Transcriptional and metabolic changes in the desiccation tolerant plant Craterostigma plantagineum during recurrent exposures to dehydration

Specific metabolic and cellular mechanisms of the vegetative desiccation tolerance in resurrection plants for adaptation to extreme dryness

MAIN CONCLUSION

Substantial advancements have been made in our comprehension of vegetative desiccation tolerance in resurrection plants, and further research is still warranted to elucidate the mechanisms governing distinct cellular adaptations. Resurrection plants are commonly referred to as a small group of extremophile vascular plants that exhibit vegetative desiccation tolerance (VDT), meaning that their vegetative tissues can survive extreme drought stress (> 90% water loss) and subsequently recover rapidly upon rehydration. In contrast to most vascular plants, which typically employ water-saving strategies to resist partial water loss and optimize water absorption and utilization to a limited extent under moderate drought stress, ultimately succumbing to cell death when confronted with severe and extreme drought conditions, resurrection plants have evolved unique mechanisms of VDT, enabling them to maintain viability even in the absence of water for extended periods, permitting them to rejuvenate without harm upon water contact. Understanding the mechanisms associated with VDT in resurrection plants holds the promise of expanding our understanding of how plants adapt to exceedingly arid environments, a phenomenon increasingly prevalent due to global warming. This review offers an updated and comprehensive overview of recent advances in VDT within resurrection plants, with particular emphasis on elucidating the metabolic and cellular adaptations during desiccation, including the intricate processes of cell wall folding and the prevention of cell death. Furthermore, this review highlights existing unanswered questions in the field, suggests potential avenues for further research to gain deeper insights into the remarkable VDT adaptations observed in resurrection plants, and highlights the potential application of VDT-derived techniques in crop breeding to enhance tolerance to extreme drought stress.

Ectopic over-expression of HaFT-1, a 14-3-3 protein from Haloxylon ammodendron, enhances acquired thermotolerance in transgenic Arabidopsis

Haloxylon ammodendron, an important shrub utilized for afforestation in desert areas, can withstand harsh ecological conditions such as drought, high salt and extreme heat. A better understanding of the stress adaptation mechanisms of H. ammodendron is vital for ecological improvement in desert areas. In this study, the role of the H. ammodendron 14-3-3 protein HaFT-1 in thermotolerance was investigated. qRT-PCR analysis showed that heat stress (HS) priming (the first HS) enhanced the expression of HaFT-1 during the second HS and subsequent recovery phase. The subcellular localization of YFP-HaFT-1 fusion protein was mainly detected in cytoplasm. HaFT-1 overexpression increased the germination rate of transgenic Arabidopsis seeds, and the survival rate of HaFT-1 overexpression seedlings was higher than that of wild-type (WT) Arabidopsis after priming-and-triggering and non-primed control treatments. Cell death staining showed that HaFT-1 overexpression lines exhibited significantly reduced cell death during HS compared to WT. Transcriptome analysis showed that genes associated with energy generation, protein metabolism, proline metabolism, autophagy, chlorophyll metabolism and reactive oxygen species (ROS) scavenging were important to the thermotolerance of HS-primed HaFT-1 transgenic plants. Growth physiology analysis indicated that priming-and-triggering treatment of Arabidopsis seedlings overexpressing HaFT-1 increased proline content and strengthened ROS scavenging activity. These results demonstrated that overexpression of HaFT-1 increased not only HS priming but also tolerance to the second HS of transgenic Arabidopsis, suggesting that HaFT-1 is a positive regulator in acquired thermotolerance.

Drought Stress Priming Improved the Drought Tolerance of Soybean

The capability of a plant to protect itself from stress-related damages is termed "adaptability" and the phenomenon of showing better performance in subsequent stress is termed "stress memory". While drought is one of the most serious disasters to result from climate change, the current understanding of drought stress priming in soybean is still inadequate for effective crop improvement. To fill this gap, in this study, the drought memory response was evaluated in cultivated soybean (Glycine max). To determine if a priming stress prior to a drought stress would be beneficial to the survival of soybean, plants were divided into three treatment groups: the unprimed group receiving one cycle of stress (1S), the primed group receiving two cycles of stress (2S), and the unstressed control group not subjected to any stress (US). When compared with the unprimed plants, priming led to a reduction of drought stress index (DSI) by 3, resulting in more than 14% increase in surviving leaves, more than 13% increase in leaf water content, slight increase in shoot water content and a slower rate of loss of water from the detached leaves. Primed plants had less than 60% the transpiration rate and stomatal conductance compared to the unprimed plants, accompanied by a slight drop in photosynthesis rate, and about a 30% increase in water usage efficiency (WUE). Priming also increased the root-to-shoot ratio, potentially improving water uptake. Selected genes encoding late embryogenesis abundant (LEA) proteins and MYB, NAC and PP2C domain-containing transcription factors were shown to be highly induced in primed plants compared to the unprimed group. In conclusion, priming significantly improved the drought stress response in soybean during recurrent drought, partially through the maintenance of water status and stronger expression of stress related genes. In sum, we have identified key physiological parameters for soybean which may be used as indicators for future genetic study to identify the genetic element controlling the drought stress priming.

