Reduced Water Loss Rate Research Articles

Short day-triggered quiescence promotes water conservation in the American dog tick, Dermacentor variabilis.

A short day-long day exposure comparison of water balance characteristics throughout the life history of the American dog tick, Dermacentor variabilis, was conducted to examine how this tick survives winter-associated dryness. Larvae, nymphs, and adults under potential diapause-inducing conditions (short day, 20 °C) have low water loss rates compared to long day exposed ticks. Above the critical equilibrium activity, these nonfed ticks show greater water gain from lower water vapor activities. In contrast to nonfed stages, eggs and blood-engorged stages (fed larva, fed nymph) were enhanced for water conservation independent of short day exposure and did not display reduced water loss rates under short day conditions. This short day-prompted response in nonfed ticks was distinguished from a genetically programmed diapause as noted in other arthropods by: (1) reduction in water loss was temporary and could be reversed quickly by transferring to long day for only a single day, then re-triggered by returning to short day conditions; (2) switch between low and high water loss rates could be induced multiple times and for multiple different stages; and (3) response by ticks was to the photoperiod they were under, not the condition that was experienced by previous fed stages or in the mother in the case of eggs. Furthermore, short day had no impact on altering body water content, dehydration tolerance limit, instar yields, or developmental stadia. We conclude that a short day-triggered quiescence reduces desiccation stress associated with overwintering in nonfed ticks. This effect on water conservation due to short day exposure is more pronounced for subadults and is likely due to their increased sensitivity to water loss as a result of their smaller size, which is critical to prevent dehydration during the dry winter periods.

A Cotton MYB Transcription Factor, GbMYB5, is Positively Involved in Plant Adaptive Response to Drought Stress

Drought stress negatively affects plant growth and limits plant productivity. Genes functioning in plant responses to drought stress are essential for the development of drought-tolerant crops. Here, we report that an R2R3-type MYB transcription factor gene in Gossypium barbadense, GbMYB5, confers drought tolerance in cotton and transgenic tobacco. Virus-induced gene silencing of GbMYB5 compromised the tolerance of cotton plantlets to drought stress and reduced the post-rewatering water recovery survival rate to 50% as compared with the 90% survival rate in the wild type (WT). Silencing GbMYB5 decreased proline content and antioxidant enzyme activities and increased malondialdehyde (MDA) content in cotton under drought stress. The expression levels of drought-inducible genes NCED3, RD22 and RD26 were not affected by the silencing of GbMYB5. However, GbMYB5-overexpressing tobacco lines displayed hypersensitivity to ABA and improved survival rates as well as reduced water loss rates under drought stress. Furthermore, stomatal size and the rate of opening of stomata were markedly decreased in transgenic tobacco. The overexpression of GbMYB5 enhanced the accumulation of proline and antioxidant enzymes while it reduced production of MDA in transgenic tobacco as compared with the WT under drought stress. The transcript levels of the antioxidant genes SOD, CAT and GST, polyamine biosynthesis genes ADC1 and SAMDC, the late embryogenesis abundant protein-encoding gene ERD10D and drought-responsive genes NCED3, BG and RD26 were generally higher in GbMYB5-overexpressing tobacco than in the WT under drought stress. Collectively, our data suggested that GbMYB5 was positively involved in the plant adaptive response to drought stress.

Over-expression of SlSHN1 gene improves drought tolerance by increasing cuticular wax accumulation in tomato.

