Plant, Cell & Environment

Thermal adaptation is critical for organismal viability, with acquired thermotolerance (AT) in Arabidopsis thaliana typically conferred by acclimation temperatures between 34{degrees}C and 37{degrees}C, while acclimation at 40{degrees}C fails to protect against lethal heat stress. To elucidate the transcriptional mechanisms underlying the loss of AT at higher temperatures we performed RNA-seq profiling of Arabidopsis seedlings across a single-degree thermal gradient from 37{degrees}C to 40{degrees}C. Our analysis reveals that all of these temperatures result in a robust heat shock response, characterized by the upregulation of genes involved in protein folding and stress responses. However, each temperature elicits a distinct transcriptional signature. These findings demonstrate that temperature-specific fine-tuning of the heat shock response occurs within this narrow range and that these differences may dictate the successful acquisition of thermotolerance. This dataset provides a valuable resource for understanding the molecular architecture of heat-stress adaptation and the distinct transcriptional states associated with different thermal regimes.

The xerophyte argan (Sideroxylon spinosum) has great ecological and socioeconomic importance for Morocco. However, it is endangered due to climate change and human overexploitation. We assess drought resistance traits of argan and test the potential of arbuscular mycorrhizal (AM) symbiosis to promote its growth and mitigate the consequences of drought. We compare ten endemic Moroccan mycorrhizal inocula with the model AM fungus Rhizophagus irregularis and with the drought-adapted isolate Diversispora omaniana. We integrated physiological phenotyping and RNA sequencing to investigate the stress resistance mechanisms of argan against drought. We show that AM symbiosis significantly mitigates drought effects on plant growth, mainly by improving water relations and photosynthetic efficiency, resulting in increased growth rates. Taken together, physiological and transcriptomic analyses show that stress markers were moderatly induced during severe drought stress irrespective of mycorrhizal status, suggesting that argan adopts a drought-coping strategy that involves both, stress avoidance and stress tolerance. Argan is highly AM-responsive, both at the phenotypic and transcriptomic level, suggesting that AM has great potential to promote argan growth under drought stress.

Transgenerational plasticity can shape plant phenotype and influence plant response to environmental changes in interaction with the current conditions. While how past stress interact with either current optimal or stress conditions is increasingly documented within a single plant, transgenerational plasticity remains particularly poorly understood especially at the metabolome level. In our study, we investigated whether heat stress induces transgenerational metabolic and phenotypic modifications along two successive clonal ramet generations of the sub-Antarctic aquatic plant Limosella australis. We performed untargeted metabolomic approaches and measured morphologic and performance traits, to assess both transgenerational plasticity of the metabolome and the phenotype. We found that heat stress remodelled the metabolic profile and influenced the foraging strategy of our clonal plant, and that some of these metabolic changes persisted into the first clonal generation. This one therefore adopted an intermediate growth strategy, even though culture conditions were optimal. By comparing differentially accumulated features between daughter ramets from heat stressed mother ramets and from unstressed mother ramets, we identified common and specific metabolites accumulation to heat stress response, belonging to diverse compound families. However, we did not observe any adaptative advantage and any metabolic imprint during another heat stress applied on the second clonal generation. This work provides especially new clues into how plant metabolome integrates and transfers previous stressed clonal generation's information.

Several tomato (Solanum lycopersicum) varieties are sensitive to heat stress, which compromises plant growth, development, and ultimately yield. Biostimulants represent a promising approach to improve crop performance, yet their widespread adoption is hindered by the incomplete understanding of their mechanisms of action. This study aimed to elucidate the physiological and molecular effects of a protein hydrolysate-based biostimulant (Leafamine(R)), in tomato, under both optimal and heat stress conditions. Leafamine(R) increased primary root length by 15 to 20% compared to the controls, independent of the tested conditions, through promotion of cell division and potentially expansion. Transcriptomic analyses revealed the upregulation of genes involved in cell division and expansion under both optimal and heat stress conditions and the downregulation of heat stress markers under heat stress conditions. Hormone and metabolite profiling showed elevated levels of jasmonic and salicylic acid, putrescine, citrulline, and GABA, after Leafamine(R) treatment, consistent with the activation of stress tolerance pathways. Leafamine(R) pre-treated seedlings exhibited reduced growth inhibition during heat exposure, suggesting a priming effect. These findings highlight Leafamine(R) as a promising biostimulant for enhancing tomato growth in the context of climate change and the potential of biostimulants generically.

