Strategies of plants to overcome abiotic and biotic stresses

In their environment, plants are exposed to a multitude of abiotic and biotic stresses that differ in intensity, duration and severity. As sessile organisms, they cannot escape these stresses, but instead have developed strategies to overcome them or to compensate for the consequences of stress exposure. Defence can take place at different levels and the mechanisms involved are thought to differ in efficiency across these levels. To minimise metabolic constraints and to reduce the costs of stress defence, plants prioritise first‐line defence strategies in the apoplastic space, involving ascorbate, defensins and small peptides, as well as secondary metabolites, before cellular processes are affected. In addition, a large number of different symplastic mechanisms also provide efficient stress defence, including chemical antioxidants, antioxidative enzymes, secondary metabolites, defensins and other peptides as well as proteins. At both the symplastic and the apoplastic level of stress defence and compensation, a number of specialised transporters are thought to be involved in exchange across membranes that still have not been identified, and information on the regeneration of different defence compounds remains ambiguous. In addition, strategies to overcome and compensate for stress exposure operate not only at the cellular, but also at the organ and whole‐plant levels, including stomatal regulation, and hypersensitive and systemic responses to prevent or reduce the spread of stress impacts within the plant. Defence can also take place at the ecosystem level by root exudation of signalling molecules and the emission of volatile organic compounds, either directly or indirectly into the rhizosphere and/or the aboveground atmosphere. The mechanisms by which plants control the production of these compounds and that mediate perception of stressful conditions are still not fully understood. Here we summarise plant defence strategies from the cellular to ecosystem level, discuss their advantages and disadvantages for plant growth and development, elucidate the current state of research on the transport and regeneration capacity of defence metabolites, and outline insufficiently explored questions for further investigation.


I. INTRODUCTION
As a consequence of anthropogenic activities and climate change, plants increasingly encounter multiple abiotic and biotic stresses.Abiotic stresses include environmental factors, such as temperature extremes, drought, flooding, high light intensity, altered salinity as well as water, air and soil pollution, while plants experience biotic stresses from root and shoot pathogens, herbivores, and resource competition with other plants (Rennenberg & Simon, 2013).As sessile organisms, plants cannot simply move to a new location under adverse environmental conditions, but rather have to overcome abiotic and biotic stresses and/or compensate for the consequences of such exposure throughout their lifetime.Consequently, plants have evolved sophisticated strategies such as special morphological structures and/or (bio)-chemical mechanisms to counteract various stresses and ensure survival, growth and reproduction under these conditions.However, these defence mechanisms require investment of resources and energy that often result in a trade-off with growth and, hence, in fundamental ecological and economic consequences, particularly for crops (Rennenberg & Simon, 2013;He, Webster & He, 2022).For instance, the development of morphological defence structures, the production of defence compounds, and investment into additional reproductive structures under harsh environmental conditions require metabolites and energy that can no longer be used for growth, development and storage (Hartmann & Trumbore, 2016).In addition, essential nutrients such as nitrogen (N), phosphorus (P), and sulfur (S) are required for stress defence reactions (Malcheska et al., 2017;Batool et al., 2018;Lambers, 2022;Du et al., 2023).However, removal of resources for defence reactions may also stimulate photosynthesis and nutrient acquisition, thereby preventing a negative impact on growth and development (Matyssek, Lüttge & Rennenberg, 2013).
At the individual plant level, defence can take place in both the apoplastic and the symplastic space, not only in the organ that experiences stress but also in the whole plant or in the ecosystem (Takahashi & Shinozaki, 2019).Some stresses, such as temperature, may be perceived directly by whole cells, while others may affect particular cellular components, for example light initially affects chloroplasts.As part of the apoplastic space, the cell wall together with the plasma membrane forms a continuous plant surface that is in direct contact with the environment (Farvardin et al., 2020).Thus, the cell wall-plasma membrane is, in many cases, the first target of abiotic and biotic stress that can sense and react to structural and metabolic disturbances (Rui & Dinneny, 2020).To minimise metabolic constraints and to reduce the costs of stress defence, plants have developed mechanisms to overcome stresses in the apoplastic space, for instance, by scavenging reactive oxygen species (ROS) (Foyer, Kyndt & Hancock, 2020b) and by ion compartmentation (Wang et al., 2019), before they cross the plasma membrane and initiate symplastic responses (Farvardin et al., 2020).Irrespective of whether apoplastic or symplastic defence is involved, stress exposure also can induce systemic reactions in plant organs not exposed to that particular stress in order to prevent its spread or in preparation for future exposure (Du et al., 2018(Du et al., , 2021b;;Hu et al., 2021).As an extension of systemic reactions, stress responses can include interactions with unaffected plants, which can prevent the propagation of stress at the ecosystem level, via root exudation of signal metabolites and emission of volatile organic compounds (Catola et al., 2018;Bouwmeester et al., 2019;Wenig et al., 2019;Vives-Peris et al., 2020;Loreto & D'Auria, 2022).