Increasing cuticular wax accumulation in plants has been associated with improving drought tolerance in plants. In this study, a cDNA clone encoding the SlSHN1 transcription factor, the closest ortholog to WIN/SHN1 gene in Arabidopsis, was isolated from tomato plant. Expression analysis of SlSHN1 indicated that it is induced in response to drought conditions. The over-expression of SlSHN1 in tomato under the control of the constitutive CaMV 35S promoter produced plants that showed mild growth retardation phenotype with shiny and dark green leaves. Scanning electron microscopy showed that the over-expression of SlSHN1 in tomato resulted in higher cuticular wax deposition on leaf epidermial tissue when compared to non-transformed plants. Expression analysis in transgenic lines over-expressing SlSHN1 indicated that several wax-related synthesis genes were induced. Transgenic tomato plants over-expressing SlSHN1 showed higher drought tolerance when compared with wild type plants; this was reflected in delayed wilting of transgenic lines, improved water status and reduced water loss rate when compared with wild type plants. In conclusion, the SlSHN1 gene can modulate wax accumulation and could be utilized to enhance drought tolerance in tomato plant.

Divergent mechanisms for water conservation inDrosophilaspecies

AbstractThe role of melanization and cuticular lipids in water conservation has been studied in manyDrosophilaspecies (Diptera:Drosophilidae). Nevertheless, a comparative approach to larval and adult stages of ecologically diverse, wildDrosophilaspecies is still required. Based upon abdominal cuticular melanization patterns, wild‐caughtDrosophilaspecies were categorized as (1) melanic, (2) fixed‐melanic, or (3) non‐melanic. At the interspecific level, the ecological significance of melanization and cuticular lipids was determined by the inverse association of melanization and cuticular water loss in melanic species, and of cuticular lipids and cuticular water loss in fixed‐melanic and non‐melanic species. Interestingly, higher amounts of cuticular lipids were also evident in fixed as well as non‐melanic species, as compared to melanic species at larval stages, which is consistent with their differences in reduced water loss rates. Moreover, fixed‐melanic and non‐melanic species exhibited comparatively higher (ca. 1.8–2.0 fold) desiccation resistance. Thus, cuticular lipids provide a better waterproofing mechanism than melanization. Furthermore, acclimation to dehydration stress in adults improved desiccation resistance in melanic species, whereas such effects were lacking in fixed‐melanic and non‐melanic species. However, there were no changes in cuticular components as a consequence of desiccation acclimation. Thus, our results indicate that melanic, fixed‐melanic, and non‐melanicDrosophilaspecies differ in the evolved physiological mechanisms of water conservation to adapt to dry conditions.

Behavioral correction to prevent overhydration and increase survival by larvae of the net-spinning caddisflies in relation to water flow.

We report behavioral regulation of body water content in caddisfly larvae, Hydropsyche morosa and Cheumatopsyche pettiti, by selecting microhabitats with different water flow rates. The purpose of our study was to examine features necessary for survival in the same apparent habitat, because the two species co-exist in riffle areas of freshwater streams. Both species are highly sensitive to water loss as a result of high water loss rates and depend on immersion in fresh water (hypo-osmotic) to maintain water stores. In contrast to C. pettiti, H. morosa is larger, retains water more effectively, and features reduced water loss rates with suppressed activation energies. When H. morosa was confined to areas of low or no water flow, overhydration led to rapid mortality, whereas the same conditions favored water balance maintenance and survival in C. pettiti. In attraction bioassays, H. morosa moved and remained within areas of high water flow and C. pettiti preferred areas with low water flow. Because water flow rates are unlikely to directly impact water gain, the mechanism responsible for increased survival and water balance maintenance is likely related to the impact of water flow on oxygen availability, differences in feeding ecology, or other underlying factors.

Overexpression of Transcription Factor OsWR2 Regulates Wax and Cutin Biosynthesis in Rice and Enhances its Tolerance to Water Deficit

