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Drought stress was evaluated with polyethylene glycol 6000 (PEG 6000) treatment in Dunaliella salina, a microalga known for its great ability to withstand salinities of more than 30%. The aim was to explore the acclimation mechanisms used by the microalga to regulate its growth and physiology during coping with drought stress. The microalga was subjected to culture mediums containing 2 and 5% PEG for 25 days and was compared with a control culture medium. Significant decrease in growth parameters such as specific growth rate, biomass and number of divisions per day was demonstrated in PEG-treated algae. During PEG treatment, chlorophylls slightly increased, while β-carotene and total protein were not affected. Osmolytes, as well as carbohydrates, were found to be significantly higher in PEG-treated algae than in control. Increased catalase and ascorbate peroxidase activities were proportionally related to PEG concentrations in the cultures. The PEG-treated cells accumulated a considerable amount of hydrogen peroxide and malondialdehyde, especially at higher PEG concentrations. Electrolyte leakage increased, regardless of the PEG concentrations applied, while DNA fragmentation was not observed after 25 days of treating with PEG. It was concluded that Dunaliella cells could respond to the drought stress, probably by using a higher accumulation of a range of osmolytes and also more stimulation of the antioxidant enzymatic system.

The 70-kDa heat shock proteins (HSP70s) are a conserved class of chaperones that play critical roles during the normal life cycle of plants. HSP70s are particularly involved in the regulation of biotic and abiotic stress responses. In this paper, the potential roles of this protein were investigated. A reverse genetic approach was employed for transient silencing of hsp70 gene in tomato (Solanum lycopersicum L.) to evaluate different growth and physiological parameters under normal conditions and during the response to drought stress. A combined ANOVA (analysis of variance) and HCA (hierarchical clustering analysis) showed that hsp70 silencing led to severe growth retardation and mortality, significant membrane damage and leakage, decline in relative water content, low rate of pigment accumulation, and reduced antioxidant enzyme activity under normal and drought stress conditions. Among the different parameters, proline was the only trait that was unaffected by gene silencing and accumulated by similar amounts to that of nonsilent plants. In conclusion, HSP70 played critical roles in maintaining the cellular homeostasis of plants during adaptation to drought and under normal plant life conditions. It was speculated that proline was, to some extent, involved in improving the loss of protein folding or function resulting from HSP70 deficiency, and played a crucial role in the adaptation of plants on exposure to stress.

Microalgae as one of the key components of food chains in aquatic environments are promising biological models for investigating gamma irradiation effects on eukaryotic organisms. Understanding resistance mechanisms in these organisms as simple models might illuminate how gamma irradiation resistance improves in these algae and even more complex organisms. The present study aimed to investigate the effects of ionizing irradiation on Chlorella vulgaris as a stress-tolerant microalga and to find how the eukaryotic cells would tolerate such stressful conditions. The physiological responses and biochemical alterations of C. vulgaris were analyzed at three different time points (0, 16, and 48 h) after 600 Gy gamma irradiation. Compared to the control, gamma-irradiated algae had slower growth rate with significantly longer lag phase, less chlorophyll and protein contents at time 0, which were compensated and recovered during the next 48 h. The results also showed spontaneous H2O2 burst accompanied by a higher rate of lipid peroxidation and electrolyte leakage, a rapid increase of catalase activity and more ferric reducing antioxidant power immediately after irradiation. During a 48-h period, most alterations stabilized. Raising trends were observed in the carotenoid contents, the ratios of carbohydrates, amide I and amide II to fatty acids. Principal component analysis and hierarchical clustering suggest two possible distinct mechanisms: a “quick” one including spontaneous responses by boosting H2O2, amplifying enzymatic antioxidant systems, and increasing compatible solutes like proline and a “delayed” responsive strategy including the increase of soluble carbohydrates, carotenoids, and stress-related proteins triggered several hours after irradiation.

The natural variation of drought tolerance among eight Iranian bermudagrass (Cynodon dactylon L.) accessions was evaluated and compared with a commonly used variety by measuring various morphological, growth, physiological and molecular responses during mild and severe drought stress conditions. A combined multivariable analysis using principal component analysis, hierarchical clustering analysis, as well as drought-tolerance ranking measurement showed that the accessions were clustered into three groups in terms of drought tolerance. This method evaluated three accessions (Maragh, Abyaneh, and Badrood) as superior drought-tolerant, three (Karshahi, Josheghan, and Kashan) as relatively drought-tolerant, and three (Ardeha, SWI-7, and Khonb) as drought-sanative genotypes. Drought-tolerant accessions generally showed improved growth and relative water content (RWC), minor electrolyte leakage (EL) and lipid peroxidation (MDA), higher content of pigments (Chlorophyll and carotenoid) and dark green color index (DGCI), more significant up-regulation of two stress-related genes (ECERIFERUM1 and Delta 1-pyrroline-5-carboxylate synthetase), as well as higher accumulation of proline content compared to sensitive accessions under drought stress. On the other hand, drought-sensitive accessions typically had more activity of antioxidant enzymes (catalase and ascorbate peroxidase), and higher amounts of soluble sugars (glucose, mannose, and rhamnose), compared to tolerant accessions under drought stress. In conclusion, different defensive strategies may be recruited by tolerant or sensitive bermudagrass genotypes during encountering to drought stress.

