Plant Drought Response Research Articles

Biochemical and physiological responses of rice (Oryza sativa L.) as influenced by Trichoderma harzianum under drought stress

Rice is one of the most important food crops drastically affected by drought in lowland rice ecosystems. In the present study, the impact of drought tolerant isolates of endophytic fungus Trichoderma harzianum on rice response to drought was investigated. Out of 43 Trichoderma isolates studied, only five isolates viz., Th 56, Th 69, Th 75, Th 82 and Th 89 were selected to be drought tolerant as these were able to colonize well on cow dung at low moisture content of 10-20 percent, though two isolates, Th 56 and Th 75, grew even at 5 percent moisture content. Trichoderma-colonized rice seedlings were slower to wilt in response to drought. Colonization delayed drought induced changes like stomatal conductance, net photosynthesis and leaf greenness. Drought conditions varying from 3 to 9 days of withholding water led to an increase in the concentration of many stress induced metabolites in rice leaves and decrease of MSI, while Trichoderma colonization caused a decrease in proline, MDA and H₂O₂ contents, and increase in phenolics concentration and MSI. Among test isolates, Th 56 induced maximum drought tolerance as treated plants recorded only 20-40 percent wilting even at 9 DDS. With or without exposure to drought, colonization by Trichoderma promoted seedling growth, with Th 56 giving the most consistent effect. The primary direct effect of Trichoderma colonization was promotion of root growth, regardless of water status, which caused delay in the drought responses of rice plants.

Long‐Term Field Drought Affects Leaf Protein Pattern and Chloroplast Ultrastructure of Winter Wheat in a Cultivar‐Specific Manner

AbstractRecurrent drought periods of varying duration often cause extensive crop damage and affect wheat production in Southern Europe. This study compares biochemical and ultrastructural responses of four wheat (Triticum aestivum L.) cultivars to long‐term field drought, and their contribution to final grain yield. Gel electrophoresis and immunoblotting analyses combined with transmission electron microscopy and grain yield evaluation were employed to assess drought susceptibility of the wheat cultivars. Two of them behaved as drought‐tolerant, the other two presented as drought sensitive. Enhanced degradation of Rubisco large subunit (RLS), Rubisco small subunit (RSS) and Rubisco activase (RA) accompanied by an increased protease activity and reduced levels of heat shock proteins (HSP70) and dehydrins (DHNs) were associated with drought sensitivity. Drought tolerance coincided with relatively stable or increased HSP70 and DHN contents, and unchanged/higher levels of RLS, RSS and RA. Sensitive cultivars were more vulnerable to ultrastructural damages, showing obvious degradation of chloroplast membrane systems and depletion of leaf starch reserves. These drought responses affected yield potential, as tolerant cultivars gave higher yield under intense drought. Thus, our results provide additional insights into the complexity of plant drought responses, identifying multiple interacting traits that may serve as indirect selection criteria for wheat drought tolerance.

The Potential of Proteomics Technologies for Crop Improvement under Drought Conditions

Sustainable food production is a major challenge to our current agricultural systems. At the global level, water stress is one of the most critical abiotic factors that affect crop productivity. Currently, a vast amount of research efforts are being laid down to generate molecular information leading to an understanding of this abiotic stress factor. Although gene expression analysis at the transcription level has enhanced our understanding regarding the response of plants to drought stress, many questions remain unanswered at the protein level. In view of the fact that the plant proteome is highly dynamic in nature, proteomics is becoming essential for the study of many different aspects of plant functions. Further, proteomics is also a key part of the –omics suite of techniques and facilitates a systems biology approach to biological problems. Although proteomics studies of drought responses in crop plants are still in their infancy, recent significant progress illustrates the promise of proteomics projects for improving drought stress responses in crop plants. This review mainly focuses on the response of various plant proteomes under drought stress conditions. The results of previous comparative proteomics investigations are analyzed, which allow us to gather information on how different plant species alter their proteome to sustain themselves under the drought stress conditions. We predict that proteomics-based projects will continue the discovery of novel gene targets for crop improvement and that a systems biology approach, that includes proteomics, holds great promise for the future of agriculture.

