Roles of lipids as signaling molecules and mitigators during stress response in plants

Authors


Summary

Lipids are the major constituents of biological membranes that can sense extracellular conditions. Lipid-mediated signaling occurs in response to various environmental stresses, such as temperature change, salinity, drought and pathogen attack. Lysophospholipid, fatty acid, phosphatidic acid, diacylglycerol, inositol phosphate, oxylipins, sphingolipid, and N–acylethanolamine have all been proposed to function as signaling lipids. Studies on these stress-inducible lipid species have demonstrated that each lipid class has specific biological relevance, biosynthetic mechanisms and signaling cascades, which activate defense reactions at the transcriptional level. In addition to their roles in signaling, lipids also function as stress mitigators to reduce the intensity of stressors. To mitigate particular stresses, enhanced syntheses of unique lipids that accumulate in trace quantities under normal growth conditions are often observed under stressed conditions. The accumulation of oligogalactolipids and glucuronosyldiacylglycerol has recently been found to mitigate freezing and nutrition-depletion stresses, respectively, during lipid remodeling. In addition, wax, cutin and suberin, which are not constituents of the lipid bilayer, but are components derived from lipids, contribute to the reduction of drought stress and tissue injury. These features indicate that lipid-mediated defenses against environmental stress contributes to plant survival.

Introduction

Plants are constantly exposed to stresses posed by unfavorable surroundings, including wounding, pathogen attack, drought, cold, salinity, excessive light exposure and nutritional limitation. To survive in these circumstances, plants must recognize environmental stresses and rapidly exert multiple defense reactions. At the same time, they also have to mitigate the cellular damage caused by the stresses. Lipids are major and vital cellular constituents because they provide the structural basis for cell membranes and provide an energy stock for metabolism. In recent years, increasing evidence has indicated that lipids are involved not only in the initiation of defense reactions, as signal mediators (Wang, 2004; Wang et al., 2006; Munnik and Testerink, 2009), but also in the mitigation processes in response to stress in plant cells (Gaude et al., 2008; Nakamura et al., 2009; Moellering et al., 2010; Gasulla et al., 2013; Okazaki et al., 2013b). The current changes in the global climate significantly affect agronomy (Karl et al., 2009), and thus studies on the stress responses of plants mediated by lipids are more important than ever.

Signaling lipids include a wide range of lipid classes, such as lysophospholipid, fatty acid, phosphatidic acid, inositol phosphate, diacylglycerol, oxylipin, sphingolipid and N–acylethanolamine (Figure 1; Table 1; Wang, 2004; Wang et al., 2006; Kang et al., 2008; Munnik and Testerink, 2009; Kilaru et al., 2011; Markham et al., 2013). These lipids are usually present in small quantities in tissues, and are quickly synthesized from pre-existing membrane lipids or biosynthetic intermediates of membrane lipids. During this process, lipid-hydrolytic enzymes such as phospholipases and esterase are critically important (Wang, 2004; Wang et al., 2006; Munnik and Testerink, 2009). In plants, there are many types of lipases and esterases with different substrate preferences, distributions and modes of induction following stress treatment. There are also many genes encoding lipoxygenases and kinases that use lipids as substrates (Li-Beisson et al., 2013), suggesting a highly complex and flexible lipid-mediated signaling system that enables adaptation to various types of environmental stresses is present in plants. In addition to their roles in signaling, lipids also play a role as mitigators to reduce the impact of stressors. For instance, membrane lipid remodeling occurs in response to stresses such as freezing, dehydration and nutrition depletion (Gaude et al., 2008; Nakamura et al., 2009; Moellering et al., 2010; Gasulla et al., 2013; Okazaki et al., 2013b), and this remodeling contributes to the maintenance of membrane properties affecting lipid dynamics, membrane integrity and membrane-bound protein functions. Surface protection through wax and cutin deposition and suberization are also an essential anti-stress function of lipids (Kolattukudy, 1981; Samuels et al., 2008). In this review, we summarized the current understanding of the roles of lipids as signal transducers and mitigators in stress responses, along with their biosynthesis and the signaling cascades in which they are involved.