Drought is the major abiotic stress limiting crop production. Plant cuticle represents the outer-most layer of the epidermis and plays an important role in decreasing plant water loss under drought stress by restricting non-stomatal transpiration. We report here that the Wax Synthesis Regulatory 2 gene (OsWR2) in rice (Oryza sativa L.) is highly expressed in epidermal tissues and contributes to the transcriptional regulation of both cuticular wax and cutin biosynthesis in rice cuticle. Overexpression of OsWR2 in rice increased the total cuticular wax level by 48.6 % in leaves and by 72.4 % in panicles. Of the major wax classes, aldehydes increased most in leaves, and alkanes increased most in panicles. Total cutin amounts were increased by 48.1 % in leaves and 65.9 % in panicles of rice overexpressing OsWR2, and these increases were due primarily to the increase in ω-OH and di-OH acids. Our results showed that 19 genes previously associated with wax and cutin biosynthesis were up-regulated in OsWR2 overexpressors. Overexpression of OsWR2 also altered cuticular wax crystallization and cuticle membrane ultrastructure. Furthermore, OsWR2 overexpression in rice decreased leaf chlorophyll leaching rate, reduced water loss rate, and enhanced tolerance to water-limited conditions. We demonstrate in this report that OsWR2 regulates wax and cutin biosynthesis differently than does the OsWR1 homologue, and plays a major role in controlling cuticle permeability. The increased resistance to water deficit conditions by OsWR2 overexpression in rice elucidates a potential new strategy for genetic improvement of plant drought tolerance.

Overexpression of the transcription factor HvSNAC1 improves drought tolerance in barley (Hordeum vulgare L.)

NAC proteins constitute a family of plant-specific transcription factors that are involved in many plant cellular processes including responses to abiotic stress. In this study, a cDNA clone encoding the HvSNAC1 transcription factor was isolated from drought-stressed barley using a bioinformatics approach based on amino acid sequence data of the stress-related SNAC1 protein from rice. Phylogenetic analysis of the deduced amino acid sequence of HvSNAC1 showed that this protein belongs to the stress clade of NAC proteins that include SNAC1 and TaNAC2. Expression analysis indicated that the HvSNAC1 gene is strongly induced by different abiotic stresses including drought. Overexpression of HvSNAC1 in barley under the control of a constitutive promoter produced plants that grew normally under well-watered conditions when compared with wild-type plants. Transgenic barley plants overexpressing HvSNAC1 showed higher drought tolerance at different growth stages when compared with wild-type plants. In addition, the constitutive overexpression of HvSNAC1 resulted in improved water status, photosynthetic activity and reduced water loss rate when compared with wild-type plants under drought conditions. Furthermore, the transgenic plants also showed significantly improved productivity, as reflected by the increase in biological yield over the wild-type plants under severe field drought conditions. In conclusion, the HvSNAC1 gene could be a useful tool for improving barley productivity under field drought conditions without impairment in growth under normal field conditions.

Increasing Nitric Oxide Content in Arabidopsis thaliana by Expressing Rat Neuronal Nitric Oxide Synthase Resulted in Enhanced Stress Tolerance

Nitric oxide (NO) plays essential roles in many physiological and developmental processes in plants, including biotic and abiotic stresses, which have adverse effects on agricultural production. However, due to the lack of findings regarding nitric oxide synthase (NOS), many difficulties arise in investigating the physiological roles of NO in vivo and thus its utilization for genetic engineering. Here, to explore the possibility of manipulating the endogenous NO level, rat neuronal NOS (nNOS) was expressed in Arabidopsis thaliana. The 35S::nNOS plants showed higher NOS activity and accumulation of NO using the fluorescent probe 3-amino, 4-aminomethyl-2', 7'-difluorescein, diacetate (DAF-FM DA) assay and the hemoglobin assay. Compared with the wild type, the 35S::nNOS plants displayed improved salt and drought tolerance, which was further confirmed by changes in physiological parameters including reduced water loss rate, reduced stomatal aperture, and altered proline and malondialdehyde content. Quantitative real-time PCR analyses revealed that the expression of several stress-regulated genes was up-regulated in the transgenic lines. Furthermore, the transgenic lines also showed enhanced disease resistance against Pseudomonas syringae pv. tomato (Pst) DC3000 by activating the expression of defense-related genes. In addition, we found that the 35S::nNOS lines flowered late by regulating the expression of CO, FLC and LFY genes. Together, these results demonstrated that it is a useful strategy to exploit the roles of plant NO in various processes by the expression of rat nNOS. The approach may also be useful for genetic engineering of crops with increased environmental adaptations.