Root hairs (RH) are excellent model systems for studying cell size and polarity since they elongate several hundred-fold their original size. Their tip growth is regulated by both intrinsic and environmental signals and is associated with the existence of a highly controlled cytoplasmic tip Ca{superscript 2} gradient, whose disruption impairs RH development. The molecular mechanisms underlying the Ca2+ homeostasis fine tuning and the Ca2+ organellar contributions to the cytoplasmic pool remain unclear. In the model plant Arabidopsis thaliana, many efflux routes are present, including those that employ Ca2+-pumps from the Autoinhibited Ca2+-ATPase (ACA) family. Here, we identified that the ER localized ACA2, and to a lower extent ACA7, are crucial ACAs required to control RH growth. By using genetically encoded Ca2+ biosensors we showed that Ca2+-dynamics are compromised in the aca2-2 mutant, having lower cytosolic Ca2+ concentration [Ca2+]cyt and growth rate, showing an altered homeostatic calcium setpoint compared to Col-0. Accordingly, the ACA2 mutation changed the dynamics of [Ca2+]cyt oscillations coupled to growth rate, inducing longer periods and more regular oscillations in the dominant high-frequency range (around 22 s), and slower oscillations (around 1 min) in the low-frequency range. Finally, expression of ACA2 with changes in four putative Ca2+ binding residues (ACA2{Delta}Ca2+) failed to rescue the RH growth phenotype in the aca2-2 mutant. Collectively, our findings indicate that ER-localized ACA2 and possibly ACA7 are crucial for modulating cytoplasmic Ca2+ signals, possibly composing a critical part of a negative feedback loop, and their absence leads to impairments in RH cell elongation.

The article introduces a new Forest Stress Index (ISF) based on a plant hydraulic modelling approach rather than classical climatic drought indices. Unlike other index like scPDSI or SPEI, ISF is grounded in xylem embolism dynamics simulated with the mechanistic SurEau model. The goal is to better link climatic anomalies to tree physiological functioning and mortality risk. ISF is defined using a locally adapted ideotype characterized by an optimal P50 value under a reference hydraulic functioning threshold. Simulations are performed across Europe and France using multiple climate datasets. The index is robust to model parameterization choices and assumptions about plant functional traits. Results show strong spatial and temporal consistency and significant correlations with SPEI and scPDSI. However, ISF more strongly highlights extreme drought years and exhibits a more skewed distribution. Future projections under SSP5-8.5 indicate a widespread increase in hydraulic stress with strong regional contrasts. Overall, ISF provides a mechanistic and complementary drought indicator more directly linked to forest mortality processes.

O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=120 SRC="FIGDIR/small/741208v1_ufig1.gif" ALT="Figure 1"> View larger version (51K): org.highwire.dtl.DTLVardef@1238a49org.highwire.dtl.DTLVardef@4bc55corg.highwire.dtl.DTLVardef@8a277aorg.highwire.dtl.DTLVardef@25c625_HPS_FORMAT_FIGEXP M_FIG O_FLOATNOGraphical AbstractC_FLOATNO C_FIG Legume-rhizobium symbiosis is constrained by carbon allocation and nutrient exchanges between the host plant and symbiotic bacteroids, emphasizing the necessity of biostimulant-based strategies to improve symbiotic efficiency. Studies suggest that plants provide carbon substrates (mainly TCA cycle intermediates) and nitrogen assimilation precursors to support bacteroid metabolism and nitrogen fixation. The GABA shunt represents a conserved bypass, linking the GS/GOGAT cycle, TCA cycle, and broader nitrogen metabolism. In the present study, the metabolic phenotypes upon rhizospheric application of exogenous {gamma}-aminobutyric acid (GABA) and succinate were investigated in pea. Application of exogenous GABA and succinate primarily altered the morpho-biochemical properties and root nodule establishment of the plant. While GABA supplementation reduced the root length and increased stomatal density, succinate treatment alone inhibited nodule organogenesis with disrupted symbiosome zoning. Metabolic rewirings associated with these phenotypic changes begin at the root nodule compartment, where exogenous GABA altered endogenous tricarboxylic acid (TCA) cycle pools, leading to higher turnover of TCA cycle intermediates, including succinate, fumarate, and malate, as well as elevated levels of sugars (sucrose, glucose, and fructose) and polyols (pinitol, mannitol, and myo-inositol). Concurrently, the plants accumulated substantially higher levels of asparagine (+3.9 log10-fold higher), accompanied by altered shoot protein content and enhanced elemental nitrogen content (+0.8-1%). Collectively, these findings demonstrate the potential of GABA as a biostimulant capable of altering the carbon-nitrogen dynamics and enhancing symbiotic functioning in pea root nodules, whereas succinate alters the process of nodule formation.