In recent years, significant progress has been made in discovering many aspects of defence strategies used by plants to cope with abiotic and biotic stress.In this review, we summarise recent progress on different strategies of stress defence and compensation at the cellular, organ, whole plant and ecosystem levels including mechanisms in the apoplastic and the symplastic space.In addition, we discuss specific examples, highlight the advantages and Plant defence strategies disadvantages of each, and identify questions to be addressed by future research.

II. DEFENCE IN THE APOPLASTIC SPACE (1) Ascorbate and glutathione
As the most abundant water-soluble plant antioxidant and the only abundant antioxidant in the apoplastic space (Foyer et al., 2020b), ascorbate (ASC) constitutes an effective scavenger of ROS generated through non-enzymatic mechanisms and as by-products of enzymatic reactions by various abiotic stresses, e.g. from heavy metals, high light intensity, temperature extremes, drought, altered salinity, and air pollutants (Du et al., 2016(Du et al., , 2018(Du et al., , 2021a;;Podg orska, Burian & Szal, 2017;Foyer et al., 2020b).A growing number of studies has revealed that ASC is also involved in plant responses to biotic stress from pathogens, but the mechanisms involved are still ambiguous and controversial (Kerchev et al., 2013;Podg orska et al., 2017;Foyer et al., 2020b).For example, Singh et al. (2020) found that priming of tomato seeds with ASC effectively induced seed germination and elicited defence mechanisms to control wilt disease in plants.By contrast, ASC deficiency enhanced the resistance of Arabidopsis thaliana to Pseudomonas syringae (Pavet et al., 2005) and aphids (Kerchev et al., 2013).Further studies are required to characterise the significance of apoplastic ASC in stress defence.
The apoplastic ASC pool is highly oxidised, and its redox state depends on the cytoplasmic ASC-dehydroascorbate (DHA) redox system in the symplastic space (Vanacker, Carver & Foyer, 1998;Veljovic-Jovanovic et al., 2001;del Carmen C ordoba-Pedregosa et al., 2003;de Pinto & De Gara, 2004).In the cytoplasm, ASC concentration can be regulated by both de novo biosynthesis and regeneration through the Foyer-Halliwell-Asada cycle (ASC-glutathione cycle) catalysed by the enzymes ASC peroxidase (APx), monodehydroascorbate reductase (MDHAR), DHA reductase (DHAR) and glutathione reductase (GR) (Foyer et al., 2020b).This dual strategy enhances response flexibility to stress, but the regeneration of oxidised ASC produced in the apoplastic space requires DHA transport into the cytoplasm and, subsequently, transport of the regenerated ASC from the cytoplasm into the apoplastic space.Therefore, the apoplastic ASC-DHA redox state presumably also depends on the presence of specific transporters (Fig. 1) (Horemans, Foyer & Asard, 2000).However, characterisation of these transporters has so far not been achieved (Smirnoff, 2018;Foyer et al., 2020b).A potential, but still poorly understood regeneration pathway of apoplastic monodehydroascorbate (MDHA) may involve plasmalemmabound proteins, such as members of the cytochrome b561 (CYB561) and CYBDOM (CYB561 associated with a dopamine β-monooxygenase redox domain) families, located on the cell wall-facing surface of the plasma membrane (Horemans et al., 2003;Picco et al., 2015).ASC can autoxidise, generating superoxide radicals and its dismutation product H 2 O 2 , but only in the presence of catalytic metal ions and in a pH-dependent reaction (Smirnoff, 2018).Thus, autoxidation of ASC will require additional capacity for ROS scavenging.
In addition to ASC, glutathione (GSH), the long-distance transport form of reduced sulfur (Rennenberg, Schmitz & Bergmann, 1979;Köstner et al., 1998), is present in the apoplastic space, but in relatively low concentrations, partly due to the high degradative activity of γ-glutamyl transpeptidase 1 (GGT1) and GGT2 (Köstner et al., 1998;Ito & Ohkama-Ohtsu, 2023).GSH plays an essential role in signalling and activating plant defences against fungal pathogens, i.e. biotrophic fungi, rather than directly detoxifying H 2 O 2 (Zechmann, 2020).The regeneration of GSH oxidised in the apoplastic space requires transport of oxidised glutathione (GSSG) into the cytoplasm and, subsequently, the transport of GSH out of the cytoplasm, across the plasmalemma.Therefore, apoplastic GSH content depends on symplastic regeneration, synthesis, and transport (Fig. 1).However, little is known about the mechanisms and transporters involved in these processes (Oestreicher & Morgan, 2019;Ito & Ohkama-Ohtsu, 2023).This is surprising, since GSH transport across the plasmalemma has been reported for several decades from studies with plant tissue cultures, where GSH can be released into the culture medium and retrieved when other sulfur sources are extinguished (Bergmann & Rennenberg, 1978).Once GSSG is transported from the apoplastic space into the cytoplasm, it can be reduced to GSH by glutathione reductase (GR) in an NADPHdependent reaction (Foyer & Noctor, 2011) (Fig. 1).
