Phaseolus vulgaris L., cv Mondragone) seedlings suffer severe water stress and wilt without any significant increase in leaf abscisic acid (ABA) content (P. Vernieri, A. Pardossi, F. Tognoni [1991] Aust J Plant Physiol 18: 25-35). Plants regain turgor after 30 to 40 h. We hypothesized that inability to rapidly synthesize ABA at low temperatures contributes to chilling-induced water stress and that turgor recovery after 30 to 40 h is mediated by changes in endogenous ABA content. Entire bean seedlings were subjected to long-term (up to 6 d) chilling (3°C, 0.2-0.4 kPa vapor pressure deficit, 100 μmol·m⁻²·s⁻¹ photosynthetic photon flux density, continuous fluorescent light). During the first 24 h, stomata remained open, and plants rapidly wilted as leaf transpiration exceeded root water absorption. During this phase, ABA did not accumulate in leaves or in roots. After 24 h, ABA content increased in both tissues, leaf diffusion resistance increased, and plants rehydrated and regained turgor. No osmotic adjustment was associated with turgor recovery. Following turgor recovery, stomata remained closed, and ABA levels in both roots and leaves were elevated compared with controls. The application of ABA (0.1 mm) to the root system of the plants throughout exposure to 3°C prevented the chilling-induced water stress. Excised leaves fed 0.1 mm ABA via the transpiration stream had greater leaf diffusion resistance at 20 and 3°C compared with non-ABA fed controls, but the amount of ABA needed to elicit a given degree of stomatal closure was higher at 3°C compared with 20°C. These findings suggest that endogenous ABA may play a role in ameliorating plant water status during chilling.     

Abscisic Acid is not the only stomatal inhibitor in the transpiration stream of wheat plants

Xylem sap was collected from the transpiration stream of wheat (Triticum aestivum L.) plants and assayed for the presence of an inhibitor of transpiration using leaves detached from well-watered plants. Transpiration of detached leaves was reduced by nearly 60% by sap collected from plants in drying soil, and to a lesser extent (about 25%) by sap from plants in well-watered soil. As the soil dried the abscisic acid (ABA) concentration in the sap increased by about 50 times to 5 × 10⁻⁸ molar. However, the ABA in the sap did not cause its inhibitory activity. Synthetic ABA of one hundred times this concentration was needed to reduce transpiration rates of detached leaves to the same extent. Furthermore, inhibitory activity of the sap was retained after its passage through an immunoaffinity column to remove ABA. Xylem sap was also collected by applying pressure to the roots of plants whose leaf water status was kept high as the soil dried. Sap collected from these plants reduced transpiration to a lesser extent than sap from nonpressurised plants. This suggests that the inhibitory activity was triggered partly by leaf water deficit and partly by root water deficit.     

Stomatal response to abscisic Acid is a function of current plant water status

We investigated, under laboratory and field conditions, the possibility that increasing abscisic acid (ABA) concentrations and decreasing water potentials can interact in their effects on stomata. One experiment was carried out with epidermal pieces of Commelina communis incubated in media with a variety of ABA and polyethylene glycol concentrations. In the media without ABA, incubation in solutions with water potentials between −0.3 and −1.5 megapascals had no significant effect on stomatal aperture. Conversely, the sensitivity of stomatal aperture to ABA was trebled in solutions at −1.5 megapascals compared with sensitivity at −0.3 megapascals. The effect of the change in sensitivity was more important than the absolute effect of ABA at the highest water potential. In a field experiment, sensitivity of maize stomatal conductance to the concentration of ABA in the xylem sap varied strongly with the time of the day. We consider that the most likely explanation for this is the influence of a change in leaf or epidermal water potential that accompanies an increase in irradiance and saturation deficit as the day progresses. These observations suggest that epidermal water relations may act as a modulator of the responses of stomata to ABA. We argue that such changes must be taken into account in studies or modeling of plant responses to drought stress.     

