Salt tolerance, salt accumulation, and ionic homeostasis in an epidermal bladder-cell-less mutant of the common ice plant Mesembryanthemum crystallinum

The aerial surfaces of the common or crystalline ice plant Mesembryanthemum crystallinum L., a halophytic, facultative crassulacean acid metabolism species, are covered with specialized trichome cells called epidermal bladder cells (EBCs). EBCs are thought to serve as a peripheral salinity and/or water storage organ to improve survival under high salinity or water deficit stress conditions. However, the exact contribution of EBCs to salt tolerance in the ice plant remains poorly understood. An M. crystallinum mutant lacking EBCs was isolated from plant collections mutagenized by fast neutron irradiation. Light and electron microscopy revealed that mutant plants lacked EBCs on all surfaces of leaves and stems. Dry weight gain of aerial parts of the mutant was almost half that of wild-type plants after 3 weeks of growth at 400 mM NaCl. The EBC mutant also showed reduced leaf succulence and leaf and stem water contents compared with wild-type plants. Aerial tissues of wild-type plants had approximately 1.5-fold higher Na(+) and Cl(-) content than the mutant grown under 400 mM NaCl for 2 weeks. Na(+) and Cl(-) partitioning into EBCs of wild-type plants resulted in lower concentrations of these ions in photosynthetically active leaf tissues than in leaves of the EBC-less mutant, particularly under conditions of high salt stress. Potassium, nitrate, and phosphate ion content decreased with incorporation of NaCl into tissues in both the wild type and the mutant, but the ratios of Na(+)/K(+) and Cl(-)/NO(3)(-)content were maintained only in the leaf and stem tissues of wild-type plants. The EBC mutant showed significant impairment in plant productivity under salt stress as evaluated by seed pod and seed number and average seed weight. These results clearly show that EBCs contribute to succulence by serving as a water storage reservoir and to salt tolerance by maintaining ion sequestration and homeostasis within photosynthetically active tissues of M. crystallinum.     

Identification of interior salt-tolerant bacteria from ice plant <Emphasis Type="Italic">Mesembryanthemum crystallinum</Emphasis> and evaluation of their promoting effects

Endophytic bacteria can stimulate host plant development. Insufficient information is available about NaCl-tolerant bacteria that colonize ice plants (Mesembryanthemum crystallinum) in their habitats. In this study, a culture-dependent method was used to isolate endophytic nitrogen-fixing bacteria from ice plants, and the resulting cultures were screened for salt-stress tolerance in vitro. A total of 17 salt-tolerant bacteria were obtained. The majority of the isolates grew well in 2.05 M NaCl with a maximum tolerance at 3.59 M. Most of the strains were Gram-positive bacteria with various plant growth-promoting traits. The 16S rRNA gene sequences revealed that the 17 isolates were distributed within three genera and corresponded to the bacterial species Halomonas sp., Bacillus sp., and Planococcus sp. Inoculation of cabbage (Brassica olereacea) seeds with selected strains showed that the strain MC1 promoted seed germination, and the same strain significantly increased root dry weight under saline stress by 24.5%. Our study suggests that ice plants naturally accommodate a variety of salt-tolerant endophytic bacteria and that these bacteria are able to relieve abiotic stress during plant growth.     

Two functionally different vacuoles for static and dynamic purposes in one plant mesophyll leaf cell

It is a common belief that plant mesophyll cells are occupied up to 95% by a single multipurpose vacuole. The common ice plant, Mesembryanthemum crystallinum L., however, requires two contrasting functions of the vacuole under salt stress. Large amounts of NaCl have to be sequestered permanently for osmotic purpose and for protecting the cytoplasm from NaCl toxicity. A dynamic exchange with the cytoplasm is required because photosynthesis proceeds under these conditions via the metabolic cycle of crassulacean acid metabolism (CAM). Nocturnally acquired CO2 must be kept as malate in the vacuole and re-mobilized in the daytime. Here, we show that two large independent types of vacuoles with different transport properties meet the requirements for the contrasting functions within the same cell.     