Thus, the abundant ASC in the apoplastic space can counter ROS production by abiotic and biotic stress and other defence reactions can be initiated by GSH at low concentrations.However, the detoxification processes involved require additional transport systems and depend on symplastic regeneration.
(2) Defensins and related small peptides/proteins Plant defensins and defensin-like proteins are a large group of small cysteine-rich proteins generally abundant in seeds and all plant organs.Class I defensins are synthesised in the cytoplasm with an N-terminal signal sequence and a mature defensin domain (Parisi et al., 2019) of about 4-6 kDa.Together with other small proteins, e.g.lipid transfer proteins, thionins, glycine-rich peptides, cyclotides, systemin and protease inhibitors, they are constitutively expressed and partially allocated into the apoplastic space (Fig. 1) with a broad spectrum of protective activities against fungi, bacteria, viruses and insects (Dang & Van Damme, 2015;Parisi et al., 2019;Farvardin et al., 2020;Mammari et al., 2021).Several possible protective mechanisms have been proposed for plant defensins, e.g.participating in innate immunity, binding and interacting with negatively charged microbial cell membranes through their conserved domains (Sher Khan et al., 2019), and ROS accumulation and membrane permeabilisation in invading pathogen cells (Aerts et al., 2011;Hegedüs & Marx, 2013).Production of some DNA fragments, peptides and systemin are triggered by wounding and herbivory attack, and the release of these small molecules into the apoplastic space is considered to constitute endogenous secondary danger signals (Erb & Reymond, 2019;Tanaka & Heil, 2021).Defensins, together with other peptides/proteins, also constitute a novel class of messengers involved in cell-to-cell communication and long-range signalling (Parisi et al., 2019;Takahashi & Shinozaki, 2019;Farvardin et al., 2020).Huang et al. (2008) found that the small cysteine-rich protein defensin SPD 1 (GenBank accession no.AY552546) from roots of sweet potato (Ipomoea batatas) had the ability to convert DHA and MDHA to ASC in the presence of GSH and NADH (Fig. 1).Therefore, they seem to be involved in mediating plant responses to various abiotic stresses, e.g.drought, salt, nutrient deficiency (Takahashi & Shinozaki, 2019;Sin, Lam & Ngai, 2022) and air pollution by contributing to cellular redox homeostasis (Hamisch et al., 2012), but further investigations are needed to elucidate these functions (Fig. 1).
(3) Secondary compounds Plants synthesise a large diversity of secondary metabolites, facilitating defence against both biotic and abiotic stresses, such as microbial pathogens, herbivores and air pollution (Du et al., 2018;Cesarino, 2019;Erb & Kliebenstein, 2020).This function is not restricted to free phenolic compounds, but is also observed for polyphenols and volatile organic compounds (VOCs) (Brglez Mojzer et al., 2016;Sharma et al., 2019;Šimpraga et al., 2019;Wedow, Ainsworth & Li, 2021), e.g. for the phenolic polymers of sporopollenin and lignin.Sporopollenin is a ubiquitous and extremely chemically inert biopolymer that constitutes the outer wall of all land-plant spores and pollen grains (Li et al., 2019).Lignin is derived from oxidative coupling of three main units of monolignols and constitutes a major component of plant cell walls, which crosslinks with polysaccharides and hydroxycinnamic residues (Barnes & Anderson, 2018).These phenolic polymers constitute physical barriers, facilitate water transport and provide protection against various stresses including enzymatic attack (He, Dai & Wu, 2016;Oliver et al., 2020;Wolf, 2022).As the most abundant polyphenol in nature, lignin has shown promising antioxidant activities with its plentiful phenolic hydroxyl and unique lignin-carbohydrate linkages (Cesarino, 2019).Increased lignin, cutin and suberin contents and well developed Casparian bands have been shown to enhance the metal tolerance of plants (Liu et al., 2020b;Philippe et al., 2020;de Silva et al., 2021;Asare, Sz akov a & Tlustoš, 2023), as well as tolerance to other abiotic and biotic stresses (Chen et al., 2022).Xu et al. (2022) showed that the network of lignification, suberisation and Casparian strip formation is well regulated in the root endodermis.Also VOCs can act as anti-oxidants in the apoplastic space and particularly can mediate scavenging of oxidative air pollutants such as ozone or nitrogen dioxide before they can attack the symplastic space (Loreto & Velikova, 2001;Šimpraga et al., 2019;Wedow et al., 2021) (Fig. 1).