Heat shock proteins (HSPs) have vital roles during plant adaptation to biotic and abiotic stresses, as well as stress-free conditions. In the present study, we used a heterologous strategy of virus induced gene silencing to investigate the role of different classes of HSPs in Nicotiana benthamiana. Different growth and physiological parameters in silenced plants were evaluated under both control and salt stress conditions. Among the treatments, silencing of HSP70, especially under salinity regime, was found to have stronger impacts on growth, protein concentration, the accumulation of photosynthetic pigments, proline and total soluble carbohydrates content, malondialdehyde, the activity of antioxidant enzymes, performance index, relative water content and the ratio of K+/Na+, suggesting a more prominent role for HSP70 in both stressful and stress-free life cycle of the plants. Principal component analysis and hierarchical clustering indicated that HSP70 gene was silenced, the plants might effectively respond to stress by a higher accumulation of compatible solutes, like proline to protect the cell membranes and proteins from damage.

The growth and physiological parameters of fourteen tomato cultivars were measured during both mild and severe drought. The tomato cultivars showed decrease in growth traits, relative water content and pigments content under both drought treatments compared with the control. By contrast, proline accumulation, malondialdehyde and electrolyte leakage increased in the cultivars. The activity of superoxide dismutase was more correlated to drought stress than those of ascorbate peroxidase or catalase. The combined approach of principal component analysis, hierarchical clustering analysis and calculating mean tolerance value showed relative differences in drought tolerance among the cultivars. The studied cultivars were clustered into four groups. Four cultivars were placed in cluster I as the drought-tolerant group. Moderately tolerant cultivars were grouped into Cluster II. The clusters III and IV represented the sensitive and highly sensitive cultivars, respectively. Early orbana, Roma and Cal-j cultivars were indicated as the highly sensitive tomato cultivars. It was concluded that such an approach could be used as a useful tool to screen the other abiotic or even biotic stress tolerance in tomato.

Myrtle (Myrtus communis L.) is an evergreen and aromatic plant species with therapeutic and ornamental values. The effect of water stress induced by three concentrations of Polyethylene glycol (PEG), 0% (control), 3% (3%PEG) and 6% (6% PEG) was evaluated on in vitro-regenerated myrtle shoots. After 30 days of culture, all regeneration, growth and physiological parameters were measured at the end of the experiment. Murashige and Skoog (MS) medium contained 1.5 mgl-1 6-benzyladenine (BAP) and 0.2 mgl-1 Naphthaleneacetic acid (NAA) was optimized for the in vitro experiment. Increasing PEG concentration in the culture media resulted in fewer regenerated shoots, and higher rate of browning in the apical tissues. Chlorophylls and total carotenoids content significantly decreased in the shoots treated with PEG, however there were no significant differences in the pigment content under moderate or severe water stress conditions. PEG-induced water stress resulted in reduced plant growth and relative water content of newly-formed shoots. Lipid peroxidation and electrolyte leakage were lower in myrtle shoots treated with 6%PEG than 3%PEG, but they were still higher than that of the control. Proline accumulation significantly increased under 3%PEG treatment, while a little difference were observed between proline content under 6%PEG and the control. Apoptosis did not occur in the myrtle shoots grown under moderate or severe stress treatments which correlated with more increased Catalase and Superoxide dismutase activities in response to higher stress levels. The results showed a possible contribution of enzymatic antioxidant system in myrtle when counteracting severe water stress.

In this work, we investigated the importance of phytoene desaturase (PDS) during salt stress in plants. For this aim, we transiently suppressed the corresponding gene of Nicotiana benthamiana using a Tobacco Rattle Virus silencing system. After salinity stress, the silenced plants were assayed for different growth and physiological parameters. The silenced plants were found to be induced more stunted and feeble shoots and roots than the negative controls. The results indicated that the concurrent incidence of salinity stress and viral infection did not have an additive influences on the plant growth and physiological parameters. Importantly, our results showed a more detrimental influence of salinity on plant growth and physiology after PDS silencing. Compared to the control, salt-stressed plants contained more proline, and malondialdehyde, less amount of photosynthetic pigments, higher Na+/K+ ratios in shoots and roots and lower relative water content after PDS silencing. The activity of antioxidant enzymes were also decreased in the plants coped with salinity or Tobacco Rattle Virus. It was also represented that simultaneous silencing of the PDS gene and salinity treatment resulted in a significant decrease of the performance index (PIABS) and an increase of dissipation per active reaction center (DIo/RC) reflecting severe injuries in photosynthetic machinery under such a condition. Overall results suggested that PDS silencing could increase the sensitivity of plants to salinity.