植物角质层对非生物逆境胁迫响应研究进展

角质层,包括角质和蜡质,是主要由脂肪酸及其衍生物构成的覆盖在植物的外表面的高度疏水层,在植物生长发育过程中起到非常重要的保护屏障作用。除了在极端温度、干旱、高盐等多种非生物逆境胁迫下起到保护作用外,还能够保护植物内部组织免受细菌、真菌病原体的侵染。现就植物角质层的组成、合成途径以及与植物抗逆性,特别是与抗旱能力的关系方面的最新研究进展进行了综述。     

Environmental and developmental regulation of the wound-induced cell wall protein WI12 in the halophyte ice plant

A wounded gene WI12 was used as a marker to examine the interaction between biotic stress (wounding) and abiotic stress (high salt) in the facultative halophyte ice plant (Mesembryanthemum crystallinum). The deduced WI12 amino acid sequence has 68% similarity to WUN1, a known potato (Solanum tuberosum) wound-induced protein. Wounding, methyl jasmonate, and pathogen infection induced local WI12 expression. Upon wounding, the expression of WI12 reached a maximum level after 3 h in 4-week-old juvenile leaves, whereas the maximum expression was after 24 h in 8-week-old adult leaves. The temporal expression of WI12 in salt-stressed juvenile leaves was similar to that of adult leaves. The result suggests that a salt-induced switch from C3 to Crassulacean acid metabolism has a great influence on the ice plant's response to wounding. The expression of WI12 and the accumulation of WI12 protein were constitutively found in phloem and in wounded mesophyll cells. At the reproductive stage, WI12 was constitutively found in petals and styles, and developmentally regulated in the placenta and developing seeds. The histochemical analysis showed that the appearance of WI12 is controlled by both environmental and developmental factors. Immunogold labeling showed WI12 preferentially accumulates in the cell wall, suggesting its role in the reinforcement of cell wall composition after wounding and during plant development.     

Endophyte-mediated adjustments in host morphology and physiology and effects on host fitness traits in grasses

Endophytic fungi have been shown to increase tolerance of hosts to biotic and abiotic stresses and in some cases alter growth and development of plants. In this article we evaluate some effects that clavicipitaceous endophytes have on development and physiology of plant tissues. We postulate that oxidative stress protection is the fundamental underlying benefit conferred by many endophytes, accounting for frequently observed enhanced disease resistance, drought tolerance, heavy metal tolerance and tolerance to numerous additional oxidative stresses. We hypothesize that endophyte-mediated oxidative stress protection of the host is the result of at least two processes, including: (1) secretion of reactive oxygen species (ROS) from endophytic mycelia into plant cells; and (2) secretion of auxin from endophytic mycelia into plant cells. Both processes result in an increase in ROS in plant tissues; and stimulate plant tissues to increase activities of antioxidant systems. Auxin is suggested to function in suppression of plant cell death and may be important in maintaining the endophyte–plant symbiosis.     

Plant cell organelle proteomics in response to abiotic stress

Proteomics is one of the finest molecular techniques extensively being used for the study of protein profiling of a given plant species experiencing stressed conditions. Plants respond to a stress by alteration in the pattern of protein expression, either by up-regulating of the existing protein pool or by the synthesizing novel proteins primarily associated with plants antioxidative defense mechanism. Improved protein extraction protocols and advance techniques for identification of novel proteins have been standardized in different plant species at both cellular and whole plant level for better understanding of abiotic stress sensing and intracellular stress signal transduction mechanisms. In contrast, an in-depth proteome study of subcellular organelles could generate much detail information about the intrinsic mechanism of stress response as it correlates the possible relationship between the protein abundance and plant stress tolerance. Although a wealth of reviews devoted to plant proteomics are available, review articles dedicated to plant cell organelle proteins response under abiotic stress are very scanty. In the present review, an attempt has been made to summarize all significant contributions related to abiotic stresses and their impacts on organelle proteomes for better understanding of plants abiotic stress tolerance mechanism at protein level. This review will not only provide new insights into the plants stress response mechanisms, which are necessary for future development of genetically engineered stress tolerant crop plants for the benefit of humankind, but will also highlight the importance of studying changes in protein abundance within the cell organelles in response to abiotic stress.     