Direct evidence of drought stress memory in mulberry from a physiological perspective: Antioxidative, osmotic and phytohormonal regulations

Drought stress commonly happens more than once during the life cycle of perennial trees. Stress memory endows better capacity to cope with repeated stresses for plants, while the underlying mechanisms are not fully elucidated. In this study, 2-month-old saplings of two mulberry cultivars (Husang32 and 7307 of Morus multicaulis) with or without an early soil water deficit were subjected to subsequent drought for 9 days. The shoot height growth, biomass production, stable carbon isotope discrimination, phytohormones, reactive oxygen species (ROS), osmotic substances and antioxidant enzymes were analyzed after the first and the second drought, respectively. Drought priming saplings sustained comparable or slightly higher biomass accumulation under the second drought than those non-priming. They also exhibited decreased levels of soluble sugars, free proline and soluble proteins, lower accumulation of malonaldehyde (MDA) and superoxide anion (O₂^•-), reduced activities of superoxide dismutase (SOD) and peroxidase (POD) compared to non-priming plants. Moreover, cultivar Husang32 exhibited elevated abscisic acid (ABA) and jasmonic acid (JA) where 7307 displayed opposite changes. PCA suggests that MDA, H₂O₂, free proline, SOD and POD in roots, and ROS, soluble sugars and glutamate reductase in leaves are dominant factors influenced by stress memory. ABA and JA in leaves also play important roles in exerting drought imprints. Collectively, stress memory can confer mulberry resistance to recurrent drought via combined regulations of antioxidative protection, osmotic adjustment and phytohormonal responses.

How do plants remember drought?

Plants develop both short-term and transgenerational memory of drought stress through epigenetic regulation of transcription for a better response to subsequent exposure. Recurrent spells of droughts are more common than a single drought, with intermittent moist recovery intervals. While the detrimental effects of the first drought on plant structure and physiology are unavoidable, if survived, plants can memorize the first drought to present a more robust response to the following droughts. This includes a partial stomatal opening in the watered recovery interval, higher levels of osmoprotectants and ABA, and attenuation of photosynthesis in the subsequent exposure. Short-term drought memory is regulated by ABA and other phytohormone signaling with transcriptional memory behavior in various genes. High levels of methylated histones are deposited at the drought-tolerance genes. During the recovery interval, the RNA polymerase is stalled to be activated by a pause-breaking factor in the subsequent drought. Drought leads to DNA demethylation near drought-response genes, with genetic control of the process. Progenies of the drought-exposed plants can better adapt to drought owing to the inheritance of particular methylation patterns. However, a prolonged watered recovery interval leads to loss of drought memory, mediated by certain demethylases and chromatin accessibility factors. Small RNAs act as critical regulators of drought memory by altering transcript levels of drought-responsive target genes. Further studies in the future will throw more light on the genetic control of drought memory and the interplay of genetic and epigenetic factors in its inheritance. Plants from extreme environments can give queues to understanding robust memory responses at the ecosystem level.

Stress memory responses and seed priming correlate with drought tolerance in plants: an overview

Environmental-friendly techniques based on plant stress memory, cross-stress tolerance, and seed priming help sustainable agriculture by mitigating negative effects of dehydration stress. The frequently uneven rainfall distribution caused by global warming will lead to more irregular and multiple abiotic stresses, such as heat stress, dehydration stress, cold stress or the combination of these stresses. Dehydration stress is one of the major environmental factors affecting the survival rate and productivity of plants. Hence, there is an urgent need to develop improved resilient varieties. Presently, technologies based on plant stress memory, cross-stress tolerance and priming of seeds represent fruitful and promising areas of future research and applied agricultural science. In this review, we will provide an overview of plant drought stress memory from physiological, biochemical, molecular and epigenetic perspectives. Drought priming-induced cross-stress tolerance to cold and heat stress will be discussed and the application of seed priming will be illustrated for different species.

Profiling of phenolic compounds in desiccation-tolerant and non-desiccation-tolerant Linderniaceae

INTRODUCTION

Craterostigma plantagineum and Lindernia brevidens are resurrection plants, so these plants can tolerate desiccation of their vegetative tissues. Different components and mechanisms contribute to desiccation tolerance and secondary plant metabolites, like phenolic compounds, may play a role during these processes.

OBJECTIVES

Secondary plant metabolites of the two resurrection plants, C. plantagineum and L. brevidens as well as the closely related desiccation sensitive species, L. subracemosa, were investigated regarding the polyphenol profile.

