The isolation of genes from the resurrection grass Sporobolus stapfianus which are induced during severe drought stress

A modification of the ‘cold plaque’ screening technique (Hodge et al., Plant Journal1992, 2, 257–260) was used to screen a cDNA library constructed from drought‐stressed leaf tissue of the desiccation tolerant (‘resurrection’) grass Sporobolus stapfianus. This technique allowed a large number of clones representing genes expressed at low abundance to be isolated. An examination of expression profiles revealed that several of these genes are induced in desiccation‐tolerant tissue experiencing severe drought stress. Further characterization indicated that the gene products encoded include an eIF1 protein translation initiation factor and a glycine‐ and proline‐rich protein which have not previously been associated with drought stress. In addition, genes encoding a serine/threonine phosphatase type 2C, a tonoplast‐intrinsic protein (TIP) and an early light‐inducible protein (ELIP) were isolated. A number of these genes are expressed differentially in desiccation‐tolerant and desiccation‐sensitive tissues, suggesting that they may be associated with the desiccation tolerance response of S. stapfianus. The results indicate that there may be unique gene regulation processes occurring during induction of desiccation tolerance in resurrection plants which allow different drought‐responsive genes to be selectively expressed at successive levels of water loss.     

Antioxidant system is essential to increase drought tolerance of sugarcane

Drought is one of the main factors affecting the productivity of agricultural crops, and plants respond to such stress by activating various physiological and biochemical mechanisms against dehydration. The present study investigated two varieties of sugarcane (Saccharum spp.) with contrasting responses to drought (RB867515, more tolerant; and RB855536, less tolerant) and subjected them to progressive drought conditions (2, 4, 6 and 8 days) followed by rehydration. Drought caused a decrease in water potential (ψ_w) and osmotic potential (ψ_os) in the leaves, which recovered to normal levels after rehydration only up to the fourth day of drought. Water stress changed the carbon metabolism of leaves by reducing starch and sucrose contents and increasing glucose and fructose contents in both varieties. Water deficit caused a significant reduction in the maximum quantum efficiency of photosystem II (F_v/F_m) and effective quantum yield (Φ_PSII) in both varieties; however, RB867515 recovered faster after rehydration. Under water stress, the more tolerant variety RB867515 exhibited increased activity of the antioxidant enzymes catalase, ascorbate peroxidase and superoxide dismutase compared with the RB855536 variety. The results suggest that RB867515 is more tolerant to drought conditions because of a more efficient antioxidant system, which results in reduced photosynthesis photoinhibition during water stress, thus revealing itself as a potential physiological marker for drought tolerance studies.     

Adaptation to high temperature mitigates the impact of water deficit during combined heat and drought stress in C3 sunflower and C4 maize varieties with contrasting drought tolerance

Heat and drought stress frequently occur together, however, their impact on plant growth and photosynthesis (P_N) is unclear. The frequency, duration and severity of heat and drought stress events are predicted to increase in the future, having severe implications for agricultural productivity and food security. To assess the impact on plant gas exchange, physiology and morphology we grew drought tolerant and sensitive varieties of C3 sunflower (Helianthus annuus) and C4 maize (Zea mays) under conditions of elevated temperature for 4 weeks prior to the imposition of water deficit. The negative impact of temperature on P_N was most apparent in sunflower. The drought tolerant sunflower retained ribulose‐1,5‐bisphosphate carboxylase/oxygenase (RubisCO) activity under heat stress to a greater extent than its drought sensitive counterpart. Maize exhibited no varietal difference in response to increased temperature. In contrast to previous studies, where a sudden rise in temperature induced an increase in stomatal conductance (G_s), we observed no change or a reduction in G_s with elevated temperature, which alongside lower leaf area mitigated the impact of drought at the higher temperature. The drought tolerant sunflower and maize varieties exhibited greater investment in root‐systems, allowing greater uptake of the available soil water. Elevated temperatures associated with heat‐waves will have profound negative impacts on crop growth in both sunflower and maize, but the deleterious effect on P_N was less apparent in the drought tolerant sunflower and both maize varieties. As C4 plants generally exhibit water use efficiency (WUE) and resistance to heat stress, selection on the basis of tolerance to heat and drought stress would be more beneficial to the yields of C3 crops cultivated in drought prone semi‐arid regions.     