Early salt stress effects on the changes in chemical composition in leaves of ice plant and Arabidopsis. A Fourier transform infrared spectroscopy study

A technique based on Fourier transform infrared (FT-IR) spectrometry was developed to detect the corresponding changes in chemical composition associated with the rapid changes in sodium and water content in 200 mM NaCl-stressed halophyte ice plants (Mesembryanthemum crystallinum). The changes in glycophyte Arabidopsis stressed with 50 mM NaCl were also examined for comparison. The obtained IR spectra were further processed by deconvolution and curve fitting to examine the chemical nature of the responding sources in the leaves. Using three stages of ice plant leaves, absorption bands corresponding to carbohydrates, cell wall pectin, and proteins were identified, with distinct IR spectra representing each developmental stage. Within 48 h of mild salt stress, the absorption band intensities in the fingerprint region increased continuously in both plants, suggesting that the carbon assimilation was not affected at the early stage of stress. The intensities of ester and amide I absorption bands decreased slightly in Arabidopsis but increased in ice plant, suggesting that the cell expansion and protein synthesis ceased in Arabidopsis but continued in ice plant. In both plants, the shift in amide I absorption band was observed hourly after salt stress, indicating a rapid conformational change of cellular proteins. Analyses of the ratio between major and minor amide I absorption band revealed that ice plant was able to maintain a higher-ordered form of proteins under stress. Furthermore, the changes in protein conformation showed a positive correlation to the leaf sodium contents in ice plant, but not in Arabidopsis.     

Possible plant mitochondria involvement in cell adaptation to drought stress. A case study: durum wheat mitochondria

Although plant cell bioenergetics is strongly affected by abiotic stresses, mitochondrial metabolism under stress is still largely unknown. Interestingly, plant mitochondria may control reactive oxygen species (ROS) generation by means of energy-dissipating systems. Therefore, mitochondria may play a central role in cell adaptation to abiotic stresses, which are known to induce oxidative stress at cellular level. With this in mind, in recent years, studies have been focused on mitochondria from durum wheat, a species well adapted to drought stress. Durum wheat mitochondria possess three energy-dissipating systems: the ATP-sensitive plant mitochondrial potassium channel (PmitoK(ATP)); the plant uncoupling protein (PUCP); and the alternative oxidase (AOX). It has been shown that these systems are able to dampen mitochondrial ROS production; surprisingly, PmitoK(ATP) and PUCP (but not AOX) are activated by ROS. This was found to occur in mitochondria from both control and hyperosmotic-stressed seedlings. Therefore, the hypothesis of a 'feed-back' mechanism operating under hyperosmotic/oxidative stress conditions was validated: stress conditions induce an increase in mitochondrial ROS production; ROS activate PmitoK(ATP) and PUCP that, in turn, dissipate the mitochondrial membrane potential, thus inhibiting further large-scale ROS production. Another important aspect is the chloroplast/cytosol/mitochondrion co-operation in green tissues under stress conditions aimed at modulating cell redox homeostasis. Durum wheat mitochondria may act against chloroplast/cytosol over-reduction: the malate/oxaloacetate antiporter and the rotenone-insensitive external NAD(P)H dehydrogenases allow cytosolic NAD(P)H oxidation; under stress this may occur without high ROS production due to co-operation with AOX, which is activated by intermediates of the photorespiratory cycle.     

Stem Cell Maintenance and Abiotic Stress Response in Shoot Apical Meristem for Developmental Plasticity

The shoot apical meristem (SAM) serves as a non-drying reservoir of pluripotent stem cells to supply new daughter cells forming above-ground tissues and organs such as leaves, stems, flowers and fruits throughout the life cycle of plants. Accordingly, the homeostasis control of stem cell division and differentiation must be an essential core mechanism for harmonic growth and development of plants as multicellular higher eukaryotes. Unlike animals, plants are sessile organisms and thus constantly face environmental factors, including abiotic stresses. Therefore, post-embryonic development derived from stem cells in the SAM likely interacts with surrounding abiotic stresses for plant adaptation and plastic development. For this reason, this review provides the most recent findings regarding comprehensive signaling networks involved in stem cell maintenance in the SAM, and then describes how stem cell signaling is related with abiotic stress response through involvement of phytohormones and reactive oxygen species in the SAM.     