Constitutive Overexpression of the Calcium Sensor CBL5 Confers Osmotic or Drought Stress Tolerance in Arabidopsis

Calcium serves as a critical messenger in many adaptation and developmental processes. Cellular calcium signals are detected and transmitted by sensor molecules such as calcium-binding proteins. In plants, the calcineurin B-like protein (CBL) family represents a unique group of calcium sensors and plays a key role in decoding calcium transients by specifically interacting with and regulating a family of CBL-interacting protein kinases (CIPKs). In this study, we report the role of Arabidopsis CBL5 gene in high salt or drought tolerance. CBL5 gene is expressed significantly in green tissues, but not in roots. CBL5 was not induced by abiotic stress conditions such as high salt, drought or low temperature. To determine whether the CBL5 gene plays a role in stress response pathways, we ectopically expressed the CBL5 protein in transgenic Arabidopsis plants (35S-CBL5) and examined plant responses to abiotic stresses. CBL5-overexpressing plants displayed enhanced tolerance to high salt or drought stress. CBL5 overexpression also rendered plants more resistant to high salt or hyperosmotic stress during early development (i.e., seed germination) but did not alter their response to abiscisic acid (ABA). Furthermore, overexpression of CBL5 alters the gene expression of stress gene markers, such as RD29A, RD29B and Kin1 etc. These results suggest that CBL5 may function as a positive regulator of salt or drought responses in plants.

Interactions between temperature, drought and stomatal opening in legumes

The effect of heat and drought on stomatal behaviour of 2–4-week-old legumes, bean ( Phaseolus vulgaris L.) and red clover ( Trifolium pratense), was investigated. Drought stress was induced by complete water deficit or with polyethylene glycol (PEG), and abscisic acid (ABA) was applied to mimic plant drought response. Heat stress was simulated by water bath (leaf segments) and infrared and halogen lighting (whole plants). Various experimental conditions were studied: high temperature alone or combined with drought, and low or high photosynthetically active radiation (PAR). Stomatal opening was either measured directly or determined using thermal imaging as a proxy. When water was not limiting, stomata opened in darkness under heat stress. At high PAR, drought and moderate heat caused increased leaf temperatures and temperature oscillations (±3–4 °C), attributed to the opening and closing of stomata. At low PAR, heat led to leaf temperature oscillations in control plants, whereas the application of drought caused stomatal closure, increasing leaf temperature to 39 °C. Stomatal opening occurred under high temperatures, despite the presence of the drought-induced hormone ABA, and was maintained into a recovery period at room temperature for 30 min. This study helps to illustrate stomatal plasticity and the interplay between leaf gas-exchange and maintaining favourable metabolic conditions (water status and temperature) within the leaf. Knowledge of how legumes are affected by two environmental stresses, heat and drought, expected to occur simultaneously with greater frequency in the future, is important in determining overall plant survival strategies.

Effects of drought on water content and photosynthetic parameters in potato plants expressing the trehalose-6-phosphate synthase gene of Saccharomyces cerevisiae

Two transgenic potato lines, T1 and T2, expressing the trehalose-6-phosphate synthase (TPS1) gene of yeast were isolated. In our experimental approach, we applied two novelties, namely the fusion of the drought-inducible promoter StDS2 to TPS1 and a marker-free transformation method. In contrast to the expected drought-induced expression, only a very low constitutive TPS1 expression was detected in the transgenic lines, probably due to chromosomal position effects. The observed expression pattern, however, was sufficient to alter the drought response of plants. Detached leaves of T1 and T2 showed an 8 h delay in wilting compared to the non-transformed control. Potted plants of T1 and T2 kept water 6 days longer than control plants and maintained high stomatal conductance and a satisfactory rate of net photosynthesis. During drought treatment, CO2 assimilation rate measured at saturating CO2 level was maintained at maximum level for 6-9 days in transgenic plants while it decreased rapidly after 3 days in the wild type plants. Under optimal growth conditions, lower CO2 fixation was detected in the transgenic than in the control plants. Stomatal densities of T1 and T2 leaves were reduced by 30-40%. This may have contributed to the lower CO2 fixation rate and altered drought response.

