Maintenance of abiotic stress memory in plants: Lessons learned from heat acclimation

Physiological and molecular response to drought in somatic plants from Pinus radiata embryonal masses induced at high temperatures

Drought and heat are among the major abiotic stresses in forest trees and are directly related with the consequences of climatic change. Many responses to abiotic stresses in plants have been associated with plant memory but mechanisms underlying this phenomenon remain unclear. Somatic embryogenesis, which is considered one of the most important methods for large-scale vegetative propagation of plants, is also used for stress induction and study the mechanisms involved in adaptation to abiotic stress. Specifically, heat stress during initiation stage of somatic embryogenesis has shown to have an impact in differential expression of stress related genes in pines. Modifications caused by a previous stress could eventually influence the stress tolerance of somatic plants years later. In this study we analysed the response to drought in 2-year-old radiata pine somatic plants, derived from embryonal masses initiated at 60 °C, at physiological, transcriptomic and amino acid accumulation level. Our results showed a more pronounce response to drought in plants coming from 60 °C treatment, which presented lower values in several physiological parameters as well as higher proline and tyrosine levels. Additionally, the transcriptomic response to drought was stronger in heat primed plants compared to control plants, suggesting a memory acquired two years before.

Priming thermotolerance: unlocking heat resilience for climate-smart crops

Rising temperatures and heat waves pose a substantial threat to crop productivity by disrupting essential physiological and reproductive processes. While plants have a genetically inherited capacity to acclimate to high temperatures, the thermotolerance capacity of many crops remains limited. This limitation leads to yield losses, which are further intensified by the increasing intensity of climate change. In this review, we explore how thermopriming enhances plant resilience by preparing plants for future heat stress (HS) events and summarize the mechanisms underlying the memory of HS (thermomemory) in different plant tissues and organs. We also discuss recent advances in priming agents, including chemical, microbial and physiological interventions, and their application strategies to extend thermotolerance beyond inherent genetic capacity. Additionally, this review examines how integrating priming strategies with genetic improvements, such as breeding and genome editing for thermotolerance traits, provides a holistic solution to mitigate the impact of climate change on agriculture. By combining these approaches, we propose a framework for developing climate-resilient crops and ensuring global food security in the face of escalating environmental challenges.This article is part of the theme issue 'Crops under stress: can we mitigate the impacts of climate change on agriculture and launch the 'Resilience Revolution'?'.

Native Australian seedlings exhibit novel strategies to acclimate to repeated heatwave events

Heatwaves are becoming more intense and frequent. Plant photosystem thermal thresholds can vary with species, but also shift in response to environmental triggers. Both upper and lower thresholds can acclimate to repeated heatwaves through ecological stress memory, where prior exposure primes them for subsequent events. The extent to which acclimation to repeated heat stress events varies among environmental origin and/or species is unknown. Different acclimation strategies might reflect biome of origin, or may be species-specific. For 12 species from two contrasting biomes-extreme desert and benign coastal temperate-we investigated responses to two simulated heatwaves, via shifts in upper and lower critical temperatures of photosystem II, and the difference between these thresholds, thermal tolerance breadth (TTB). Biome of origin had no effect on thermal tolerance. Observed differences among species following heat events suggested two possible acclimatory strategies. In some cases, species increased thermal thresholds during the first heatwave, but at the cost of reduced thermal tolerance during the second heatwave, a sprinter strategy. Other species acclimated to the first heatwave and further increased thermal tolerance to a second heatwave, indicative of ecological stress memory, a marathoner strategy. Synthesis: these among-species responses to heatwaves could suggest distinct vulnerabilities and resilience to repeat heat stress events, with some species having limited capacity to tolerate consecutive heatwaves, possibly as the cost of acclimation is too great, with other species having the advantage of increased tolerance via stress memory, helping them survive future stress, at least in the short-term.

A thermosensor FUST1 primes heat-induced stress granule formation via biomolecular condensation in Arabidopsis

The ability to sense cellular temperature and induce physiological changes is pivotal for plants to cope with warming climate. Biomolecular condensation is emerging as a thermo-sensing mechanism, but the underlying molecular basis remains elusive. Here we show that an intrinsically disordered protein FUST1 senses heat via its condensation in Arabidopsis thaliana. Heat-dependent condensation of FUST1 is primarily determined by its prion-like domain (PrLD). All-atom molecular dynamics simulation and experimental validation reveal that PrLD encodes a thermo-switch, experiencing lock-to-open conformational changes that control the intermolecular contacts. FUST1 interacts with integral stress granule (SG) components and localizes in the SGs. Importantly, FUST1 condensation is autonomous and precedes condensation of several known SG markers and is indispensable for SG assembly. Loss of FUST1 significantly delays SG assembly and impairs both basal and acquired heat tolerance. These findings illuminate the molecular basis for thermo-sensing by biomolecular condensation and shed light on the molecular mechanism of heat stress granule assembly.

Is the redox state of the PQ pool involved in regulating the ET biosynthesis pathway of CAM facultative semi-halophytes?

The mechanisms and factors regulating ethylene (ET) biosynthesis and its action remain largely unexplored, particularly in CAM-facultative and (semi)halophytic plants. The use of Mesembryanthemum crystallinum L. (ice plant) provides a unique opportunity to examine plastoquinone (PQ) - ET interactions in semi-halophytes and CAM-facultative plants simultaneously. Here, we present the results of an analysis of the common ice plant's response to prolonged (14-day) salinity stress and DCMU, which maintains the PQ pool in a more oxidised state, thereby mimicking darkness conditions. Differentially expressed gene (DEG) analysis showed that the expression of genes involved in ET regulation was not significantly altered after DCMU application. However, in C3 plants not affected by salinity stress, the expression of genes related to both photosystems, photosynthesis, and the photosynthetic electron transport chain was significantly affected by DCMU. We propose that sustained salinity stress and the occurrence of CAM photosynthesis render physiological processes insensitive to disruptions caused by a modified PQ pool redox state. The UPLC-MS analysis of the ET biosynthesis pathway central intermediate - 1-aminocyclopropane-1-carboxylic acid (ACC) - confirmed the molecular analysis results, as ACC content was similarly affected in salinity untreated and treated plants. Moreover, the analysis of key antioxidative system components, namely catalase and superoxide dismutases, suggests that PQ pool redox state does not directly regulate them. Instead, an alternative regulation mechanism involving reactive oxygen species (ROS) accumulation and a ROS-induced signalling cascade has been proposed.