Table 1. Typical functions of signaling lipids
Lipid speciesStressDescriptionRelated references
lysoPCElicitorYeast-derived elicitor induced a transient accumulation of lysoPC in Californian poppy, leading to phytoalexin productionViehweger et al. (2002)
lysoPCSymbiosisApplication of root extract of mycorrhizal plants containing lysoPC or solely lysoPC induced phosphate transporter that is normally induced by arbuscular mycorrhizal fungal colonization of cortex cellsDrissner et al. (2007)
Fatty acidH2O2Fatty acid-modulated activity of PLD, leading to the suppression of H2O2-induced cell deathZhang et al. (2003)
PAPathogenInhibition of a PLD isoform by either gene knock-out or an inhibitor (n–butanol) deactivated defense-related reactionsKalachova et al. (2013), Pinosa et al. (2013)
PALow temperatureRapid accumulation of PA was observed at low temperaturesArisz et al. (2013), Ruelland et al. (2002)
PASalinityRapid accumulation of PA was observed rapidly under saline conditionsMcLoughlin et al. (2013)
PADroughtPLD and PA are involved in ABA and H2O2 signaling. Inhibition of PLD inhibited ABA-dependent stomatal closureGuo et al. (2012a), Katagiri et al. (2005), Uraji et al. (2012), Zhang et al. (2009)
IP3Low temperatureLow temperature induced the transient accumulation of IP3, whereas DAG produced from phosphoinositide was also used for enhanced PA production. Genetic ablation of phosphatidylinositol 4–kinase resulted in the suppression of inducible PA accumulation and hypersensitive response to chilling at the germination stageDelage et al. (2013, 2012), Ruelland et al. (2002)
IP3Drought and ABAPhosphatidylinositol-4-phosphate 5–kinase required for IP3 production was induced by ABA treatment and droughtMikami et al. (1998)
IP3HyperosmolarityRapid accumulation of IP3 was observed after treatment with NaCl, KCl and sorbitolDeWald et al. (2001)
JAPathogen and insectsJA induced the expression of a wide range of defense-related genes for the production of poisonous chemicals. Plants with defective JA biosynthesis or JA reception showed altered responses to insects and pathogen attacksBlechert et al. (1995), Howe et al. (1996), McConn et al. (1997), Stintzi et al. (2001)
JAUVUV irradiation caused JA-dependent accumulation of phenolics and anti-herbivore defenseConconi et al. (1996), Demkura et al. (2010)
OPDAWound and related cellular redox changeOPDA functions as a signaling molecule in the wounding response. Unlike signaling via JAs, OPDA signaling was CORONATINE INSENSITIVE 1-independent. Wound-responsive regulation of cellular redox signals is relayed by the OPDA–CYP20–3 complex in plastidPark et al. (2013), Taki et al. (2005)
CeramidePathogenMutants with disrupted sphingolipid biosynthetic genes exhibited enhanced SA-dependent cell death and accumulated more ceramideGan et al. (2009), Liang et al. (2003), Shi et al. (2007), Wang et al. (2008)
LCB phosphateDrought and ABALCB phosphate is implicated in ABA-dependent stomatal closure. LCB kinase is activated by another signaling lipid, PACoursol et al. (2003), Guo et al. (2012b), Ng et al. (2001), Worrall et al. (2008)
Figure 1.

The relationship between signaling lipids based on their biosynthesis. Various signaling lipids are produced from membrane lipids, including monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), sulfoquinovosyldiacylglycerol (SQDG), phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylglycerol (PG) and phosphatidylinositol (PI).

