Effect of drought stress on lipid metabolism in the leaves of Arabidopsis thaliana (ecotype Columbia)

BACKGROUND AND AIMS: Cell membranes are major targets of environmental stresses. Lipids are important membrane components, and changes in their composition may help to maintain membrane integrity and preserve cell compartmentation under water stress conditions. The aim of this work was to investigate the effects of water stress on membrane lipid composition and other aspects of lipid metabolism in the leaves of the model plant, Arabidopsis thaliana. METHODS: Arabidopsis thaliana (ecotype Columbia) plants were submitted to progressive drought stress by withholding irrigation. Studies were carried out in plants with hydration levels ranging from slight to very severe water deficit. Enzymatic activities hydrolysing MGDG, DGDG and PC were measured. Expression of several genes essential to lipid metabolism, such as genes coding for enzymes involved in lipid biosynthesis (MGDG synthase, DGDG synthase) and degradation (phospholipases D, lipoxygenase, patatin-like lipolytic-acylhydrolase), was studied. KEY RESULTS: In response to drought, total leaf lipid contents decreased progressively. However, for leaf relative water content as low as 47.5 %, total fatty acids still represented 61 % of control contents. Lipid content of extremely dehydrated leaves rapidly increased after rehydration. The time-course of the decrease in leaf lipid contents correlated well with the increase in lipolytic activities of leaf extracts and with the expression of genes involved in lipid degradation. Despite a decrease in total lipid content, lipid class distribution remained relatively stable until the stress became very severe. CONCLUSIONS: Arabidopsis leaf membranes appeared to be very resistant to water deficit, as shown by their capacity to maintain their polar lipid contents and the stability of their lipid composition under severe water loss conditions. Moreover, arabidopsis displayed several characteristics indicative of a so far unknown adaptation capacity to drought-stress at the cellular level, such as an increase in the DGDG : MGDG ratio and fatty acid unsaturation.     

Does proline accumulated in leaves of water deficit stressed barley plants confine cell membrane injury? I. Free proline accumulation and membrane injury index in drought and osmotically stressed plants

The aim of this work was to examine the relationship between proline accumulation and membrane injury in barley leaves suffering from the effects of water deficit. Water deficit stress was induced by water withholding or by immersing the roots in polyethylene glycol (PEG 6000) solution of osmotic potential −1.5 MPa. The effect of water stress on proline accumulation and on membrane injury was evaluated in leaf blades of several barley genotypes. Substantial differences in proline accumulation and membrane injury indices among most of the genotypes investigated were observed. It was found that in drought stressed plants a higher ability to accumulate proline positively correlates with lower membrane injury. Whereas, in osmotically stressed plants the highest proline accumulation in the leaves was noticed in genotype with the largest membrane injury. The possible role of proline in membrane protection under conditions of slow-acting drought or shock-acting osmotic stress is discussed.     

Jasmonic acid is involved in the water-stress-induced betaine accumulation in pear leaves

Jasmonic acid (JA) is known to be involved in the response of plants to environmental stresses such as drought, and betaine (glycinebetaine) is an osmopretectant accumulated in plants under environmental stresses including drought. However, it remains currently unclear whether JA is involved in the water‐stress‐induced betaine accumulation in plant leaves. The present experiment, performed with the whole pear plant (Pyrus bretschneideri Redh. cv. Suli), revealed that the exogenously applied JA induced a significant increase of the betaine level in the pear leaves when the plants were not yet stressed by drought, and when the plants were subjected to water stress, the ‘JA plus drought’ treatment induced a significant higher betaine level than did the drought treatment alone. Meanwhile, the ‘JA plus drought’ treatment induced higher levels of betaine aldehyde dehydrogenase (BADH, E C 1.2.1.8) and activities in the leaves than did the drought treatment alone. These results obtained in the whole plant experiments were supported by the results of detached leaf experiments. In detached leaves JA induced significant increases in betaine levels, BADH activities and BADH protein amounts in a time‐ and concentration‐dependent manner. These data demonstrate that JA is involved in the drought‐induced betaine accumulation in pear leaves.     

