Does a terminal drought tolerance QTL contribute to differences in ROS scavenging enzymes and photosynthetic pigments in pearl millet exposed to drought?

The control of reactive oxygen species (ROS) and the stability of photosynthetic pigments under stress conditions are hypothesized to contribute to drought tolerance. Here we studied how ascorbic peroxidase (APX), superoxide dismutase (SOD), catalase (CAT) isozyme activities and chlorophyll a, b (Chl a, b) and carotenoids (Car) contents responded to water stress and whether they related to presence of a terminal drought tolerance QTL in pearl millet. We used PRLT2/89-33 (QTL donor), H77/833-2 (sensitive), and near-isogenic lines (QTL-NILs) introgressed with the QTL in H77/833-2 background. Under water stress there was no significant change in the total APX activity; only the proportional APX5 activity increased, with higher band intensity in tolerant genotypes. There were no significant changes in total activities of CAT and SOD under water stress, with similar band intensities in all genotypes, and a new CAT isozyme was induced in all genotypes. The photosynthetic pigment content decreased under water stress, although not differently in any genotype. Under water stress, the activities of most APX, CAT and SOD isozymes were closely related to the total chlorophyll/carotenoids ratio. Overall, besides APX5, water stress did not lead to major changes in the profile of isoenzymes involved in ROS scavenging. Similarly, the pigment content under stress did not discriminate genotypes according to the presence/absence of the QTL. This absence of discrimination for the ROS scavenging enzymes and for the pigment content under stress suggests that these traits may not play a key role in terminal drought tolerance in pearl millet.     

Which seed origin provides better tolerance to flooding and drought when restoring to face climate change?

Our goal was to establish the tolerance to flooding and drought of seedlings from a hydric gradient of different seed sources to provide recommendations for forest restoration in the face of climate change. We used Drimys winteri var. chilensis, a tree species that grows from extreme arid zones to wetlands along Chile, as the study subject. We expected that seedlings of xeric origin would perform better in drought conditions than populations from moist environments, and vice versa for flooding tolerance. We collected D. winteri seeds from xeric, mesic and wet environments. Seedlings at two development stages were submitted to an extreme flooding and drought treatment during 2 or 4 months in a common garden. After the flooding and drought assays finished, the number of surviving and damaged seedlings, lenticels and adventitious root presence, height, new leaves and specific leaf area, shoot/root ratio, water potential and/or chlorophyll fluorescence (Fv/Fm), were recorded. We found that flooding and drought affected almost all the parameters studied negatively. The xeric population seedlings, at both development stages studied, were the most tolerant to the drought and, unexpectedly, also to the flooding treatment. We recommend restoring with seedlings of xeric origin especially in arid areas where sudden flooding is frequent, as occurs in the Andes Mountains. In the face of climate change, we recommend carrying out common garden and field studies before advising which population origin should be used for restoration, since they do not always respond in accordance with expected patterns of local adaptation.     

Role of κ1 opioid receptors and cAMP in regulation of cardiac tolerance to ischemia and reperfusion

Cardioprotective, inotropic, and antiarrhythmic effects of the selective agonist of κ₁ opioid receptors (κ₁-ORs) U-50.488H have been studied after 45-min global ischemia and 30-min reperfusion of isolated perfused rat hearts. The heart κ₁-ORs were stimulated by adding 0.1 or 1 μmol/l U-50.488H to the perfusion solution. The opioid did not affect the frequency of reperfusion arrhythmias. At a concentration of 0.1 μmol/l, it induced a twofold decrease in the reperfusion release of creatine phosphokinase (CPK), which positively correlated with a decrease in the myocardial cAMP level (r = 0.89, p < 0.01). Application of U-50.488H at a final concentration of 1 μmol/l did not change the cAMP level and CPK release. These results suggest that the cardioprotective effect of U-50.488H is due to a decrease in the level of cAMP in cardiomyocytes. Activation of κ₁-ORs decreased the frequency and force of myocardial contractions. It has been shown that the negative inotropic and chronotropic effects of U-50.488H are independent of changes in the myocardial cAMP level. A hypothesis is proposed that the absence of cardioprotective effect of 1μM U-50.488H is a result of activation of nonopioid receptors in cardiomyocytes.     