Roles of lipid signaling in stress responses

Lysophospholipids and free fatty acids

Whereas glycerolipids are known to be the major components of biological membranes, they also serve many other diverse functions such as the release of partially deacylated glycerolipids, lysophospholipids and free fatty acids, which have been implicated in cell signaling associated with various stress-related responses, growth and development (Wang, 2004; Munnik and Testerink, 2009). The enzyme phospholipase A (PLA) is critically important as it is involved in the formation of both lysophospholipids and fatty acids. PLA can be classified in terms of its reaction specificity: PLA1 removes acyl moieties from the sn–1 position of phospholipids, and PLA2 does so from the sn–2 position, whereas some PLAs have hydrolytic activity at both positions. Lysophospholipids are usually present in trace quantities in plant tissues, but under stress conditions such as freezing, the level of these lipids substantially increases (Welti et al., 2002). The enhanced production of lysophospholipids and free fatty acids is also often evoked in plants by the application of some elicitors, which is sometimes accompanied by a transient increase in PLA activity (Roy et al., 1995; Chandra et al., 1996; Narvaez-Vasquez et al., 1999; Scherer et al., 2002).

There have been several reports suggesting physiological roles of lysophospholipids in stress responses and plant–environment interactions. The transient increase of lysophosphatidylcholine generated by phospholipase A2 (PLA2) at the plasma membrane has been implicated in the induced phytoalexin production in elicitor-treated cultured cells of Eschscholzia californica (California poppy; Viehweger et al., 2002). Lysophosphatidylcholine and lysophosphatidylethanolamine also act as signal transducers, respectively, in arbuscular mycorrhizal symbiosis in Solanum tuberosum (potato; Drissner et al., 2007) and pollen germination (Kim et al., 2011).

Fatty acids have also been demonstrated to be involved in stress responses (Kachroo and Kachroo, 2009). Oleic acid can function as a stimulator of the signaling enzyme phospholipase D (PLD), which has an anti-cell death function (Zhang et al., 2003). Oleic acid also modulates levels of nitric oxide-associated protein, thereby regulating nitric oxide-mediated defense signaling in Arabidopsis (Mandal et al., 2012). Recently, patatin-related PLAs have been well studied with respect to their role in the production of fatty acid and their oxidized products, oxylipins, demonstrating the importance of fatty acid metabolites produced in response to drought and pathogen infection, and in organ development (Yang et al., 2007, 2012; Scherer et al., 2010; Li et al., 2011). Free fatty acids also regulate salt, drought and heavy metal tolerance, as well as wound-induced responses and defenses against insect/herbivore feeding in plants (Upchurch, 2008). Fatty acid derivatives are also involved in stress responses in plants. Azelaic acid, a saturated dicarboxylic acid containing nine carbon atoms, has been reported to prime plants in order to accumulate salicylic acid (SA), a known defense signal for systemic acquired resistance (SAR) upon pathogen infection (Jung et al., 2009).

Lysophospholipids and free fatty acids are likely to be involved in the signaling of various stress responses and cell regulation. Thus, the suppression of the expression of particular PLA genes by reverse genetics or by the inhibition of PLA activities by inhibitors is a popular approach to investigate the role of these lipid species in vivo; however, plant PLA does not solely function as a hydrolytic enzyme of phospholipids. Recently, in Arabidopsis, transcription factor MYB30 and the secretory PLA2–α have been reported to physically interact in vivo, following MYB30-mediated translocation of PLA2–α from cytoplasmic vesicles to the plant cell nucleus (Froidure et al., 2010). Although the translocation of PLA2–α possibly modulates lysophospholipid and/or fatty acid-mediated signaling, this report indicates that the effect of PLA gene silencing in plants is not straightforward.

Phosphatidic acid

Phosphatidic acid (PA) is the essential intermediate for the de novo biosynthesis of all glycerolipids. This lipid class is usually maintained at low levels in plants, but accumulation of PA can be induced via two major pathways: the direct hydrolysis of phospholipids by PLD or the combined action of phospholipase C (PLC) and diacylglycerol kinase (Ruelland et al., 2002; Wang, 2004; Testerink and Munnik, 2005; Wang et al., 2006; Arisz et al., 2009, 2013; Peters et al., 2010; Pokotylo et al., 2014). An increased accumulation of PA has been reported in plants treated with various stresses (Wang, 2004), including chilling, freezing (Welti et al., 2002), wounding (Ryu and Wang, 1998; Wang, 2004) and other stresses.