Proliferation of red velvet mite Balaustium sp. nr. putmani (Erythraeidae) in Ohio based on water relations of active instars

ABSTRACT Population bursts, numbering in thousands of individuals, are characteristic of red velvet mites, Balaustium sp. nr. putmani, in the springtime in Central Ohio. We determined water-balance characteristics for each of the mobile instars (larva, deutonymph, adult female; males are seemingly absent from this population) to link dehydration tolerance with heightened activity in the same habitat. This mite loses water rapidly: larva > deutonymph > adult. None of these active instars could absorb water vapor, unlike many terrestrial mites, indicating that this mite must imbibe liquid as free water or from food sources to maintain water balance. As a voracious predator, most of the liquid is probably obtained from feeding. Developmentally, larvae are the most sensitive and vulnerable to water stress because of their high net transpiration rate and low dehydration tolerance limit. Additionally, urnulae glands are only present in the post-larval instars; hence, larvae lack the protective waterproofing secretion that is provided by these glands. Deutonymphs and adults are much more resistant to desiccation, are capable of losing approximately 40% of their water content before succumbing to death, and have reduced water loss rates. We concluded that population bursts of B. sp. nr. putmani might be a response to abundance of plant pests in the landscape. Deutonymphs and adults are modified for water conservation and are correspondingly active for longer periods of time. Larvae survive for a brief time, which minimizes their exposure to desiccating habitats, and use the cooler temperatures experienced early in the season, which suppress their high rate of water loss.

Transgenic expression of MYB15 confers enhanced sensitivity to abscisic acid and improved drought tolerance in Arabidopsis thaliana

Abiotic stresses cause serious crop losses. Knowledge on genes functioning in plant responses to adverse growth conditions is essential for developing stress tolerant crops. Here we report that transgenic expression of MYB15, encoding a R2R3 MYB transcription factor in Arabidopsis thaliana, conferred hypersensitivity to exogenous abscisic acid (ABA) and improved tolerance to drought and salt stresses. The promoter of MYB15 was active in not only vegetative and reproductive organs but also the guard cells of stomata. Its transcript level was substantially upregulated by ABA, drought or salt treatments. Compared with wild type (WT) control, MYB15 overexpression lines were hypersensitive to ABA in germination assays, more susceptible to ABA-elicited inhibition of root elongation, and more sensitive to ABA-induced stomatal closure. In line with the above findings, the transcript levels of ABA biosynthesis (ABA1, ABA2), signaling (ABI3), and responsive genes (AtADH1, RD22, RD29B, AtEM6) were generally higher in MYB15 overexpression seedlings than in WT controls after treatment with ABA. MYB15 overexpression lines displayed improved survival and reduced water loss rates than WT control under water deficiency conditions. These overexpression lines also displayed higher tolerance to NaCl stress. Collectively, our data suggest that overexpression of MYB15 improves drought and salt tolerance in Arabidopsis possibly by enhancing the expression levels of the genes involved in ABA biosynthesis and signaling, and those encoding the stress-protective proteins.

The effect of respiratory pattern on water loss in desiccation-resistant Drosophila melanogaster.

We measured CO2 and H2O release from individual fruit flies from five populations of Drosophila melanogaster selected for resistance to desiccation (D flies). Our previous work found that these flies survive for an extended period in dry air, have an increase in the peak height and frequency of CO2 release, as measured by the standard error of a linear regression (SER) of CO2 release for the entire survival period, and have reduced water loss rates (VH2O) compared with their control or ancestor populations. In the present study, we examined the following respiratory characteristics: VCO2, VH2O, the SER of CO2 release and the ratio of VCO2 to VH2O in the D flies. Correlations between these characters were calculated in order to determine the effect of respiratory pattern on water loss. We found that, within the D flies, neither periodic release of CO2 nor an increased SER for CO2 release was associated with reduced water loss. In addition, an increased SER was positively correlated with both an increased water loss rate and a decreased survival time. Therefore, although selection for desiccation resistance leads to both an increased SER and a decreased rate of water loss in the D flies, the increased SER does not significantly reduce respiratory water loss.