On the one hand, defence against biotic and abiotic stress provided by secondary metabolites may have relatively low costs due to their multi-functionality (Erb & Kliebenstein, 2020), on the other hand, it may be costly intensive if the polymerisation products of ROS scavenging cannot be regenerated.However, some secondary metabolites induced upon environmental stress can at least partially be recycled back into primary metabolism once the stress is removed (Erb & Kliebenstein, 2020;Wolf, 2022).This is unlikely to be the case for the majority of secondary metabolites in the apoplastic space which mainly include secondary cell wall constituents such as lignin and its metabolic precursors.Thus, stress-induced defence by secondary metabolites will interact with cell wall development (Wolf, 2022) and their synthesis and regeneration take place at the expense of allocation of resources to growth and development (Neilson et al., 2013).Still, counteracting biotic and abiotic stress by using compounds already in the apoplastic space may represent an efficient way to prevent cellular damage caused by stress exposure.
(4) Enzymatic activities Several enzymes are known to facilitate defence against both biotic and abiotic stress.One very prominent and large family is the apoplastic class III peroxidases (Prxs) which catalyse a wide variety of redox reactions via hydroxylic and peroxidative cycles (Kidwai, Ahmad & Chakrabarty, 2020;Farvardin et al., 2020).These proteins are produced within the cell, glycosylated during the endoplasmic reticulum (ER)-Golgi pathway and secreted into the apoplastic space using an N-terminal-located signal peptide (Passardi et al., 2007).Their functions differ among various members of the family; some seem to be multi-functional (Kidwai et al., 2020).Common roles include (re-)organisation of the cell wall structure and participation in hormone-controlled developmental processes (Francoz et al., 2015;Kidwai et al., 2020).Moreover, plant Prxs are involved in pathogen defence, and are described to be important for ozone and sulfite detoxification (Francoz et al., 2015;Lüthje & Martinez-Cortes, 2018;Baillie et al., 2019;Kidwai et al., 2020).Pfanz et al. (1990) demonstrated increased activity of apoplastic Prxs in sweet potato after fumigation with SO 2 .In A. thaliana, peroxidase 34 (At3G49120) showed the most significant transcriptional up-regulation in response to SO 2 (Hamisch et al., 2012).Baillie et al. (2019) proved that apoplastic Prx oxidise sulfite to sulfate, in the presence of phenolic compounds and hydrogen peroxide, thus acting as a first-line biochemical defence against toxic sulfite.Apoplastic and symplastic processes often act together in stress responses.For sulfite detoxification, in addition to apoplastic processes, the key enzyme sulfite oxidase in the symplastic space, is localised in the peroxisomes (Nowak et al., 2004).Prxs are abundant in all organs and almost all tissues, where they are involved in a broad range of physiological and developmental processes acting against both biotic and abiotic stress (Francoz et al., 2015).In addition, the plasma membrane-located NADPH oxidase is another recognised ROS producer and a key player in systemic ROS signalling involved in abiotic and biotic stress responses (Hu et al., 2020;Liu et al., 2020a;Pfeilmeier et al., 2021).Together with the plasma membrane-located NADPH oxidase, Prxs catalyse O 2 •− production in the apoplastic space, which is then further converted to H 2 O 2 by superoxide dismutase (SOD) (Fig. 1) (Kidwai et al., 2020;Farvardin et al., 2020).Pfeilmeier et al. (2021) recently found that NADPH oxidase is required for microbiota homeostasis in A. thaliana leaves.However, the functional characterisation as well as the molecular and genetic basis of peroxidase and NADPH oxidase responses to stress have still not been uncovered in detail (Hu et al., 2020;Kidwai et al., 2020).

III. DEFENCE IN THE SYMPLASTIC SPACE (1) Chemical antioxidants and antioxidative enzymes
If apoplastic strategies fail to overcome biotic or abiotic stresses completely, ROS generated from stress exposure in intracellular compartments, i.e. chloroplasts, mitochondria, peroxisomes and the cytosol, need to be detoxified by antioxidants to prevent potential damage caused by ROS accumulation (Dumanovi c et al., 2021).ASC and GSH, present in millimolar and micromolar concentrations, respectively (Du et al., 2016(Du et al., , 2021a)), are the most abundant intracellular antioxidants that scavenge ROS in chemical reactions.ROS scavenging by the Foyer-Halliwell-Asada cycle is particularly important when stomata are closed under stressed conditions, since the enzymatic reduction process depends upon the reducing power of NAD + and NADP + , and these pyridine nucleotides are at the junction of metabolism and ROS detoxification (Gakière et al., 2018).Over-reduction of the NADP + pool to NADPH in response to stress may cause either an electron flow under strong light or lack of availability of NADP + for metabolic reactions, and consequently accelerated ROS formation due to deflected electron transport (Scheibe et al., 2005;Gakière et al., 2018).