Drought-induced increases in abscisic Acid levels in the root apex of sunflower

Abscisic acid (ABA) levels in 3-mm apical root segments of slowly droughted sunflower plants (Helianthus annuus L. cv Russian Giant) were analyzed as the methyl ester by selected ion monitoring gas chromatography-mass spectrometry using characteristic ions. An internal standard, hexadeuterated ABA (d6ABA) was used for quantitative analysis. Sunflower seedlings, grown in aeroponic chambers, were slowly droughted over a 7-day period. Drought stress increased ABA levels in the root tips at 24, 72, and 168 hour sample times. Control plants had 57 to 106 nanograms per gram ABA dry weight in the root tips (leaf water potential, −0.35 to −0.42 megapascals). The greatest increase in ABA, about 20-fold, was found after 72 hours of drought (leaf water potential, −1.34 to −1.47 megapascals). Levels of ABA also increased (about 7− to 54-fold) in 3-mm apical root segments which were excised and then allowed to dessicate for 1 hour at room temperature.     

Water deficits and reproduction in maize : response of the reproductive tissue to water deficits at anthesis and mid-grain fill

Reproductive development in maize (Zea mays L.) is vulnerable to plant water deficits during anthesis but becomes less sensitive as reproduction progresses. To determine whether changes in tissue water status correlated with the change in sensitivity, we examined the water potential (Ψ_w), osmotic potential (Ψ_s), and turgor of reproductive tissues during a short-term water deficit imposed at anthesis or mid-grain fill. Plants were grown in controlled environments in soil. At anthesis, leaf, husk, silk, and ovary Ψ_w of control plants was similar (−0.5 to −0.65 megapascal) at midday. When water was withheld, Ψ_w decreased to −1.75, −1.3, −1.2, and −1.0 megapascal in these tissues. Net water uptake by the ovaries was inhibited, but final dry weight, solute content, and total extractable carbohydrates were similar to the controls. At mid-grain fill, leaf, husk, grain, and embryo Ψ_w of control plants were −0.55, −0.35, −0.75, and −0.80 megapascal at midday. When water was withheld, leaf and husk Ψ_w decreased to −2.4 and −1.4 megapascal within 6 days. However, grain and embryo Ψ_w remained within 0.15 megapascal of control values. The grain continued to accumulate dry matter despite a net loss of water and a reduction in total solute content. These results indicate that the response of the reproductive tissues to plant water deficits varies with stage of grain development. The maintenance of a favorable water status only after grain filling is under way may explain, at least in part, the high sensitivity to plant water deficits early in reproductive development and the decrease in sensitivity as reproduction progresses.     

Does Water Deficit Stress Promote Ethylene Synthesis by Intact Plants?

The effect of plant water deficit on ethylene production by intact plants was tested in three species, beans (Phaseolus vulgaris L.), cotton (Gossypium hirsutum L.) and miniature rose (Rosa hybrida L., cv Bluesette). Compressed air was passed through glass, plant-containing cuvettes, ethylene collected on chilled columns, and subsequently assayed by gas chromatography. The usual result was that low water potential did not promote ethylene production. When plants were subjected to cessation of irrigation, ethylene production decreased on a per plant or dry weight basis of calculation. No significant promotion of ethylene production above control levels was detected when water deficit-treated bean or cotton plants were rewatered. The one exception to this was for cotton subjected to a range of water deficits, plants subjected to deficits of −1.4 to −1.6 MPa exhibited a transient increase of ethylene production of 40 to 50% above control levels at 24 or 48 hours. Ethylene was collected from intact leaves while plants developed a water deficit stress of −2.9 megapascals after rewatering, and no significant promotion of ethylene production was detected. The shoots of fruited, flowering cotton plants produced less ethylene when subjected to cessation of irrigation. In contrast, the ability of bench drying of detached leaves to increase ethylene production several-fold was verified for both beans and cotton. The data indicate that detached leaves react differently to rapid drying than intact plants react to drying of the soil with regard to ethylene production. This result suggests the need for additional attention to ethylene as a complicating factor in experiments employing excised plant parts and the need to verify the relevance of shock stresses in model systems.     