Drought responses of six ornamental herbaceous perennials

Abstract Although low water use landscaping is becoming common in arid regions, little is known about drought tolerance and drought responses of many ornamental plants, especially herbaceous perennials. Drought responses were assessed for six herbaceous ornamental landscape perennials in a 38 l pot-in-pot system in northern Utah over a 2-year period. The first year was an establishment period. During the second year, drought responses were evaluated for established Echinacea purpurea (L.) Moench, Gaillardia aristata Pursh, Lavandula angustifolia P. Mill., Leucanthemum × superbum (J.W. Ingram) Berg. ex Kent, ‘Alaska’, Penstemon barbatus Roth var. praecox nanus rondo , and Penstemon × mexicali Mitch. ‘Red Rocks’. Plants were irrigated at frequencies of 1 (control), 2, or 4 weeks between June and September, simulating well-watered conditions, moderate drought, or severe drought. Osmotic potential ( Ψ s ), gas exchange, visual quality, leaf area, and dry weight were assessed. In a confined root zone, P. barbatus showed the greatest tolerance to all levels of drought, avoiding desiccation by increasing root:shoot ratio and decreasing stomatal conductance as water became limiting. L. angustifolia and P . × mexicali showed tolerance to moderate drought conditions, but died after exposure to the first episode of severe drought. Neither G. aristata nor L. superbum were able to regulate shoot water loss effectively. Instead, both species displayed drought avoidance mechanisms, dying back when water was limiting and showing new growth after they were watered. Compared to control plants, G. aristata shoot dry weight was reduced by 50% and 84%, and L. superbum shoot dry weight was reduced by 47% and 99% for the 2- and 4-week irrigation intervals, respectively. Root dry weights were affected similarly for both species. E. purpurea exhibited poor visual quality at all irrigation intervals, in particular wilting severely in both drought treatments, but regaining turgor when watered again. P. barbatus is recommended for ornamental landscapes that receive little or no supplemental irrigation, while E. purpurea is not recommended for low water landscapes because of low visual quality under even mild drought.

Identification of Features Regulating OST1 Kinase Activity and OST1 Function in Guard Cells

The phytohormone abscisic acid (ABA) mediates drought responses in plants and, in particular, triggers stomatal closure. Snf1-related kinase 2 (SnRK2) proteins from several plant species have been implicated in ABA-signaling pathways. In Arabidopsis (Arabidopsis thaliana) guard cells, OPEN STOMATA 1 (OST1)/SRK2E/SnRK2-6 is a critical positive regulator of ABA signal transduction. A better understanding of the mechanisms responsible for SnRK2 protein kinase activation is thus a major goal toward understanding ABA signal transduction. Here, we report successful purification of OST1 produced in Escherichia coli: The protein is active and autophosphorylates. Using mass spectrometry, we identified five target residues of autophosphorylation in recombinant OST1. Sequence analysis delineates two conserved boxes located in the carboxy-terminal moiety of OST1 after the catalytic domain: the SnRK2-specific box (glutamine-303 to proline-318) and the ABA-specific box (leucine-333 to methionine-362). Site-directed mutagenesis and serial deletions reveal that serine (Ser)-175 in the activation loop and the SnRK2-specific box are critical for the activity of recombinant OST1 kinase. Targeted expression of variants of OST1 kinase in guard cells uncovered additional features that are critical for OST1 function in ABA signaling, although not required for OST1 kinase activity: Ser-7, Ser-18, and Ser-29 and the ABA-specific box. Ser-7, Ser-18, Ser-29, and Ser-43 represent putative targets for regulatory phosphorylation and the ABA-specific box may be a target for the binding of signaling partners in guard cells.

Mycorrhizal symbiosis and response of sorghum plants to combined drought and salinity stresses

Arbuscular mycorrhizal (AM) symbiosis can confer increased host resistance to drought stress, although the effect is unpredictable. Since AM symbiosis also frequently increases host resistance to salinity stress, and since drought and salinity stress are often linked in drying soils, we speculated that the AM influence on plant drought response may be partially the result of AM influence on salinity stress. We tested the hypothesis that AM-induced effects on drought responses would be more pronounced when plants of comparable size are exposed to drought in salinized soils. In two greenhouse experiments, several water relations characteristics were measured in sorghum plants colonized by Glomus intraradices (Gi), Gigaspora margarita (Gm) or a mixture of AM species, during a sustained drought following exposure to salinity treatments (NaCl stress, osmotic stress via concentrated macronutrients, or soil leaching). The presence of excess salt in soils widened the difference in drought responses between AM and nonAM plants in just two instances. Days required for plants to reach stomatal closure were similar for Gi and nonAM plants exposed to drought alone, but with exposure to combined NaCl and drought stress, stomates of Gi plants remained open 17-22% longer than in nonAM plants. Promotion of stomatal conductance by Gm occurred with exposure to NaCl/drought stress but not with drought alone or with soil leaching before drought. In other instances, however, the addition of salt tended to nullify an AM-induced change in drought response. Our findings confirm that AM fungi can alter host response to drought but do not lend much support to the idea that AM-induced salt resistance might help explain why AM plants can be more resilient to drought stress than their nonAM counterparts.