The roles of PA during various stress responses have been demonstrated through the study of knock-out mutants of the PLD or PLC genes, or by the application of a PLD inhibitor (n–butanol), which leads to an inactive phosphatidyl alcohol (Munnik et al., 1995; Hong et al., 2009). Thus far, PA and PLD have been shown to be involved in basal defense and non-host resistance against pathogenic fungi (Pinosa et al., 2013), signal transduction of the SA-mediated signaling cascade (Kalachova et al., 2013), low-temperature stress response (Ruelland et al., 2002; Arisz et al., 2013), salt-stress response (Yu et al., 2010; McLoughlin et al., 2013), microtubule organization in response to salt stress (Zhang et al., 2012), transduction of hydrogen peroxide signals through interaction with cytosolic glyceraldehyde-3-phosphate dehydrogenase (Guo et al., 2012a), suppression of cell death induced by hydrogen peroxide (Zhang et al., 2003), response to Nod factor (den Hartog et al., 2003), and response to abscisic acid (ABA), drought (Katagiri et al., 2005; Zhang et al., 2005c; Hong et al., 2008; Uraji et al., 2012) and nitrogen signaling (Hong et al., 2009). Although it is sometimes difficult to discriminate the functions of PA as a signaling molecule and as a metabolic intermediate for other membrane lipids, PA and PLD are essential in the lipid remodeling that occurs during phosphorus starvation (Misson et al., 2005), and in normal root hair development, where PLD is directly regulated by the transcription factor GL2 (Ohashi et al., 2003).

As PA is implicated in various stress-signaling pathways, there are several reports on downstream targets suggesting a relationship between PA and ABA responses based on the investigation of ABA-dependent stomatal closure. PLDα1 produces PA, which binds to the ABI1 protein phosphatase 2C, a negative regulator of ABA effects (Zhang et al., 2004). PA also affects ABA signaling through the activation of the kinase of long-chain bases of sphingolipid (SPHK) (Guo et al., 2011, 2012b). Activation of PLDα1 requires SPHK activity, thus, SPHK and PLDα1 are suggested to be co-dependent in the amplification of the response to ABA (Guo et al., 2012b). PA also regulates NADPH oxidase activity and the production of reactive oxygen species in ABA-mediated stomatal closure in Arabidopsis (Zhang et al., 2009). There are also many other PA-binding proteins that are not described here (Janda et al., 2013).

Inositol polyphosphates and diacylglycerol

Inositol phosphates, including inositol-1,4,5-triphosphate (IP3) and diacylglycerol (DAG), are well-known secondary messenger molecules involved in signal transduction and lipid signaling in animals (Munnik and Testerink, 2009). These molecules are produced from phosphatidylinositol-4,5-bisphosphate (PIP2) by phospholipase C (PI–PLC). DAG can also be supplied from other phospholipids by non-specific phospholipase C (NPC; Gaude et al., 2008; Peters et al., 2010; Pokotylo et al., 2014). Signaling mediated by IP3 is involved in numerous cellular processes in plants, including the responses to environmental stimuli such as temperature changes (DeWald et al., 2001; Ruelland et al., 2002; Delage et al., 2012, 2013), water stress (drought and salt stress) and ABA (Mikami et al., 1998; DeWald et al., 2001). IP3 is also implicated in stomatal guard-cell closure (Gilroy et al., 1990), pollen-tube elongation (Franklin-Tong et al., 1996) and pollen dormancy (Wang et al., 2012). Disruption of NPC genes has demonstrated that DAG and related PA derived from NPC-product DAG are involved in plant stress responses to ABA (Hunt et al., 2003; Peters et al., 2010), salt (Peters et al., 2010) and phosphorus deprivation (Gaude et al., 2008; Pokotylo et al., 2013).

Several electrophysiological studies support the existence of ligand-gated channels in the tonoplast, and suggest the regulation of these channels by IP3 (Kudla et al., 2010); however, the signaling cascades involving IP3 and DAG are considered to be rather different in plants compared with animals because of the absence of animal-type IP3 receptors and PKC, which is the most important DAG target in plant genomes (Krinke et al., 2007; Munnik and Testerink, 2009). Moreover, the level of PIP2 is very low in plant tissues (Munnik and Zarza, 2013). Thus, the precise signaling cascade involving IP3 in plants remains the subject of much debate and speculation.