OsGL1-3 is Involved in Cuticular Wax Biosynthesis and Tolerance to Water Deficit in Rice

Cuticular wax covers aerial organs of plants and functions as the outermost barrier against non-stomatal water loss. We reported here the functional characterization of the Glossy1(GL1)-homologous gene OsGL1-3 in rice using overexpression and RNAi transgenic rice plants. OsGL1-3 gene was ubiquitously expressed at different level in rice plants except root and its expression was up-regulated under ABA and PEG treatments. The transient expression of OsGL1-3–GFP fusion protein indicated that OsGL1-3 is mainly localized in the plasma membrane. Compared to the wild type, overexpression rice plants exhibited stunted growth, more wax crystallization on leaf surface, and significantly increased total cuticular wax load due to the prominent changes of C₃₀–C₃₂ aldehydes and C₃₀ primary alcohols. While the RNAi knockdown mutant of OsGL1-3 exhibited no significant difference in plant height, but less wax crystallization and decreased total cuticular wax accumulation on leaf surface. All these evidences, together with the effects of OsGL1-3 on the expression of some wax synthesis related genes, suggest that OsGL1-3 is involved in cuticular wax biosynthesis. Overexpression of OsGL1-3 decreased chlorophyll leaching and water loss rate whereas increased tolerance to water deficit at both seedling and late-tillering stages, suggesting an important role of OsGL1-3 in drought tolerance.     

Involvement of abscisic Acid in potato cold acclimation

Upon exposure to 2°C day/night (D/N), leaves of Solanum commersonii (Sc) began acclimating on the 4th day from a −5°C (killing temperature) hardy level to −12°C by the 15th day. Leaves of S. tuberosum L. (St) cv `Red Pontiac' typically failed to acclimate and were always killed at −3°C. Leaves of control (20/15°C, D/N) and treated plants (2°C, D/N) of St showed similar levels of free abscisic acid (ABA) during a 15-day sampling period. In treated Sc plants, however, free ABA contents increased 3-fold on the 4th day and then declined to their initial level thereafter. The increase was not observed in leaves of Sc control plants.

Treated St plants showed a slightly higher content of leaf soluble protein than controls. In Sc, leaves of controls maintained relatively constant soluble proteins, but leaves of treated plants showed a distinct increase. This significant increase was initiated on the 4th day, peaked on the 5th day, and remained at a high level throughout the 15-day sampling period.

Exogenously applied ABA induced frost hardiness in leaves of Sc plants whether plants were grown under a 20°C or 2°C temperature regime. When cycloheximide was added to the medium of stem-cultured plants at the beginning of 2°C acclimation, or at the beginning of the ABA treatment in the 20°C regime, it completely inhibited the development of frost hardiness. However, when cycloheximide was added to plants on the 5th day during 2°C acclimation, the induction of frost hardiness was not inhibited. The role of ABA in triggering protein synthesis needed to induce frost hardiness is discussed.

     

Stomatal responses to water stress and to abscisic Acid in phosphorus-deficient cotton plants

Cotton (Gossypium hirsutum L.) plants were grown in sand culture on nutrient solution containing adequate or growth-limiting levels of P. When water was withheld from the pots, stomata of the most recently expanded leaf closed at leaf water potentials of approximately −16 and −12 bars in the normal and P-deficient plants, respectively. Pressure-volume curves showed that the stomata of P-deficient plants closed when there was still significant turgor in the leaf mesophyll. Leaves of P-deficient plants accumulated more abscisic acid (ABA) in response to water stress, but the difference was evident only at low water potentials, after initiation of stomatal closure. In leaves excised from unstressed plants, P deficiency greatly increased stomatal response to ABA applied through the transpiration stream. Kinetin blocked most of this increase in apparent sensitivity to ABA. The effect of P nutrition on stomatal behavior may be related to alterations of the balance between ABA and cytokinins.     