Tropospheric ozone (O) is a major atmospheric pollutant that affects plant carbon metabolism, redox homeostasis, and secondary metabolism, including the biosynthesis and emission of biogenic volatile organic compounds (BVOCs). However, the contribution of BVOCs to O3 tolerance, particularly in tropical woody species, remains poorly understood. Here, we investigated whether acute O exposure (cumulative AOT40 of 3497.82 ppb h) induces alterations in photosynthetic performance, redox homeostasis, and BVOC partitioning in Eugenia uniflora. We evaluated gas exchange, photosynthetic pigments, ascorbate and glutathione pools, emitted BVOCs, modeled intercellular BVOC concentrations, and the relative carbon cost associated with BVOC emissions. O exposure significantly increased net CO2 assimilation without affecting stomatal conductance, transpiration, leaf water status, or chlorophyll concentrations, indicating maintenance of photosynthetic performance. Carotenoid concentrations and total glutathione decreased, whereas glutathione redox status was maintained. O induced marked compound-specific changes in BVOC composition and partitioning. Several monoterpenes appeared exclusively under O exposure, {gamma}-elemene emission increased significantly, and the relative distribution of individual BVOCs between the modeled intercellular and emitted pools was altered. These findings show that the response of E. uniflora to acute O exposure was characterized by interplay among carbon assimilation, glutathione redox regulation, and BVOC partitioning rather than by increased total volatile emission. Enhanced carbon assimilation occurred without additional carbon loss through BVOC release, while changes in the modeled intercellular pool indicate that part of the volatile response remained within the leaf. Our findings highlight BVOC partitioning as an important dimension of the plant response to oxidative stress and demonstrate that emission measurements alone may not fully capture the fate and potential physiological role of volatile carbon under O exposure. O_FIG O_LINKSMALLFIG WIDTH=178 HEIGHT=200 SRC="FIGDIR/small/743946v1_ufig1.gif" ALT="Figure 1"> View larger version (52K): org.highwire.dtl.DTLVardef@653af1org.highwire.dtl.DTLVardef@ca5forg.highwire.dtl.DTLVardef@1e641bforg.highwire.dtl.DTLVardef@1e68fae_HPS_FORMAT_FIGEXP M_FIG C_FIG BVOC Partitioning Contributes to Oxidative Stress Defence Under Acute O Exposure

Due to climate change, extreme weather events such as droughts are becoming more frequent and intense. This has a profound impact on plant performance and ecological interactions, including those involving herbivorous insects. The combined impact of drought stress and insect herbivory on plant metabolism has rarely been studied, particularly in woody plants. In this study, we investigated the influence of varying degrees of drought, both alone and in combination with herbivory by the leaf beetle Chrysomela tremulae, on the morphological and chemical characteristics of black poplar (Populus nigra) trees using a full factorial experimental design. We quantified morphological traits, volatile organic compound (VOC) emissions, phytohormone and amino acid concentrations, and phenolic profiles. Drought conditions increased the concentrations of salicylic acid (SA) and abscisic acid (ABA), while feeding induced ABA and SA. Amino acid profiles shifted significantly under drought conditions, particularly in beetle-infested plants. In contrast, salicinoids, which are the most important phenolic defense compounds in poplars, remained relatively stable. We also observed significant compound-specific effects on both constitutive and herbivore-induced VOC emissions. Our results demonstrate that drought and insect herbivory exert a joint influence on the chemical responses of P. nigra across multiple metabolic pathways. These findings highlight how the interaction between abiotic and biotic stresses can influence the defense chemistry of trees, which will consequently affect ecological interactions in forest ecosystems in the face of climate change.