In plant cells, H 2 O 2 is efficiently removed without depleting the ASC and GSH pool.DHA and GSSG produced in these reaction can be regenerated by enzymatic reactions catalysed by GR and DHAR, respectively (Foyer et al., 2020b).In chloroplasts, mitochondria and the cytosol these reactions are also part of the Foyer-Haliwell-Asada cycle of ROS scavenging that additionally includes APx and MDHAR.
Among the four identified biosynthetic pathways of ASC synthesis, the D-mannose/L-galactose pathway is the most widespread and physiologically relevant pathway in higher plants (Foyer et al., 2020b).GDP-L-galactose phosphorylase, catalysing the first committed step of the D-mannose/ L-galactose pathway of ASC synthesis, requires inorganic phosphate and may be regulated by light (Foyer et al., 2020b).Therefore, ASC synthesis depends on sufficient phosphorus and light availability.As a consequence, ASC synthesis may be limited in plants growing on lowphosphorus and phosphorus-depleted soil.In addition, extensive intracellular and intercellular ASC transport is required, however, few transporters responsible for ASC transport have been identified (Foyer et al., 2020b).ASC can be synthesised in both green and non-green tissues.
The versatile tripeptide GSH can be synthesised in the cytosol and the chloroplast via the action of two ATPdependent enzymes, γ-glutamylcysteine synthetase (γ-ECS) and GSH synthase (GS), with a requirement for sulfur and nitrogen (Zechmann, 2020).GSH transporters are also needed to distribute GSH among all cell compartments, but little is known about the molecular identity of the transporters involved (Oestreicher & Morgan, 2019).Recently, a low GSH/GSSG ratio was shown to be adjusted by GSSG sequestration in the vacuole (Ito & Ohkama-Ohtsu, 2023) mediated by tonoplastic ATP-binding cassette (ABC) transporters (Halkier & Xu, 2022).
Subcellular ROS detoxification can further be achieved by enzymatic reactions catalysed by SOD, catalase (CAT), APx, GSH peroxidase (GPx), peroxiredoxins (Prrxs) and thioredoxins (Trx) (Dumanovi c et al., 2021) (Fig. 1).The advantage of these processes is the direct scavenging of ROS at the sites of generation, thereby preventing damage by immediate detoxification.On the other hand, they are costly in terms of resource use for metabolite and enzyme biosynthesis as well as energy consumption.Moreover, removal of ROS via these antioxidative processes has the potential to disturb cellular redox balance and hence impact the activity and function of proteins involved in essential metabolic reactions (Dumont & Rivoal, 2019;Foyer et al., 2020a) (2) Secondary metabolites Phenolic compounds are the most abundant secondary metabolites in plants with antimicrobial and antioxidant properties against a broad range of biotic and abiotic stresses (Erb & Kliebenstein, 2020;Dumanovi c et al., 2021) (Fig. 1).Moreover, a large number of phenolic compounds such as stilbenes, flavonoids, vanilloids, hydroxycinnamic acids, lignans, tannin and glucosinolates, are attractant or repellent to many organisms (Huang et al., 2020;Eugui et al., 2022).Alkaloids are one of the largest groups of secondary metabolites widely distributed in vascular plants against biotic (Thawabteh et al., 2019) and abiotic stress (Frick et al., 2018).Toxic cyanides and cyanogenic glucosides can defeat insect herbivores (Yadav, Singh & Singh, 2023).Also the non-proteinogenic sulfur-containing amino acids methiin, alliin, and isoalliin found in the genus Allium possess antioxidative and antibacterial activity (Gruhlke, 2019).Emerging evidence indicates that unsaturated fatty acids are not only constituents of membrane lipids, but also defence metabolites against various biotic and abiotic stresses, since they act as membrane modulators, oxylipin, jasmonate and nitro-alkene precursors, ROS scavengers, and molecular chaperones, as well as compatible solutes (He & Ding, 2020).