Inositol phosphates also include other types of compounds such as inositol hexakisphosphate (IP6, phytate) and inositol pentakisphosphate (IP5). There are some reports that imply that IP6 is also a potential signaling messenger (Lemtiri-Chlieh et al., 2003). Recently, IP6 and IP5 were found in the crystal structures of the receptors of auxin and jasmonic acid (JA), respectively (Tan et al., 2007; Sheard et al., 2010). Although the roles of these inositol phosphates in hormone signaling remain unknown, they may be associated with the potentiation of signaling in plants.

Oxylipins

The phytohormone JA is the best-known oxylipin involved in the activation of various defense responses (Browse, 2009; Ballare, 2011). JA biosynthesis is believed to start in chloroplasts via the hydrolytic cleavage of linolenic acid from membrane lipids by PLAs (Ishiguro et al., 2001; Hyun et al., 2008). More than 15 derivatives of JA have been reported (Kombrink, 2012), and among them, the conjugation of JA and isoleucine (JA–Ile) by jasmonate resistant 1 is detected by the COI1–JAZ receptor complex (Staswick and Tiryaki, 2004; Yan et al., 2009). COI1 is part of the ubiquitin E3 complex, that, upon binding of JA-Ile, targets a number of jasmonate-ZIM domain repressor proteins for degradation (Chini et al., 2007; Thines et al., 2007; Yan et al., 2007), leading to the relief of the inhibition of transcription factors and the activation of a large number of genes (Pauwels and Goossens, 2011). JA contributes to plant defense against necrotrophic pathogens by accumulating JA-induced proteins (Howe et al., 1996), and by the activation of the biosynthesis of poisonous compounds (Gundlach et al., 1992; Mueller et al., 1993; Blechert et al., 1995). Thus, mutants in Arabidopsis and other species that are defective in JA biosynthesis or perception are highly susceptible to insect and necrotic pathogen attack (Howe et al., 1996; McConn et al., 1997; Thomma et al., 1998; Vijayan et al., 1998; Stintzi et al., 2001; Lorenzo et al., 2004), whereas these mutants of Arabidopsis are more resistant or not less susceptible to biotrophic and hemi-biotrophic pathogens, compared with the wild type (Feys et al., 1994; Thomma et al., 1998; McDonald et al., 2007; Thines et al., 2007; Melotto et al., 2008; Thatcher et al., 2009). JA is also involved in signaling in response to other stresses such as UV damage (Conconi et al., 1996; Demkura et al., 2010) and ozone-induced hypersensitive cell death (Mackerness et al., 1999).

Other oxylipins are also implicated in signaling (Bottcher and Pollmann, 2009). 12–oxo-phytodienoic acid (OPDA) is an intermediate of JA biosynthesis, and there is specific regulation of pathways by OPDA itself, separate from JA (Devoto et al., 2005), and in addition not all JA-regulated genes are COI1-dependent (Taki et al., 2005). Recently, OPDA was shown to regulate stress-responsive cellular redox homeostasis via the activation of thiol metabolism in chloroplasts (Park et al., 2013). OPDA and dinor-OPDA have been sometimes found in acyl moieties in galactolipids (Stelmach et al., 2001; Hisamatsu et al., 2003, 2005; Andersson et al., 2006; Buseman et al., 2006; Nakajyo et al., 2006; Kourtchenko et al., 2007; Vu et al., 2012), although little is known about the functions of these unusual galactolipids for the biosynthesis of free oxylipins in the cell. Galactolipids bound with OPDA and dinor-OPDA could be the intermediates of biosynthesis of free OPDA, dinor-OPDA and jasmoates, and/or the pools of excess OPDA and dinor-OPDA. There are other types of oxylipins such as azelaic acid that primes plants to accumulate SA, a known defense signal for SAR upon pathogen infection (Jung et al., 2009). In addition, several volatiles derived from oxylipin synthetic pathways have been suggested to be involved in plant–microbe interactions (Matsui, 2006; Howe and Jander, 2008; Junker and Tholl, 2013).