(319) Mycorrhizal Symbiosis and Response of Sorghum Plants to Combined Drought and Salt Stresses

Colonization of roots by arbuscular mycorrhizal (AM) fungi can increase host resistance to drought stress, although the effect is unpredictable. Since AM symbiosis also frequently increases host resistance to salt stress, and since drought and salt stress are often linked in drying soils, we speculated that the AM influence on plant drought response may be linked to AM influence on salt stress. We tested the hypothesis that AM-induced effects on drought responses would be more pronounced when plants of comparable size are exposed to drought in salinized soils. In two greenhouse experiments, several water relations characteristics were measured in sorghum plants colonized by Glomus intraradices, Gigaspora margarita, or a mixture of AM species during a sustained drought following exposure to salt treatments (NaCl stress, osmotic stress, or soil leaching). The presence of excess salt in soils widened the difference in drought responses between AM and non-AM plants in just two instances: days needed for plants to reach stomatal closure, and promotion of stomatal conductance. In other instances, the addition of salt tended to nullify an AM-induced change in drought response; e.g., an AM effect on the decline in leaf or soil water potential required to cause stomatal closure disappeared when soils were salinized. Our findings confirm that AM fungi can alter host response to drought but do not lend much support to the idea that AM-induced salt resistance might help explain why AM plants can be more resilient to drought stress than their non-AM counterparts.

Development of In-ground Container Plants of Mexican Elders Exposed to Drought

Mexican elder (Sambucus mexicana Presl.) is used in arid landscapes of the Southwest, but the plant is known for its unpredictable performance in those landscapes. We studied drought responses of mexican elder plants grown in an arid environment using an in-ground nursery production system. Plants were maintained as well-irrigated controls or exposed to cyclic drought and irrigated based on evapotranspiration. Drought treatment lasted 165 days. Plants exposed to drought had more negative predawn and midday water potentials than well-watered plants. The ratio of variable to maximal fluorescence (Fv/Fm) for the drought group (0.76), was near the optimum value of 0.8, suggesting that chloroplasts of drought-stressed plants maintained high levels of activity. Drought cycle, but not drought treatment affected stomatal conductance. Drought-stressed plants had lower transpiration rates than controls except at drought cycle five when transpiration rates were similar between irrigation treatments. Relative water content was higher in controls (76%) than plants exposed to drought (66%). Leaf area of well-irrigated plants was over four times higher than that of plants exposed to drought. Leaf area to root dry weight ratio of drought-stressed plants was 79% lower than control plants. Severely reduced leaf area of drought-stressed plants might be one reason why landscape personnel conclude that mexican elder plants perform poorly in arid landscapes.

Identification of a rice zinc finger protein whose expression is transiently induced by drought, cold but not by salinity and abscisic acid

A C2H2-type zinc finger protein gene ZFP245 was cloned by RT-PCR approach from cold treated rice seedlings. ZFP245 is 712 bp in length and encodes a 24.5 KDa protein, which has 35% identity in amino acid with SCOF-1, a cold-inducible zinc finger protein from soybean. By database search, ZFP245 was mapped on chromosome 7 and clustered together with another C2H2 zinc finger gene. Tissue expression analysis showed that ZFP245 was constitutively expressed in various rice tissues including roots, stems, leaves and spikes. The semi-quantitative-RT-PCR assay revealed ZFP245 was strongly induced after 6 h exposure to cold and drought stresses, and then reduced to the baseline. However, ZFP245 was not regulated by high salt or abscisic acid treatment. The promoter sequence of 1017 nucleotides, upstream of ZFP245, was cloned directly by PCR approach. Sequence analysis revealed a CRT/DRE element was found within the ZFP245 promoter region. Taken together, ZFP245, as the first identified C2H2-type zinc finger protein involved in stress response in monocots probably plays a role as a transcription regulator in plant cold and drought responses through an ABA-independent pathway.