The sequence of leaf physiological thresholds underlying plant responses to water deficit is thought to be functionally coordinated; yet, to what extent this coordination is maintained across genotypes and environments remains poorly documented at the intraspecific level. We characterized the sequence of stomatal closure, turgor loss and xylem embolism in the leaves of two genotypes of the riparian species Populus nigra (DRA-038 vs. PG-31) subjected to control, additional nitrogen or additional potassium treatments. Under control conditions, embolism measurements using the optical vulnerability method showed that DRA-038 was more vulnerable than PG-31, in agreement with measurements performed on stems with the reference Cavitron method. Stomatal closure consistently preceded xylem embolism, while bulk leaf turgor loss was typically observed once xylem embolism had already reached 50%. Hydraulic thresholds responded to treatments in a genotype-dependent manner, the intrinsically more vulnerable genotype DRA-038 being typically more plastic. However, despite variations across genotypes and treatments, the trait sequence remained tightly coordinated such that stomatal safety margins (SSMs) remained virtually null. These findings support a strong mechanistic integration of leaf hydraulic thresholds in poplar across genetic units and varying environments, questioning whether to favour intrinsic tolerance or plastic capacities in breeding future drought-tolerant genotypes.

Xylem and bark properties influence tree growth and drought resistance, yet their functional coordination and their environmental drivers remain unclear. We assessed xylem-bark coordination in branches of eight temperate woody Rosaceae species spanning different ecological preferences. We quantified xylem hydraulic efficiency and safety alongside bark traits governing permeability, hygroscopic water exchange, water storage, and anatomy, and evaluated phylogenetic signal and climatic associations. Bark water vapor conductance (Gbark) increased with maximum xylem hydraulic conductivity (Kh) and with xylem water potential at 50% loss of conductivity (P50), indicating species with more efficient but more embolism-vulnerable xylem developed more permeable bark. Species with higher Gbark showed reduced hygroscopic absorption time, consistent with faster rehydration from atmospheric water vapor. Both Gbark and P50 were phylogenetically conserved and covaried with climatic factors, namely air temperature, vapor pressure deficit (VPD), and isothermality. Species from warmer, high-VPD climates with greater diurnal temperature variability combined higher bark permeability with more vulnerable xylem, implying a shift from embolism avoidance to embolism tolerance strategies. Overall, xylem and bark hydraulics in Rosaceae evolved in concert along diurnal and annual gradients of evaporative demand, showing that drought resistance in woody angiosperms cannot be understood without considering bark traits alongside xylem function. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=104 SRC="FIGDIR/small/730605v1_ufig1.gif" ALT="Figure 1"> View larger version (41K): org.highwire.dtl.DTLVardef@14872c2org.highwire.dtl.DTLVardef@1662c8corg.highwire.dtl.DTLVardef@f6b4aaorg.highwire.dtl.DTLVardef@cf1aef_HPS_FORMAT_FIGEXP M_FIG C_FIG Caption: This study shows that xylem and bark in Rosaceae species form an integrated functional system in which xylem hydraulic safety, efficiency, and bark permeability are jointly tuned along diurnal and annual gradients of air temperature and evaporative demand.

