Received 23 September 2014; revised 22 October 2014; accepted 8 November 2014

ABSTRACT

Late Embryogenesis Abundant (LEA) proteins, a group of hydrophilic proteins, have been linked to survival in plants and animals in periods of stress, putatively through safeguarding enzymatic function and prevention of aggregation in times of dehydration/heat. Yet despite decades of effort, the molecular-level mechanisms defining this protective function remain unknown. In this paper, we summarize and review research discoveries of the classification of the LEA protein groups based on their amino acid sequence similarity and on the presence of distinctive conserved motifs. Moreover, we focus on high correlation between their accumulation and water deficit, reinforcing their functional relevance under abiotic stresses. We also discuss the biochemical properties of LEA proteins arising from their hydrophilic nature and by amino acid composition. Although significant similarities have not been found between the members of the different groups, a unifying and outstanding feature of most of them is their high hydrophilicity and high content of glycine. Therefore, we have highlighted the biotechnological applications of LEA genes, and the effects of over-expressing LEA genes from all LEA groups from different species of origin into different plant hosts. Apart from agronomical purposes, LEA proteins could be useful for other biotechnological applications in relation to their capacity to prevent aggregation of proteins.

Keywords:

LEA Proteins, Abiotic Stress, Protein Aggregation, Intrinsically Unstructured Proteins (IUPs)

1. Introduction

Plants are exposed to multiple environmental stresses along their life cycle. Abiotic stresses such as drought, high salinity and freezing temperatures affect most areas of the world and they impact in plants by directly reducing its survival in the natural environment and its productivity in agriculture. About half of the annual world crop production is lost due to abiotic stresses, especially drought [1] [2] . Most plants encounter transient decreases in relative water content at some stages of their life, and many produce a highly desiccation tolerant structure, such as seeds, spores or pollen. Although in a few vascular plants i.e. resurrection plants, desiccation tolerance also occurs in the vegetative tissues [3] , in general plant vegetative tissues, leaves and roots, are very sensitive to water deficit and they can only withstand reduced and transient water losses [4] [5] . Drought stress induces a range of physiological, cellular and molecular responses in plants towards stress tolerance.

Cellular turgor is not the only important transducer of plant water stress. The growth regulating hormone abscisic acid (ABA) is produced in response to desiccation, causing many of the known expressions and consequences of plant water deficit such as arrested growth, stomatal closure and reproductive failure [4] . Drought and high salinity result in strong increases of plant ABA levels which induce the expression of stress-related genes and adaptive physiological responses [6] [7] . Abiotic stress produces extensive changes in the regulation of gene expression, gene activation/gene suppression, signal transduction pathways biochemical modulation and proteomic machinery which lead to the survival or death of the affected plants [8] -[10] .

Upon water deficit a common mechanism is the synthesis of osmotically active compounds compatible with metabolism. These are low-molecular weight solutes that accumulate in the intracellular compartment for osmotic adjustment. Among the many existing osmoprotectants, the non-reducing disaccharide trehalose is most widespread in nature, in anhydrobiotic bacteria, fungi, invertebrates and resurrection plants. However, in most plants systems, sucrose and certain oligosaccharides such as raffinose, stachyose and cyclitols, instead of trehalose, accumulate in large quantities during seed maturation. Non-reducing disacharides function as water replacement molecules and vitrification agents and contribute to the formation of bioglasses [11] . The LEA proteins are a family of hydrophilic proteins that are presumed to play a protective role during exposure to different abiotic stresses [12] . The study of results of physiological assays under drought and salt treatments revealed that the LEA gene confers significantly enhanced tolerance to drought and salt in transgenic Arabidopsis plants [13] .