Alterations in photosynthetic pigments, protein and osmotic components in cotton genotypes subjected to short-term drought stress followed by recovery

In order to assess drought tolerance mechanism in cotton, short-term drought-induced biochemical responses were monitored in two cotton (Gossypium hirsutum L.) genotypes contrasting their tolerance to water deficit. The seeds of two genotypes, namely GM 090304 (moderately drought tolerant) and Ca/H 631 (drought sensitive), were sown in pots containing soil, sand and peat in the ratio of 1:1:1, and irrigated every alternate day up to 45 days after sowing when each genotype was subjected to a cycle of water stress by withholding irrigation for 7 days. The stress cycle was terminated by re-watering the stressed plants for 7 days. The leaf of the drought tolerant genotype (GM 090304) maintained higher relative water content under water stress than that of the drought sensitive genotype (Ca/H 631). The levels of biochemical components, such as chlorophylls, carotenoids, total protein, free proline, total free amino acids, sugars, starch and polyphenols, were measured during the stress as well as the recovery periods. The chlorophylls, carotenoids, protein and starch contents decreased in drought stressed plants as compared to control and tended to increase when the plants were recovered from stress. The degree of decrease in chlorophylls, carotenoids and protein contents under drought was higher in the sensitive genotype (Ca/H 631) as compared to the moderately tolerant genotype (GM 090304). However, proline, total free amino acids, total sugars, reducing sugars and polyphenol contents were increased in drought stressed plants and tended to decrease during the period of recovery. Drought-induced increases in total free amino acids, proline, sugars and polyphenols were significantly higher in the moderately tolerant genotype (GM 090304) than in the sensitive genotype (Ca/H 631). These results suggest that proline, sugars and polyphenols act as main compatible solutes in cotton in order to maintain osmotic balance, to protect cellular macromolecules, to detoxify the cells, and to scavenge free radicals under water stress condition.     

Intra-specific variability in biomass partitioning and carbon isotopic discrimination under moderate drought stress in seedlings from four <Emphasis Type="Italic">Pinus pinaster</Emphasis> populations

The response to drought of 20 open-pollinated families from four Pinus pinaster Ait. populations covering a latitudinal cline (France, Central and Southern Spain, and Morocco) was assessed. The seedlings were cultivated in a greenhouse under controlled temperature and vapor pressure deficit for 120 days, and subjected to two watering regimes for 60 days. Different biomass partitioning variables, pre-dawn water potential, and isotopic discrimination of ¹³C in needles (Δ) as surrogate of long-term water use efficiency were estimated for each seedling at the end of the experiment. In response to the imposed drought, there was no change in the root biomass partitioning, but the overall Δ values decreased in response to water stress. All the families of the population from Morocco showed the highest investment in roots, regardless of the watering regime imposed. Inter-family differences within populations were also significant for most parameters as confirmed by the heritability values estimated (higher under the well-watered treatment). The studied P. pinaster populations showed different strategies of response to drought. This may represent an important mechanism by local populations in facing future climatic change. The results could be of value in forest conservation and breeding programs of maritime pine in the future.     

Sorghum (Sorghum bicolor) varieties adopt strongly contrasting strategies in response to drought

Sorghum is one of the most drought tolerant crops but surprisingly, little is known about the mechanisms achieving this. We have compared physiological and biochemical responses to drought in two sorghum cultivars with contrasting drought tolerance. These closely related cultivars have starkly contrasting responses to water deficit. In the less tolerant Samsorg 40, drought induced progressive loss of photosynthesis. The more drought tolerant Samsorg 17 maintained photosynthesis, transpiration and chlorophyll content until the most extreme conditions. In Samsorg 40, there was a highly specific down‐regulation of selected proteins, with loss of PSII and Rubisco but maintenance of PSI and cytochrome b₆f, allowing plants to maintain ATP synthesis. The nitrogen released allows for accumulation of glycine betaine and proline. To the best of our knowledge, this is the first example of specific reengineering of the photosynthetic apparatus in response to drought. In contrast, in Samsorg 17 we detected no substantial change in the photosynthetic apparatus. Rather, plants showed constitutively high soluble sugar concentration, enabling them to maintain transpiration and photosynthesis, even in extremely dry conditions. The implications for these strikingly contrasted strategies are discussed in relation to agricultural and natural systems.     

Nitric oxide increases tolerance responses to moderate water deficit in leaves of Phaseolus vulgaris and Vigna unguiculata bean species

Drought stress is one of the most intensively studied and widespread constraints, and nitric oxide (NO) is a key signaling molecule involved in the mediation of abiotic stresses in plants. We demonstrated that a sprayed solution of NO from donor sodium nitroprusside increased drought stress tolerance responses in both sensitive (Phaseolus vulgaris) and tolerant (Vigna unguiculata) beans. In intact plants subjected to halting irrigation, NO increased the leaf relative water content and stomatal conductance in both species. After cutting leaf discs and washing them, NO induced increased electrolyte leakage, which was more evident in the tolerant species. These leaf discs were then subjected to different water deficits, simulating moderate and severe drought stress conditions through polyethylene glycol solutions. NO supplied at moderate drought stress revealed a reduced membrane injury index in sensitive species. In hydrated discs and at this level of water deficit, NO increased the electron transport rate in both species, and a reduction of these rates was observed at severe stress levels. Taken together, it can be shown that NO has an effective role in ameliorating drought stress effects, activating tolerance responses at moderate water deficit levels and in both bean species which present differential drought tolerance.     

