Priming xylem for stress recovery depends on coordinated activity of sugar metabolic pathways and changes in xylem sap pH

A Solanum lycopersicum polyamine oxidase contributes to the control of plant growth, xylem differentiation, and drought stress tolerance

Polyamines are involved in several plant physiological processes. In Arabidopsis thaliana, five FAD-dependent polyamine oxidases (AtPAO1 to AtPAO5) contribute to polyamine homeostasis. AtPAO5 catalyzes the back-conversion of thermospermine (T-Spm) to spermidine and plays a role in plant development, xylem differentiation, and abiotic stress tolerance. In the present study, to verify whether T-Spm metabolism can be exploited as a new route to improve stress tolerance in crops and to investigate the underlying mechanisms, tomato (Solanum lycopersicum) AtPAO5 homologs were identified (SlPAO2, SlPAO3, and SlPAO4) and CRISPR/Cas9-mediated loss-of-function slpao3 mutants were obtained. Morphological, molecular, and physiological analyses showed that slpao3 mutants display increased T-Spm levels and exhibit changes in growth parameters, number and size of xylem elements, and expression levels of auxin- and gibberellin-related genes compared to wild-type plants. The slpao3 mutants are also characterized by improved tolerance to drought stress, which can be attributed to a diminished xylem hydraulic conductivity that limits water loss, as well as to a reduced vulnerability to embolism. Altogether, this study evidences conservation, though with some significant variations, of the T-Spm-mediated regulatory mechanisms controlling plant growth and differentiation across different plant species and highlights the T-Spm role in improving stress tolerance while not constraining growth.

Searching for mechanisms driving root pressure in Zea mays-a transcriptomic approach

While there are many theories and a variety of innovative datasets contributing to our understanding of the mechanism generating root pressure in vascular plants, we are still unable to produce a specific cellular mechanism for any species. To discover these mechanisms, we used RNA-Seq to explore differentially expressed genes in three different tissues between individual Zea mays plants expressing root pressure and those producing none. Working from the perspective that roots cells are utililizing a combination of osmotic exudation and hydraulic pressure mechanisms to generate positively-pressured flow of water into the xylem from the soil, we hypothesized that differential expression analysis would yield candidate genes coding for membrane transporters, ion channels, ATPases, and hormones with clear relevance to root pressure generation. In basal stem and coarse root tissue, we observed these classes of differentially expressed genes and more, including a strong cytoskeletal remodeling response. Fine roots displayed remarkably little differential expression relevant to root pressure, leading us to conclude that they either do not contribute to root pressure generation or are constitutively expressing root pressure mechanisms regardless of soil water content.

Seasonal dynamics of cell-to-cell transport in angiosperm wood

This study describes the seasonal changes in cell-to-cell transport in three selected angiosperm tree species, Acer pseudoplatanus (maple), Fraxinus excelsior (ash), and Populus tremula × tremuloides (poplar), with an emphasis on the living wood component, xylem parenchyma cells (XPCs). We performed anatomical studies, dye loading through the vascular system, measurements of non-structural carbohydrate content, immunocytochemistry, inhibitory assays and quantitative real-time PCR to analyse the transport mechanisms and seasonal variations in wood. The abundance of membrane dye in wood varied seasonally along with seasonally changing tree phenology, cambial activity, and non-structural carbohydrate content. Moreover, dyes internalized in vessel-associated cells and 'trapped' in the endomembrane system are transported farther between other XPCs via plasmodesmata. Finally, various transport mechanisms based on clathrin-mediated and clathrin-independent endocytosis, and membrane transporters, operate in wood, and their involvement is species and/or season dependent. Our study highlights the importance of XPCs in seasonally changing cell-to-cell transport in both ring-porous (ash) and diffuse-porous (maple, poplar) tree species, and demonstrates the involvement of both endocytosis and plasmodesmata in intercellular communication in angiosperm wood.

Seasonal variation in the xylem sap composition of six Australian trees and shrubs

In recent years, xylem sap composition has been shown to affect xylem hydraulics. However, information on how much xylem sap composition can vary across seasons and specifically under drought stress is still limited. We measured xylem sap chemical composition ([Ca2+], [K+], [Na+], electrical conductivity EC and pH) and surface tension (γ) of six Australian angiosperm trees and shrubs over 1 year, which comprised of exceptional dry and wet periods. Percentage losses of hydraulic conductivity and predawn leaf water potential were also monitored. In all species, measured parameters changed considerably over the annual time course. Ions and pH tended to decrease during winter months whereas γ showed a slight increase. No clear correlation was found between sap and hydraulic parameters, except for pH that was higher when plants suffered higher drought stress levels. Results indicate xylem sap composition to be complex and dynamic, where most variation in its composition seems to be dictated by season, even under severe dry conditions. However, pH might play a role as signals of drought stress.