Characterization of the β-Carotene Hydroxylase Gene DSM2 Conferring Drought and Oxidative Stress Resistance by Increasing Xanthophylls and Abscisic Acid Synthesis in Rice

Drought is a major limiting factor for crop production. To identify critical genes for drought resistance in rice (Oryza sativa), we screened T-DNA mutants and identified a drought-hypersensitive mutant, dsm2. The mutant phenotype was caused by a T-DNA insertion in a gene encoding a putative β-carotene hydroxylase (BCH). BCH is predicted for the biosynthesis of zeaxanthin, a carotenoid precursor of abscisic acid (ABA). The amounts of zeaxanthin and ABA were significantly reduced in two allelic dsm2 mutants after drought stress compared with the wild type. Under drought stress conditions, the mutant leaves lost water faster than the wild type and the photosynthesis rate, biomass, and grain yield were significantly reduced, whereas malondialdehyde level and stomata aperture were increased in the mutant. The mutant is also hypersensitive to oxidative stresses. The mutant had significantly lower maximal efficiency of photosystem II photochemistry and nonphotochemical quenching capacity than the wild type, indicating photoinhibition in photosystem II and decreased capacity for eliminating excess energy by thermal dissipation. Overexpression of DSM2 in rice resulted in significantly increased resistance to drought and oxidative stresses and increases of the xanthophylls and nonphotochemical quenching. Some stress-related ABA-responsive genes were up-regulated in the overexpression line. DSM2 is a chloroplast protein, and the response of DSM2 to environmental stimuli is distinctive from the other two BCH members in rice. We conclude that the DSM2 gene significantly contributes to control of the xanthophyll cycle and ABA synthesis, both of which play critical roles in the establishment of drought resistance in rice.Abiotic stresses such as drought, salinity, and adverse temperatures are major limiting factors for plant growth and reproduction. To respond to environmental cues, plants have evolved a variety of biochemical and physiological mechanisms to adapt to adverse conditions during their growth and development (Boyer, 1982). Abscisic acid (ABA) has been recognized as a stress hormone that coordinates the complex networks of stress responses. Under drought or salt stress conditions, plant endogenous ABA level can rise to about 40-fold, triggering the closure of stomata and accumulating reactive oxygen species (ROS), dehydrins, and late embryogenesis abundant proteins for osmotic adjustment (Verslues et al., 2006). The endogenous ABA level is determined by ABA biosynthesis, catabolism, and release of ABA from ABA-Glc conjugates (Nambara and Marion-Poll, 2005; Lee et al., 2006). Therefore, identification of all the components affecting active ABA content is essential for a complete understanding of the action of the hormone.Numerous ABA biosynthetic genes have been identified through mutant analysis, such as maize (Zea mays) viviparous mutants vp2, vp5, vp7, vp9, vp14, w3, y3, and y9 (Schwartz et al., 1997; Hable et al., 1998; Singh et al., 2003); rice (Oryza sativa) preharvest-sprouting mutants psh1, psh2, psh3, and psh4 (Fang et al., 2008); sunflower (Helianthus annuus) nondormant mutant nd-1 (Conti et al., 2004); Arabidopsis (Arabidopsis thaliana) ABA- and nonphotochemical quenching (NPQ)-deficient mutants aba1, aba2, aba3, aba4, npq1, npq2, b1, b2, and nced3 (Havaux et al., 2000; Xiong et al., 2001; Tian et al., 2003; Barrero et al., 2005; Kim and DellaPenna, 2006; North et al., 2007); and tomato (Solanum lycopersicum) white-flower mutant wf (Galpaz et al., 2006; Supplemental Fig. S1). The mutants unable to biosynthesize carotenoid precursors for endogenous ABA synthesis often produced preharvest-sprouting seeds and wilted or white leaves (Gubler et al., 2005; Nambara and Marion-Poll, 2005; Finch-Savage and Leubner-Metzger, 2006).ABA biosynthesis initiates with the synthesis of a C5 building block, isopentenyl pyrophosphate, and its isomer dimethylallyl pyrophosphate through a plastid methylerythritol phosphate pathway (Eisenreich et al., 2001; Hunter, 2007). The three isopentenyl pyrophosphate molecules are then added to dimethylallyl pyrophosphate by geranylgeranyl