Sugar exchange at the root interface is a pivotal process governing the establishment and stability of plant-fungal symbioses. Precise regulation of sugar exchange determines the success of this ecologically significant interaction. Sugar Will Eventually be Exported proteins (SWEETs) constitute a family of regulatory, energy-independent bidirectional sugar transporters that influence plant development, stress resilience, and survival. However, how specific SWEET transporters coordinate systemic carbon allocation and immune regulation during beneficial plant-fungal interactions remains poorly understood. In this study, we examined the role of the systemically induced Arabidopsis sugar transporter SWEET11 during association with the beneficial endophytic fungus Serendipita indica and following treatment with its elicitor, cellotriose (CT). Expression profiling of SWEET family members revealed a rapid and preferential induction of SWEET11 in aerial tissues upon fungal colonization and CT treatment. Loss-of-function of SWEET11 compromises key mutualistic outcomes, including plant growth enhancement, fungal colonization efficiency, penetration ability, carbohydrate distribution, and the regulation of defense-related phytohormones such as jasmonic acid and abscisic acid. Global transcriptome analysis further demonstrated that SWEET11 regulates whole-plant responses by orchestrating genes involved in central metabolism, secondary metabolite production, sesquiterpenoid and triterpenoid pathways, as well as defense signaling and nutrient transport systems. We show that SWEET11 interacts with a stress associated SNF1-related protein kinase (SnRK2.8) and plays a crucial role in enabling fungal establishment while mitigating host defense responses, and supporting plant growth. Our data shows that SWEET11 functions as a shoot-derived sugar exporter that directs carbon toward roots, facilitating sugar unloading to S. indica. This controlled carbon supply allows the fungus to meet its metabolic demands without disrupting host sugar balance, thereby maintaining a stable and well-regulated symbiotic association under immune constraints.

Photorespiration significantly influences terrestrial carbon fluxes, yet empirical measurements of its variability across tree species and temperature conditions remain limited, constraining predictions of vegetation and climate models. We quantified apparent photorespiratory CO2 loss (Lapp) and its temperature response for seven temperate broadleaf tree species in northern Europe, using in situ O2-shift measurements in Uppsala, Sweden during peak summer. Apparent loss was derived as the difference between net CO2 assimilation under ambient (Anet) and O2-free conditions at three leaf temperatures (25, 30, and 35 {degrees}C), spanning typical and heat-wave scenarios. Apparent photorespiratory CO2 loss showed pronounced interspecific variation and increased with temperature, while net photosynthesis remained relatively stable. The ratio of apparent loss to net photosynthesis ({phi} = Lapp/Anet) rose sharply with temperature, reaching species-mean values up to 0.94 at 35 {degrees}C, indicating that photorespiration can represent nearly the entirety of net carbon gain under heat stress even when leaves remain net CO2 sinks. Suppression of photorespiration under N2 and associated changes in leaf temperature systematically reallocated photosynthetic electron transport: the fraction of ambient electron transport rate (ETR) allocated to net CO2 assimilation declined with temperature, whereas the complementary fraction allocated to apparent photorespiratory loss and other O2-dependent sinks increased, with ETR-based apparent loss and its proportional expression rising steeply across the 25-35 {degrees}C range. Together, these in situ flux and partitioning measurements reveal high variability and strong temperature sensitivity in apparent photorespiration among temperate trees. Compared to crop-based parameterisations, the {phi} values we report for temperate trees are substantially higher and more temperature-dependent, providing species-specific constraints that can improve Farquhar-von Caemmerer-Berry-type vegetation model representations of photorespiration in forest ecosystems.

The root systems of wild tomatoes (S. Pimpinellifolium) can be understood as biological networks in which the lateral roots branch from a single main root and together balance two competing objectives: minimizing the material cost of building the network (wiring cost) and minimizing the transport time from the root tips to the shoot (conduction delay). Our prior work showed that S. Pimpinellifolium root architectures cluster near the Pareto-optimal front defined by these two objectives, with morphological diversity resolving into four qualitative topologies ((Chandrasekhar and Julkowska, 2022)). That framework assumed lateral roots grow as straight lines - ignoring gradual onset of lateral root gravitropism, the tendency of roots to curve toward the gravity vector. Because curved trajectories are longer than straight lines, gravitropism directly increases both wiring cost and conduction delay, constraining which architectures are physically realizable and thereby reshaping the Pareto front itself. Here we extend the model to explicitly incorporate lateral root gravitropism, producing predicted architectures that align much more closely with observed S. Pimpinellifolium root systems. We present a computational method to infer gravitropic sensitivity directly from anatomical tracing data - without reorientation assays - and apply it to 2423 arbors across different root topologies, growth conditions, and hormone treatments. Incorporating gravitropism reveals variation invisible to the straight-line model:notably, lateral roots show reduced gravitropic sensitivity under salt stress, mirroring a phenomenon previously described only for main roots and overlooked in lateral roots until now.