Carbohydrate dynamics in Populus trees under drought: An expression atlas of genes related to sensing, translocation, and metabolism across organs

In trees, nonstructural carbohydrates (NSCs) serve as long-term carbon storage and long-distance carbon transport from source to sink. NSC management in response to drought stress is key to our understanding of drought acclimation. However, the molecular mechanisms underlying these processes remain unclear. By combining a transcriptomic approach with NSC quantification in the leaves, stems, and roots of Populus alba under drought stress, we analyzed genes from 29 gene families related to NSC signaling, translocation, and metabolism. We found starch depletion across organs and accumulation of soluble sugars (SS) in the leaves. Activation of the trehalose-6-phosphate/SNF1-related protein kinase (SnRK1) signaling pathway across organs via the suppression of class I TREHALOSE-PHOSPHATE SYNTHASE (TPS) and the expression of class II TPS genes suggested an active response to drought. The expression of SnRK1α and β subunits, and SUCROSE SYNTHASE6 supported SS accumulation in leaves. The upregulation of active transporters and the downregulation of most passive transporters implied a shift toward active sugar transport and enhanced regulation over partitioning. SS accumulation in vacuoles supports osmoregulation in leaves. The increased expression of sucrose synthesis genes and reduced expression of sucrose degradation genes in the roots did not coincide with sucrose levels, implying local sucrose production for energy. Moreover, the downregulation of invertases in the roots suggests limited sucrose allocation from the aboveground organs. This study provides an expression atlas of NSC-related genes that respond to drought in poplar trees, and can be tested in tree improvement programs for adaptation to drought conditions.

Contrasting Responses of Two Grapevine Cultivars to Drought: The Role of Non-structural Carbohydrates in Xylem Hydraulic Recovery

Xylem embolism is one of the possible outcomes of decreasing xylem pressure when plants face drought. Recent studies have proposed a role for non-structural carbohydrates (NSCs) in osmotic pressure generation, required for refilling embolized conduits. Potted cuttings of grapevine Grenache and Barbera, selected for their adaptation to different climatic conditions, were subjected to a drought stress followed by re-irrigation. Stem embolism rate and its recovery were monitored in vivo by X-ray micro-computed tomography (micro-CT). The same plants were further analyzed for xylem conduit dimension and NSC content. Both cultivars significantly decreased Ψpd in response to drought and recovered from xylem embolism after re-irrigation. However, although the mean vessel diameter was similar between the cultivars, Barbera was more prone to embolism. Surprisingly, vessel diameter was apparently reduced during recovery in this cultivar. Hydraulic recovery was linked to sugar content in both cultivars, showing a positive relationship between soluble NSCs and the degree of xylem embolism. However, when starch and sucrose concentrations were considered separately, the relationships showed cultivar-specific and contrasting trends. We showed that the two cultivars adopted different NSC-use strategies in response to drought, suggesting two possible scenarios driving conduit refilling. In Grenache, sucrose accumulation seems to be directly linked to embolism formation and possibly sustains refilling. In Barbera, maltose/maltodextrins could be involved in a conduit recovery strategy via the formation of cell-wall hydrogels, likely responsible for the reduction of conduit lumen detected by micro-CT.

Alkaline soil primes the recovery from drought in Populus nigra plants through physiological and chemical adjustments

Perennial plants are frequently exposed to severe and prolonged drought, and when the balance between water transport and transpirational demand is compromised trees are in danger of embolism formation. To maintain the physiological balance, plants can rely on mechanisms to quickly recover the lost xylem hydraulic capacity and reduce the prolonged impact on photosynthetic activity upon rehydration. Among factors helpful for plants to sustain acclimation and adaptation responses to drought and promote recovery, maintaining an optimal nutritional status is crucial for plant survival. This study aimed to investigate the physiological and biochemical responses under drought and recovery of Populus nigra plants grown in soil with impaired nutrient bioavailability obtained by adding calcium oxide (CaO) to the substrate. Although the CaO treatment did not affect plant growth, in well-watered conditions, treated poplars displayed an impaired inorganic ions profile in tissues. Under drought, although CaO-treated and untreated plants showed similar physiological responses, the former closed the stomata earlier. During water stress relief, the CaO-treated poplars exhibited a faster stomatal opening and a higher capacity to restore xylem hydraulic conductivity compared to not-treated plants, probably due to the higher osmolyte accumulation during drought. The content of some inorganic ions (e.g, Ca2+ and Cl-) was also higher in the xylem sap collected from stressed CaO-treated plants, thus contributing to increase the osmotic gradient necessary for the recovery. Taken together, our results suggest that CaO treatment promotes a faster and more efficient plant recovery after drought due to a modulation of ions homeostasis.

Unveiling resilience mechanisms of Quercus ilex seedlings to severe water stress: Changes in non-structural carbohydrates, xylem hydraulic functionality and wood anatomy