Suppression of polyphenol oxidases increases stress tolerance in tomato

Polyphenol oxidases (PPOs) have been proposed to function in the Mehler reaction, photoreduction of molecular oxygen by PSI [Physiol. Plant.72 (1988) 659]. Under drought stress where photosynthesis is reduced, the Mehler reaction may provide a nondestructive sink for absorbed light energy not used in photochemistry [Plant Physiol. 112 (1996) 265]. If so, water-stressed plants with suppressed PPO are expected to exhibit photooxidative damage and plants with elevated PPO may show increased stress tolerance. However, in experiments examining drought response of tomato plants with different levels of PPO expression, we report more favorable water relations and decreased photoinhibition in transgenic plants with suppressed PPO (SP plants) compared to nontransformed controls (NT plants) and transgenic plants overexpressing PPO (OP plants). In addition, chlorophyll content is higher in water-stressed SP plants compared to NT and OP plants indicating lower photooxidative stress in SP plants. These results suggest that PPO may have a role in the development of plant water stress and potential for photoinhibition and photooxidative damage that may be unrelated to any effects on the Mehler reaction. As a preliminary attempt to understand the role PPO may play in potentiating stress-induced damage, we examined the expression of PPOs during these experiments. Among the seven-member PPO gene family, we found that in response to water stress, two members, PPO B and D, are transcriptionally upregulated in abscission zones of leaf petioles. In addition, PPO B is induced in leaf veins throughout foliar development and in stem vascular tissues, cortical and pith cells. Similarly, tissue printing of stem tissues following drought stress showed increased immunoreactive PPO in vascular tissues, cortical and pith cells of NT and OP plants. Foliar PPO activity of NT plants also increased throughout development in response to water stress. The physiological role(s) of drought-induced PPO activity remains to be determined.

Salicylic acid induced physiological and biochemical changes in wheat seedlings under water stress

Salicylic acid (SA) is an important signal molecule modulating plantresponses to stress. It is recently reported to induce multiple stresstolerancein plants including drought. An experiment was, therefore, conducted toascertain the effect of salicylic acid on the growth and metabolic profile ofwheat seedlings under water stress. Irrespective of the SA concentration(1–3 mM) and water stress, SA treated plants showed, ingeneral, a higher moisture content, dry mass, carboxylase activity of Rubisco,superoxide dismutase (SOD) activity and total chlorophyll compared to those ofuntreated seedlings. SA treatment, under water stress, protected nitratereductase (NR) activity and maintained, especially at 3 mM SAconcentration, the protein and nitrogen content of leaves compared to watersufficient seedlings. Results signify the role of SA in regulating the droughtresponse of plants and suggest that SA could be used as a potential growthregulator, for improving plant growth under water stress.

Drought Responses of Mycorrhizal and Nonmycorrhizal Neem Plants of Comparable Size and Tissue Nutrition

Mycorrhizal (VAM) and phosphorus (P)-supplemented nonmycorrhizal neem plants (non-VAM) of comparable size and tissue nutrition were subjected to a slowly developing drought. VAM and non-VAM plants responded to drought similarly. However, mycorrhiza compensated for low P supply, allowing VAM plants to have comparable growth, tissue P, and other physiological parameters as non-VAM plants, which received higher P supply. Drought decreased growth, transpiration (E), photosynthetic rate (A), stomatal conductance (gs), and plant water status. Osmotic adjustment did not occur, but the relatively low osmotic potential of this species helped maintain turgor during drought. Plant water relations and A of stressed plants fully recovered in 24 hours after rehydration, while gs and E partially recovered. Instantaneous water use efficiency (A/E) increased during drought and recovery, except for a decrease at peak stress due to very low A. Carbon isotope discrimination (D) values of mature leaves remained constant regardless of mycorrhiza or drought. However, D decreased in expanding leaves that developed during a drought period, indicating an increased long-term water use efficiency of these leaves.