Differential transpiration occurs in soybean under a wide range of water deficit and heat stress combination conditions

Differential transpiration is a newly discovered acclimation strategy of annual plants that mitigates the negative impacts of combined water deficit (WD) and heat stress (HS) on plant reproduction. Under conditions of WD + HS, transpiration of vegetative tissues is suppressed in plants such as soybean and tomato, while transpiration of reproductive tissues is not (termed 'Differential Transpiration'; DT). This newly identified acclimation process enables the cooling of reproductive organs under conditions of WD + HS, limiting HS-induced damage to plant reproduction. However, the thresholds at which DT remains active and effectively cools reproductive tissues, as well as the developmental stages at which it is activated in soybean, remain unknown. Here, we report that DT occurs at most nodes (leaf developmental stages) of soybean plants subjected to WD + HS, and that it can function under extreme conditions of WD + HS (i.e., 18% of field water capacity and 42°C combined). Our findings reveal that DT is an effective acclimation strategy that protects reproductive processes from extreme conditions of WD + HS at almost all developmental stages. In addition, our findings suggest that, under field conditions, DT could also be active in plants subjected to low or mild levels of WD during a heat wave.

Transcriptional memories mediate the plasticity of sulfide stress responses to enable acclimation in Urechis unicinctus

To cope with environmental stresses, organisms often adopt a memory response upon primary stress exposure to facilitate a quicker and/or stronger reaction to recurring stresses. Somatic stress memory is essential in dealing with contemporary stress. The earliest sign of somatic stress memory is a change in gene transcription levels, which alters physiology and phenotype to better cope with stress. Sulfide is a common environmental pollutant; however, some organisms have successfully colonized sulfur-rich environments. Whether stress memory plays important role in sulfide stress adaptation remains unclear. In this study, to determine whether Urechis unicinctus, a sulfur-tolerant organism, retains the memory of previous sulfide stress, we simulated a repetitive sulfide stress/recovery system. The results showed that the tolerance of U. unicinctus to sulfide stress was significantly increased after priming with 50 µM sulfide. Further, transcriptional memory genes (TMGs) involved in regulating sulfide stress memory were identified, classified according to their expression patterns, and functionally analyzed. TMGs involved in sulfide metabolism, sugar metabolism, and protein homeostasis pathway showed an enhanced response, whereas those related to DNA repair pathway demonstrated a modified response pattern. Our study indicated that U. unicinctus retains memory of sulfide stress priming, which mediates plasticity to accelerate sulfide stress adaptation.

Stress resilience in plants: the complex interplay between heat stress memory and resetting

Heat stress (HS) poses a major challenge to plants and agriculture, especially during climate change-induced heatwaves. Plants have evolved mechanisms to combat HS and remember past stress. This memory involves lasting changes in specific stress responses, enabling plants to better anticipate and react to future heat events. HS memory is a multi-layered cellular phenomenon that, in addition to epigenetic modifications, involves changes in protein quality control, metabolic pathways and broader physiological adjustments. An essential aspect of modulating stress memory is timely resetting, which restores defense responses to baseline levels and optimizes resource allocation for growth. Balancing stress memory with resetting enables plants to withstand stress while maintaining growth and reproductive capacity. In this review, we discuss mechanisms and regulatory layers of HS memory and resetting, highlighting their critical balance for enhancing stress resilience and plant fitness. We primarily focus on the model plant Arabidopsis thaliana due to the limited research on other species and outline key areas for future study.

Impact of Nutrient Stress on Plant Disease Resistance

Plants are constantly exposed to abiotic and biotic stresses that seriously affect crop yield and quality. A coordinated regulation of plant responses to combined abiotic/biotic stresses requires crosstalk between signaling pathways initiated by each stressor. Interconnected signaling pathways further finetune plant stress responses and allow the plant to respond to such stresses effectively. The plant nutritional status might influence disease resistance by strengthening or weakening plant immune responses, as well as through modulation of the pathogenicity program in the pathogen. Here, we discuss advances in our understanding of interactions between nutrient stress, deficiency or excess, and immune signaling pathways in the context of current agricultural practices. The introduction of chemical fertilizers and pesticides was a major component of the Green Revolution initiated in the 1960s that greatly boosted crop production. However, the massive application of agrochemicals also has adverse consequences on the environment and animal/human health. Therefore, an in-depth understanding of the connections between stress caused by overfertilization (or low bioavailability of nutrients) and immune responses is a timely and novel field of research with important implications for disease control in crop species. Optimizing nutrient management practices tailored to specific environmental conditions will be crucial in maximizing crop production using environmentally friendly systems.