Over the last few decades, extensive dieback and mortality episodes of Quercus ilex L. have been documented after severe drought events in many Mediterranean forests. However, the underlying physiological, anatomical, and biochemical mechanisms remain poorly understood. We investigated the physiological and biochemical processes linked to embolism formation and non-structural carbohydrates (NSCs) dynamics in Q. ilex seedlings exposed to severe water stress and rewatering. Measurements of leaf gas exchange, water relations, non-structural carbohydrates, drought-related gene expression, and anatomical changes in wood parenchyma were assessed. Under water stress, the midday stem water potential dropped below - 4.5 MPa corresponding to a ~ 50 % loss of hydraulic conductivity. A 70 % reduction in stomatal conductance led to a strong depletion of wood NSCs. Starch consumption, resulting from the upregulation of the β-amylase gene BAM3, together with the downregulation of glucose (GPT1) and sucrose (SUC27) transport genes, suggests glucose utilization to sustain cellular metabolism in the wood parenchyma. After rewatering, the presence of residual xylem embolism led to an incomplete recovery of leaf gas exchanges. However, the partial restoration of photosynthesis allowed the accumulation of new starch reserves in the wood parenchyma and the production of new narrower vessels. In addition, changes in the cell wall composition of the wood parenchyma fibers were observed. Our findings indicate that thirty days of rewatering were sufficient to restore the NSCs reserves and growth rates of Q. ilex seedlings and that the carryover effects of water stress were primarily caused by hydraulic dysfunction.

Responses to Drought Stress in Poplar: What Do We Know and What Can We Learn?

Poplar (Populus spp.) is a high-value crop for wood and biomass production and a model organism for tree physiology and genomics. The early release, in 2006, of the complete genome sequence of P. trichocarpa was followed by a wealth of studies that significantly enriched our knowledge of complex pathways inherent to woody plants, such as lignin biosynthesis and secondary cell wall deposition. Recently, in the attempt to cope with the challenges posed by ongoing climate change, fundamental studies and breeding programs with poplar have gradually shifted their focus to address the responses to abiotic stresses, particularly drought. Taking advantage from a set of modern genomic and phenotyping tools, these studies are now shedding light on important processes, including embolism formation (the entry and expansion of air bubbles in the xylem) and repair, the impact of drought stress on biomass yield and quality, and the long-term effects of drought events. In this review, we summarize the status of the research on the molecular bases of the responses to drought in poplar. We highlight how this knowledge can be exploited to select more tolerant genotypes and how it can be translated to other tree species to improve our understanding of forest dynamics under rapidly changing environmental conditions.

The effects of previous summer drought and fertilization on winter non-structural carbon reserves and spring leaf development of downy oak saplings

It is still unknown whether the previous summer season drought and fertilization will affect the winter non-structural carbohydrate (NSC) reserves, spring leaf development, and mortality of trees in the next year. We, therefore, conducted an experiment with Quercus pubescens (downy oaks) saplings grown under four drought levels from field capacity (well-watered; ~25% volumetric water content) to wilting point (extreme drought; ~6%), in combination with two fertilizer treatments (0 vs. 50 kg/ha/year blended) for one growing season to answer this question. We measured the pre- and post-winter NSC, and calculated the over-winter NSC consumption in storage tissues (i.e. shoots and roots) following drought and fertilization treatment, and recorded the spring leaf phenology, leaf biomass, and mortality next year. The results showed that, irrespective of drought intensity, carbon reserves were abundant in storage tissues, especially in roots. Extreme drought did not significantly alter NSC levels in tissues, but delayed the spring leaf expansion and reduced the leaf biomass. Previous season fertilization promoted shoot NSC use in extreme drought-stressed saplings over winter (showing reduced carbon reserves in shoots after winter), but it also showed positive effects on survival next year. We conclude that: (1) drought-stressed downy oak saplings seem to be able to maintain sufficient mobile carbohydrates for survival, (2) fertilization can alleviate the negative effects of extreme drought on survival and recovery growth of tree saplings.

Tonoplast Sucrose Trafficking Modulates Starch Utilization and Water Deficit Behavior in Poplar Leaves

Leaf osmotic adjustment by the active accrual of compatible organic solutes (e.g. sucrose) contributes to drought tolerance throughout the plant kingdom. In Populus tremula x alba, PtaSUT4 encodes a tonoplast sucrose-proton symporter, whose downregulation by chronic mild drought or transgenic manipulation is known to increase leaf sucrose and turgor. While this may constitute a single drought tolerance mechanism, we now report that other adjustments which can occur during a worsening water deficit are damped when PtaSUT4 is constitutively downregulated. Specifically, we report that starch use and leaf relative water content (RWC) dynamics were compromised when plants with constitutively downregulated PtaSUT4 were subjected to a water deficit. Leaf RWC decreased more in wild-type and vector control lines than in transgenic PtaSUT4-RNAi (RNA-interference) or CRISPR (clustered regularly interspersed short palindromic repeats) knockout (KO) lines. The control line RWC decrease was accompanied by increased PtaSUT4 transcript levels and a mobilization of sucrose from the mesophyll-enriched leaf lamina into the midvein. The findings suggest that changes in SUT4 expression can increase turgor or decrease RWC as different tolerance mechanisms to reduced water availability. Evidence is presented that PtaSUT4-mediated sucrose partitioning between the vacuole and the cytosol is important not only for overall sucrose abundance and turgor, but also for reactive oxygen species (ROS) and antioxidant dynamics. Interestingly, the reduced capacity for accelerated starch breakdown under worsening water-deficit conditions was correlated with reduced ROS in the RNAi and KO lines. A role for PtaSUT4 in the orchestration of ROS, antioxidant, starch utilization and RWC dynamics during water stress and its importance in trees especially, with their high hydraulic resistances, is considered.