Plants have a high production capacity of secondary metabolites, predominantly of lignin precursors (Cesarino, 2019), and plant vacuoles are important sites of storage for various secondary metabolites and their precursors, such as phenolic compounds and alkaloids (Shitan & Yazaki, 2020), but also glucosinolates and cyanides, which can be released to the cytosol (Kuliahsari, Sari & Estiasih, 2021;Eugui et al., 2022;Xu et al., 2023) Plant defence strategies form of phenoxyl radicals can be reduced by ASC to avoid DNA damage (Holopainen et al., 2018), oxidation is mostly connected with polymerisation and many polymerised phenolic compounds are unlikely to be regenerated.As a consequence, the use of phenolic compounds to overcome biotic or abiotic stress can be more costly than the use of antioxidants such as ASC or GSH.
Defensins and related small proteins have a broad spectrum of biotic and abiotic stress defence, are produced by all plant species, and expressed in all tissues at every developmental stage (Shafee et al., 2017;Parisi et al., 2019).In general, plant defensins and related small proteins are non-toxic to plant cells.However, negative effects such as interference with cell wall biosynthesis, reduced cell growth and regeneration efficiency, inhibited fertility and abnormal morphology have been reported when defensins are continuously expressed (Parisi et al., 2019) or strongly over-expressed (Omidvar et al., 2021).The targets of some defensins are species and tissue specific (Parisi et al., 2019).Little is known about the recycling of the oxidised defensins, where such regeneration takes place and how they are transported intra-and extracellularly, particularly in woody plants (Wei et al., 2020).The activity and stability of defensins and related peptides can be impacted not only by some bacterial pathogens of vertebrates, but also by strains of the plant pathogen Sinorhizobium meliloti (Koprivnjak & Peschel, 2011;Price et al., 2015) indicating that the function of these peptides in compensating biotic stress can be circumvented.

(4) Defence proteins
Plants produce a wide variety of defence proteins against both abiotic and biotic stresses (Jain et al., 2022).Lectins were one of the first groups of proteins purified from plants and currently 12 plant lectin families have been identified, not only in seeds but also in other plant organs, where they bind carbohydrates in vacuoles (Dang & Van Damme, 2015).These proteins are involved in plant development, immunity, stress signalling and gene expression regulation in response to insect and pathogen attacks as well as various abiotic stresses (Dang & Van Damme, 2015;Jain et al., 2022).Another example of plant defence proteins is heat shock proteins (HSPs), which play a pivotal role in conferring biotic and abiotic stress tolerance (ul Haq et al., 2019).However, many features of plant HSPs are far from being understood, e.g.signal sensing and regulation, the interaction of structure and function in their mechanism of action and the localisation of HSPs in plant cells (ul Haq et al., 2019).Many other proteins, e.g. late embryogenesis abundant (LEA) proteins, storage proteins, tuber proteins, and osmotin take part in defence mechanisms (Anil Kumar et al., 2015;Oliver et al., 2020;Chowdhary & Tank, 2022).More recently, Medina-Puche et al. (2020) found a viral protein that moved from the plasma membrane to chloroplasts upon plant sensing of biotic threats, suggesting a protein-mediated regulatory defence pathway linking the plasma membrane and chloroplasts.Moreover, protein-protein interactions such as folding, stabilisation, degradation and activation play important roles in ROS scavenging within diverse metabolic and physiological processes, but are understudied (Melicher et al., 2022).
Plant defence proteins can cope with both biotic and abiotic stresses with high specificity.For instance, some lectins possess specific toxicity to insect species (Jain et al., 2022) and HSPs act as molecular chaperones in a tissue-and genotype-specific way (ul Haq et al., 2019).Nosenko et al. (2021) recently highlighted the potential of using particular stress-induced plant defence proteins as potential markers for early detection of forest damage and diseases.For plants, the disadvantages of defence proteins are their high costs of biosynthesis and their potential toxicity to the host plant.For example, lectins and ribosome-inactivating proteins are toxic to the producing cells of the host plant and their synthesis requires strong regulatory restriction to stress events and cellular compartmentation (Jain et al., 2022).In addition, protein synthesis is one of the most expensive cellular process and is strictly regulated even under stressful conditions (Nelson & Millar, 2015).

IV. DEFENCE AT ORGAN AND WHOLE-PLANT LEVEL
(1) Stomatal closure in response to environmental constraints Exposure of plants to heavy metals, air pollution and drought frequently results in stomatal closure as a mechanism of defence that reduces pollution influx from the atmosphere into the leaves, water vapour efflux from the leaves into the atmosphere, and heavy metal allocation from the roots to the leaves (Heath, 1994;Malcheska et al., 2017;Paoletti et al., 2021;Guo et al., 2023).In addition, emerging evidence shows that the stomatal aperture also participates in interactions between plants and herbivores (Lin et al., 2022).However, this response to stress has whole-plant implications, because it also reduces CO 2 influx into the leaves and, hence, photosynthetic CO 2 fixation and carbohydrate allocation to the roots (Arab et al., 2022).Thus, stomatal closure in response to environmental constraints will always involve a trade-off between stress defence and growth.The mechanism of stomatal closure has received particular attention in response to drought, since roots rather than leaves constitute the initial site of low water availability.Therefore, this environmental constraint frequently experienced by plants must be signalled from the roots to the leaves to induce stomatal closure before wilting.Recent investigations showed that xylem-derived sulfate was a chemical signal of drought that induces stomatal closure by tuning abscisic acid (ABA) biosynthesis in leaves (Malcheska et al., 2017;Batool et al., 2018).