MATERIAL AND METHODS

Secondary plant compounds were extracted with acidified methanol and analysed with ultra-high-performance liquid chromatography electrospray ionisation mass spectrometry (UHPLC-ESI-MS). Phenolic compounds were identified by comparing of ultraviolet (UV) and MSⁿ -spectra with published data. All compounds were quantified as verbascoside equivalents by external calibration at the compound specific wavelength.

RESULTS

In total, eight compounds that belong to the subclass of phenylethanoid glycosides and one flavone, luteolin hexoside pentoside, were identified. Two of these compounds exhibited a fragmentation pattern, which is closely related to phenylethanoid glycosides. The predominantly synthesised phenylethanoid in all of the three plant species and in every stage of hydration was verbascoside. The total content of phenolic compounds during the three stages of hydration, untreated, desiccated, and rehydrated revealed differences especially between C. plantagineum and L. brevidens as the latter one lost almost all phenolic compounds during rehydration.

CONCLUSION

The amount of verbascoside correlates with the degree of desiccation tolerance and verbascoside might play a role in the protective system in acting as an antioxidant.

Seed 'primeomics': plants memorize their germination under stress

Seed priming is a pre-germination treatment administered through various chemical, physical and biological agents, which induce mild stress during the early phases of germination. Priming facilitates synchronized seed germination, better seedling establishment, improved plant growth and enhanced yield, especially in stressful environments. In parallel, the phenomenon of 'stress memory' in which exposure to a sub-lethal stress leads to better responses to future or recurring lethal stresses has gained widespread attention in recent years. The versatility and realistic yield gains associated with seed priming and its connection with stress memory make a critical examination useful for the design of robust approaches for maximizing future yield gains. Herein, a literature review identified selenium, salicylic acid, poly-ethylene glycol, CaCl₂ and thiourea as the seed priming agents (SPRs) for which the most studies have been carried out. The average priming duration for SPRs generally ranged from 2 to 48 h, i.e. during phase I/II of germination. The major signalling events for regulating early seed germination, including the DOG1 (delay of germination 1)-abscisic acid (ABA)-heme regulatory module, ABA-gibberellic acid antagonism and nucleus-organelle communication are detailed. We propose that both seed priming and stress memory invoke a 'bet-hedging' strategy in plants, wherein their growth under optimal conditions is compromised in exchange for better growth under stressful conditions. The molecular basis of stress memory is explained at the level of chromatin reorganization, alternative transcript splicing, metabolite accumulation and autophagy. This provides a useful framework to study similar mechanisms operating during seed priming. In addition, we highlight the potential for merging findings on seed priming with those of stress memory, with the dual benefit of advancing fundamental research and boosting crop productivity. Finally, a roadmap for future work, entailing identification of SPR-responsive varieties and the development of dual/multiple-benefit SPRs, is proposed for enhancing SPR-mediated agricultural productivity worldwide.

Unexplored dimensions of variability in vegetative desiccation tolerance

Desiccation tolerance has evolved recurrently across diverse land plant lineages as an adaptation for survival in regions where seasonal rainfall drives periodic drying of vegetative tissues. Growing interest in this phenomenon has fueled recent physiological, biochemical, and genomic insights into the mechanistic basis of desiccation tolerance. Although, desiccation tolerance is often viewed as binary and monolithic, substantial variation exists in the phenotype and underlying mechanisms across diverse lineages, heterogeneous populations, and throughout the development of individual plants. Most studies have focused on conserved responses in a subset desiccation-tolerant plants under laboratory conditions. Consequently, the variability and natural diversity of desiccation-tolerant phenotypes remains largely uncharacterized. Here, we discuss the natural variation in desiccation tolerance and argue that leveraging this diversity can improve our mechanistic understanding of desiccation tolerance. We summarize information collected from ~600 desiccation-tolerant land plants and discuss the taxonomic distribution and physiology of desiccation responses. We point out the need to quantify natural diversity of desiccation tolerance on three scales: variation across divergent lineages, intraspecific variation across populations, and variation across tissues and life stages of an individual plant. We conclude that this variability should be accounted for in experimental designs and can be leveraged for deeper insights into the intricacies of desiccation tolerance.