Stem xylem traits and wood formation affect sex-specific responses to drought and rewatering in Populus cathayana

The increased frequency and intensity of drought pose great threats to the survival of trees, especially in dioecious tree species with sexual differences in mortality and biased sex ratios. The sex-specific mechanisms underlying stem xylem anatomy and function and carbon metabolism in drought resistance and recovery were investigated in dioecious Populus cathayana Rehder. The sex-specific drought resistance and subsequent recovery were linked to the xylem anatomy and carbon metabolism. Females had a greater xylem vessel area per vessel, biomass and theoretically hydraulic efficiency under well-watered conditions. Conversely, males had a lower xylem lumen area, but greater vessel numbers, and a higher cell wall thickness, suggesting a theoretically conservative water-use strategy and drought resistance. The recovery of photosynthetic ability after drought in males was largely dependent on the recovery of xylem function and the regulation of the xylem carbohydrate metabolism. Additionally, the number of upregulated genes related to xylem cell wall biogenesis was greater in males relative to females under drought stress and subsequent rewatering, which facilitated drought resistance and xylem function restoration in males. These results suggested that sex-specific drought resistance and restoration were related to xylem anatomy and function, carbohydrate metabolism and cell turgor maintenance.

Testing the limits of plant drought stress and subsequent recovery in four provenances of a widely distributed subtropical tree species

Drought-induced tree mortality may increase with ongoing climate change. Unraveling the links between stem hydraulics and mortality thresholds, and the effects of intraspecific variation, remain important unresolved issues. We conducted a water manipulation experiment in a rain-out shelter, using four provenances of Schima superba originating from a gradient of annual precipitation (1124-1796 mm) and temperature (16.4-22.4°C). Seedlings were droughted to three levels of percentage loss of hydraulic conductivity (i.e., P₅₀ , P₈₈  and P₉₉₎ and subsequently rewatered to field capacity for 30 days; traits related to water and carbon relations were measured. The lethal water potential associated with incipient mortality was between P₅₀ and P₈₈ . Seedlings exhibited similar drought responses in xylem water potential, hydraulic conductivity and gas exchange. Upon rehydration, patterns of gas exchange differed among provenances but were not related to the climate at the origin. The four provenances exhibited a similar degree of stem hydraulic recovery, which was correlated with the magnitude of antecedent drought and stem soluble sugar at the end of the drought. Results suggest that there were intraspecific differences in the capacity of S. superba seedlings for carbon assimilation during recovery, indicating a decoupling between gas exchange recovery and stem hydraulics across provenances.

Do the ends justify the means? Impact of drought progression rate on stress response and recovery in Vitis vinifera

Plants are frequently exposed to prolonged and intense drought events. To survive, species must implement strategies to overcome progressive drought while maintaining sufficient resources to sustain the recovery of functions. Our objective was to understand how stress rate development modulates energy reserves and affects the recovery process. Grenache Vitis vinifera cultivar was exposed to either fast-developing drought (within few days; FDD), typical of pot experiments, or slow-developing drought (few weeks, SDD), more typical for natural conditions. FDD was characterized by fast (2-3 days) stomatal closure in response to increased stress level, high abscisic acid (ABA) accumulation in xylem sap (>400 μg L^-1 ) without the substantial changes associated with stem priming for recovery (no accumulation of sugar or drop in xylem sap pH). In contrast, SDD was characterized by gradual stomatal closure, low ABA accumulation (<100 μg L^-1 ) and changes that primed the stem for recovery (xylem sap acidification from 6 to 5.5 pH and sugar accumulation from 1 to 3 g L^-1 ). Despite FDD and SDD demonstrating similar trends over time in the recovery of stomatal conductance, they differed in their sensitivity to xylem ABA. Grenache showed near-isohydric and near-anisohydric behavior depending on the rate of drought progression, gauging the risk between hydraulic integrity and photosynthetic gain. The isohydry observed during FDD could potentially provide protection from large sudden swings in tension, while transitioning to anisohydry during SDD could prioritize the maintenance of photosynthetic activity over hydraulic security.

Phenoloxidases in Plants-How Structural Diversity Enables Functional Specificity

The metabolism of polyphenolic polymers is essential to the development and response to environmental changes of organisms from all kingdoms of life, but shows particular diversity in plants. In contrast to other biopolymers, whose polymerisation is catalysed by homologous gene families, polyphenolic metabolism depends on phenoloxidases, a group of heterogeneous oxidases that share little beyond the eponymous common substrate. In this review, we provide an overview of the differences and similarities between phenoloxidases in their protein structure, reaction mechanism, substrate specificity, and functional roles. Using the example of laccases (LACs), we also performed a meta-analysis of enzyme kinetics, a comprehensive phylogenetic analysis and machine-learning based protein structure modelling to link functions, evolution, and structures in this group of phenoloxidases. With these approaches, we generated a framework to explain the reported functional differences between paralogs, while also hinting at the likely diversity of yet undescribed LAC functions. Altogether, this review provides a basis to better understand the functional overlaps and specificities between and within the three major families of phenoloxidases, their evolutionary trajectories, and their importance for plant primary and secondary metabolism.