(2) Hypersensitive responses The hypersensitive response constitutes an extremely effective immune system component to prevent further spreading of biotic stress caused by biotrophic pathogens.It is a remarkably widespread phenomenon across the plant kingdom and is characterised by rapid localised cell death in and around the initial area of pathogen infection, including areas damaged by viruses, fungi, insects and nematodes (Balint-Kurti, 2019).The localised response can send signals to distal parts of the plant to activate systemic defences for subsequent attacks (Fu & Dong, 2013).Understanding of the mechanisms behind this effector-triggered programmed cell death still remains incomplete, but it is probably regulated by proteases, ion fluxes, ROS and nitric oxide (NO) in different cell compartments (Dalio et al., 2021) (Fig. 2).
The hypersensitive response is tightly regulated due to its high cost and is suppressed under non-disease conditions as well as when biotic stress is relieved to prevent further spontaneous cell death (Nogueira Júnior et al., 2020).In addition, it is temperature sensitive and light dependent (Negeri et al., 2013;Lukan et al., 2018).Once a hypersensitive response takes place, it determines pathogen susceptibility, but often mediates a decline of photosynthesis at the directly injured spot and, hence, growth retardation (Balint-Kurti, 2019;Dalio et al., 2021) even though compensatory responses from neighbouring zones and the rest of the leaf through increased photosynthesis may occur (Moustaka, Meyling & Hauser, 2021).In some cases, it can be less effective and may be beneficial to the host only in early plantpathogen interaction (Balint-Kurti, 2019).
Systemic responses initiate defence strategies as a precaution in uninfected plant tissues, and this can be rapid (within minutes; Zandalinas et al., 2020), and may play a crucial role in acclimation to, and subsequent survival in, harsh environmental conditions (Takahashi & Shinozaki, 2019).Long-range responses require a parallel machinery for signalling, exchanging and integrating activities across the entire plant body (Fichman & Mittler, 2020) and may lead to unnecessarily high costs in the case of successful local defence or when the stress is removed.Emissions of VOCs by the leaves can constitute a systemic response to oxidative pollutants in the atmosphere ( Šimpraga et al., 2019).VOCs not oxidised in the apoplastic space and emitted by leaves will react with oxidative pollutants in the atmosphere such as ozone or nitrogen oxides, thereby reducing the interaction of these pollutants with other plant organs ( Šimpraga et al., 2019;Wedow et al., 2021).

V. DEFENCE AT ECOSYSTEM LEVEL
(1) Root exudation of signalling compounds that counteract pathogens Plants release a variety of biologically active compounds into the rhizosphere to counteract stresses either directly or indirectly through the rhizosphere microbiome (Vives-Peris et al., 2020).Although up to 40% of photosynthesis-derived carbon can be released in the form of exudates, lysates, mucilage, secretions, dead cell material, and respiratory CO 2 (Lynch & Whipps, 1990), the significance of root exudates as belowground defence substances has long been underestimated (Canarini et al., 2019).Several mechanisms are known, including solubilising nutrients, chelating toxic compounds, attracting beneficial microbiota, and releasing substances toxic to pathogens, nematodes and root-feeding arthropods (Canarini et al., 2019;Vives-Peris et al., 2020).Root exudates can be constitutive and inducible; for instance, constitutive antimicrobial compounds such as phytoanticipins (mainly including saponins, avenacin, and tomatine) are synthesised before infection or produced from preformed precursors, whereas, inducible antimicrobial phytoalexins are not detectable in healthy plants, but are produced only in response to biotic and abiotic stresses (Tiku, 2020).Recently, Li et al. (2023) found that root-secreted (−)-loliolide can modulate both belowground defence and aboveground flowering in Arabidopsis and tobacco (Nicotiana benthamiana).Wen et al. (2021) also found that root exudated long chain fatty acids and amino acids induce systemic resistance against foliar pathogen attack and facilitate recruitment of beneficial microbes (Fig. 2).