Different classes of proteases are involved in the response to drought of Phaseolus vulgaris L. cultivars differing in sensitivity

Protein breakdown and recycling, which depend on the levels of proteolytic enzymes, are an essential part of the plant response to environmental stress. In order to study changes in the activity of proteases in Phaseolus vulgaris L. subjected to water deficit, three cultivars of European origin that exhibit different degrees of sensitivity to drought were chosen on the basis of changes in water potential, psiw, water and protein contents of leaves during progressive water deficit, and loss of membrane integrity after osmotic stress. Twenty-day-old plants were subjected to water deficit by withholding irrigation. Specific enzyme activities in leaf extracts were determined for plants under different degrees of drought stress using different substrates and protease inhibitors. Proteolytic activities were partially characterized by gel exclusion chromatography. Activities of two of the three identified serine proteinases changed under water deficit. The activity of the one with apparent molecular mass of approximately 65 kDa was observed to increase progressively with increasing withdrawal of water in the more sensitive cultivars, but to decrease in the more resistant cultivar. The same activity was elevated in senescent leaves. Under conditions of severe water deficit, the most sensitive cultivar exhibited a marked increase in the activity of two different aminopeptidases, while the more resistant cultivar showed a significant decrease in the activity of these aminopeptidases. These results point to complex and probably specific roles for different proteases in the plant response to drought.     

Role of nitric oxide dependence on nitric oxide synthase-like activity in the water stress signaling of maize seedling

Nitric oxide (NO) has been known as an important signal in plant antioxidative defense but its production and roles in water stress are less known. The present study investigated whether NO dependence on a NO synthase-lika (NOS) activity is involved in the signaling of drought-induced protective responses in maize seedlings. NOS activity, rate of NO release and drought responses were analyzed when NO donor sodium nitroprusside (SNP), NO scavenger c-PTIO (2-(4-carboxyphenyl)-4,4,5,5-tetramathylimidazoline-1-oxyl-3-oxide) and NOS inhibitor L-NAME (NG-nitro-L-arginine methyl ester) were applied to both detached maize leaves and whole plants. Both NOS activity and the rate of NO release increased substantially under dehydration stress. The high NOS activity induced by c-PTIO as NO scavenger and NO accumulation Inhibited by NOS inhibitor L-NAME In dehydration-treated maize seedlings Indicated that most NO production under water deficit stress may be generated from NOS-like activity. After dehydration stress for 3 h, detached maize leaves pretreated with NO donor SNP maintained more water content than that of control leaves pretreated with water. This result was consistent with the decrease in the transpiration rate of SNP-treated leaves subjected to drought treatment for 3 h. Membrane permeability, a cell injury index, was lower in SNP-trested maize leaves under dehydration stress for 4 h when compared with the control leaves. Also, superoxide dismutsse (SOD) activity of SNP combined drought treatment maize leaves was higher than that of drought treatment alone, indicating that exogenous NO treatment alleviated the water loss and oxidative damage of maize leaves under water deficit stress. When c-PTIO as a specific NO scavenger was applied, the effects of applied SNP were overridden. Treatment with L-NAME on leaves also led to higher membrane permeability, higher transpiration rate and lower SOD activities than those of control leaves, indicating that NOS-like activity was involved in the antioxidative defense under water stress. These results suggested that NO dependence on NOS-like activity serves as a signaling component in the induction of protective responses and is associated with drought tolerance in maize seedlings.     

