Stimulation of Plant Growth and Drought Tolerance by Native Microorganisms (AM Fungi and Bacteria) from Dry Environments: Mechanisms Related to Bacterial Effectiveness

In this study we tested whether rhizosphere microorganisms can increase drought tolerance to plants growing under water-limitation conditions. Three indigenous bacterial strains isolated from droughted soil and identified as Pseudomonas putida, Pseudomonas sp., and Bacillus megaterium were able to stimulate plant growth under dry conditions. When the bacteria were grown in axenic culture at increasing osmotic stress caused by polyethylene glycol (PEG) levels (from 0 to 60%) they showed osmotic tolerance and only Pseudomonas sp. decreased indol acetic acid (IAA) production concomitantly with an increase of osmotic stress (PEG) in the medium. P. putida and B. megaterium exhibited the highest osmotic tolerance and both strains also showed increased proline content, involved in osmotic cellular adaptation, as much as increased osmotic stress caused by NaCl supply. These bacteria seem to have developed mechanisms to cope with drought stress. The increase in IAA production by P. putida and B. megaterium at a PEG concentration of 60% is an indication of bacterial resistance to drought. Their inoculation increased shoot and root biomass and water content under drought conditions. Bacterial IAA production under stressed conditions may explain their effectiveness in promoting plant growth and shoot water content increasing plant drought tolerance. B. megaterium was the most efficient bacteria under drought (in successive harvests) either applied alone or associated with the autochthonous arbuscular mycorrhizal fungi Glomus coronatum, Glomus constrictum or Glomus claroideum. B. megaterium colonized the rhizosphere and endorhizosphere zone. We can say, therefore, that microbial activities of adapted strains represent a positive effect on plant development under drought conditions.     

The role of abscisic acid and salination in the adaptive response of plants to reduced root aeration

Cessation of root aeration to tobacco plants (Nicotiana rustica)growing in half-Hoagland nutrient solution resulted in rapidwilting of the shoot. Plants growing in salinated solutions,however, retained their turgor despite lack of aeration. Thisresistance to aeration stress was associated with increasedconcentration of abscisic acid in the leaves. Maximum amountsof abscisic acid in leaves were reached by the 4th day of salinationand coincided with the maximum resistance to lack of aeration.When the plants were returned to half-Hoagland, leaf-abscisicacid decreased to the presalination level. Resistance to lackof aeration also resulted when plants were pretreated with abscisicacid. We tentatively conclude that resistance to lack of rootaeration reflects adaptation to osmotic stress and is partlydue to an increase in the content of leaf-abscisic acid. ¹ Volcani Institute of Agricultural Research, Bet Dagan, Israel.Present address: Department of Horticulture, Michigan StateUniversity, East Lansing, Michigan 48823, U. S. A. (Received June 12, 1971; )     

Overexpression of wheat dehydrin DHN-5 enhances tolerance to salt and osmotic stress in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

Late Embryogenesis Abundant (LEA) proteins are associated with tolerance to water-related stress. A wheat (Triticum durum) group 2 LEA proteins, known also as dehydrin (DHN-5), has been previously shown to be induced by salt and abscisic acid (ABA). In this report, we analyze the effect of ectopic expression of Dhn-5 cDNA in Arabidopsis thaliana plants and their response to salt and osmotic stress. When compared to wild type plants, the Dhn-5 transgenic plants exhibited stronger growth under high concentrations of NaCl or under water deprivation, and showed a faster recovery from mannitol treatment. Leaf area and seed germination rate decreased much more in wild type than in transgenic plants subjected to salt stress. Moreover, the water potential was more negative in transgenic than in wild type plants. In addition, the transgenic plants have higher proline contents and lower water loss rate under water stress. Also, Na⁺ and K⁺ accumulate to higher contents in the leaves of the transgenic plants. Our data strongly support the hypothesis that Dhn-5, by its protective role, contributes to an improved tolerance to salt and drought stress through osmotic adjustment.     