Plant response to intermittent heat stress involves modulation of mRNA translation efficiency

Acquired thermotolerance (also known as priming) is the ability of cells or organisms to survive acute heat stress if preceded by a milder one. In plants, acquired thermotolerance has been studied mainly at the transcriptional level, including recent descriptions of sophisticated regulatory circuits that are essential for this learning capacity. Here, we tested the involvement of polysome-related processes [translation and cotranslational mRNA decay (CTRD)] in Arabidopsis (Arabidopsis thaliana) thermotolerance using two heat stress regimes with and without a priming event. We found that priming is essential to restore the general translational potential of plants shortly after acute heat stress. We observed that mRNAs not involved in heat stress suffered from reduced translation efficiency at high temperatures, whereas heat stress-related mRNAs were translated more efficiently under the same condition. We also showed that the induction of the unfolded protein response (UPR) pathway in acute heat stress is favored by a previous priming event and that, in the absence of priming, ER-translated mRNAs become preferential targets of CTRD. Finally, we present evidence that CTRD can specifically regulate more than a thousand genes during heat stress and should be considered as an independent gene regulatory mechanism.

Does Pre-Acclimation Enhance the Tolerance of Quercus ilex and Arbutus unedo Seedlings to Drought?

Mediterranean forests are severely threatened by increasing seedling mortality due to harsh environmental conditions, especially drought. In this study, we investigate whether seedlings of Quercus ilex and Arbutus unedo, previously exposed to water deficit, acquired tolerance to summer drought. Seedlings of the two species were grown from April to September in a plastic tunnel greenhouse and exposed to two irrigation regimes (control, 100% water holding capacity; water-stressed, 50% of control). In mid-August, the irrigation of all plants was suspended for three weeks. The response of the species was analyzed to evaluate survival, growth, ecological, and anatomical traits of wood produced under stressful conditions and marked through the pinning technique. The results suggest that both species show pre-acclimation to drought, with Q. ilex demonstrating a marked increase in survival percentage. This is likely due to a reduction in vessel size in response to previous water stress. In contrast, in A. unedo, the higher frequency of narrower vessels allowed safer water transport compared to Q. ilex, thus explaining the slight increase in survival. Overall results indicated that the two species adopt different strategies to overcome drought, providing valuable insights for managing seedlings in natural ecosystems and urban green spaces.

Effects of thermopriming and bacteria-mediated heat-stress acclimation strategies on seed yield and quality criteria in Brassica napus cv Aviso and Camelina sativa cv Calena

The effects of intense heat during the reproductive phase of two Brassica species-B. napus and C. sativa-could be alleviated by a prior gradual increase exposure and/or PGPR inoculation. Abct. Among extreme weather events caused by climate change, heat waves are one of the most threatening issues for food security. Heat stress is known to be particularly penalizing at the reproductive stage for oleaginous crops, such as oilseed rape and camelina, and is responsible for crop failures as a consequence of yield losses and lower quality of harvest plants parts. In this context, our study aims to analyze two acclimation strategies that rely on the induction of signals prior to an intense heat stress event, i.e., thermopriming (herein, a gradual increase in temperature) and bacteria inoculations (herein, two Plant Growth-Promoting Rhizobacteria (PGPR) were tested). In the two experiments, we assessed the expected beneficial effects of these two acclimation strategies on yield components, seed quality criteria (nutritional and related to dormancy). While thermopriming improved heat stress tolerance in B. napus cv Aviso by maintaining yield, seed nutritional quality and seed dormancy, the effects of the gradual increase prior to the heat stress were even more negative than the later intense heat stress event in C. sativa cv Calena which resulted in cumulated negative effects. The experimentation based on PGPR inoculation highlighted similar trends to thermopriming in B. napus cv Aviso but to a lesser extent. However, in C. sativa cv Calena, very weak effects of PGPR inoculation upon heat stress were observed. Finally, these two acclimation strategies were shown to help alleviate the impacts of intense heat stress but in a species-dependent manner. This study should be deepened by exploring the behaviors of more cultivars of oilseed rape and camelina in the perspective to generalize these results at the species scale.

Chromatin modifications and memory in regulation of stress-related polyphenols: finding new ways to control flavonoid biosynthesis

The influence of epigenetic factors on plant defense responses and the balance between growth and defense is becoming a central area in plant biology. It is believed that the biosynthesis of secondary metabolites can be regulated by epigenetic factors, but this is not associated with the formation of a "memory" to the previous biosynthetic status. This review shows that some epigenetic effects can result in epigenetic memory, which opens up new areas of research in secondary metabolites, in particular flavonoids. Plant-controlled chromatin modifications can lead to the generation of stress memory, a phenomenon through which information regarding past stress cues is retained, resulting in a modified response to recurring stress. How deeply are the mechanisms of chromatin modification and memory generation involved in the control of flavonoid biosynthesis? This article collects available information from the literature and interactome databases to address this issue. Visualization of the interaction of chromatin-modifying proteins with the flavonoid biosynthetic machinery is presented. Chromatin modifiers and "bookmarks" that may be involved in the regulation of flavonoid biosynthesis through memory have been identified. Through different mechanisms of chromatin modification, plants can harmonize flavonoid metabolism with: stress responses, developmental programs, light-dependent processes, flowering, and longevity programs. The available information points to the possibility of developing chromatin-modifying technologies to control flavonoid biosynthesis.

Chitosan nanoparticles alleviate chromium toxicity by modulating metabolic homeostasis and promoting chromium sequestration in Zea mays L

Chitosan nanoparticles (CSNPs) have been proposed as a potential alternative in alleviating chromium (Cr) toxicity. However, the mechanisms underlying remains poorly understood. This study investigates the effects of CSNPs on carbon/nitrogen metabolism, cell wall Cr binding capacity, and antioxidant activity in Zea mays L. under Cr stress. Cr stress decreased the total dry weight (DW) by 48.5 %. By contrast, the total DW was reduced by only 26.2 % in CSNPs-treated plants. Analysis of transcriptomic, enzyme activity, and metabolite content data, CSNPs-treated plants exhibited a higher level of relatively stable Carbon and Nitrogen metabolism than untreated plants. CSNPs application resulted in a substantial increase in the levels of sucrose and soluble protein by 78.0 % and 19.4 % in the leaves, and 60.0 % and 59.7 % in the roots, respectively. Meanwhile, CSNPs increased the contents of glutathione, phytochelatin, and cell wall polysaccharide. This increase resulted in a higher retention of Cr in vacuole and cell wall. Additionally, CSNPs alleviated the oxidative damage by improving antioxidant activity. Overall, our results suggest that CSNPs alleviates Cr toxicity by modulating metabolic homeostasis and promoting Cr sequestration in maize plants. This study provides new insights into the mechanisms underlying CSNPs-mediated Cr stress response with potential implications for crop production.