Overexpression of dehydrin tas14 gene improves the osmotic stress imposed by drought and salinity in tomato

One strategy to increase the level of drought and salinity tolerance is the transfer of genes codifying different types of proteins functionally related to macromolecules protection, such as group 2 of late embryogenesis abundant (LEA) proteins or dehydrins. The TAS14 dehydrin was isolated and characterized in tomato and its expression was induced by osmotic stress (NaCl and mannitol) and abscisic acid (ABA) [Godoy et al., Plant Mol Biol 1994;26:1921-1934], yet its function in drought and salinity tolerance of tomato remains elusive. In this study, transgenic tomato plants overexpressing tas14 gene under the control of the 35SCaMV promoter were generated to assess the function of tas14 gene in drought and salinity tolerance. The plants overexpressing tas14 gene achieved improved long-term drought and salinity tolerance without affecting plant growth under non-stress conditions. A mechanism of osmotic stress tolerance via osmotic potential reduction and solutes accumulation, such as sugars and K(+) is operating in tas14 overexpressing plants in drought conditions. A similar mechanism of osmotic stress tolerance was observed under salinity. Moreover, the overexpression of tas14 gene increased Na(+) accumulation only in adult leaves, whereas in young leaves, the accumulated solutes were K(+) and sugars, suggesting that plants overexpressing tas14 gene are able to distribute the Na(+) accumulation between young and adult leaves over a prolonged period in stressful conditions. Measurement of ABA showed that the action mechanism of tas14 gene is associated with an earlier and greater accumulation of ABA in leaves during short-term periods. A good feature for the application of this gene in improving drought and salt stress tolerance is the fact that its constitutive expression does not affect plant growth under non-stress conditions, and tolerance induced by overexpression of tas14 gene was observed at the different stress degrees applied to the long term.     

5-Aminolevulinic acid enhances photosynthetic gas exchange, chlorophyll fluorescence and antioxidant system in oilseed rape under drought stress

This study evaluates the role of exogenous foliar application of 5-aminolevulinic acid (ALA) on water relations, gas exchange, chlorophyll fluorescence, and the activities and gene expression patterns of antioxidant enzymes in leaves of oilseed rape under drought stress and recovery conditions. Seedlings at four-leaf stage were imposed to well-watered condition (80 % of water-holding capacity) or drought stress (40 % of water-holding capacity) and subsequently foliar sprayed with water or ALA (30 mg l^?1). Drought suppressed the accumulation of plant biomass and decreased chlorophyll content and leaf water status (relative water content and water potential). The actual quantum yield of photosystem II and electron transport rates were hampered in parallel to net photosynthetic rate. However, drought stress induced the accumulation of malondialdehyde (MDA) and hydrogen peroxide, enhanced the activities of catalase (CAT), peroxidase (POD), ascorbate peroxidase (APX), glutathione reductase (GR) and superoxide dismutase and up-regulated the expression of APX and GR. After rehydration for 4 days, the growth of drought-treated seedlings was restored to normal level for most of the physiological parameters. Foliar application of ALA maintained relatively higher leaf water status and enhanced chlorophyll content, net photosynthetic rate, actual quantum yield of photosystem II, photochemical quenching, non-photochemical quenching and electron transport rates in stressed leaves. Exogenous ALA also alleviated the accumulation of MDA and hydrogen peroxide, increased the activities of antioxidant enzymes and enhanced the expression of CAT and POD in drought-treated plants. These results indicate that ALA may effectively protect rapeseed seedlings from damage induced by drought stress.     

Drought stress and rehydration affect the balance between MGDG and DGDG synthesis in cowpea leaves

Membranes are main targets of drought, and there is growing evidence for the involvement of membrane lipid in plant adaptation to such an environmental stress. Biosynthesis of the galactosylglycerolipids, monogalactosyl-diacylglycerol (MGDG) and digalactosyl-diacylglycerol (DGDG), which are the main components of chloroplast envelope and thylakoid membranes, could be important for plant tolerance to water deficit and for recovery after rehydration. In this study, galactolipid (GL) biosynthesis in cowpea ( Vigna unguiculata L. Walp) leaves was analysed during drought stress and subsequent rewatering. Comparison of two cowpea cutivars, one drought tolerant and the other drought susceptible submitted to moderate drought stress, revealed patterns associated with water-deficit tolerance: increase in DGDG leaf content, stimulation of DGDG biosynthesis in terms of ¹⁴C-acetate incorporation and messenger accumulation corresponding to four genes coding for GL synthases ( MDG1 , MGD2 , DGD1 and DGD2 ). Similar to phosphate starvation, lack of water enhanced DGDG biosynthesis and it was hypothesized that the drought-induced DGDG accumulated in extrachloroplastic membranes, and thus contributes to plant tolerance to arid environments.     