A new protein hydrolysate-based biostimulant applied by fertigation promotes relief from drought stress in Capsicum annuum L

Recently, biostimulants have been used in sustainable agriculture as priming agents able to increase crop tolerance to abiotic stressors. Here, a soil application of GHI_16_VHL, a plant protein hydrolysate-based biostimulant, was tested for its capability to mitigate severe water stress effects on Capsicum annuum at flowering time. The biostimulant influence on plant physiological status was monitored upon stress and its relief, by measuring chlorophyll levels, stomatal density, stem water potential, leaf gas exchanges and plant growth. Moreover, leaf osmoregulation and oxidative stress levels were also evaluated by quantifying free proline, total non-structural carbohydrates (NSC), ROS-scavenging activity and H₂O₂ level. Although biostimulant-primed plants showed a quicker decrease of stem water potential with respect to untreated plants upon drought imposition, they recovered faster probably due to the higher leaf osmolyte accumulation, namely NSC during drought. Moreover, leaf gas exchange recovery was prompted in biostimulant-treated plants, which showed an incremented stomatal density and the same chlorophyll level of well-watered plants at the end of the recovery phase. Hydrogen peroxide level was significantly lower during stress and early recovery in biostimulant primed plants, probably due to the higher catalase activity in treated plants before drought or to the higher level of non-enzymatic antioxidant scavengers in primed stressed plants. Finally, the biostimulant priming increased aboveground relative growth rate and final fruit yield of stressed plants. Taken together, our data suggest that the biostimulant priming treatment promotes a faster and more efficient plant recovery after drought.

Sucrose Metabolism and Transport in Grapevines, with Emphasis on Berries and Leaves, and Insights Gained from a Cross-Species Comparison

In grapevines, as in other plants, sucrose and its constituents glucose and fructose are fundamentally important and carry out a multitude of roles. The aims of this review are three-fold. First, to provide a summary of the metabolism and transport of sucrose in grapevines, together with new insights and interpretations. Second, to stress the importance of considering the compartmentation of metabolism. Third, to outline the key role of acid invertase in osmoregulation associated with sucrose metabolism and transport in plants.

Shade-induced reduction of stem nonstructural carbohydrates increases xylem vulnerability to embolism and impedes hydraulic recovery in Populus nigra

Nonstructural carbohydrates (NSCs) have been suggested to affect xylem transport under fluctuating water availability, but conclusive evidence is still lacking. We tested the effect of shade-induced NSC depletion on xylem vulnerability to embolism and hydraulic recovery on Populus nigra saplings. Vulnerability was assessed in light-exposed (L) and shaded (S) plants with the hydraulic method, and in vivo with the optical method and X-ray micro-computed tomography. Plants were stressed to 80% loss of hydraulic conductance (PLC) and re-irrigated to check for possible recovery. We measured PLC, bark and wood NSC content, as well as xylem sap pH, surface tension (γ_sap ) and sugar concentration, before, during and after drought. Shading induced depletion of stem NSC (mainly starch) reserves. All methods converged in indicating higher xylem vulnerability in S than in L plants. This difference was not explained by xylem vessel and pit anatomy or by γ_sap . Shading impeded sap acidification and sugar accumulation during drought in S plants and prevented hydraulic recovery, which was observed in L plants. Our results highlight the importance of stem NSCs to sustain xylem hydraulic functioning during drought and suggest that light and/or adequate stem NSC thresholds are required to trigger xylem sap chemical changes involved in embolism recovery.

Stem metabolism under drought stress - a paradox of increasing respiratory substrates and decreasing respiratory rates

Metabolic changes underpinning drought-induced variations in stem respiration (R_s ) are unknown. We measured R_s rates and metabolite and gene expression profiles in Ulmus minor Mill. and Quercus ilex L. seedlings subjected to increasing levels of drought stress to better understand how carbon, nitrogen and energy metabolism interact during drought. In both species, only plants showing extreme stress symptoms - i.e. negligible rates of leaf stomatal conductance and photosynthesis, and high stem dehydration (30-50% of maximum water storage) and contraction (50-150 μm week^-1 ) - exhibited lower R_s rates than well-watered plants. Abundance of low-molecular weight sugars (e.g. glucose and fructose) and sugar alcohols (e.g. mannitol) increased with drought, at more moderate stress and to a higher extent in Q. ilex than U. minor. Abundance of amino acids increased at more severe stress, more abruptly, and to a higher extent in U. minor, coinciding with leaf senescence, which did not occur in Q. ilex. Organic acids changed less in response to drought: threonate and glycerate increased, and citrate decreased although slightly in both species. Transcripts of genes coding for enzymes of the Krebs cycle decreased in Q. ilex and increased in U. minor in conditions of extreme drought stress. The maintenance of R_s under severe growth and photosynthetic restrictions reveals the importance of stem mitochondrial activity in drought acclimation. The eventual decline in R_s diverts carbon substrates from entering the Krebs cycle that may help to cope with osmotic and oxidative stress during severe drought and to recover hydraulic functionality afterwards.