Sphingolipids

Sphingolipids are a class of lipids containing a backbone of sphingoid bases. A set of aliphatic amino alcohols that includes sphingosine is referred to as a long chain base (LCB). In plants, there are various types of sphingolipids, such as LCB, LCB phosphate, ceramide (N–acyl LCB), glucosylceramide and glycosyl inositolphosphoceramide. Sphingolipids are known to play roles in various environmental stress responses (Berkey et al., 2012; Markham et al., 2013). It should be especially noted that sphingolipids are an essential component in the regulation of cell death, and that the disruption of sphingolipid biosynthetic genes in plants often induces programmed cell death (PCD; Liang et al., 2003; Shi et al., 2007), and enhanced pathogen-induced hypersensitive responses (Liang et al., 2003; Wang et al., 2008; Gan et al., 2009). These reports suggest that elevated ceramide might be linked with enhanced cell death phenotypes through SA-dependent signaling pathways; however, the relationship among ceramide, SA and enhanced cell death seems to be rather complex, as a hydroxylase-deficient mutant that leads to elevated ceramide and SA does not display a PCD-like phenotype (Konig et al., 2012), and a mutant that accumulates glycosphingolipid with an abnormal glycosylation pattern has a constitutive hypersensitive response phenotype without elevated ceramide accumulation (Mortimer et al., 2013). Thus, other metabolites such as LCB may be the cause of enhanced cell death (Shi et al., 2007; Markham et al., 2013). With respect to signaling, sphingolipid-mediated enhanced cell death is mediated by a mitogen-activated protein kinase (MPK6; Saucedo-Garcia et al., 2011) and a Ca2+-dependent protein kinase regulated by 14-3-3-proteins (Lachaud et al., 2013).

Another type of sphingolipid, the LCB phosphates, is implicated in signaling during drought (Ng et al., 2001; Coursol et al., 2003; Worrall et al., 2008). It has been reported that LCB kinase activity is transiently stimulated by ABA, and that heteromeric G proteins are the downstream component of LCB phosphate in the ABA signaling pathway (Coursol et al., 2003; Worrall et al., 2008). Recently, there was a report on the activation of LCB kinase by PA, as described above, suggesting an interplay between PLD-dependent PA signaling and sphingolipid-dependent signaling (Guo et al., 2012b). LCB phosphates are also implicated in the cold-stress response (Cantrel et al., 2011; Dutilleul et al., 2012; Guillas et al., 2013).

Other lipids

Fatty acid derivatives other than oxylipins are sometimes implicated in stress signaling. N–acylethanolamine (NAE) comprises a class of fatty acid amides. In the endocannabinoid pathway in mammalian brain, NAE binds to a G protein-coupled receptor. Although NAE has not been investigated in plants to the extent it has in mammals, it has been implicated in the stress responses in plants (Tripathy et al., 1999; Teaster et al., 2007; Kang et al., 2008). The catabolic pathway for NAE has also been proposed (Kilaru et al., 2011). Alkamides other than NAE also induce defense reactions in plants through the activation of JA biosynthesis and other signaling pathways (Mendez-Bravo et al., 2011). A group of elicitors derived from some larvae, such as volicitin, also have the same long-chain fatty acid amide moiety as found in NAE (Howe and Jander, 2008).

Although the precise signaling molecules have not been identified, there have been several reports suggesting an involvement of lipid-like molecules in signaling during stress responses. A lipid transfer protein Defective in Induced Resistance 1 (DIR1) has been reported to be associated with SAR in Arabidopsis (Maldonado et al., 2002). Recently, radioactive tracer experiments have shown that the translocation of a glycerol-3-phosphate derivative to distal tissues requires the lipid transfer protein DIR1 (Chanda et al., 2011).