A coherent feed-forward loop in the Arabidopsis root stem cell organizer regulates auxin biosynthesis and columella stem cell maintenance

Stem cells in plant meristems are kept undifferentiated by signals from surrounding cells and provide the basis for continuous organ formation. In the stem cell organizer of the Arabidopsis thaliana root, the quiescent centre (QC), the WOX5 transcription factor, functions as a central hub in regulating columella stem cell (CSC) homoeostasis. However, the processes mediating WOX5 function are only poorly understood. Here we identify the transcription factor HAN as a central mediator of WOX5-regulated stem cell maintenance. HAN is required for mitotic quiescence of QC and CSC maintenance and is sufficient to induce ectopic stem cells. WOX5 and HAN repress transcription of the differentiation factor gene CDF4 in a coherent feed-forward loop (cFFL), one output of which is the expression of the auxin biosynthesis gene TAA1 and maintenance of auxin response maxima in the organizer. These findings and mathematical modelling provide a mechanistic framework for WOX5 function in the root stem cell niche.

Involvement of epigenetic factors in flavonoid accumulation during plant cold adaptation

Plant responses to cold stress include either induction of flavonoid biosynthesis as part of defense responses or initially elevated levels of these substances to mitigate sudden temperature fluctuations. The role of chromatin modifying factors and, in general, epigenetic variability in these processes is not entirely clear. In this work, we review the literature to establish the relationship between flavonoids, cold and chromatin modifications. We demonstrate the relationship between cold acclimation and flavonoid accumulation, and then describe the cold adaptation signaling pathways and their relationship with chromatin modifying factors. Particular attention was paid to the cold signaling module OST1-HOS1-ICE1 and the novel function of the E3 ubiquitin protein ligase HOS1 (a protein involved in chromatin modification during cold stress) in flavonoid regulation.

Contribution of the regulatory miR156-SPL9 module to the drought stress response in pigmented potato (Solanum tuberosum L.)

Potato (Solanum tuberosum L.) is nowadays an important component of diversified cropping systems due to its adaptability, yielding capacity, and nutrition contribution. Breeding programs aiming at raising potato's nutritional value have mainly focused on the accumulation in potato tubers of health-promoting phytochemicals such as anthocyanins. In different plant species, increased amounts of anthocyanins in vegetative tissues have been associated with enhanced tolerance to abiotic and biotic stresses that challenge agrifood systems in the current context of global climate change. In the present study, we aimed at gaining insight into the effect of anthocyanin accumulation on the potato plants response to drought stress using three different potato genotypes with differential canopy and tuber pigmentation: the purple fleshed commercial variety Bleuet; the red fleshed breeding clone DAR170; and the non-pigmented commercial variety Monalisa. The varieties Bleuet and DAR170 exhibiting higher anthocyanin content in vegetative tissues than the Monalisa variety showed a remarkable inhibition of stem growth development under drought stress treatment suggestive of an anthocyanin-mediated physiological shift from growth to resilience as a mechanism of stress tolerance. The results of the expression analysis of stu-miR156a and its target StSPL9 gene in the potato plants with different anthocyanin content, as well as their change in response to drought stress support the participation of the conserved miR156-SPL9 regulatory module in coordinating potato plants development and plant responses to drought stress, involving precise fine-tuning of anthocyanin biosynthesis.

Heat Stress and Plant-Biotic Interactions: Advances and Perspectives

Climate change presents numerous challenges for agriculture, including frequent events of plant abiotic stresses such as elevated temperatures that lead to heat stress (HS). As the primary driving factor of climate change, HS threatens global food security and biodiversity. In recent years, HS events have negatively impacted plant physiology, reducing plant's ability to maintain disease resistance and resulting in lower crop yields. Plants must adapt their priorities toward defense mechanisms to tolerate stress in challenging environments. Furthermore, selective breeding and long-term domestication for higher yields have made crop varieties vulnerable to multiple stressors, making them more susceptible to frequent HS events. Studies on climate change predict that concurrent HS and biotic stresses will become more frequent and severe in the future, potentially occurring simultaneously or sequentially. While most studies have focused on singular stress effects on plant systems to examine how plants respond to specific stresses, the simultaneous occurrence of HS and biotic stresses pose a growing threat to agricultural productivity. Few studies have explored the interactions between HS and plant-biotic interactions. Here, we aim to shed light on the physiological and molecular effects of HS and biotic factor interactions (bacteria, fungi, oomycetes, nematodes, insect pests, pollinators, weedy species, and parasitic plants), as well as their combined impact on crop growth and yields. We also examine recent advances in designing and developing various strategies to address multi-stress scenarios related to HS and biotic factors.

Comparative proteomics in tall fescue to reveal underlying mechanisms for improving Photosystem II thermotolerance during heat stress memory

BACKGROUND

The escalating impacts of global warming intensify the detrimental effects of heat stress on crop growth and yield. Among the earliest and most vulnerable sites of damage is Photosystem II (PSII). Plants exposed to recurring high temperatures develop heat stress memory, a phenomenon that enables them to retain information from previous stress events to better cope with subsequent one. Understanding the components and regulatory networks associated with heat stress memory is crucial for the development of heat-resistant crops.

