Critical water contents at leaf, stem and root level leading to irreversible drought-induced damage in two woody and one herbaceous species.

Plant water content is a simple and promising parameter for monitoring drought-driven plant mortality risk. However, critical water content thresholds leading to cell damage and plant failure are still unknown. Moreover, it is unclear whether whole-plant or a specific organ water content is the most reliable indicator of mortality risk. We assessed differences in dehydration thresholds in leaf, stem and root samples, hampering the organ-specific rehydration capacity and increasing the mortality risk. We also tested eventual differences between a fast experimental dehydration of uprooted plants, compared to long-term water stress induced by withholding irrigation in potted plants. We investigated three species with different growth forms and leaf habits i.e., Helianthus annuus (herbaceous), Populus nigra (deciduous tree) and Quercus ilex (evergreen tree). Results obtained by the two dehydration treatments largely overlapped, thus validating bench dehydration as a fast but reliable method to assess species-specific critical water content thresholds. Regardless of the organ considered, a relative water content value of 60% induced significant cell membrane damage and loss of rehydration capacity, thus leading to irreversible plant failure and death.

Delaying drought-driven leaf cell damage may be the key trait of invasive trees ensuring their success in the Mediterranean basin.

Invasive alien species (IAS) threaten the biodiversity richness of the Mediterranean basin, a drought-prone region. However, our knowledge on the adaptive strategies of IAS for facing Mediterranean drought summers is still incomplete. The aim of the present study is to compare the water relations and the critical relative water content (RWC) values leading to loss of cell rehydration capacity of two Mediterranean basin IAS (i.e., Ailanthus altissima (Mill.) Swingle and Robinia pseudoacacia L.) versus two co-occurring native species (i.e., Fraxinus ornus L. and Quercus pubescens Willd.). Study IAS showed higher values of water potential at turgor loss point and osmotic potential at full turgor, lower values of modulus of elasticity and leaf mass area but higher photosynthesis rate, even during the summer, with respect to the Mediterranean native species. These findings supported the hypothesis that IAS are characterized by a resource acquisitive strategy coupled with a safety-efficiency trade-off, compared with Mediterranean native species. However, similar leaf RWC thresholds leading to loss of cell rehydration capacity were recorded in the two groups of species. Moreover, IAS showed higher saturated water content and capacitance values compared with the co-occurring species. Overall, our results suggest that the success of Mediterranean IAS is driven by their ability to delay dehydration damage of mesophyll cells during Mediterranean summer drought, thereby supporting their distinctive high carbon assimilation rate.

When Water Availability Is Low, Two Mediterranean Salvia Species Rely on Root Hydraulics.

Increase in severity and frequency of drought events is altering plant community composition, exposing biomes to a higher risk of biodiversity losses. This is exacerbated in the most fragile areas as Mediterranean biome. Thus, identifying plant traits for forecasting species with a high risk of drought-driven mortality is particularly urgent. In the present study, we investigated the drought resistance strategy of two Mediterranean native species: Salvia ceratophylloides Ard. (Sc) and Salvia officinalis L. (So) by considering the impact of drought-driven water content decline on plant hydraulics. Well-watered samples of Sc displayed higher leaf and stemsaturated water content and lower shoot biomass than So samples, but similar root biomass. In response to drought, Sc showed a conservative water use strategy, as the prompt stomatal closure and leaves shedding suggested. A drought-tolerant mechanism was confirmed in So samples. Nevertheless, Sc and So showed similar drought-driven plant hydraulic conductance (Kplant) recover ability. Root hydraulic traits played a key role to reach this goal. Relative water content as well as loss of cell rehydration capability and membrane damages, especially of stem and root, were good proxies of drought-driven Kplant decline.

Too dry to survive: Leaf hydraulic failure in two Salvia species can be predicted on the basis of water content.

Global warming is exposing plants to increased risks of drought-driven mortality. Recent advances suggest that hydraulic failure is a key process leading to plant death, and the identification of simple and reliable proxies of species-specific risk of irreversible hydraulic damage is urgently required. We assessed the predictive power of leaf water content and shrinkage for monitoring leaf hydraulic failure in two Mediterranean native species, Salvia ceratophylloides (Sc) and S. officinalis (So). The study species showed significant differences in relative water content (RWC) thresholds inducing loss of rehydration capacity, as well as leaf hydraulic conductance (KL) impairment. Sc turned out to be more resistant to drought than So. However, Sc and So showed different leaf saturated water content values, so that different RWC values actually corresponded to similar absolute leaf water content. Our findings suggest that absolute leaf water content and leaf water potential, but not RWC, are reliable parameters for predicting the risk of leaf hydraulic impairment of two Salvia species, and their potential risk of irreversible damage under severe drought. Moreover, the lack of any KL decline until the turgor loss point in Sc, coupled to consistent leaf shrinkage, rejects the hypothesis to use leaf shrinkage as a proxy to predict KL vulnerability, at least in species with high leaf capacitance. Robust linear correlations between KL decline and electrolyte leakage measurements suggested a role of membrane damage in driving leaf hydraulic collapse.