Seed biopriming with drought tolerant isolates of Trichoderma harzianum promote growth and drought tolerance in Triticum aestivum

Green house study was aimed to investigate the effect of seed biopriming with drought tolerant isolates of Trichoderma harzianum, viz. Th 56, 69, 75, 82 and 89 on growth of wheat under drought stress and to explore the mechanism underlying plant water stress resilience in response to Trichoderma inoculation. Measurements of relative water content, osmotic potential, osmotic adjustment, leaf gas exchange, chlorophyll fluorescence and membrane stability index were performed. In addition, analysis of the phenolics, proline, lipid peroxidation and measurements of phenylalanine ammonia‐lyase activity were carried out. Seed biopriming enhanced drought tolerance of wheat as drought induced changes like stomatal conductance, net photosynthesis and chlorophyll fluorescence were delayed. Drought stress from 4 to 13 days of withholding water induced an increase in the concentration of stress induced metabolites in leaves, while Trichoderma colonisation caused decrease in proline, malondialdehyde (MDA) and hydrogen peroxide (H₂O₂), and an increase in total phenolics. A common factor that negatively affects plants under drought stress conditions is accumulation of toxic reactive oxygen species (ROS), and we tested the hypothesis that seed biopriming reduced damages resulting from accumulation of ROS in stressed plants. The enhanced redox state of colonised plants could be explained by higher l ‐phenylalanine ammonia‐lyase (PAL) activity in leaves after 13 days of drought stress in Trichoderma treated plants. Similar activity was induced in untreated plants in response to drought stress but to a lower extent in comparison to treated plants. Our results support the hypothesis that seed biopriming in wheat with drought tolerant T. harzianum strains increased root vigour besides performing the process of osmoregulation. It ameliorates drought stress by inducing physiological protection in plants against oxidative damage, due to enhanced capacity to scavenge ROS and increased level of PAL, a mechanism that is expected to augment tolerance to abiotic stresses.     

OsASR5 enhances drought tolerance through a stomatal closure pathway associated with ABA and H2O2 signalling in rice

Drought is one of the major abiotic stresses that directly implicate plant growth and crop productivity. Although many genes in response to drought stress have been identified, genetic improvement to drought resistance especially in food crops is showing relatively slow progress worldwide. Here, we reported the isolation of abscisic acid, stress and ripening (ASR) genes from upland rice variety, IRAT109 (Oryza sativa L. ssp. japonica), and demonstrated that overexpression of OsASR5 enhanced osmotic tolerance in Escherichia coli and drought tolerance in Arabidopsis and rice by regulating leaf water status under drought stress conditions. Moreover, overexpression of OsASR5 in rice increased endogenous ABA level and showed hypersensitive to exogenous ABA treatment at both germination and postgermination stages. The production of H₂O₂, a second messenger for the induction of stomatal closure in response to ABA, was activated in overexpression plants under drought stress conditions, consequently, increased stomatal closure and decreased stomatal conductance. In contrast, the loss‐of‐function mutant, osasr5, showed sensitivity to drought stress with lower relative water content under drought stress conditions. Further studies demonstrated that OsASR5 functioned as chaperone‐like protein and interacted with stress‐related HSP40 and 2OG‐Fe (II) oxygenase domain containing proteins in yeast and plants. Taken together, we suggest that OsASR5 plays multiple roles in response to drought stress by regulating ABA biosynthesis, promoting stomatal closure, as well as acting as chaperone‐like protein that possibly prevents drought stress‐related proteins from inactivation.     

Overexpression of AtABCG36 improves drought and salt stress resistance in Arabidopsis

Drought and salt are major abiotic stresses that adversely affect crop productivity. Thus, identification of factors that confer resistance to these stresses would pave way to increasing agricultural productivity. When grown on soil in green house longer than 5 weeks, transgenic Arabidopsis plants that overexpress an ATP‐binding cassette (ABC) transporter, AtABCG36/AtPDR8, produced higher shoot biomass and less chlorotic leaves than the wild‐type. We investigated whether the improved growth of AtABCG36‐overexpressing plants was due to their improved resistance to abiotic stresses, and found that AtABCG36‐overexpressing plants were more resistant to drought and salt stress and grew to higher shoot fresh weight (FW) than the wild‐type. On the contrary, T‐DNA insertional knockout lines were more sensitive to drought stress than wild‐type and were reduced in shoot FW. To understand the mechanism of enhanced salt and drought resistance of the AtABCG36 overexpressing plants, we measured sodium contents and found that AtABCG36 overexpressing plants were lower in sodium content than the wild‐type. Our data suggest that AtABCG36 contributes to drought and salt resistance in Arabidopsis by a mechanism that includes reduction of sodium content in plants.     