RESULTS

Physiological assays revealed that heat priming (HP) enabled tall fescue to possess higher Photosystem II photochemical activity when subjected to trigger stress. To investigate the underlying mechanisms of heat stress memory, we performed comparative proteomic analyses on tall fescue leaves at S0 (control), R4 (primed), and S5 (triggering), using an integrated approach of Tandem Mass Tag (TMT) labeling and Liquid Chromatography-Mass Spectrometry. A total of 3,851 proteins were detected, with quantitative information available for 3,835 proteins. Among these, we identified 1,423 differentially abundant proteins (DAPs), including 526 proteins that were classified as Heat Stress Memory Proteins (HSMPs). GO and KEGG enrichment analyses revealed that the HSMPs were primarily associated with the "autophagy" in R4 and with "PSII repair", "HSP binding", and "peptidase activity" in S5. Notably, we identified 7 chloroplast-localized HSMPs (HSP21, DJC77, EGY3, LHCA4, LQY1, PSBR and DEGP8, R4/S0 > 1.2, S5/S0 > 1.2), which were considered to be effectors linked to PSII heat stress memory, predominantly in cluster 4. Protein-protein interaction (PPI) analysis indicated that the ubiquitin-proteasome system, with key nodes at UPL3, RAD23b, and UCH3, might play a role in the selective retention of memory effectors in the R4 stage. Furthermore, we conducted RT-qPCR validation on 12 genes, and the results showed that in comparison to the S5 stage, the R4 stage exhibited reduced consistency between transcript and protein levels, providing additional evidence for post-transcriptional regulation in R4.

CONCLUSIONS

These findings provide valuable insights into the establishment of heat stress memory under recurring high-temperature episodes and offer a conceptual framework for breeding thermotolerant crops with improved PSII functionality.

The role of priming and memory in rice environmental stress adaptation: Current knowledge and perspectives

Plant responses to abiotic stresses are dynamic, following the unpredictable changes of physical environmental parameters such as temperature, water and nutrients. Physiological and phenotypical responses to stress are intercalated by periods of recovery. An earlier stress can be remembered as 'stress memory' to mount a response within a generation or transgenerationally. The 'stress priming' phenomenon allows plants to respond quickly and more robustly to stressors to increase survival, and therefore has significant implications for agriculture. Although evidence for stress memory in various plant species is accumulating, understanding of the mechanisms implicated, especially for crops of agricultural interest, is in its infancy. Rice is a major food crop which is susceptible to abiotic stresses causing constraints on its cultivation and yield globally. Advancing the understanding of the stress response network will thus have a significant impact on rice sustainable production and global food security in the face of climate change. Therefore, this review highlights the effects of priming on rice abiotic stress tolerance and focuses on specific aspects of stress memory, its perpetuation and its regulation at epigenetic, transcriptional, metabolic as well as physiological levels. The open questions and future directions in this exciting research field are also laid out.

From environmental responses to adaptation: the roles of plant lncRNAs

As sessile organisms, plants are continuously exposed to heterogeneous and changing environments and constantly need to adapt their growth strategies. They have evolved complex mechanisms to recognize various stress factors, activate appropriate signaling pathways, and respond accordingly by reprogramming the expression of multiple genes at the transcriptional, post-transcriptional, and even epigenome levels to tolerate stressful conditions such as drought, high temperature, nutrient deficiency, and pathogenic interactions. Apart from protein-coding genes, long non-coding RNAs (lncRNAs) have emerged as key players in plant adaptation to environmental stresses. They are transcripts larger than 200 nucleotides without protein-coding potential. Still, they appear to regulate a wide range of processes, including epigenetic modifications and chromatin reorganization, as well as transcriptional and post-transcriptional modulation of gene expression, allowing plant adaptation to various environmental stresses. LncRNAs can positively or negatively modulate stress responses, affecting processes such as hormone signaling, temperature tolerance, and nutrient deficiency adaptation. Moreover, they also seem to play a role in stress memory, wherein prior exposure to mild stress enhances plant ability to adapt to subsequent stressful conditions. In this review, we summarize the contribution of lncRNAs in plant adaptation to biotic and abiotic stresses, as well as stress memory. The complex evolutionary conservation of lncRNAs is also discussed and provides insights into future research directions in this field.

The complex transcriptional regulation of heat stress response in maize

As one of the most important food and feed crops worldwide, maize suffers much more tremendous damages under heat stress compared to other plants, which seriously inhibits plant growth and reduces productivity. To mitigate the heat-induced damages and adapt to high temperature environment, plants have evolved a series of molecular mechanisms to sense, respond and adapt high temperatures and heat stress. In this review, we summarized recent advances in molecular regulations underlying high temperature sensing, heat stress response and memory in maize, especially focusing on several important pathways and signals in high temperature sensing, and the complex transcriptional regulation of ZmHSFs (Heat Shock Factors) in heat stress response. In addition, we highlighted interactions between ZmHSFs and several epigenetic regulation factors in coordinately regulating heat stress response and memory. Finally, we laid out strategies to systematically elucidate the regulatory network of maize heat stress response, and discussed approaches for breeding future heat-tolerance maize.

Heat-Induced Cross-Tolerance to Salinity Due to Thermopriming in Tomatoes

Global plant production is challenged by unpredictable (a)biotic stresses that occur individually, simultaneously or staggered. Due to an increasing demand for environmentally friendly plant production, new sustainable, universal, and preventive measures in crop protection are needed. We postulate thermopriming as a suitable procedure that fulfills these requirements. Therefore, we performed thermopriming as a pre-conditioning on tomato transplants in combination with two subsequent salt stress treatments to evaluate their single and combined physiological effects on leaves and fruits with regard to plant performance, fruit yield and quality. We identified a cross-tolerance to salinity that was triggered by the preceding thermopriming treatment and resulted in an accumulation of phenols and flavonols in the leaves. Plant growth and fruit yield were initially delayed after the stress treatments but recovered later. In regard to fruit quality, we found an increase in carotenoid and starch contents in fruits due to thermopriming, while sugars and titratable acidity were not affected. Our results indicate that thermopriming can mitigate the impact of subsequent and recurrent stress events on plant performance and yield under production-like conditions.