Unique Attributes of the Laurel Wilt Fungal Pathogen, Raffaelea lauricola, as Revealed by Metabolic Profiling

Raffaelea lauricola is the causative agent of laurel wilt, a devastating disease of lauraceous trees. R. lauricola is also an obligate nutritional symbiont of several ambrosia beetle species who act as vectors for the pathogen. Here, we sought to establish the baseline “phenome” of R. lauricola with knowledge concerning its metabolic capability, expanding our understanding of how these processes are impacted by environmental and host nutrients. Phenotypic screening using a microarray of over one thousand compounds was used to generate a detailed profile of R. lauricola substrate utilization and chemical sensitivity. These data revealed (i) relatively restricted carbon utilization, (ii) broad sulfur and phosphate utilization, and (iii) pH and osmotic sensitivities that could be rescued by specific compounds. Additional growth profiling on fatty acids revealed toxicity on C10 substrates and lower, with robust growth on C12–C18 fatty acids. Conditions for lipid droplet (LD) visualization and LD dynamics were examined using a series of lipid dyes. These data provide unique insights regarding R. lauricola metabolism and physiology, and identify distinct patterns of substrate usage and sensitivity which likely reflect important aspects of the host-microbe interface and can be exploited for the development of strategies for mitigating the spread of laurel wilt.

Trade-offs between xylem water and carbohydrate storage among 24 coexisting subtropical understory shrub species spanning a spectrum of isohydry

Hydraulic capacitance and carbohydrate storage are two drought adaptation strategies of woody angiosperms. However, we currently lack information on their associations and how they are associated with species' degree of isohydry. We measured total stem xylem nonstructural carbohydrate (NSC) concentration in the dry and wet seasons, xylem hydraulic capacitance, native leaf water potentials, pressure-volume curve parameters and photosynthetic performance in 24 woody understory species differing in their degree of isohydry. We found a trade-off between xylem water and carbohydrate storage both in storage capacitance and along a spectrum of isohydry. Species with higher hydraulic capacitance had lower native NSC storage. The less isohydric species tended to show greater NSC depletion in the dry season and have more drought-tolerant leaves. In contrast, the more isohydric species had higher hydraulic capacitance, which may enhance their drought avoidance capacity. In these species, leaf flushing in the wet season and higher photosynthetic rates in the dry season resulted in accumulation rather than depletion of NSC in the dry season. Our results provide new insights into the mechanisms through which xylem storage functions determine co-occurring species' drought adaptation strategies and improve our capacity to predict community assembly processes under drought.

Modelling the physiological relevance of sucrose export repression by an Flowering Time homolog in the long-distance phloem of potato

Yield of harvestable plant organs depends on photosynthetic assimilate production in source leaves, long-distance sucrose transport and sink-strength. While photosynthesis optimization has received considerable interest for optimizing plant yield, the potential for improving long-distance sucrose transport has received far less attention. Interestingly, a recent potato study demonstrates that the tuberigen StSP6A binds to and reduces activity of the StSWEET11 sucrose exporter. While the study suggested that reducing phloem sucrose efflux may enhance tuber yield, the precise mechanism and physiological relevance of this effect remained an open question. Here, we develop the first mechanistic model for sucrose transport, parameterized for potato plants. The model incorporates SWEET-mediated sucrose export, SUT-mediated sucrose retrieval from the apoplast and StSP6A-StSWEET11 interactions. Using this model, we were able to substantiate the physiological relevance of the StSP6A-StSWEET11 interaction in the long-distance phloem for potato tuber yield, as well as to show the non-linear nature of this effect.

Xylem and Phloem Formation Dynamics in Quercus ilex L. at a Dry Site in Southern Italy

Quercus ilex L. dieback has been recently reported at numerous Mediterranean sites. Wood and phloem formation dynamics and tree-ring series of anatomical traits can be used to evaluate growth conditions of trees. We monitored cambial activity in Q. ilex trees growing at a site in southern Italy in order to assess how xylem and phloem production are affected by harsh seasonal climatic variation during a dry year. We followed xylogenesis by counting the number of cambial cells and detecting the occurrence of post-cambial cells throughout the year. As phloem did not show clear growth rings and boundaries between them, we followed the development of phloem fibres—their morphological traits during development and the distance from the cambium served as a reference point to evaluate the phloem production during the year. We detected a multimodal pattern in cambial activity, with wood production in three periods of the year and consequent formation of intra-annual density fluctuations (IADFs). The lowest production of xylem cells was observed in the dry late spring and summer period (likely due to the low water availability), while the highest occurred in autumn (the wettest period). Although we could not differentiate between early and late phloem, the analysis of phloem traits was useful to follow the dynamics of phloem production, which is generally difficult in Mediterranean tree species. We found cambial production of phloem throughout the year, even in the periods without xylem production. The results showed that if tree growth was constrained by environmental limitations, the ratio between xylem to phloem cells decreased and, in the most severely affected trees, more cells were formed preferentially in the phloem compared to xylem. We also briefly report the way in which to solve technical problems with tissue preparation due to extreme hardness and to the peculiar structure of Q. ilex wood and outer bark.