Abscisic acid, indole-3-acetic acid and mineral–nutrient changes induced by drought and salinity in longan (Dimocarpus longan Lour.) plants

Longan species (Dimocarpus longan Lour.) exhibit a high agronomic potential in many subtropical regions worldwide; however, little is known about its responses to abiotic stress conditions. Drought and salinity are the most environmental factors inducing negative effects on plant growth and development. In order to elucidate the responses of longan to drought and salinity, seedlings were grown under conditions of drought and salt stresses. Drought was imposed by suspending water supply leading to progressive soil dehydration, and salinity was induced using two concentrations of NaCl, 100 and 150 mM in water solution, for 64 days. Data showed that salt concentrations increased foliar abscisic acid (ABA) and only 150 mM NaCl reduced indole-3-acetic acid (IAA) and increased proline levels. NaCl treatments also increased Na⁺ and Cl^? content in plant organs proportionally to salt concentration. Drought increased leaf ABA but did not change IAA concentrations, and also increased proline synthesis. In addition, drought and salt stresses reduced the photosynthesis performance; however, only drought decreased leaf growth and relative leaf water content. Overall, data indicate that under severe salt stress, high ABA accumulation was accompanied by a reduction of IAA levels; however, drought strongly increased ABA but did not change IAA concentrations. Moreover, drought and high salinity similarly increased (or maintained) ion levels and proline synthesis. Data also suggest that ABA accumulation may mitigate the impact of salt stress through inducing stomatal closure and delaying water loss, but did not mediate the effects of long-term drought conditions probably because leaves reached a strong dehydration and the role of ABA at this stage was not effective to detain leaf injuries.     

Photosynthetic CO<Subscript>2</Subscript>/H<Subscript>2</Subscript>O gas exchange and dynamics of carbohydrates content in maize leaves under drought

We studied the temporal sequence of changes in the photosynthetic CO₂/H₂O gas exchange intensity, as well as leaf water status, contents of soluble carbohydrates, starch, proline, pigments, and MDA, in maize seedlings (Zea mays L., cv. Luchistaya) under adaptation to increasing water deficit. The duration of drought was 2, 3, 5, and 6 days. Withholding water from maize plants caused gradual increase in the intensity of water deficit: from mild (2 or 3 days) to moderate (5 days) and nearly severe (6 days) water stress. After 6 days, relative leaf water content decreased by 19.8% as compared to the control. On the second day after the onset of drought, slight reduction in the photosynthetic CO₂/H₂O gas exchange intensity of the treated plants was observed. After 6 days, photosynthesis and transpiration of leaves synchronously reduced almost threefold due to stomatal closure. The progressive soil drought had substantial impact on the carbohydrate metabolism. After 2 days of water deficit, the content of reducing sugars and sucrose increased slightly, whereas after 6 days, it increased ten and four times, respectively. After 2, 3, and 5 days of drought, the starch content declined slightly; however, under severe drought (6 days), it increased by 30% as compared to the control. Simultaneously with the increase in the content of soluble sugars, proline content increased significantly and it was the highest on the sixth day of drought. At all stages of water deficit, the proline content increased more significantly than the content of reducing carbohydrates and sucrose. Under increasing water deficit (5 and 6 days), the content of MDA was found to rise. At the initial drought stage (2 or 3 days) and under severe water deficit (6 days), no significant changes in the pigment content were observed. Thus, at the initial stages of progressive drought, in the leaves of this maize cultivar, a decline in photosynthetic activity proceeded simultaneously with accumulation of reducing sugars, sucrose, and proline. The results obtained showed that, at the first stages of adaptation of maize seedlings to drought, the changes in carbohydrate and proline metabolism have been observed, which have increased upon further plant dehydration.