Roles of lipids in stress responses as mitigators

Lipids also play essential roles other than as signal mediators. For instance, the well-defined relationship between membrane composition and chilling sensitivity has been verified by genetic modification that changed the saturation rate of a particular lipid class (Murata et al., 1992; Wolter et al., 1992; Nishida and Murata, 1996). Recently, a detailed analysis of changes in glycerolipid metabolism in response to temperature change was also reported (Burgos et al., 2011; Degenkolbe et al., 2012). Some lipids, including oxylipin, function as protective compounds by having antimicrobial properties (Blee, 2002; Prost et al., 2005; Nalam et al., 2012; Shimada et al., 2014). In addition to these roles, plant lipids also function as stress mitigators to reduce the severity of environmental stresses. For example, polyunsaturated fatty acids attenuate cell damage upon stress by scavenging reactive oxygen species (Mene-Saffrane et al., 2009). In this section, we will summarize recent reports on stress mitigation based on the plant lipid metabolism.

Temperature stress

Freezing is a notorious stress affecting crop yields through the initial growth of extracellular ice formation, resulting in cellular dehydration and loss of membrane integrity. Oligogalactolipids were recently reported to be responsible for the mitigation of freezing stress (Figure 2a; Moellering et al., 2010). The mutant sfr2 (sensitive freezing 2) of Arabidopsis was found to be highly sensitive to freezing that resulted in the rupture of chloroplasts (Warren et al., 1996; Thorlby et al., 2004). Detailed biochemical characterization revealed that SFR2 is a galactosyltransferase that progressively transfers a galactose moiety from a monogalactosyldiacylglycerol (MGDG) molecule to other galactolipids including MGDG, resulting in the formation of oligogalactolipids in the chloroplast envelope and the removal of MGDG from the envelope membrane, changing the ratio of bilayer- to non-bilayer-forming membrane lipids (Moellering et al., 2010). The importance of lipid metabolism in the protection against freezing was also suggested by the increased tolerance of an Arabidopsis mutant with a disruption in PLDα (Welti et al., 2002). Regarding the mitigation of freezing stress, Δ8 unsaturation of LCB in sphingolipids is also required for the alleviation of damage caused by freezing (Chen et al., 2012).

Figure 2.

Lipidic stress mitigators accumulated following their induction against stresses. (a) βββ-type trigalactosyldiacylglycerol (TGDG) plays essential role in Arabidopsis exposed to freezing temperatures, and βαβTGDG is reported to be accumulated in a desiccation-tolerant plant, Craterostigma plantagineum. (b) Glucuronosyldiacylglycerol.

Drought and desiccation stress

Almost all of the aerial parts of plants are covered with a hydrocarbon layer composed mainly of wax and cutin, which helps mitigate the stress derived from water deficiency by restricting transpiration. Wax and cutin are considered to play a major role in plant drought tolerance (Samuels et al., 2008), and drought, salt stress and ABA treatment often leads to an increase in these hydrocarbon layers (Williams et al., 1999; Cameron et al., 2006; Kosma et al., 2009; Seo et al., 2011). Several lines that have disrupted wax synthesis accumulate less wax and are less tolerant to drought (Qin et al., 2011; Seo et al., 2011; Mao et al., 2012; Zhu and Xiong, 2013), whereas the over-accumulation of wax by overexpression of transcription factors or wax biosynthetic genes can increase tolerance to drought stress (Aharoni et al., 2004; Broun et al., 2004; Zhang et al., 2005a; Cameron et al., 2006; Yang et al., 2011; Luo et al., 2013; Zhou et al., 2013a,b). The biosynthesis of cutin and wax is activated by wound injuries (Aharoni et al., 2004; Panikashvili et al., 2007; Lulai et al., 2008), and this may be related to a reduction of cellular dehydration caused by tissue injury.

Desiccation is a more severe stress for plants. A detailed comparison of plants with different susceptibilities to desiccation stress recently demonstrated that a reduction in MGDG levels coincided with an increase in oligogalactolipids (Figure 2b), PI and PA. Upon desiccation, these changes in lipid profile were more significantly observed in desiccation-tolerant species than in desiccation-sensitive ones (Gasulla et al., 2013). There are also other potential mitigation mechanisms that have been proposed: for example, an overexpression of ω–3 desaturases in Nicotiana tabacum (tobacco) results in altering the tolerance to salt and drought (Zhang et al., 2005b).