Elevated temperatures compromise plant immunity and increase susceptibility to bacterial pathogens through extensive reprogramming of hormone signaling pathways. Although autophagy contributes to both stress adaptation and pathogen defense, its role in hormone-dependent immune regulation under warm conditions remains unclear. Here, we investigated the contribution of NBR1 (NEIGHBOR OF BRCA1 GENE 1)-mediated selective autophagy to Arabidopsis immunity against Pseudomonas cannabina pv. alisalensis at elevated temperature. Bacterial infection under warming enhanced autophagic flux and promoted NBR1 turnover, indicating increased autophagic activity. Analysis of atg5 and nbr1 mutants, and NBR1-overexpressing lines, demonstrated that both core autophagy and NBR1-mediated selective autophagy contribute to bacterial immunity under warm conditions. Hormone and gene expression analyses indicated that NBR1 negatively regulates abscisic acid (ABA)-associated transcriptional responses during infection, while salicylic acid signaling was largely unaffected. Mechanistically, NBR1 physically associated with the ABA-responsive transcription factor ABI5 (ABA INSENSITIVE 5) and promoted its autophagy-dependent turnover in planta. ABI5 turnover was strongly reduced under warm conditions, leading to its accumulation in nbr1 and atg5 plants. Consistent with a functional role for ABI5 in this phenotype, genetic disruption of ABI5 largely reversed the increased susceptibility of nbr1 mutants at elevated temperature, whereas ABI5 overexpression increased susceptibility to bacterial infection. Together, our results identify NBR1-mediated selective autophagy as a regulatory mechanism that restrains ABA-associated susceptibility through the autophagy-dependent turnover of ABI5. These findings reveal a previously unrecognized connection between selective autophagy and ABA-dependent immune regulation and identify NBR1-mediated ABI5 turnover as a temperature-dependent mechanism that prevents stronger bacterial susceptibility under warm conditions.

Hydroxylated monoterpenes (HMTPs) are emitted during resistant tomato-Pseudomonas syringae interactions and confer antibacterial resistance, yet their integration into immune signalling remains poorly understood. Here we show that HMTPs act as endogenous mimics of pathogen attack that engage canonical defence pathways in tomato. Using -terpineol as a representative HMTP, we demonstrate that this volatile activates MPK kinase-, calcium- and reactive oxygen species (ROS)-dependent signalling, promotes jasmonate and salicylic acid accumulation, and induces pathogen-like stomatal immunity independently of abscisic acid. Functional analyses revealed that HMTPs biosynthesis depends on ROS and jasmonate signalling, and HMTPs further promote their own accumulation, establishing a self-reinforcing feed-forward mechanism. Moreover, in vitro oxidative conditions drive chemical remodelling and selective interconversion among HMTPs, contributing to volatile diversification and favouring the accumulation of highly bioactive hydroxylated forms. Consistently, deuterium-labelled linalool is incorporated into plant metabolism and converted into deuterated -terpineol in planta, providing direct evidence for volatile interconversion. Together, our findings establish HMTPs as dynamic amplifiers of tomato antibacterial immunity and key actors in pathogen-associated signalling.

Seed quality is a complex trait shaped by morphological, biochemical and microbiological properties that are rarely characterised simultaneously, limiting our ability to identify robust predictive indicators of germination speed and seedling emergence across varieties. Here, we performed a multi-factor characterisation of eight common bean (Phaseolus vulgaris L.) varieties, combining seed morphometrics, untargeted GC-MS metabolomics on three seed organs, and amplicon sequencing of bacterial and fungal communities, to identify indicators of germination speed and emergence percentage. The eight varieties showed substantial variation in both traits, used as physiological seed quality proxies. Seed weight and size variation between varieties were correlated with germination speed. The intravariety variance of seed weight was independently correlated with emergence performance. Metabolome composition differed strongly across seed organs, with variety as the dominant driver. Individual-seed metabolomic profiles in the plumule and cotyledon were associated with germination speed but not emergence, yielding 16 plumule and three cotyledon candidate metabolite markers. Fungal community composition was associated with both germination speed and emergence, while bacterial communities were associated with emergence only. Nine fungal and four bacterial taxa were identified as candidate indicators. Inter-kingdom co-occurrence network analysis revealed that fungi with similar germination speed associations tend to cluster in the same modules, suggesting that community-level modules rather than individual taxa may constitute more robust microbial indicators. These results demonstrate that germination speed and emergence capacity are governed by distinct seed properties, and provide morphological, metabolic and microbial candidate indicators for integration into targeted seed quality assessment frameworks for common bean.