2. Late Embryogenesis Abundant (LEA) Proteins

As the name suggests, Late Embryogenesis Abundant proteins were originally discovered in the late stages of embryo development in cotton seeds [14] [15] . In plants, most of LEA proteins and their mRNAs accumulate to high concentrations in embryo tissues during the last stages of seed development when desiccation occurs [3] [16] -[20] . Embryogenesis in flowering plants represents a series of stages to develop a miniature plant within the seed. In this process the formed embryo will undergo a cellular expansion stage with dry mass increase in order to provide energy for the process of germination. In most plants the final stage of seed development, maturation, is initiated by a reduction in seed water content, which will eventually drop to about 10%. During this stage and preceded by an increase in ABA content, gene expression and protein profiles change greatly and are associated with the acquisition of desiccation tolerance and development of capacity for seed germination [21] [22] . LEA proteins are accumulated in this final stage, in contrast to storage proteins which appear earlier. Moreover, their mRNAs are maintained at high levels in the dehydrated mature embryos, while transcripts of storage protein genes are completely degraded during the last embryogenesis stage [21] . Since orthodox seeds acquire the ability to withstand severe dehydration at this stage, LEA proteins have been associated with desiccation tolerance [14] [15] [23] . LEA proteins accumulate in vegetative tissues exposed to dehydration, osmotic, and/or low temperature stress [5] [17] [18] [24] . They are also found in anydrobiotic resurrection plants upon drying [3] [25] . Members of the LEA family seem to be ubiquitous in the plant kingdom. Since their first description, hundreds of LEA proteins from vascular to nonvascular plants have been isolated. Their presence has been confirmed not only in angiosperms and gymnosperms [5] but also in seedless vascular plants (e.g. Selaginella) [26] [27] , bryo- phytes (e.g. Tortula, Physcomitrella) [28] , pteridophytes (e.g. ferns) [29] and algae [30] . Some LEA-like genes are only induced by ABA or other environmental clues [19] [20] [31] . In addition they have now also been identified in some microorganisms [32] [33] , fungi [34] [35] , protozoa, rotifers, nematodes [36] [37] , insects and crustacean [20] [38] [39] . The correlation of LEA proteins in seed maturation stages, during water stress in vegetative plant organs, and in anydrobiotic animals suggests that LEA proteins represent a widespread adaptation to water deficit; however, their precise functions remain unclear.

3. Classification of LEA Proteins: Sequence Motifs

LEA proteins were initially classified in six subgroups on the basis of specific domains [24] . With increasing information available on new described members, differences on expression profiles, description in organisms other that plants and especially with the new bioinformatic tools, the classification has been subjected to different rearrangements [17] -[19] [40] [41] . Here, we will adopt a modification by Covarrubias’s group [19] of the classification initially introduced by Dure et al. [24] in which LEA proteins from cotton were categorized by virtue of similarities in their deduced amino acid sequences. This classification is based on the presence of specific motifs conserved across species which are unique to each family. Based on these characteristics and considering all available sequence information from different plant species LEA proteins are grouped into seven distinctive groups or families; nevertheless, groups 1, 2 and 3 are considered the major LEA groups containing most members of the protein family. The first inventory of LEA proteins was performed in the Arabidopsis thaliana genome where fifty LEA genes have been identified [17] . In rice the lea genome comprises 33 genes, 36 in grapevine and 33 in poplar [18] . Over 100 entries for each group 1, 2 and 3 are found in public databases, Gen Bank (http://www.ncbi.nlm.nih.gov) and TIGR database (http://planta.tigr.org) [31] . LEA protein database (LEAPdb) is publicly available at http://forge.info.univ-angers.fr/~gh/Leadb/index.php, [42] . These authors resume the different classification for LEA protein (Table 1). Jaspard et al. [43] using physico-chemical properties of LEAPs and amino acid computed and statistically analyzed, led to the classification of 710 LEAPs into 12 non-overlapping classes with distinct properties.

3.1. LEA Group 1

This LEA group (Pfam PF00477) originally represented by the D-19 and D-132 proteins from cotton seeds contain an internal 20-mer sequence (TRKEQ [L/M] G [T/E] EGY [Q/K] EMGRKGG [L/E]). This motif may be

present in several copies arranged in tandem, from one to four in plant species, and up to eight in other organisms [18] [44] -[46] . The wheat Em proteins belong to this group. In the maize EmB564, and EMB5 proteins belong to this group [47] [48] . Group 1 LEA proteins have also been found in Bacillus subtilis [33] , in other soil bacterial species and in the crustacean Artemia franciscana.