Integrated drought responses of black poplar: how important is phenotypic plasticity?

Climate change is expected to increase drought frequency and intensity which will threaten plant growth and survival. In such fluctuating environments, perennial plants respond with hydraulic and biomass adjustments, resulting in either tolerant or avoidant strategies. Plants' response to stress relies on their phenotypic plasticity. The goal of this study was to explore physiology of young Populus nigra in the context of a time‐limited and progressive water deficit in regard to their growth and stress response strategies. Fourteen French 1‐year‐old black poplar genotypes, geographically contrasted, were subjected to withholding water during 8 days until severe water stress. Water fluxes (i.e. leaf water potentials and stomatal conductance) were analyzed together with growth (i.e. radial and longitudinal branch growth, leaf senescence and leaf production). Phenotypic plasticity was calculated for each trait and response strategies to drought were deciphered for each genotype. Black poplar genotypes permanently were dealing with a continuum of adjusted water fluxes and growth between two extreme strategies, tolerance and avoidance. Branch growth, leaf number and leaf hydraulic potential traits had contrasted plasticities, allowing genotype characterization. The most tolerant genotype to water deficit, which maintained growth, had the lowest global phenotypic plasticity. Conversely, the most sensitive and avoidant genotype ceased growth until the season's end, had the highest plasticity level. All the remaining black poplar genotypes were close to avoidance with average levels of traits plasticity. These results underpinned the role of plasticity in black poplar response to drought and calls for its wider use into research on plants' responses to stress.     

Improving water use efficiency by changing hydraulic and stomatal characteristics in soybean exposed to drought: the involvement of nitric oxide

A variety of cellular responses is needed to ensure the plants survival during drought, but little is known about the signaling mechanisms involved in this process. Soybean cultivars (EMBRAPA 48 and BR 16, tolerant and sensitive to drought, respectively) were exposed to the following treatments: control conditions (plants in field capacity), drought (20% of available water in the soil), sodium nitroprusside (SNP) treatment (plants irrigated and treated with 100-µM SNP [SNP–nitric oxide (NO) donor molecule], and Drought + SNP (plants subjected to drought and SNP treatment). Plants remained in these conditions until the reproductive stage and were evaluated for physiological (photosynthetic pigments, chlorophyll a fluorescence and gas exchange rates), hydraulic (water potential, osmotic potential and leaf hydraulic conductivity) and morpho-anatomical traits (biomass, venation density and stomatal characterization). Exposure to water deficit considerably reduced water potential in both cultivars and resulted in decrease in photosynthesis and biomass accumulation. The addition of the NO donor attenuated these damaging effects of water deficit and increased the tolerance index of both cultivars. The results showed that NO was able to reduce plant's water loss, while maintaining their biomass production through alteration in stomatal characteristics, hydraulic conductivity and the biomass distribution pattern. These hydraulic and morpho-anatomical alterations allowed the plants to obtain, transport and lose less water to the atmosphere, even in water deficit conditions.     

Expression profiling of rice cultivars differing in their tolerance to long-term drought stress

Understanding the molecular basis of plant performance under water-limiting conditions will help to breed crop plants with a lower water demand. We investigated the physiological and gene expression response of drought-tolerant (IR57311 and LC-93-4) and drought-sensitive (Nipponbare and Taipei 309) rice (Oryza sativa L.) cultivars to 18 days of drought stress in climate chamber experiments. Drought stressed plants grew significantly slower than the controls. Gene expression profiles were measured in leaf samples with the 20 K NSF oligonucleotide microarray. A linear model was fitted to the data to identify genes that were significantly regulated under drought stress. In all drought stressed cultivars, 245 genes were significantly repressed and 413 genes induced. Genes differing in their expression pattern under drought stress between tolerant and sensitive cultivars were identified by the genotype x environment (G x E) interaction term. More genes were significantly drought regulated in the sensitive than in the tolerant cultivars. Localizing all expressed genes on the rice genome map, we checked which genes with a significant G x E interaction co-localized with published quantitative trait loci regions for drought tolerance. These genes are more likely to be important for drought tolerance in an agricultural environment. To identify the metabolic processes with a significant G x E effect, we adapted the analysis software MapMan for rice. We found a drought stress induced shift toward senescence related degradation processes that was more pronounced in the sensitive than in the tolerant cultivars. In spite of higher growth rates and water use, more photosynthesis related genes were down-regulated in the tolerant than in the sensitive cultivars.