The extra-vascular water pathway regulates dynamic leaf hydraulic decline and recovery in Populus nigra.

Leaf hydraulic conductance (Kleaf ) is highly dynamic and typically responds to changes in water status and irradiance. However, the relative contribution of vascular (Kx ) and extra-vascular (Kox ) water pathways to Kleaf changes in response to water potential decline and recovery in function of light conditions remains poorly investigated. We investigated the dynamic responses of leaf hydraulics in Populus nigra L. by measuring Kleaf , Kx , and Kox changes under drought and upon recovery. Measurements were done at both low and high irradiance (LI and HI, respectively). Kleaf increased and became more vulnerable to dehydration under HI conditions than LI, due to marked changes of Kox . After re-watering, Kleaf recovered in parallel with Kox recovery, but Kleaf response to irradiance remained inhibited. Strong correlations between Kleaf and drought-induced membrane damage demonstrated the relevance of the cell-to-cell water pathway in driving the dynamic responses of Kleaf under drought and recovery. Our findings highlight the importance of coordination between water and light availability in modulating the overall Kleaf response to environmental conditions.

Characterizing the interplay between multiple levels of organization within bacterial sigma factor regulatory networks.

Bacteria contain multiple sigma factors, each targeting diverse, but often overlapping sets of promoters, thereby forming a complex network. The layout and deployment of such a sigma factor network directly impacts global transcriptional regulation and ultimately dictates the phenotype. Here we integrate multi-omic data sets to determine the topology, the operational, and functional states of the sigma factor network in Geobacter sulfurreducens, revealing a unique network topology of interacting sigma factors. Analysis of the operational state of the sigma factor network shows a highly modular structure with σ(N) being the major regulator of energy metabolism. Surprisingly, the functional state of the network during the two most divergent growth conditions is nearly static, with sigma factor binding profiles almost invariant to environmental stimuli. This first comprehensive elucidation of the interplay between different levels of the sigma factor network organization is fundamental to characterize transcriptional regulatory mechanisms in bacteria.

Exploiting adaptive laboratory evolution of Streptomyces clavuligerus for antibiotic discovery and overproduction.

Adaptation is normally viewed as the enemy of the antibiotic discovery and development process because adaptation among pathogens to antibiotic exposure leads to resistance. We present a method here that, in contrast, exploits the power of adaptation among antibiotic producers to accelerate the discovery of antibiotics. A competition-based adaptive laboratory evolution scheme is presented whereby an antibiotic-producing microorganism is competed against a target pathogen and serially passed over time until the producer evolves the ability to synthesize a chemical entity that inhibits growth of the pathogen. When multiple Streptomyces clavuligerus replicates were adaptively evolved against methicillin-resistant Staphylococcus aureus N315 in this manner, a strain emerged that acquired the ability to constitutively produce holomycin. In contrast, no holomycin could be detected from the unevolved wild-type strain. Moreover, genome re-sequencing revealed that the evolved strain had lost pSCL4, a large 1.8 Mbp plasmid, and acquired several single nucleotide polymorphisms in genes that have been shown to affect secondary metabolite biosynthesis. These results demonstrate that competition-based adaptive laboratory evolution can constitute a platform to create mutants that overproduce known antibiotics and possibly to discover new compounds as well.

Directed formation of lipid membrane microdomains as high affinity sites for His-tagged proteins.

Lipid membranes composed of an iminodiacetic acid functionalized lipid, DSIDA, in a POPC matrix exhibited switchable properties via Cu(2+) recognition to rapidly assemble microdomains that act as high affinity sites for His-tagged proteins. The microdomains demonstrated an order of magnitude enhanced affinity for the proteins compared to homogeneously functionalized POPC membranes with Ni(2+)-NTA DOGS or Cu(2+)-DOIDA, while a rapid release and restoration of the original membrane was accomplished with micromolar concentrations of EDTA.

Lipid nanotube formation from streptavidin-membrane binding.

A novel transformation of giant lipid vesicles to produce nanotubular structures was observed upon the binding of streptavidin to biotinylated membranes. Unlike membrane budding and tubulation processes caused by proteins involved with endocytosis and vesicle fusion, streptavidin is known to crystallize at near the isoelectric point (pI 5 to 6) into planar sheets against biotinylated films. We have found, however, that at neutral pH membranes of low bending rigidity (<10kT), such as 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), spontaneously produce tubular structures with widths ranging from micrometers to below the diffraction limit (<250 nm) and lengths spanning up to hundreds of micrometers. The nanotubes were typically held taut between surface-bound vesicles suggesting high membrane tension, yet the lipid nanotubes exhibited a fluidic nature that enabled the transport of entrained vesicles. Confocal microscopy confirmed the uniform coating of streptavidin over the vesicles and nanotubes. Giant vesicles composed of lipid membranes of higher bending energy exhibited only aggregation in the presence of streptavidin. Routes toward the development of these highly curved membrane structures are discussed in terms of general protein-membrane interactions.