Abiotic stress signalling in extremophile land plants

Plant life relies on complex arrays of environmental stress sensing and signalling mechanisms. Extremophile plants develop and grow in harsh environments with extremes of cold, heat, drought, desiccation, or salinity, which have resulted in original adaptations. In accordance with their polyphyletic origins, extremophile plants likely possess core mechanisms of plant abiotic stress signalling. However, novel properties or regulations may have emerged in the context of extremophile adaptations. Comparative omics of extremophile genetic models, such as Arabidopsis lyrata, Craterostigma plantagineum, Eutrema salsugineum, and Physcomitrella patens, reveal diverse strategies of sensing and signalling that lead to a general improvement in abiotic stress responses. Current research points to putative differences of sensing and emphasizes significant modifications of regulatory mechanisms, at the level of secondary messengers (Ca2+, phospholipids, reactive oxygen species), signal transduction (intracellular sensors, protein kinases, transcription factors, ubiquitin-mediated proteolysis) or signalling crosstalk. Involvement of hormone signalling, especially ABA signalling, cell homeostasis surveillance, and epigenetic mechanisms, also shows that large-scale gene regulation, whole-plant integration, and probably stress memory are important features of adaptation to extreme conditions. This evolutionary and functional plasticity of signalling systems in extremophile plants may have important implications for plant biotechnology, crop improvement, and ecological risk assessment under conditions of climate change.

Physiological and PIP Transcriptional Responses to Progressive Soil Water Deficit in Three Mulberry Cultivars

Although mulberry cultivars Wubu, Yu711, and 7307 display distinct anatomical, morphological, and agronomic characteristics under natural conditions, it remains unclear if they differ in drought tolerance. To address this question and elucidate the underlying regulatory mechanisms at the whole-plant level, 2-month old saplings of the three mulberry cultivars were exposed to progressive soil water deficit for 5 days. The physiological responses and transcriptional changes of PIPs in different plant tissues were analyzed. Drought stress led to reduced leaf relative water content (RWC) and tissue water contents, differentially expressed PIPs, decreased chlorophyll and starch, increased soluble sugars and free proline, and enhanced activities of antioxidant enzymes in all plant parts of the three cultivars. Concentrations of hydrogen peroxide (H2O2), superoxide anion (O2 •-), and malonaldehyde (MDA) were significantly declined in roots, stimulated in leaves but unaltered in wood and bark. In contrast, except the roots of 7307, soluble proteins were repressed in roots and leaves but induced in wood and bark of the three cultivars in response to progressive water deficit. These results revealed tissue-specific drought stress responses in mulberry. Comparing to cultivar Yu711 and 7307, Wubu showed generally slighter changes in leaf RWC and tissue water contents at day 2, corresponding well to the steady PIP transcript levels, foliar concentrations of chlorophyll, O2 •-, MDA, and free proline. At day 5, Wubu sustained higher tissue water contents in green tissues, displayed stronger responsiveness of PIP transcription, lower concentrations of soluble sugars and starch, lower foliar MDA, higher proline and soluble proteins, higher ROS accumulation and enhanced activities of several antioxidant enzymes. Our results indicate that whole-plant level responses of PIP transcription, osmoregulation through proline and soluble proteins and antioxidative protection are important mechanisms for mulberry to cope with drought stress. These traits play significant roles in conferring the relatively higher drought tolerance of cultivar Wubu and could be potentially useful for future mulberry improvement programmes.

Desiccation Tolerance: Avoiding Cellular Damage During Drying and Rehydration

Desiccation of plants is often lethal but is tolerated by the majority of seeds and by vegetative tissues of only a small number of land plants. Desiccation tolerance is an ancient trait, lost from vegetative tissues following the appearance of tracheids but reappearing in several lineages when selection pressures favored its evolution. Cells of all desiccation-tolerant plants and seeds must possess a core set of mechanisms to protect them from desiccation- and rehydration-induced damage. This review explores how desiccation generates cell damage and how tolerant cells assuage the complex array of mechanical, structural, metabolic, and chemical stresses and survive.Likewise, the stress of rehydration requires appropriate mitigating cellular responses. We also explore what comparative genomics, both structural and responsive, have added to our understanding of cellular protection mechanisms induced by desiccation, and how vegetative desiccation tolerance circumvents destructive, stress-induced cell senescence.

Common and Specific Mechanisms of Desiccation Tolerance in Two Gesneriaceae Resurrection Plants. Multiomics Evidences

Environmental stress, especially water deficiency, seriously limits plant distribution and crop production worldwide. A small group of vascular angiosperm plants termed "resurrection plants," possess desiccation tolerance (DT) to withstand dehydration and to recover fully upon rehydration. In recent years, with the rapid development of life science in plants different omics technologies have been widely applied in resurrection plants to study DT. Boea hygrometrica is native in East and Southeast Asia, and Haberlea rhodopensis is endemic to the Balkans in Europe. They are both resurrection pants from Gesneriaceae family. This paper reviews recent advances in transcriptome and metabolome, and discusses the differences and similarities of DT features between both species. Finally, we believe we provide novel insights into understanding the mechanisms underlying the acquisition and evolution of desiccation tolerance of the resurrection plants that could substantially contribute to develop new approaches for agriculture to overcome water deficiency in future.