Rapid starch degradation in the wood of olive trees under heat and drought is permitted by three stress-specific beta amylases

Carbon reserve use is a major drought response in trees, enabling tree survival in conditions prohibiting photosynthesis. However, regulation of starch metabolism under drought at the whole-tree scale is still poorly understood. To this end, we combined measurements of nonstructural carbohydrates (NSCs), tree physiology and gene expression. The experiment was conducted outside on olive trees in pots under 90 d of seasonal spring to summer warming. Half of the trees were also subjected to limited water conditions for 28 d. Photosynthesis decreased in dehydrating trees from 19 to 0.5 µmol m^-2  s^-1 during the drought period. Starch degradation and mannitol production were a major drought response, with mannitol increasing to 71% and 41% out of total NSCs in shoots and roots, respectively. We identified the gene family members potentially relevant either to long-term or stress-induced carbon storage. Partitioning of expression patterns among β amylase and starch synthase family members was observed, with three β amylases possibly facilitating the rapid starch degradation under heat and drought. Our results suggest a group of stress-related, starch metabolism genes, correlated with NSC fluctuations during drought and recovery. The daily starch metabolism gene expression was different from the stress-mode starch metabolism pattern, where some genes are uniquely expressed during the stress-mode response.

Water 'on the rocks': a summer drink for thirsty trees?

Drought-induced tree mortality frequently occurs in patches with different spatial and temporal distributions, which is only partly explained by inter- and intraspecific variation in drought tolerance. We investigated whether bedrock properties, with special reference to rock water storage capacity, affects tree water status and drought response in a rock-dominated landscape. We measured primary porosity and available water content of breccia (B) and dolostone (D) rocks. Saplings of Fraxinus ornus were grown in pots filled with soil or soil mixed with B and D rocks, and subjected to an experimental drought. Finally, we measured seasonal changes in water status of trees in field sites overlying B or D bedrock. B rocks were more porous and stored more available water than D rocks. Potted saplings grown with D rocks had less biomass and suffered more severe water stress than those with B rocks. Trees in sites with B bedrock had more favourable water status than those on D bedrock which also suffered drought-induced canopy dieback. Bedrock represents an important water source for plants under drought. Different bedrock features translate into contrasting below-ground water availability, leading to landscape-level heterogeneity of the impact of drought on tree water status and dieback.

Chemical inhibition of xylem cellular activity impedes the removal of drought-induced embolisms in poplar stems - new insights from micro-CT analysis

In drought-stressed plants a coordinated cascade of chemical and transcriptional adjustments occurs at the same time as embolism formation. While these processes do not affect embolism formation during stress, they may prime stems for recovery during rehydration by modifying apoplast pH and increasing sugar concentration in the xylem sap. Here we show that in vivo treatments modifying apoplastic pH (stem infiltration with a pH buffer) or reducing stem metabolic activity (infiltration with sodium vanadate and sodium cyanide; plant exposure to carbon monoxide) can reduce sugar accumulation, thus disrupting or delaying the recovery process. Application of the vanadate treatment (NaVO_3, an inhibitor of many ATPases) completely halted recovery from drought-induced embolism for up to 24 h after re-irrigation, while partial recovery was observed in vivo in control plants using X-ray microcomputed tomography. Our results suggest that stem hydraulic recovery in poplar is a biological, energy-dependent process that coincides with accumulation of sugars in the apoplast during stress. Recovery and damage are spatially coordinated, with embolism formation occurring from the inside out and refilling from the outside in. The outside-in pattern highlights the importance of xylem proximity to the sugars within the phloem to the embolism recovery process.

Wood day capacitance is related to water content, wood density, and anatomy across 30 temperate tree species

Water released from wood during transpiration (capacitance) can meaningfully affect daily water use and drought response. To provide context for better understanding of capacitance mechanisms, we investigated links between capacitance and wood anatomy. On twigs of 30 temperate angiosperm tree species, we measured day capacitance (between predawn and midday), water content, wood density, and anatomical traits, that is, vessel dimensions, tissue fractions, and vessel-tissue contact fractions (fraction of vessel circumference in contact with other tissues). Across all species, wood density (WD) and predawn lumen volumetric water content (VWC_L-pd ) together were the strongest predictors of day capacitance (r²_adj = .44). Vessel-tissue contact fractions explained an additional ~10% of the variation in day capacitance. Regression models were not improved by including tissue lumen fractions. Among diffuse-porous species, VWC_L-pd and vessel-ray contact fraction together were the best predictors of day capacitance, whereas among semi/ring-porous species, VWC_L-pd , WD and vessel-fibre contact fraction were the best predictors. At predawn, wood was less than fully saturated for all species (lumen relative water content = 0.52 ± 0.17). Our findings imply that day capacitance depends on the amount of stored water, tissue connectivity and the bulk wood properties arising from WD (e.g., elasticity), rather than the fraction of any particular tissue.