Plant growth-promoting bacteria enhance plant performance, yet how different modes of plant-microbe interaction shape nutrient-specific host responses remains poorly understood. In particular, it is unclear how direct bacterial contact and volatile-mediated interactions originating from the same bacterial community differentially regulate plant nutrient acquisition pathways. Here, we investigated how a 16-member synthetic bacterial community (16SC) affects plant growth, nutrient status, signaling, and metabolite profiles under full nutrient supply as well as nitrogen (N), sulfur (S), and phosphorus (P) limitation in Arabidopsis thaliana. We show that volatile organic compounds (VOCs) emitted by the 16SC promote shoot growth under nitrate limitation and full nutrient conditions, whereas this growth promotion is lost under sulfur- and phosphorus-limiting conditions. In contrast, direct interaction (DBC) between plants and the 16SC abolishes growth promotion under all three nutrient-limiting conditions. These nutrient-dependent phenotypes correlate with distinct regulation of nutrient transporters and key transcriptional regulators involved in N (NRT1;1 / NLP7), S (SULTR1;2 / SLIM1/EIL3), and P (PHO2 / PHR1) signaling pathways. Genetic analyses using nutrient transporter mutants revealed that VOC-induced growth promotion requires functional NRT1;1 and SULTR1;2 transporters, whereas growth promotion mediated by direct bacterial contact is retained in the corresponding mutants. This uncoupling of VOC- and contact-dependent effects indicates that distinct host regulatory pathways underlie bacterial community growth promotion depending on the interaction mode. Together, our findings demonstrate that bacterial community-mediated plant growth promotion is strongly shaped by nutrient context and interaction mode, and that volatile-mediated and contact-dependent mechanisms engage separable host nutrient regulatory networks.

O_LITerrestrial biosphere models commonly use empirical scaling factors to represent soil moisture constraints on carbon and water fluxes, but these lack mechanistic grounding and produce inconsistent estimates of soil moisture limitations on primary production and transpiration across models. C_LIO_LIHere we extended the least-cost hypothesis for optimal stomatal conductance to account for soil moisture limitations by allowing soil water availability to modulate the carbon cost of water transport, drawing on the observed temperature dependence of stem respiration, and derived a simple empirical approximation to the theory using global {delta}13C and eddy covariance data. C_LIO_LIThe empirical analysis shows moderated thermal acclimation of stem respiration and a weak increase in water transport costs with aridity, supporting the interpretation that the decline in light-use efficiency (LUE) under arid conditions is primarily attributable to non-stomatal limitations. C_LIO_LIValidation against an independent global dataset of sapflow-derived canopy conductance and transpiration shows that the revised scheme significantly improves the predictive power of the least-cost hypothesis, offering a more mechanistically coherent alternative to existing soil moisture parameterisations. C_LI

Direct heat damage has been considered secondary as a cause of climate change-induced tree mortality and productivity declines in forests compared with climate change effects on tree water relations. However, evidence from temperate and tropical forests is accumulating that direct heat damage in the photosystem II (PS II) that is independent of water relations is also a realistic scenario under climate change. We analyzed PS II heat tolerance in Larix sibirica, which represents a dominant boreal tree species in Siberia and northern Central Asia in cold environments with subzero or near-zero mean annual temperatures, but nevertheless warm summers. Thermal imaging was applied to relate heat thresholds found in the laboratory to canopy temperatures in forests on north-facing mountain slopes, which are the main habitat of L. sibirica. L. sibirica showed slight decreases of the maximum quantum yield of PS II (Fv/Fm) at 35{degrees}C and 40{degrees}C after up to 4 h, but strong reductions at [≥]45{degrees}C and minor increases in Fv/Fm in late summer, which could be interpretation as heat acclimation. Canopy temperatures in the study year did not reach the thresholds for serious PS II heat damage. However, L. sibirica was more strongly sensitive to heat than temperate conifers. This first combined study of heat tolerance and canopy temperatures from boreal forests points to the possibility of low heat tolerance of boreal tree species, but such conclusion would require the study of more tree species.