diphosphate synthase to produce C20 geranylgeranyl diphosphate. Two geranylgeranyl diphosphates are condensed by a committing enzyme, phytoene synthase, to produce colorless C40 carotenoid phytoene, which is then desaturated and isomerized into red-colored lycopene by phytoene desaturase (PDS), ζ-carotene desaturase (ZDS), and Z-ISO and CRTISO isomerases in plants (Isaacson et al., 2002; Park et al., 2002). Subsequently, several cyclization and hydroxylation reactions take place to yield α-carotene and β-carotene (Li et al., 1996; Hable et al., 1998; Park et al., 2002; Miki and Shimamoto, 2004; Fang et al., 2008). Heme-type cytochrome P450-type CYP97 and non-heme-type β-carotene hydroxylase (BCH) are primarily responsible for the hydroxylation of α-carotene and β-carotene to produce lutein and zeaxanthin, respectively. Zeaxanthin, an important component of the xanthophyll cycle, is epoxidated by zeaxanthin epoxidase to produce violaxanthin, and this reaction can be reversed by violaxanthin deepoxidase to increase the xanthophyll cycle for plants to adapt to high-light stress (Johnson et al., 2008). Neoxanthin synthase converts violaxanthin into neoxanthin (North et al., 2007). In chloroplast, 9-cis-epoxycarotenoid dioxygenase (NCED) cleaves violaxanthin and neoxanthin to produce xanthoxin, the direct substrate for ABA synthesis via ABA aldehyde (Schwartz et al., 1997, 2003; Xiong and Zhu, 2003). Increasing evidence suggest that the endogenous ABA level is fine-tuned by differential regulation of the multiple steps of ABA biosynthesis (Seo and Koshiba, 2002; Nambara and Marion-Poll, 2005; Destefano-Beltrán et al., 2006; Thompson et al., 2007; Rodríguez-Gacio et al., 2009; Supplemental Fig. S1).The xanthophyll cycle (light-dependent reversible conversion between violaxanthin and zeaxanthin) is involved in photoprotection in PSII by regulating the nonradiative dissipation of excess absorbed light energy as heat (Gilmore et al., 1994). Mutants with defects in the xanthophyll cycle exhibit a weak photoprotective ability and produce ROS such as hydrogen peroxide (H₂O₂) when the absorption of light energy exceeds that consumed for photosynthesis (Niyogi, 1999). Under dehydration stress, electrons at a high energy state can easily form ROS, which are toxic to proteins, DNA, and lipids (Mittler, 2002; Apel and Hirt, 2004). However, plants have evolved a variety of biochemical and physiological mechanisms to scavenge ROS, thus maintaining a balance between ROS production and scavenging (Mittler et al., 2004).An association between the xanthophyll cycle and stress tolerance has been reported in plants. In Arabidopsis, overexpression of a bacterial BCH gene caused a specific 2-fold increase in the size of the xanthophyll cycle and enhanced photooxidative tolerance (Davison et al., 2002). Constitutive overexpression of a bacterial BCH gene, crtZ, in tobacco (Nicotiana tabacum) led to increased zeaxanthin synthesis and enhanced UV light tolerance (Götz et al., 2002). In Arabidopsis, zeaxanthin synthesis can be catalyzed by both heme-type CYP97 hydroxylases LUT1 and LUT5 and non-heme-type hydroxylases BCH1 and BCH2, and these two types exhibit some overlapping activities (Tian et al., 2003, 2004; Kim and DellaPenna, 2006). In contrast to the intensive molecular and genetic studies of BCH in Arabidopsis, the counterpart in economically important crops such as rice has not been identified.In this study, we characterized the rice drought-sensitive mutant dsm2, impaired in the gene DSM2 encoding a BCH. Our results demonstrate that DSM2 acts as a putative enzyme catalyzing the biosynthesis of zeaxanthin, one of the precursors of ABA that participates in the process of NPQ. Decreases of NPQ, maximal efficiency of PSII photochemistry (F_v/F_m), xanthophylls, and ABA in the dsm2 mutant suggest that the drought hypersensitivity of dsm2 is due to the combination of impairments in the xanthophyll cycle and ABA synthesis under drought stress conditions. DSM2 overexpression lines, possessing high F_v/F_m and NPQ, showed significantly improved drought resistance at both seedling and reproductive stages. Furthermore, our results imply that DSM2 may be the major member of the BCH family in rice for controlling zeaxanthin synthesis in response to dehydration stresses.     