Role of κ1 opioid receptors and cAMP in regulation of cardiac tolerance to ischemia and reperfusion

The cardioprotective, inotropic, and antiarrhythmic effects of U-50.488, a selective agonist of κ₁ opioid receptors (κ₁ ORs), was studied using the model of 45-min total ischemia and 30-min reperfusion of isolated rat heart. Cardiac κ₁ ORs were stimulated by adding U-50.488 to the perfusing solution up to the final concentration of 0.1 or 1 μmol/l. The opioid had no influence on the incidence of reperfusion arrhythmias. The addition of 0.1 μmol/l U-50.488 reduced the reperfusion release of creatine phosphokinase (CPK) by half, which positively correlated with the decrease in the myocardial cAMP content (r = 0.89, p < 0.01). At the same time, the addition of U-50.488 in the higher concentration (1 μmol/l) had no effect on either cAMP level or CPK release. These results indicate that the cardioprotective effect of U-50.488 may be connected with the reduction of myocardial cAMP content. Activation of κ₁ ORs caused a decrease in both frequency and amplitude of myocardial contractions. The negative inotropic and chronotropic effect of U-50.488 was shown to be independent of changes in the myocardial cAMP content. A hypothesis is proposed that the absence of any cardioprotective effect of U-50.488 at the higher concentration (1 μmol/l) is accounted for by its interaction with unknown nonopioid receptors of cardiac myocytes.     

The physiology and proteomics of drought tolerance in maize: early stomatal closure as a cause of lower tolerance to short-term dehydration?

Understanding the response of a crop to drought is the first step in the breeding of tolerant genotypes. In our study, two maize (Zea mays L.) genotypes with contrasting sensitivity to dehydration were subjected to moderate drought conditions. The subsequent analysis of their physiological parameters revealed a decreased stomatal conductance accompanied by a slighter decrease in the relative water content in the sensitive genotype. In contrast, the tolerant genotype maintained open stomata and active photosynthesis, even under dehydration conditions. Drought-induced changes in the leaf proteome were analyzed by two independent approaches, 2D gel electrophoresis and iTRAQ analysis, which provided compatible but only partially overlapping results. Drought caused the up-regulation of protective and stress-related proteins (mainly chaperones and dehydrins) in both genotypes. The differences in the levels of various detoxification proteins corresponded well with the observed changes in the activities of antioxidant enzymes. The number and levels of up-regulated protective proteins were generally lower in the sensitive genotype, implying a reduced level of proteosynthesis, which was also indicated by specific changes in the components of the translation machinery. Based on these results, we propose that the hypersensitive early stomatal closure in the sensitive genotype leads to the inhibition of photosynthesis and, subsequently, to a less efficient synthesis of the protective/detoxification proteins that are associated with drought tolerance.     

Purification and characterization of three β-glycosidases exhibiting high glucose tolerance from Aspergillus niger ASKU28

Production and utilization of cellulosic ethanol has been limited, partly due to the difficulty in degradation of cellulosic feedstock. β-Glucosidases convert cellobiose to glucose in the final step of cellulose degradation, but they are inhibited by high concentrations of glucose. Thus, in this study, we have screened, isolated, and characterized three β-glycosidases exhibiting highly glucose-tolerant property from Aspergillus niger ASKU28, namely β-xylosidase (P1.1), β-glucosidase (P1.2), and glucan 1,3-β-glucosidase (P2). Results from kinetic analysis, inhibition study, and hydrolysis of oligosaccharide substrates supported the identification of these enzymes by both LC/MS/MS analysis and nucleotide sequences. Moreover, the highly efficient P1.2 performed better than the commercial β-glucosidase preparation in cellulose saccharification, suggesting its potential applications in the cellulosic ethanol industry. These results shed light on the nature of highly glucose-tolerant β-glucosidase activities in A. niger, whose kinetic properties and identities have not been completely determined in any prior investigations.     