Systemic stomatal responses in plants: Coordinating development, stress, and pathogen defense under a changing climate

To successfully survive, develop, grow and reproduce, multicellular organisms must coordinate their molecular, physiological, developmental and metabolic responses among their different cells and tissues. This process is mediated by cell-to-cell, vascular and/or volatile communication, and involves electric, chemical and/or hydraulic signals. Within this context, stomata serve a dual role by coordinating their responses to the environment with their neighbouring cells at the epidermis, but also with other stomata present on other parts of the plant. As stomata represent one of the most important conduits between the plant and its above-ground environment, as well as directly affect photosynthesis, respiration and the hydraulic status of the plant by controlling its gas and vapour exchange with the atmosphere, coordinating the overall response of stomata within and between different leaves and tissues plays a cardinal role in plant growth, development and reproduction. Here, we discuss different examples of local and systemic stomatal coordination, the different signalling pathways that mediate them, and the importance of systemic stomatal coordination to our food supply, ecosystems and weather patterns, under our changing climate. We further discuss the potential biotechnological implications of regulating systemic stomatal responses for enhancing agricultural productivity in a warmer and CO2 -rich environment.

Exploring priming strategies to improve stress resilience of Posidonia oceanica seedlings

Seagrasses' ability to store information after exposure to stress (i.e. stress memory) and to better respond to further stress (i.e. priming) have recently been observed, although the temporal persistence of the memory and the mechanisms for priming induction remain to be defined. Here, we explored three priming strategies in Posidonia oceanica seedlings, each inducing a different level of stress, for temperature and salinity. We investigated changes in morphometry, growth rate and biomass between primed and non-primed seedlings. The results showed similar behaviour of seedlings when exposed to an acute stress event, regardless of whether they had been primed or not and of the priming strategy received. This opens the debate on the level of stress necessary for inducing a priming status and the persistence of the stress memory in P. oceanica seedlings. Although no priming-induced stress resistance was observed, seedlings showed unexpectedly high resilience to extreme levels of both abiotic stressors.

Heat stress memory differentially regulates the expression of nitrogen transporter genes in the filamentous red alga 'Bangia' sp. ESS1

Introduction

To withstand high temperatures that would be lethal to a plant in the naïve state, land plants must establish heat stress memory. The acquisition of heat stress tolerance via heat stress memory in algae has only been observed in the red alga 'Bangia' sp. ESS1.

Methods

In this study, we further evaluated the intrinsic ability of this alga to establish heat stress memory by monitoring hydrogen peroxide (H2O2) production and examining the relationship between heat stress memory and the expression of genes encoding nitrogen transporters, since heat stress generally reduces nitrogen absorption. Next, genes encoding nitrogen transporters were selected from our unpublished transcriptome data of 'Bangia' sp. ESS1.

Results

We observed a reduction in H2O2 content when heat stress memory was established in the alga. In addition, six ammonium transporter genes, a single-copy nitrate transporter gene and two urea transporter genes were identified. Two of these nitrogen transporter genes were induced by heat stress but not by heat stress memory, two genes showed heat stress memory-dependent expression, and one gene was induced by both treatments. Heat stress memory therefore differentially regulated the expression of the nitrogen transporter genes by reducing heat stress-inducible gene expression and inducing heat stress memory-dependent gene expression.

Discussion

These findings point to the functional diversity of nitrogen transporter genes, which play different roles under various heat stress conditions. The characteristic effects of heat stress memory on the expression of individual nitrogen transporter genes might represent an indispensable strategy for reducing the threshold of sensitivity to recurrent high-temperature conditions and for maintaining nitrogen absorption under such conditions in 'Bangia' sp. ESS1.

Revisiting plant stress memory: mechanisms and contribution to stress adaptation

Highly repetitive adverse environmental conditions are encountered by plants multiple times during their lifecycle. These repetitive encounters with stresses provide plants an opportunity to remember and recall the experiences of past stress-associated responses, resulting in better adaptation towards those stresses. In general, this phenomenon is known as plant stress memory. According to our current understanding, epigenetic mechanisms play a major role in plants stress memory through DNA methylation, histone, and chromatin remodeling, and modulating non-coding RNAs. In addition, transcriptional, hormonal, and metabolic-based regulations of stress memory establishment also exist for various biotic and abiotic stresses. Plant memory can also be generated by priming the plants using various stressors that improve plants' tolerance towards unfavorable conditions. Additionally, the application of priming agents has been demonstrated to successfully establish stress memory. However, the interconnection of all aspects of the underlying mechanisms of plant stress memory is not yet fully understood, which limits their proper utilization to improve the stress adaptations in plants. This review summarizes the recent understanding of plant stress memory and its potential applications in improving plant tolerance towards biotic and abiotic stresses.

Systemic analysis of metabolome reconfiguration in Arabidopsis after abiotic stressors uncovers metabolites that modulate defense against pathogens

Understanding plant immune responses is complex because of the high interdependence among biological processes in homeostatic networks. Hence, the integration of environmental cues causes network rewiring that interferes with defense responses. Similarly, plants retain molecular signatures configured under abiotic stress periods to rapidly respond to recurrent stress, and these can alter immunity. Metabolome changes imposed by abiotic stressors are persistent, although their impact on defense remains to be clarified. In this study, we profiled metabolomes of Arabidopsis plants under several abiotic stress treatments applied individually or simultaneously to capture temporal trajectories in metabolite composition during adverse conditions and recovery. Further systemic analysis was performed to address the relevance of metabolome changes and extract central features to be tested in planta. Our results demonstrate irreversibility in major fractions of metabolome changes as a general pattern in response to abiotic stress periods. Functional analysis of metabolomes and co-abundance networks points to convergence in the reconfiguration of organic acid and secondary metabolite metabolism. Arabidopsis mutant lines for components related to these metabolic pathways showed altered defense capacities against different pathogens. Collectively, our data suggest that sustained metabolome changes configured in adverse environments can act as modulators of immune responses and provide evidence for a new layer of regulation in plant defense.