Increased endogenous abscisic Acid maintains primary root growth and inhibits shoot growth of maize seedlings at low water potentials

Roots of maize (Zea mays L.) seedlings continue to grow at low water potentials that cause complete inhibition of shoot growth. In this study, we have investigated the role of abscisic acid (ABA) in this differential growth sensitivity by manipulating endogenous ABA levels as an alternative to external applications of the hormone. An inhibitor of carotenoid biosynthesis (fluridone) and a mutant deficient in carotenoid biosynthesis (vp 5) were used to reduce the endogenous ABA content in the growing zones of the primary root and shoot at low water potentials. Experiments were performed on 30 to 60 hour old seedlings that were transplanted into vermiculite which had been preadjusted to water potentials of approximately −1.6 megapascals (roots) or −0.3 megapascals (shoots). Growth occurred in the dark at near-saturation humidity. Results of experiments using the inhibitor and mutant approaches were very similar. Reduced ABA content by either method was associated with inhibition of root elongation and promotion of shoot elongation at low water potentials, compared to untreated and wild-type seedlings at the same water potential. Elongation rates and ABA contents at high water potential were little affected. The inhibition of shoot elongation at low water potential was completely prevented in fluridone-treated seedlings during the first five hours after transplanting. The results indicate that ABA accumulation plays direct roles in both the maintenance of primary root elongation and the inhibition of shoot elongation at low water potentials.     

Pseudomonas syringae naturally lacking the canonical type III secretion system are ubiquitous in nonagricultural habitats,are phylogenetically diverse and can be pathogenic

The type III secretion system (T3SS) is an important virulence factor of pathogenic bacteria, but the natural occurrence of variants of bacterial plant pathogens with deficiencies in their T3SS raises questions about the significance of the T3SS for fitness. Previous work on T3SS-deficient plant pathogenic bacteria has focused on strains from plants or plant debris. Here we have characterized T3SS-deficient strains of Pseudomonas syringae from plant and nonplant substrates in pristine nonagricultural contexts, many of which represent recently described clades not yet found associated with crop plants. Strains incapable of inducing a hypersensitive reaction (HR⁻) in tobacco were detected in 65% of 126 samples from headwaters of rivers (mountain creeks and lakes), snowpack, epilithic biofilms, wild plants and leaf litter and constituted 2 to 100% of the P. syringae population associated with each sample. All HR⁻ strains lacked at least one gene in the canonical hrp/hrc locus or the associated conserved effector locus, but most lacked all six of the genes tested (hrcC, hrpL, hrpK1, avrE1 and hrpW1) and represented several disparate phylogenetic clades. Although most HR⁻ strains were incapable of causing symptoms on cantaloupe seedlings as expected, strains in the recently described TA-002 clade caused severe symptoms in spite of the absence of any of the six conserved genes of the canonical T3SS according to PCR and Southern blot assays. The phylogenetic context of the T3SS variants we observed provides insight into the evolutionary history of P. syringae as a pathogen and as an environmental saprophyte.     

Asic3?/? Female Mice with Hearing Deficit Affects Social Development of Pups

Background

Infant crying is an important cue for mothers to respond adequately. Inappropriate response to infant crying can hinder social development in infants. In rodents, the pup-mother interaction largely depends on pup''s calls. Mouse pups emit high frequency to ultrasonic vocalization (2–90 kHz) to communicate with their dam for maternal care. However, little is known about how the maternal response to infant crying or pup calls affects social development over the long term.

Methodology/Principal Findings

Here we used mice lacking acid-sensing ion channel 3 (Asic3^−/−) to create a hearing deficit to probe the effect of caregiver hearing on maternal care and adolescent social development. Female Asic3^−/− mice showed elevated hearing thresholds for low to ultrasonic frequency (4–32 kHz) on auditory brain stem response, which thus hindered their response to their pups'' wriggling calls and ultrasonic vocalization, as well as their retrieval of pups. In adolescence, pups reared by Asic3^−/− mice showed a social deficit in juvenile social behaviors as compared with those reared by wild-type or heterozygous dams. The social-deficit phenotype in juvenile mice reared by Asic3^−/− mice was associated with the reduced serotonin transmission of the brain. However, Asic3^−/− pups cross-fostered to wild-type dams showed rescued social deficit.