Drought-stress responses of two lowland rice cultivars to soil water status

The physiological and morphological responses of two semi-dwarf lowland rice cultivars to transient drought were studied in three greenhouse experiments. These responses were related to root-zone soil water status for use in a rainfed-rice simulation model. Results were very similar for both varieties. Drought responses in young plants occurred at a lower soil water status than in older plants. The first observed effect in a drought period in the vegetative phase was a decline in leaf expansion rate compared to well-watered plants. Leaf expansion stopped completely with root-zone soil water pressure potential h in the range −50 to −250 kPa, depending on crop age and growing season. The rate of transpiration, corrected for differences in LAI, remained roughly equal to that of well-watered plants in the range 0 > h > −100 kPa, depending on crop age. As the soil water status declined further, relative transpiration rate decreased with increasing values of log(| h|), following a logistic function. Leaf rolling and early senesence started at h < −200 kPa or lower and were linearly related with log(| h|). Yield differences between plants that were transiently stressed in the early vegetative phase and well-watered plants were not significant. However, flowering and maturity were delayed. Severe drought in the reproductive phase resulted in large yield reductions, mainly caused by an increase in the percentage of unfilled grains and also in grain weight.

How Do Chemical Signals Work in Plants that Grow in Drying Soil?

We have known for many years that the dehydration of plant cells can lead to accumulation of the plant growth regulator ABA. Application of this compound to well-watered plants mimics many of the effects of soil drymg on gene expression, physiology, growth, and development, making this compound a strong candidate for a role in the droughted plant. Dehydration of leaves can result in massive accumulations of ABA, and roots also synthesize the compound in increased amounts as they are exposed to drier and drier soil. Davies and Zhang (1991) argued that an important component of the drought responses of many plants can be an ABA signal moving from the roots to the shoots to regulate physiology and development as a function of soil water status/availability. Many recent reports show relationships between stomatal conductance and soil water status or xylem ABA concentration, which seem to support this view. Nevertheless, critica1 examination of the ABA-signaling hypothesis must show that enough extra ABA moves in the transpiration stream to the shoots to account for the changes in functioning that are recorded. Many other chemicals moving in the xylem to shoots can also provide shoots with 'information conceming root functioning, and we must consider the nature of such signals. We should also be concemed with the nature of the information that might be transmitted by a root signal (eg. a measure of soil water status or soil water availability) and the form that such a chemical signal might take (e.g. the concentration of the signal molecule in the transpiration stream or the flux of signal molecules to the site of action in the leaf).

Studies on the Status of Unburnt Eucalyptus Woodland at Ocean Grove, Victoria. V. The Interactive Effects of Droughting and Shading on Seedlings Under Competition.

Casuarina littoralis seedlings are inherently more drought-resistant than Eucalyptus ovata seedlings over a wide range of environmental conditions. Moderate shade pre-treatment (30% of full daylight) decreased the drought resistance of seedlings of E. ovata, Acacia pycnantha and C. stricta but not that of C. littoralis seedlings. Deep shade pre-treatment (8 % of full daylight) decreased the drought resistance of all species and was associated with decreased rootlshoot ratios. Both shaded and non-shaded C. littoralis seedlings closed stomata at higher relative water contents (about 80% and 88 % respectively) than did E. ovata seedlings (about 36 % and 63 % respectively). Shading decreased the relative water content at which E. ovata closed stomata and reduced the relative decrease in water potential which occurred with unit decreases in relative water content. When E. ovata and C. littoralis seedlings were grown in competition, the larger E. ovata dominated the drought response of plants under both high and low light conditions. E. ovata rapidly depleted moisture supplies thereby subjecting C. littoralis to greater stress and earlier death than it experienced in monoculture. C. littoralis seedlings grown and droughted in competition with E. ovata exhibited smaIIer decreases in water potential per unit decrease in relative water content than seedlings grown in monoculture. The height growth of E. ovata grown in monoculture and in competition with C. littoralis was reduced for at least 10-15 weeks after the wilting treatment, but height growth of C. littoralis was not affected. Eucalypts wilted at higher water potentials (-4.3 MPa) than did C. littoralis seedlings (- 6.3 MPa). It is suggested that the replacement of E. ovata by C. littoralis at Ocean Grove, Vic. may be partly due to the differential effects of shading on the drought resistance of seedlings which become established in the grass sward of canopy gaps.