Physiological mechanisms of evolved desiccation resistance in Drosophila melanogaster.

We investigated physiological characters associated with water balance in laboratory populations of Drosophila melanogaster selected for resistance to desiccating conditions for over 100 generations. Five replicate, outbred, desiccation-selected (D) populations were compared with their control (C) populations. Water loss rates of female D flies were approximately 40% lower than those of C females. Although excretory water loss was reduced in desiccation-selected flies, it comprised less than 10% of total water loss, indicating that the D populations have evolved reduced cuticular and/or respiratory water loss rates. Total surface lipid amounts did not differ between the C and D flies. Cuticular hydrocarbons from D flies were longer than those from C flies and melted at slightly higher temperatures, possibly contributing to reduced water loss rates. Desiccation-selected flies contained approximately 30% more bulk water than controls, as well as more glycogen. However, total metabolic water stores did not differ between the stocks owing to higher lipid levels in the C populations. The ability to tolerate water loss, as measured by water content at the time of death, did not differ between D and C flies. Thus, evolution of increased desiccation resistance has occurred by multiple physiological mechanisms, but some potential adaptive differences have not evolved.

Reduced Rates of Water Loss and Chemical Properties of Skin Secretions of the Frogs Litoria caerulea and Cyclorana australis

We measured the rates of water loss in two Australian hylid frogs: the arboreal Litoria caerulea and the terrestrial burrowing frog Cyclorana australis. We measured the latter species with and without cocoons. Both species showed reduced rates of water loss compared with ‘typical’ amphibians that lose water as if from a free surface. Cocooned C. australis had very low rates of water loss. We examined the chemical composition of skin secretions rinsed (using only high-pure water) from both species and the cocoon material from C. australis. The chemical composition of the material from these three sources was generally similar and consisted of 5–10% neutral lipids and 78–85% proteinaceous material. The fact that the terrestrial species has a high resistance to water loss is unusual given that almost all other known species of non-cocooned frogs with reduced rates of water loss are arboreal. The chemical similarity of the skin secretions and cocoons from this species suggest that the reduced rate of water loss in this species is linked to its ability to form a cocoon. Amino acid composition of the material indicated that a sclerotisation process may occur upon oxidation of the secretions. This would result in a physical barrier to water loss in the cocoons and possibly a thin physical proteinaceous barrier on the skin of both species in the absence of cocoons. We suggest that the high proportion of proteins in the skin secretions cannot be ignored, and that it may, in conjunction with the lipids, produce an effective waterproofing barrier in both species. We suggest that chemical components other than lipids also may be important in frogs from other continents, and complete compositional analyses of frog ‘mucus’ are required before we can fully understand the nature of the mechanisms involved in reduced rates of water loss in amphibians with and without cocoons.

Postharvest Browning of Rambutan is a Consequence of Water Loss

Rambutan (Nephelium lappaceum L.) rapidly lose their attractive appearance after harvest due to a superficial pericarp browning. Storage at high humidity minimizes fruit desiccation and may, therefore, delay browning onset. This paper examines the effect of reduced water loss rate on browning that may occur with time. Rambutan fruit pericarp browning beyond a commercially saleable level occurred at a weight loss of 25% to 40%. This depended on duration and storage relative humidity (RH). Skin browning was 50% greater on the red (R 134) than the yellow (R 156) cultivar at 60% RH. There was a storage time × RH interaction in the development of browning such that browning was observed earlier at lower RHs. Skin browning and spintern (soft spine) browning developed independently. Cracks appeared on the surface of fruit with increased weight loss. Browning occurrence was consistent with increased total phenolic compound levels in the pericarp. Water loss precedes browning occurrence and, over time, water loss is related to browning. Water stress appeared to affect rambutan pericarp tissue in much the same manner as senescence.