Conclusions/Significance

Inadequate response to pups'' calls as a result of ASIC3-dependent hearing loss confers maternal deficits in caregivers and social development deficits in their young.     

The presence of ribulose 1,5-diphosphate carboxylase in the nonphotosynthetic endosperm of germinating castor beans

Ribulose 1,5-diphosphate carboxylase was detected in extracts of germinating castor bean (Ricinus communis var. Hale) endosperms. This is the first report of this enzyme in a nonphotosynthetic (no chlorophyll) plant tissue. Radioactive 3-phosphoglyceric acid has been identified as the principle product resulting from the enzymatic condensation of ¹⁴C-bicarbonate and ribulose-1,5-diP in endosperm extracts. The Km values of bicarbonate and ribulose-1,5-diP for the endosperm carboxylase are 1.14 × 10⁻²m and 7.5 × 10⁻⁵m, respectively. The carboxylase activity peaks at 4 days in endosperms of castor beans germinated in the dark. The specific activity of the carboxylase at this stage of germination is 4.3 μmoles of 3-phosphoglycerate formed/mg protein·hr. The presence of ribulose-1,5-diP carboxylase and other enzymes of the reductive pentose phosphate pathway show the potential of this pathway in castor bean endosperms.     

Effect of water deficits on seed development in soybean : I. Tissue water status

Water deficits during seed filling often decrease seed size in soybean (Glycine max L.). The physiological basis for this response is not known but may result from direct effects of low seed water potential (Ψ_w) on the seed filling process. To determine whether low Ψ_w occurred in reproductive tissues of soybean, we monitored the water status (Ψ_w, Ψ_s, and Ψ_p) of leaf, pericarp, and seed (embryo and testa) tissue of greenhouse-grown plants subjected to a brief water deficit during the linear period of seed growth. Water deficits were imposed by withholding water and monitored in the reproductive tissues by thermocouple psychrometry. When water was abundant, leaf, pericarp, and seed Ψ_w were −0.5 to −0.7 megapascal at midday. When water was withheld, leaf Ψ_w decreased to −2.3 megapascals within 6 days. Pericarp Ψ_w also decreased to −1.9 megapascal during this time. Pericarp Ψ_s followed the decline in Ψ_w, but osmotic adjustment was not evident as the pericarp lost turgor completely by day 6. However, seed Ψ_w, Ψ_s, and Ψ_p were not significantly different from the controls. These results indicate that the water status of the developing seeds of soybean is not altered by short-term water deficits severe enough to inhibit the metabolic activity of the maternal plant. Maintenance of a favorable water status may be important for the conservation of seed growth rate exhibited by soybean under dry conditions.     

Nucleoside Diphosphate Sugar-Starch Glucosyl Transferase Activity of wx Starch Granules

Starch granule preparations from the endosperm tissue of all waxy maize (Zea mays L.) mutants tested have low and approximately equal capability to incorporate glucose from adenosine diphosphate glucose into starch. As the substrate concentration is reduced, however, the activity of waxy preparations relative to nonmutant increases until, at the lowest substrate concentration utilized (0.1 μM), the activity of the waxy preparations is nearly equal to that of the nonmutant preparation. The apparent K_m (adenosine diphosphate glucose) for starch granule preparations from wx-C/wx-C/wx-C endosperms was 7.1 × 10⁻⁵ M, which is compared to 3 × 10⁻³ M for preparations from nonwaxy endosperms. Starch granule preparations from three other waxy mutants of independent mutational origin have levels of enzymic activity approximately equal to wx-C at a given substrate concentration giving rise to similar apparent K_m estimates. We conclude that there is in maize endosperm starch granules a second starch granule-bound glycosyl transferase, whose presence is revealed when mutation eliminates activity of the more active glucosyl transferase catalyzing the same reaction.