盐旱复合胁迫对小麦幼苗生长和水分吸收的影响

为明确盐害、干旱及盐旱复合胁迫对小麦幼苗生长和水分吸收的影响,从而为盐害和干旱胁迫下栽培调控提供理论依据。以2个抗旱性不同的小麦品种(扬麦16和耐旱型洛旱7号)为材料,采用水培试验,以NaCl和PEG模拟盐旱复合胁迫,研究了盐旱复合胁迫下小麦幼苗生长、根系形态、光合特性及水分吸收特性的变化。结果表明,盐、旱及复合胁迫下小麦幼苗的生物量、叶面积、总根长与根系表面积、叶绿素荧光和净光合速率均显著下降,但是复合胁迫处理的降幅却显著低于单一胁迫。盐旱复合胁迫下根系水导速率和根系伤流液强度显著大于单一胁迫,从而提高了小麦幼苗叶片水势和相对含水量。盐胁迫下小麦幼苗Na~+/K~+显著大于复合胁迫,但复合胁迫下ABA含量却显著小于单一的盐害和干旱胁迫。因此,盐旱复合胁迫可以通过增强根系水分吸收及降低根叶中ABA含量以维持较高光合能力,这是盐旱复合胁迫提高小麦适应性的重要原因。洛旱7号和扬麦16对盐及盐旱复合胁迫的响应基本一致,但在干旱胁迫下洛旱7号表现出明显的耐性。     

Effect of drought on sorbitol and sucrose metabolism in sinks and sources of peach

In peach (Prunus persica [L.] Batsch.), sorbitol and sucrose are the two main forms of photosynthetic and translocated carbon and may have different functions depending on the organ of utilization and its developmental stage. The role and interaction of sorbitol and sucrose metabolism was studied in mature leaves (source) and shoot tips (sinks) of ‘Nemaguard’ peach under drought stress. Plants were irrigated daily at rates of 100, 67, and 33% of evapotranspiration (ET). The relative elongation rate (RER) of growing shoots was measured daily. In mature leaves, water potential (Ψ_w), osmotic potential (Ψ_s), sorbitol‐6‐phosphate dehydrogenase (S6PDH, EC 1.1.1.200), and sucrose‐phosphate synthase (SPS, EC 2.4.1.14) activities were measured weekly. Measurements of Ψ_s, sorbitol dehydrogenase (SDH, 1.1.1.14), sucrose synthase (SS, EC 2.4.1.13), acid invertase (AI, EC 3.2.1.26), and neutral invertase (NI, EC 3.2.1.27) activities were taken weekly in shoot tips. Drought stress reduced RER and Ψ_w of plants in proportion to water supply. Osmotic adjustment was detected by the second week of treatment in mature leaves and by the third week in shoot tips. Both SDH and S6PDH activities were reduced by drought stress within 4 days of treatment and positively correlated with overall Ψ_w levels. However, only SDH activity was correlated with Ψ_s. Among the sucrose enzymes, only SS was affected by drought, being reduced after 3 weeks. Sorbitol accumulation in both mature leaves and shoot tips of stressed plants was observed starting from the second week of treatment and reached up to 80% of total solutes involved in osmotic adjustment. Sucrose content was up to 8‐fold lower than sorbitol content and accumulated only occasionally. We conclude that a loss of SDH activity in sinks leads to osmotic adjustment via sorbitol accumulation in peach. We propose an adaptive role of sorbitol metabolism versus a maintenance role of sucrose metabolism in peach under drought stress.     

Does habitat of <Emphasis Type="Italic">Atriplex halimus</Emphasis> L. affect plant strategy for osmotic adjustment?

The impact of water stress was analysed in the xero-halophyte Mediterranean shrub Atriplex halimus using two Tunisian populations originating from a sub-humid coastal site (Monastir) or from a semi-arid area (Kairouan). Seedlings were exposed for 10 days to nutrient solution containing either 0 or 15% polyethylene glycol. Water potential (Ψ_w), osmotic potential (Ψ_s), osmotic potential at full turgor [Ψ_s(100)], relative water content (RWC), shoot dry weight (DW) and changes in solute concentrations were quantified every 2 days throughout the stress period and inorganic solutes contents were determined at the end of the treatment. The water deficit induced a decrease in Ψ_w, Ψ_s and RWC in both populations, recorded changes being higher in plants of Monastir than those of Kairouan while the shoot dry weight was reduced in a similar extent in stressed plants from both populations. Water deficit induced an increase in proline, glycinebetaine and sugar concentrations. Proline accumulated as early as after the 24-h stress treatment while, glycinebetaine required more than 6 days of stress to accumulate. At the end of the stress period, the plants of Kairouan population accumulated higher amounts of proline than those of Monastir, while an opposite trend was reported for glycinebetaine. Both populations specifically accumulated Na⁺ in response to drought stress, suggesting that this element could play a physiological role in the stress response of this xero-halophyte species. Presented results suggest that the non-recyclable osmotic solute glycinebetaine does not necessarily preferentially accumulates in population facing permanent water stress and that other strategy than osmotic adjustment might be involved in drought tolerance of A. halimus.     