Maintenance of stem cell activity in plant development and stress responses

Stem cells residing in plant apical meristems play an important role during postembryonic development. These stem cells are the wellspring from which tissues and organs of the plant emerge. The shoot apical meristem (SAM) governs the aboveground portions of a plant, while the root apical meristem (RAM) orchestrates the subterranean root system. In their sessile existence, plants are inextricably bound to their environment and must adapt to various abiotic stresses, including osmotic stress, drought, temperature fluctuations, salinity, ultraviolet radiation, and exposure to heavy metal ions. These environmental challenges exert profound effects on stem cells, potentially causing severe DNA damage and disrupting the equilibrium of reactive oxygen species (ROS) and Ca2+ signaling in these vital cells, jeopardizing their integrity and survival. In response to these challenges, plants have evolved mechanisms to ensure the preservation, restoration, and adaptation of the meristematic stem cell niche. This enduring response allows plants to thrive in their habitats over extended periods. Here, we presented a comprehensive overview of the cellular and molecular intricacies surrounding the initiation and maintenance of the meristematic stem cell niche. We also delved into the mechanisms employed by stem cells to withstand and respond to abiotic stressors.

Dynamic physiological and transcriptomic changes reveal memory effects of salt stress in maize

BACKGROUND

Pre-exposing plants to abiotic stresses can induce stress memory, which is crucial for adapting to subsequent stress exposure. Although numerous genes involved in salt stress response have been identified, the understanding of memory responses to salt stress remains limited.

RESULTS

In this study, we conducted physiological and transcriptional assays on maize plants subjected to recurrent salt stress to characterize salt stress memory. During the second exposure to salt stress, the plants exhibited enhanced salt resistance, as evidenced by increased proline content and higher POD and SOD activity, along with decreased MDA content, indicative of physiological memory behavior. Transcriptional analysis revealed fewer differentially expressed genes and variations in response processes during the second exposure compared to the first, indicative of transcriptional memory behavior. A total of 2,213 salt stress memory genes (SMGs) were identified and categorized into four response patterns. The most prominent group of SMGs consisted of genes with elevated expression during the first exposure to salt stress but reduced expression after recurrent exposure to salt stress, or vice versa ([+ / -] or [- / +]), indicating that a revised response is a crucial process in plant stress memory. Furthermore, nine transcription factors (TFs) (WRKY40, WRKY46, WRKY53, WRKY18, WRKY33, WRKY70, MYB15, KNAT7, and WRKY54) were identified as crucial factors related to salt stress memory. These TFs regulate over 53% of SMGs, underscoring their potential significance in salt stress memory.

CONCLUSIONS

Our study demonstrates that maize can develop salt stress memory, and the genes identified here will aid in the genetic improvement of maize and other crops.

Role of Epigenetic Factors in Response to Stress and Establishment of Somatic Memory of Stress Exposure in Plants

All species are well adapted to their environment. Stress causes a magnitude of biochemical and molecular responses in plants, leading to physiological or pathological changes. The response to various stresses is genetically predetermined, but is also controlled on the epigenetic level. Most plants are adapted to their environments through generations of exposure to all elements. Many plant species have the capacity to acclimate or adapt to certain stresses using the mechanism of priming. In most cases, priming is a somatic response allowing plants to deal with the same or similar stress more efficiently, with fewer resources diverted from growth and development. Priming likely relies on multiple mechanisms, but the differential expression of non-coding RNAs, changes in DNA methylation, histone modifications, and nucleosome repositioning play a crucial role. Specifically, we emphasize the role of BRM/CHR17, BRU1, FGT1, HFSA2, and H2A.Z proteins as positive regulators, and CAF-1, MOM1, DDM1, and SGS3 as potential negative regulators of somatic stress memory. In this review, we will discuss the role of epigenetic factors in response to stress, priming, and the somatic memory of stress exposures.

Mitotic inheritance of genetic and epigenetic information via the histone H3.1 variant

The replication-dependent histone H3.1 variant, ubiquitous in multicellular eukaryotes, has been proposed to play key roles during chromatin replication due to its unique expression pattern restricted to the S phase of the cell cycle. Here, we describe recent discoveries in plants regarding molecular mechanisms and cellular pathways involving H3.1 that contribute to the maintenance of genomic and epigenomic information. First, we highlight new advances concerning the contribution of the histone chaperone CAF-1 and the TSK-H3.1 DNA repair pathway in preventing genomic instability during replication. We then summarize the evidence connecting H3.1 to specific roles required for the mitotic inheritance of epigenetic states. Finally, we discuss the recent identification of a specific interaction between H3.1 and DNA polymerase epsilon and its functional implications.