Is there a cost to resprouting? Seedling growth rate and drought tolerance in sprouting and nonsprouting Ceanothus (Rhamnaceae)

Many woody plant species that depend upon fire-cued seed germination lack the ability to resprout. As the ability to resprout is widely assumed to be the ancestral condition in most plant groups, the failure to sprout is an evolutionary derived trait. Models for the evolutionary loss of sprouting assume a trade-off between seedling success and vegetative resprouting ability of adults. Such models require higher seedling success rates in nonsprouters than in sprouters. On the other hand, there seem to be few a priori reasons why a strong sprouter might not also have highly competitive post-fire seedlings. To test the hypothesis that nonsprouting plants have higher growth rates and/or drought survival, we grew seedlings of Ceanothus tomentosus from sprouting and nonsprouting populations in a common garden experiment. Each of these C. tomentosus populations was paired with a sympatric Ceanothus species that differed in resprouting ability. Sprouters exhibited greater allocation to root carbohydrate storage than did nonsprouters, but overall relative growth rates did not differ. Nonsprouters had earlier onset of flowering. These results provide mixed support for models of a sprouting/nonsprouting allocation trade-off.     

Hormonal and metabolic regulation of source–sink relations under salinity and drought: From plant survival to crop yield stability

Securing food production for the growing population will require closing the gap between potential crop productivity under optimal conditions and the yield captured by farmers under a changing environment, which is termed agronomical stability. Drought and salinity are major environmental factors contributing to the yield gap ultimately by inducing premature senescence in the photosynthetic source tissues of the plant and by reducing the number and growth of the harvestable sink organs by affecting the transport and use of assimilates between and within them. However, the changes in source–sink relations induced by stress also include adaptive changes in the reallocation of photoassimilates that influence crop productivity, ranging from plant survival to yield stability. While the massive utilization of -omic technologies in model plants is discovering hundreds of genes with potential impacts in alleviating short-term applied drought and salinity stress (usually measured as plant survival), only in relatively few cases has an effect on crop yield stability been proven. However, achieving the former does not necessarily imply the latter. Plant survival only requires water status conservation and delayed leaf senescence (thus maintaining source activity) that is usually accompanied by growth inhibition. However, yield stability will additionally require the maintenance or increase in sink activity in the reproductive structures, thus contributing to the transport of assimilates from the source leaves and to delayed stress-induced leaf senescence. This review emphasizes the role of several metabolic and hormonal factors influencing not only the source strength, but especially the sink activity and their inter-relations, and their potential to improve yield stability under drought and salinity stresses.     

Is hemiepiphytism an adaptation to high irradiance? Testing seedling responses to light levels and drought in hemiepiphytic and non‐hemiepiphytic Ficus

The epiphytic growth habit in many Ficus species during their juvenile stages has commonly been hypothesized to be an adaptation for avoiding deep shade in the forest understory, but this has never been tested experimentally. We examined growth and ecophysiology in seedlings of three hemiepiphytic (Hs) and three non‐hemiepiphytic (NHs) Ficus species grown under different irradiance levels. Both Hs and NHs exhibited characteristics of high light requiring species, such as high plasticity to growth irradiance and relatively high maximum photosynthetic assimilation rates. Diurnal measurements of leaf gas exchange showed that Hs have much shorter active photosynthetic periods than NHs; moreover, leaves of Hs have lower xylem hydraulic conductivity but stronger drought tolerance as indicated by much lower rates of leaf diebacks during the drought treatment. Seedlings of NHs had 3.3‐ and 13.3‐fold greater height and biomass than those of Hs species after growing in the nursery for 5 months, indicating a trade‐off between growth and drought tolerance due to the conflicting requirements for xylem conductivity and cavitation resistance. This study does not support the shade‐avoidance hypothesis; rather, it suggests that the canopy regeneration in Hs is an adaptation to avoid alternative terrestrial growth‐related risks imposed to tiny Ficus seedlings. The NHs with terrestrial regeneration reduce these risks by having an initial burst of growth to rapidly gain relatively large seedling sizes, while in Hs seedlings more conservative water use and greater drought tolerance for surviving the canopy environment are intrinsically associated with slow growth.     