Defoliation-induced compensatory transpiration is compromised in SUT4-RNAi Populus

The tonoplast sucrose transporter PtaSUT4 is well expressed in leaves of Populus tremula × Populus alba (INRA 717-IB4), and its inhibition by RNA-interference (RNAi) alters leaf sucrose homeostasis. Whether sucrose partitioning between the vacuole and the cytosol is modulated by PtaSUT4 for specific physiological outcomes in Populus remains unexplored. In this study, partial defoliation was used to elicit compensatory increases in photosynthesis and transpiration by the remaining leaves in greenhouse-grown poplar. Water uptake, leaf gas exchange properties, growth and nonstructural carbohydrate abundance in source and sink organs were then compared between wild-type and SUT4-RNAi lines. Partial defoliation increased maximum photosynthesis rates similarly in all lines. There was no indication that source leaf sugar levels changed differently between wild-type and RNAi plants following partial defoliation. Sink levels of hexose (glucose and fructose) and starch decreased similarly in all lines. Interestingly, plant water uptake after partial defoliation was not as well sustained in RNAi as in wild-type plants. While the compensatory increase in photosynthesis was similar between genotypes, leaf transpiration increased less robustly in RNAi than wild-type plants. SUT4-RNAi and wild-type source leaves differed constitutively in their bulk modulus of elasticity, a measure of leaf turgor, and storage water capacitance. The data demonstrate that reduced sucrose partitioning due to PtaSUT4-RNAi altered turgor control and compensatory transpiration capacity more strikingly than photosynthesis and sugar export. The results are consistent with the interpretation that SUT4 may control vacuolar turgor independently of sink carbon provisioning.

Changes in Non-Structural Carbohydrates, Wood Properties and Essential Oil During Chemically-Induced Heartwood Formation in Dalbergia odorifera

The highly valuable heartwood of Dalbergia odorifera T. Chen, known as Jiang Xiang in traditional Chinese medicine, is formed very slowly, and there is a need to better understand the process and promote heartwood formation. Chemical induction is considered to be one of the promising methods to induce heartwood formation. However, to date no method has been proved effective for D. odorifera as little is known about biochemical and physiological changes during heartwood development. Three potential heartwood induction substances viz. acetic acid, sodium chloride, and hydrogen peroxide solutions were injected into the trunk of D. odorifera to determine the effect on heartwood formation and physiological activity. Non-structural carbohydrates, lipids, wood properties, and essential oil were assessed in the post-treatment period. As also observed in the formation of natural heartwood, chemical-induced Jiang Xiang production was accompanied by sapwood dehydration, non-structural carbohydrates consumption, and synthesis of heartwood substances. As the heartwood substances accumulated, basic density and essential oil content increased gradually, thereby Jiang Xiang was finally produced. In this process, physiological parameters of discolored sapwood gradually evolved to resemble those of natural heartwood. Hydrogen peroxide-induced Jiang Xiang was closest to natural heartwood, and the essential oil components met the standards for high-quality Jiang Xiang, while the induction effects of acetic acid and sodium chloride were unsatisfactory. Thus, this study indicates that hydrogen peroxide has the potential to induce Jiang Xiang production in Dalbergia odorifera.

Analysis of Non-Structural Carbohydrates and Xylem Anatomy of Leaf Petioles Offers New Insights in the Drought Response of Two Grapevine Cultivars

In grapevine, the anatomy of xylem conduits and the non-structural carbohydrates (NSCs) content of the associated living parenchyma are expected to influence water transport under water limitation. In fact, both NSC and xylem features play a role in plant recovery from drought stress. We evaluated these traits in petioles of Cabernet Sauvignon (CS) and Syrah (SY) cultivars during water stress (WS) and recovery. In CS, the stress response was associated to NSC consumption, supporting the hypothesis that starch mobilization is related to an increased supply of maltose and sucrose, putatively involved in drought stress responses at the xylem level. In contrast, in SY, the WS-induced increase in the latter soluble NSCs was maintained even 2 days after re-watering, suggesting a different pattern of utilization of NSC resources. Interestingly, the anatomical analysis revealed that conduits are constitutively wider in SY in well-watered (WW) plants, and that water stress led to the production of narrower conduits only in this cultivar.

The Possible Role of Non-Structural Carbohydrates in the Regulation of Tree Hydraulics

The xylem is a complex system that includes a network of dead conduits ensuring long-distance water transport in plants. Under ongoing climate changes, xylem embolism is a major and recurrent cause of drought-induced tree mortality. Non-structural carbohydrates (NSC) play key roles in plant responses to drought and frost stress, and several studies putatively suggest their involvement in the regulation of xylem water transport. However, a clear picture on the roles of NSCs in plant hydraulics has not been drawn to date. We summarize the current knowledge on the involvement of NSCs during embolism formation and subsequent hydraulic recovery. Under drought, sugars are generally accumulated in xylem parenchyma and in xylem sap. At drought-relief, xylem functionality is putatively restored in an osmotically driven process involving wood parenchyma, xylem sap and phloem compartments. By analyzing the published data on stem hydraulics and NSC contents under drought/frost stress and subsequent stress relief, we found that embolism build-up positively correlated to stem NSC depletion, and that the magnitude of post-stress hydraulic recovery positively correlated to consumption of soluble sugars. These findings suggest a close relationship between hydraulics and carbohydrate dynamics. We call for more experiments on hydraulic and NSC dynamics in controlled and field conditions.