Wounding

Suberin is a waxy, waterproof substrate present in the cell wall that is comprised of polyaromatic and polyaliphatic domains (Kolattukudy, 1981). The polyaliphatic domains are derived from fatty alcohols, hydroxy fatty acids and dicarboxylic acids. Along with lignifications, suberization is a typical response to tissue injury. Analysis of loss-of-function mutants and biochemical analysis of enzymes has identified the long chain fatty acid reductases, fatty acid elongases and fatty acid hydroxylases required for wound-induced suberization (Agrawal and Kolattukudy, 1977; Hofer et al., 2008; Franke et al., 2009; Serra et al., 2009a,b; Domergue et al., 2010).

Nutrition starvation stress

Lipids also play mitigating roles during nutrition depletion. Phosphorus (P) mobilization by membrane lipid remodeling, i.e. changing the compositions of the membrane lipids rich in phospholipids, is one of the general adaptation mechanisms for P limitation in plants (Härtel et al., 2000; Andersson et al., 2003, 2005; Jouhet et al., 2004; Benning and Ohta, 2005). Replacement of membrane phospholipids with non-phosphorus glycerolipids, such as sulfoquinovosyldiacylglycerol (SQDG) and digalactosyldiacylglycerol (DGDG), which promotes the remobilization of P, is a typical metabolic signature associated with lipid remodeling during P deprivation (Härtel et al., 2000; Andersson et al., 2003; Jouhet et al., 2004; Benning and Ohta, 2005). To date, several biosynthetic enzymes involved in the enhanced production of these non-phosphorus glycolipids under P limitation, mainly glycosyltransferases and sugar-donor synthetic enzymes, have been characterized (Essigmann et al., 1998; Awai et al., 2001; Kelly and Dörmann, 2002; Yu et al., 2002; Kobayashi et al., 2006, 2009; Gaude et al., 2008; Okazaki et al., 2009). Phospholipid degradation and P recycling have been well studied by the characterization of several phospholipases and glycerophosphodiesterases (Nakamura et al., 2005, 2009; Cruz-Ramirez et al., 2006; Gaude et al., 2008; Cheng et al., 2011). In addition, extensive lipid analysis on lipid remodeling under P-starved conditions has been reported (Li et al., 2006). Recently, lipidomic analysis of P–starved Arabidopsis revealed an inducible accumulation of a new plant lipid class, glucuronosyldiacylglycerol (GlcADG) (Figure 2b; Okazaki et al., 2013a,b). Under P-limited conditions, the GlcADG-deficient mutant of Arabidopsis showed a severely damaged phenotype compared with wild-type plants, indicating that GlcADG plays a critical role in the protection of plants against stress caused by P limitation. GlcADG is also found in Oryza sativa (rice), and its concentration significantly increases following P limitation, suggesting a shared physiological significance of this novel lipid against P depletion in plants.

Lipid metabolism is also involved in the response to nitrogen (N) starvation. PLDε has been suggested to be involved in nitrogen signaling (Hong et al., 2009). Nitrogen starvation also leads to the breakdown of galactolipids and chlorophylls, with deposition of specific fatty acid phytyl esters in thylakoids and plastoglobules of chloroplasts (Gaude et al., 2007; Lippold et al., 2012).

Conclusion

In this review, we summarized various reports describing the roles of plant lipids as signaling molecules and mitigators of stress responses. Recent improvements in analytical techniques have improved the ability to detect small changes in metabolite levels. More detailed metabolic profiling data on minor signaling lipid species may afford new insights into the detailed regulation and metabolism of these signaling lipids, and expand the understanding of stress response mechanisms in plants, which have primarily been revealed by reverse genetics and the use of inhibitors.

Acknowledgements

This work was supported in part by grant aid from the Strategic International Collaborative Research Program (SICORP) of Japan Science Technology (Metabolomics for a Low Carbon Society, JST-NSF).

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