Soil fungal communities are compositionally resistant to drought manipulations – Evidence from culture-dependent and culture-independent analyses

Current environmental change predictions forecast intensified drought conditions. It is becoming increasingly evident that plant communities are sensitive to drought and that soil-inhabiting microbial communities vary along precipitation gradients. However, the drought sensitivity of microbial communities in general and that of soil fungi in particular remains unclear, even though understanding their responses to adverse environmental conditions is vital for better understanding of ecosystem service provisioning. We sampled soils at two sites with established experiments that imposed extreme, chronic drought to assess fungal community responses. We analyzed fungal communities using both culture-dependent and -independent tools and MiSeq-sequenced communities from colony forming units (CFU-PCR) on a drought simulating medium and from environmental DNA (ePCR), to compare the conclusions derived from these two methods. Our data from the two approaches consistently indicate that the composition of fungal communities is not affected by the drought treatment, whereas – based on the CFU-PCR but not ePCR data – their richness and diversity increased under drought conditions at the more mesic of the two sites. Further, based on the direct comparisons of CFU-PCR and ePCR, we estimate that more than 10% of the fungal community and more than 20% of the ascomycetes were culturable. We conclude that although recent research indicates that plant and bacterial communities respond to drought, fungal community responses are more variable, particularly in experiments that impose chronic drought under field conditions.     

Zooplankton dynamics in response to the transition from drought to flooding in four Murray–Darling Basin rivers affected by differing levels of flow regulation

A review of stratigraphic, radiocarbon, pollen, and aerial photographic data on the Swan Coastal Plain, south-western Australia, allows interpretation of long-term changes in climate and its effects on wetlands during the Holocene, whereas monitoring wetland hydrology and vegetation provides a measure of shorter-term changes. The information provides models for basin wetland response to changing climate. Drying climates shift wetlands to drier conditions, turning lakes into seasonally inundated or waterlogged basins, or resulting in an overall loss of wetlands, and favours more saline conditions, and development of carbonate deposits. Wetter conditions results in more frequent inundation, shifting damplands to sumplands or lakes, and resulting in fresher water conditions, and development of peat and/or organic matter enriched deposits. Examples of wetland basin responses to climate change across the Swan Coastal Plain show differential responses depending on setting, spatial distribution, hydrology, hydrochemistry and geochemistry, different temporal frameworks, and biological resilience.     

Inoculation of plant-growth-promoting rhizobacteria in Myracrodruon urundeuva Allemão supports in tolerance to drought stress

This study evaluated the influence of Azospirillum lipoferum on the growth of Myracroduon urundeuva (Anacardiaceae) plants under drought stress, by means of biometric, physical–chemical and biochemical parameters. The association of A. lipoferum with the roots of the plants provided increases of 30% root length, 50% root dry weight, 34% shoot dry weight and 10% soluble protein content. The inoculated plants still maintained 5% higher leaf water potential than those not inoculated and lower membrane damage. Furthermore, the inoculated plants shown less leaf fall and dark green leaves, confirmed by maintenance of the highest levels of chlorophyl a, b and total. On the other hand, superoxide dismutase activity was significantly lower in the inoculated plants, possibly due to the induction of a non-enzymatic protective feature. In this way, the inoculation of PGPR in M. urundeuva can be an alternative for the production of plants that are more tolerant to drought stress.