Effect of high salinity on tracheary element differentiation in light-grown callus of Mesembryanthemum crystallinum

This study investigated the inhibitory effects of NaCl on tracheary element (TE) differentiation in light-grown callus of ice plant Mesembryanthemum crystallinum L., a halophyte which adaptes well to saline environments. When ice plant callus was grown in a modified Linsmaier-Bednar and Skoog culture medium containing no NaCl (control medium), up to 20% of ice plant cells differentiated into tracheary elements during in vitro culture. Close examination of callus tissues stained with potassium permanganate revealed that tracheary elements were aggregated as discrete nodules. Some strikingly elongated tracheary elements were found in the macerated tissues. Experimental results indicated that adding 200 mM NaCl to the control medium reversibly inhibited the formation of tracheary element in the halophytic cells. The rate of tracheary element formation increased accordingly as the rate of cell growth in control medium. In the presence of high salt, the degree of tracheary element differentation remained low through the growth cycle. The inhibitory effect of salt on tracheary element differentiation was overcome by adding 10 mg l⁻¹ salicylic acid, a known signaling compound that induces a diverse group of defense-related genes, including genes involved in reinforcing the host cell wall. Furthermore, microscopic examination revealed that most tracheary elements formed under this treatment (200 mM NaCl plus 10 mg l⁻¹ salicylic acid) were round shaped. The results suggest that high salt inhibits both the biosynthesis of secondary wall components and cell elongation ice plant in vitro culture. This revised version was published online in June 2006 with corrections to the Cover Date.     

Cyclin dependent protein kinases and stress responses in plants

Plants have to adjust, grow and establish themselves in various changing environmental conditions. Additionally, the sessile life-style of plants requires the development of response mechanisms for their adaptation in such environmental cues. Under biotic and abiotic stress, plant growth is negatively affected mainly as a result of cell cycle inhibition. The perception of stress involves the activation of signaling cascades that result in a prolonged S-phase and delayed entry into mitosis. Although the molecular interactions that link the cell cycle machinery to perception of stress are not fully understood, recent studies indicated the involvement of Cyclin Dependent Kinases (CDKs) in the plant response machinery. CDKs are core cell cycle regulators but their activity has been implicated in additional diverse cellular processes. Here we review the impact of different types of abiotic stress on plant cell cycle progression and CDK activity, and discuss the contribution of CDK function in the signaling control of stress tolerance.Key words: abiotic stress, cell cycle, CDK, cyclin     

Salt Stress Perception and Plant Growth Regulators in the Halophyte Mesembryanthemum crystallinum

We selected indicators of four different metabolic processes (Crassulacean acid metabolism [CAM], amino acid and nitrogen mobilization metabolism, osmoprotection, and plant defense mechanisms) to study the relationship between salt-stress-mediated and plant growth regulator (PGR)-induced responses in Mesembryanthemum crystallinum (ice plant). Nacl and PGRs (cytokinin and abscisic acid [ABA]) are efficient elicitors of the well-studied Nacl stress responses: induction of the CAM form of phosphoenolpyruvate carboxylase, proline pinitol accumulation, and the increase of an osmotin-like protein. NaCl and cytokinin are more effective than ABA in stimulating accumulation of proline and an osmotin-like protein before the plants are committed to flowering. The results are consistent with a plant defense-induction model, in which environmental stress and PGRs are distinct signals whose subsequent effects lead to overlapping responses, the magnitude of which depends on plant developmental status.     

Environment or development? Lifetime net CO2 exchange and control of the expression of Crassulacean acid metabolism in Mesembryanthemum crystallinum

The relative influence of plant age and environmental stress signals in triggering a shift from C(3) photosynthesis to Crassulacean acid metabolism (CAM) in the annual halophytic C(3)-CAM species Mesembryanthemum crystallinum was explored by continuously monitoring net CO(2) exchange of whole shoots from the seedling stage until seed set. Plants exposed to high salinity (400 mm NaCl) in hydroponic culture solution or grown in saline-droughted soil acquired between 11% and 24% of their carbon via net dark CO(2) uptake involving CAM. In contrast, plants grown under nonsaline, well-watered conditions were capable of completing their life cycle by operating in the C(3) mode without ever exhibiting net CO(2) uptake at night. These observations are not consistent with the widely expressed view that the induction of CAM by high salinity in M. crystallinum represents an acceleration of preprogrammed developmental processes. Rather, our study demonstrates that the induction of the CAM pathway for carbon acquisition in M. crystallinum is under environmental control.