水分胁迫下内生真菌球毛壳ND35对冬小麦苗期生长和抗旱性的影响

为明确不同水分条件下内生真菌对冬小麦苗期生长和抗旱性的影响,以抗旱型小麦品种山农16和水分敏感型小麦品种山农22为材料,利用荧光定量PCR技术检测小麦干旱诱导基因脱水素wzy2的表达量来了解冬小麦在干旱胁迫下相关基因的表达差异,通过测定相关生理指标与酶活性来判断小麦发育及其在干旱胁迫下的生理响应状况。结果表明,与正常水分ND35组相比,接种球毛壳菌(Chaetomium globosum)ND35的干旱处理组小麦的根冠比、总蛋白含量、脯氨酸含量及丙二醛含量等指标显著提高,小麦叶片含水量和可溶性糖含量有所降低。在干旱处理组中,球毛壳菌ND35可以显著提高小麦山农16的根长和山农22的株高,接种球毛壳ND35的山农16脯氨酸含量、可溶性糖含量、过氧化氢酶活性比对照组均显著提高,丙二醛含量比对照组降低9.0%,但差异不显著;山农22脯氨酸含量和过氧化氢酶活性比对照组显著提高,丙二醛含量和可溶性糖含量比对照组有所降低,但可溶性糖含量差异不显著;相对定量检测数据显示,接种球毛壳ND35后,两种小麦脱水素wzy2基因的表达量较对照组均能够显著提高。综合分析说明内生真菌球毛壳ND35可以促进冬小麦苗期根系和植株发育,小麦提前进入三叶期,增强小麦避旱性,同时提高小麦根系活力,增强小麦耐旱性;提高个体细胞内水分、糖分、脯氨酸含量,降低丙二醛的氧化性损伤,增强过氧化氢酶活性,从而提高两种冬小麦对干旱胁迫的耐受能力;球毛壳ND35促进小麦干旱诱导相关基因wzy2的表达量,进而提高抗旱相关蛋白的表达,从而提高两种冬小麦耐脱水性和对干旱胁迫的适应性。     

Stomatal response to drying soil in relation to changes in the xylem sap composition of Helianthus annuus. II. Stomatal sensitivity to abscisic acid imported from the xylem sap

Sunflower plants [Helianthus annuus L.) were subjected to soil drought. Leaf conductance declined with soil water content even when the shoot was kept turgid throughout the drying period. The concentration of abscisic acid in the xylem sap increased with decreasing soil water content. No general relation could be established between abscisic acid concentration in the xylem sap and leaf conductance due to marked differences in the sensitivity of leaf conductance of individual plants to abscisic acid from the xylem sap. The combination of these results with data from Gollan, Schurr & Schulze (1992, see pp. 551–559, this issue) reveals close connection of the effectiveness of abscisic acid as a root to shoot signal to the nutritional status of the plant.     

Improved drought resistance in a wheat stay-green mutant tasg1 under field conditions

We investigated the drought resistance of a wheat (Triticum aestivum L.) stay-green mutant tasg1 and its wild-type (WT) in field experiments conducted for two years. Drought stress was imposed by controlling irrigation and sheltering the plants from rain. Compared with the WT, tasg1 exhibited a distinct delayed senescence under both normal and drought stress conditions, as indicated by slower degradation of chlorophyll and decrease in net photosynthetic rate than in WT. At the same time, tasg1 mutants maintained more integrated chloroplasts and thylakoid ultrastructure than did WT plants under drought stress. Lower malondialdehyde content and higher antioxidative enzyme activities in tasg1, compared to WT, may be involved in the stay-green phenotype and drought resistance of tasg1.     

A Novel ABA-Responsive TaSRHP Gene from Wheat Contributes to Enhanced Resistance to Salt Stress in Arabidopsis thaliana

A microarray analysis of the salt-resistant wheat mutant, RH8706-49, revealed a salt-induced gene containing a conserved DUF581 domain. The gene was cloned and designated as Triticum aestivum salt-related hypothetical protein (TaSRHP) and submitted to GenBank (accession no. GQ476575). Over-expression of TaSRHP in wild-type Arabidopsis thaliana cv. Columbia resulted in enhanced resistance to both salt and drought stresses. The sensitivity of the transgenic A. thaliana to abscisic acid (ABA) was also increased compared to that of wild-type plants. Furthermore, transgenic plants accumulated more K⁺ and proline and had a higher osmotic potential and lower Na⁺ content than untransformed plants. Real-time quantitative PCR analysis indicated that expression of TaSRHP was affected by salt, drought, cold, ABA, and other stresses, and expression of other stress-related genes in the transgenic plants differed from those of the control. Results indicate that the wheat TaSRHP gene may enhance the tolerance of plants to multiple abiotic stresses.     