Effects of drought stress memory on the accumulation of stress-protective compounds in naturally grown pine and spruce

Forest trees are subjected to multiple stressors during their long lifetime and therefore require effective and finely regulated stress-protective systems. Stressors can induce protective systems either directly or with the involvement of stress memory mechanisms. Stress memory has only begun to be uncovered in model plants and is unexplored in coniferous species. Therefore, we studied the possible role of stress memory in the regulation of the accumulation of stress-protective compounds (heat shock proteins, dehydrins, proline) in the needles of naturally grown Scots pine and Norway spruce trees subjected to the subsequent action of long-term (multiyear) and short-term (seasonal) water shortages. Although the water deficit was relatively mild, it significantly influenced the pattern of expression of stress memory-related heat shock factor (HSF) and SWI/SNF genes, indicating the formation of stress memory in both species. In spruce, dehydrin accumulation was increased by water shortage in a manner compatible with Type II stress memory. The accumulation of HSP40 in spruce needles was positively influenced by long-term water shortage, but this increase was unlikely to be of biological importance due to the concomitant decrease in HSP70, HSP90 and HSP101 accumulation. Finally, proline accumulation was negatively influenced by short-term water deficit in spruce. In pine, no one protective compound accumulated in response to water stress. Taken together, the results indicate that the accumulation of stress-protective compounds was generally independent of stress memory effects both in pine and in spruce.

Cytokinins act synergistically with heat acclimation to enhance rice thermotolerance affecting hormonal dynamics, gene expression and volatile emission

Heat stress is a frequent environmental constraint. Phytohormones can significantly affect plant thermotolerance. This study compares the effects of exogenous cytokinin meta-topolin-9-(tetrahydropyran-2-yl)purine (mT9THP) on rice (Oryza sativa) under control conditions, after acclimation by moderate temperature (A; 37 °C, 2h), heat stress (HS; 45 °C, 6h) and their combination (AHS). mT9THP is a stable cytokinin derivative that releases active meta-topolin gradually, preventing the rapid deactivation reported after exogenous cytokinin application. Under control conditions, mT9THP negatively affected jasmonic acid in leaves and abscisic and salicylic acids in crowns (meristematic tissue crucial for tillering). Exogenous cytokinin stimulated the emission of volatile organic compounds (VOC), especially 2,3-butanediol. Acclimation upregulated trans-zeatin, expression of stress- and hormone-related genes, and VOC emission. The combination of acclimation and mT9THP promoted the expression of stress markers and antioxidant enzymes and moderately increased VOC emission, including 2-ethylhexyl salicylate or furanones. AHS and HS responses shared some common features, namely, increase of ethylene precursor aminocyclopropane-1-carboxylic acid (ACC), cis-zeatin and cytokinin methylthio derivatives, as well as the expression of heat shock proteins, alternative oxidases, and superoxide dismutases. AHS specifically induced jasmonic acid and auxin indole-3-acetic acid levels, diacylglycerolipids with fewer double bonds, and VOC emissions [e.g., acetamide, lipoxygenase (LOX)-derived volatiles]. Under direct HS, exogenous cytokinin mimicked some positive acclimation effects. The combination of mT9THP and AHS had the strongest thermo-protective effect, including a strong stimulation of VOC emissions (including LOX-derived ones). These results demonstrate for the first time the crucial contribution of volatiles to the beneficial effects of cytokinin and AHS on rice thermotolerance.

Priming Potato Plants with Melatonin Protects Stolon Formation under Delayed Salt Stress by Maintaining the Photochemical Function of Photosystem II, Ionic Homeostasis and Activating the Antioxidant System

Melatonin is among one of the promising agents able to protect agricultural plants from the adverse action of different stressors, including salinity. We aimed to investigate the effects of melatonin priming (0.1, 1.0 and 10 µM) on salt-stressed potato plants (125 mM NaCl), by studying the growth parameters, photochemical activity of photosystem II, water status, ion content and antioxidant system activity. Melatonin as a pleiotropic signaling molecule was found to decrease the negative effect of salt stress on stolon formation, tissue water content and ion status without a significant effect on the expression of Na⁺/H⁺-antiporter genes localized on the vacuolar (NHX1 to NHX3) and plasma membrane (SOS1). Melatonin effectively decreases the accumulation of lipid peroxidation products in potato leaves in the whole range of concentrations studied. A melatonin-induced dose-dependent increase in Fv/Fm together with a decrease in uncontrolled non-photochemical dissipation Y(NO) also indicates decreased oxidative damage. The observed protective ability of melatonin was unlikely due to its influence on antioxidant enzymes, since neither SOD nor peroxidase were activated by melatonin. Melatonin exerted positive effects on the accumulation of water-soluble low-molecular-weight antioxidants, proline and flavonoids, which could aid in decreasing oxidative stress. The most consistent positive effect was observed on the accumulation of carotenoids, which are well-known lipophilic antioxidants playing an important role in the protection of photosynthesis from oxidative damage. Finally, it is possible that melatonin accumulated during pretreatment could exert direct antioxidative effects due to the ROS scavenging activity of melatonin molecules.

Grape Berry Responses to Sequential Flooding and Heatwave Events: A Physiological, Transcriptional, and Metabolic Overview

Grapevine cultivation, such as the whole horticulture, is currently challenged by several factors, among which the extreme weather events occurring under the climate change scenario are the most relevant. Within this context, the present study aims at characterizing at the berry level the physiological response of Vitis vinifera cv. Sauvignon Blanc to sequential stresses simulated under a semi-controlled environment: flooding at bud-break followed by multiple summer stress (drought plus heatwave) occurring at pre-vèraison. Transcriptomic and metabolomic assessments were performed through RNASeq and NMR, respectively. A comprehensive hormone profiling was also carried out. Results pointed out a different response to the heatwave in the two situations. Flooding caused a developmental advance, determining a different physiological background in the berry, thus affecting its response to the summer stress at both transcriptional levels, with the upregulation of genes involved in oxidative stress responses, and metabolic level, with the increase in osmoprotectants, such as proline and other amino acids. In conclusion, sequential stress, including a flooding event at bud-break followed by a summer heatwave, may impact phenological development and berry ripening, with possible consequences on berry and wine quality. A berry physiological model is presented that may support the development of sustainable vineyard management solutions to improve the water use efficiency and adaptation capacity of actual viticultural systems to future scenarios.