However, root exudation clearly represents a considerable carbon loss to the plant (Canarini et al., 2019).It is a complex process encompassing several steps, e.g.long-distance transport of metabolites from leaf sources to root sinks in the phloem, including loading and unloading, and transport across the plasma membrane from the root symplast to the root apoplast by specific transmembrane proteins and diffusion (Canarini et al., 2019;Vives-Peris et al., 2020).However, many of these aspects have not been extensively studied.In addition, the control of root exudation and the circumstances under which plants benefit from exudation remain unclear (Canarini et al., 2019).
(2) Communication within the ecosystem through volatile organic compounds Plants produce and emit a variety of VOCs, such as terpenoids, benzenoids and phenylpropanoids, nitrogen-and sulfur-containing compounds, and fatty acid and amino acid derivatives, which play significant roles in plant environmental adaptation and survival (Bouwmeester et al., 2019;Brosset & Blande, 2022) (Fig. 2).Both constitutive and induced VOCs can repel herbivores and pathogens and/or attract natural enemies of herbivores (Bouwmeester et al., 2019;Loreto & D'Auria, 2022).For instance, higher constitutive monoterpene concentrations and rapidly induced specific monoterpenes are associated with the host's resistance to spruce beetle colonisation (Ott, Davis & Mercado, 2021).VOCs also mediate plant tolerance to various abiotic stresses (Bouwmeester et al., 2019;Duan et al., 2019;Werner et al., 2020;Paoletti et al., 2021), and promote the acquisition of beneficial bacteria in the rhizosphere (Liu & Brettell, 2019).Pollastri et al. (2023) recently found that gene expression and metabolic regulation of isoprene biosynthesis may be involved in the activation of general stress defence mechanisms in Nicotiana tabacum.
VOCs emitted from attacked/stressed plants can be detected and used as a warning signal by non-attacked/nonstressed plants (Loreto & D'Auria, 2022) as observed in tomato (Solanum lycopersicum) (Catola et al., 2018) against aphid attacks under water-limited conditions, and tea (Camellia sinensis) plants against drought (Adedeji & Babalola, 2020).Moreover, volatiles can be used to attract natural predators of herbivores (Catola et al., 2018;Bouwmeester et al., 2019).Volatilemediated plant-plant interactions can occur both above and below ground (Bouwmeester et al., 2019;Loreto & D'Auria, 2022), and either actively or passively depend on whether a physiological change in receiver plants is needed or not (Brosset & Blande, 2022).However, enhanced investment into de novo synthesis of VOCs under harsh conditions will result in reduced availability of secondary compounds for other defence reactions and may shift plants into a negative carbon balance (Werner et al., 2020).Storage of VOCs requires highly localised partial cell wall lysis, cell wall expansion, and deposition of cell wall material with low VOC permeability (Tissier, Morgan & Dudareva, 2017).For emission, these stored VOCs must cross any membrane(s), the aqueous cell wall, and sometimes the cuticle, before moving into the gas phase (Loreto & D'Auria, 2022).Increased emission of VOCs such as isoprene and monoterpenes can increase O 3 pollution due to the reaction of peroxy radicals with nitric oxide, and consequently impact atmospheric pollution (Wedow et al., 2021).

VI. CONCLUSIONS
(1) This review summarises current knowledge on plant defence strategies at different levels: extracellular (apoplast space), intracellular (symplast space), organ, whole plant and ecosystem.To minimise the costs of defence and metabolic disturbance, plants are likely under selection to overcome stresses initially in the apoplastic space before cellular processes are affected.
(2) We discuss the advantages and disadvantages of different defence strategies in terms of regeneration of defence compounds, inter-and intracellular as well as long-distance transport, defence costs and stress compensation.
(3) Outstanding questions that require further investigation are highlighted throughout this review, including the regeneration, exchange and transport of defence compounds, the crosstalk between cellular, organ and whole-plant defence strategies, as well as the release and perception mechanisms of signalling compounds in response to stress.Integrative multi-omics approaches could contribute to a more precise picture of plant defence strategies.(4) In addition to understanding how defence strategies operate at the different spatial levels, more attention should be given to differences in responses between acute and long-term stress exposure as well as to plant responses to the combined (a) biotic stresses that frequently occur in nature.

Fig. 2 .
Fig. 2. Plant defences against biotic and abiotic stresses at organ, whole-plant and ecosystem levels.GSNO, S-nitrosoglutathione; NO, nitric oxide; NOx, nitric oxides; ROS, reactive oxygen species; VOCs, volatile organic compounds.Photograph of hypersensitive response to fungal infection on a mountain maple (Acer spicatum) leaf was provided by Birgitta Rennenberg.
. Although the oxidised Biological Reviews 99 (2024) 1524-1536 © 2024 The Authors.Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.