Effect of potassium on drought resistance of Hibiscus rosa-sinensis cv. Leprechaun: Plant growth,leaf macro- and micronutrient content and root longevity

As competition for the limited water supply available for irrigation of horticultural crops increases, research into crop management practices that enhance drought resistance, plant water-use efficiency and plant growth when water supply is limited has become increasingly essential. This experiment was conducted to determine the effect of potassium (K) nutritional status on the drought resistance of Hibiscus rosa-sinensis L. cv. Leprechaun (Hibiscus). All the treatments were fertilized with Hoagland's nutrient solution, modified to supply K as K₂SO₄, at 0 mM K (K₀), 2.5 mM K (K_2.5), and 10 mM K (K₁₀), under two irrigation regimes (drought stressed [DS] and non-drought stressed [non-DS]). Regular irrigation and fertigation were adopted for 54 days, and drought stress treatment (initiated on day 55) lasted for 21 days; while non-DS control plants continued to receive regular irrigation and fertigation. Following the 21-day drought stress period, plants were labeled with ⁸⁶Rb⁺ to determine the percentage of post-drought stress live roots. Both K deficiency (K₀) and drought stress reduced shoot growth, but drought stress increased root growth and thus the root:shoot ratio. At K₀, plants were K-deficient and had the lowest leaf K, Fe, Mn, Zn, Cu, B, Mo and Al, and highest Ca concentrations. Although the percentage of live roots was decreased by drought stress, K_2.5 and K₁₀ plants (with similar percent live roots) had greater root survival ratio after drought treatment than the K-deficient plants. These observations indicate that adequate K nutrition can improve drought resistance and root longevity in Hibiscus rosa-sinensis.     

The pepper late embryogenesis abundant protein CaLEA1 acts in regulating abscisic acid signaling,drought and salt stress response

As sessile organisms, plants are constantly challenged by environmental stresses, including drought and high salinity. Among the various abiotic stresses, osmotic stress is one of the most important factors for growth and significantly reduces crop productivity in agriculture. Here, we report a function of the CaLEA1 protein in the defense responses of plants to osmotic stress. Our analyses showed that the CaLEA1 gene was strongly induced in pepper leaves exposed to drought and increased salinity. Furthermore, we determined that the CaLEA1 protein has a late embryogenesis abundant (LEA)_3 homolog domain highly conserved among other known group 5 LEA proteins and is localized in the processing body. We generated CaLEA1‐silenced peppers and CaLEA1‐overexpressing (OX) transgenic Arabidopsis plants to evaluate their responses to dehydration and high salinity. Virus‐induced gene silencing of CaLEA1 in pepper plants conferred enhanced sensitivity to drought and salt stresses, which was accompanied by high levels of lipid peroxidation in dehydrated and NaCl‐treated leaves. CaLEA1‐OX plants exhibited enhanced sensitivity to abscisic acid (ABA) during seed germination and in the seedling stage; furthermore, these plants were more tolerant to drought and salt stress than the wild‐type plants because of enhanced stomatal closure and increased expression of stress‐responsive genes. Collectively, our data suggest that CaLEA1 positively regulates drought and salinity tolerance through ABA‐mediated cell signaling.     

Differences in osmotic adjustment,foliar abscisic acid dynamics,and stomatal regulation between an isohydric and anisohydric woody angiosperm during drought

Species are often classified along a continuum from isohydric to anisohydric, with isohydric species exhibiting tighter regulation of leaf water potential through stomatal closure in response to drought. We investigated plasticity in stomatal regulation in an isohydric (Eucalyptus camaldulensis) and an anisohydric (Acacia aptaneura) angiosperm species subject to repeated drying cycles. We also assessed foliar abscisic acid (ABA) content dynamics, aboveground/belowground biomass allocation and nonstructural carbohydrates. The anisohydric species exhibited large plasticity in the turgor loss point (Ψ_TLP), with plants subject to repeated drying exhibiting lower Ψ_TLP and correspondingly larger stomatal conductance at low water potential, compared to plants not previously exposed to drought. The anisohydric species exhibited a switch from ABA to water potential‐driven stomatal closure during drought, a response previously only reported for anisohydric gymnosperms. The isohydric species showed little osmotic adjustment, with no evidence of switching to water potential‐driven stomatal closure, but did exhibit increased root:shoot ratios. There were no differences in carbohydrate depletion between species. We conclude that a large range in Ψ_TLP and biphasic ABA dynamics are indicative of anisohydric species, and these traits are associated with exposure to low minimum foliar water potential, dense sapwood and large resistance to xylem embolism.