3.2. LEA Group 2

This group of LEA proteins (Pfam PF00257), also known as “dehydrins”, was originally identified as the D-11 family in cotton embryos. Group 2 LEA protein is the most characterized group of LEA proteins. A distinctive feature of group 2 LEA proteins is a conserved, Lys-rich 15-residue motif, EKKGIMDKIKEKLPG, named the K-segment [49] which can be found in one to 11 copies within a single polypeptide. An additional motif also found in this group is the Y-segment, whose conserved consensus sequence is VTD [E/Q] YGNP [19] , usually found in one to 35 tandem copies in the N-terminus of the protein; this motif has similar amino acid sequence to the nucleotide binding domain found in chaperones of plants and bacteria. Many proteins of this group also contain a tract of Ser residues, called the S-segment, acting as a site for protein phosphorylation [50] . Less conserved motifs, the ø-segments are usually rich in polar amino acids and lay interspersed between K-segments [24] [51] . The K-segments of the wheat dehydrin DHN-5 are essential for the protection of lactate dehydrogenase and β-glucosidase activities in vitro [52] . The presence and arrangement of these different motifs in a single polypeptide allow the classification of group 2 LEA proteins into five subgroups [53] [54] . Proteins that only contain the K-segment are in the K-subgroup, and those that include the S-segment followed by K-segment are in the SK-subgroup. In addition, there are the YSK-, YK-, and KS-subgroups [31] .

3.3. LEA Group 3

Group 3 LEA proteins (Pfam PF02987) are characterized by a repeating motif of 11 amino acids [55] . Differences found in the molecular mass in this group of proteins are usually a consequence of the number of repetitions of this 11-mer motif. The variability in the 11-mer motif leads to a sub-classification of the group 3 LEA proteins into two subgroups: 3A, represented by the cotton D-7 LEA protein; and 3B, represented by the cotton D-29 LEA protein. The first subgroup is highly conserved; two of the motifs characteristic of these proteins correspond to almost the same 11-mer described originally for this subgroup, with some variation at positions 9 and 10 (TAQ [A/S] AK [D/E] KT[S/Q] E). The other subgroup (3B) is more heterogeneous [19] ; four variations of the 11-mer were found. Interestingly, proteins similar to plant group 3 LEA proteins accumulate in several non plant organisms in response to dehydration, in fungi, microbial and animal kingdoms. They have been found in prokaryotes Deinococcus radiodurans [56] , Haemophilus influenza [57] and in Caenorhabditis elegans (CeLEA-1), whose expression is correlated with the survival of this nematode under conditions of desiccation, osmotic, and heat stress [58] . Other anhydrobiotic organisms such as the nematodes Steinernema feltiae [36] [59] and Aphelencus avenae [37] , as well as the bdelloid rotifer Philodina roseola [60] , the chironomid Polypedilum vanderplanki [61] and the crustacean A. franciscana [62] [63] also accumulate group 3 LEA proteins in their desiccated states.

3.4. LEA Group 4

Group 4 LEA proteins (Pfam PF03760) are of widespread occurrence in the plant kingdom, including nonvascular plants and vascular plants. A motif that has characterized the proteins in this group is motif 1, located at the N-terminal region with the following consensus sequence: AQEKAEKMTA [R/H] DPXKEMAHERK [E/K] [A/E] [K/R] [19] . However, four additional motifs can be distinguished in many group 4 LEA proteins. The presence or absence of motif defines two subgroups within the family. The first subgroup (group 4A) consists of small proteins (80 - 124 residues long). The other subgroup (group 4B) has longer representatives (108 - 180 residues). D-113 protein from cotton, the first discovered of this group [64] .

3.5. LEA Group 5

The first proteins described for this group (Pfam PF04927) were D-34, D-73, and D-95 from cotton [16] [23] . They represent an atypical LEA subgroup because they contain a higher proportion of hydrophobic residues. These proteins are not soluble after boiling, suggesting that they adopt a globular conformation [16] [23] [44] . They include maize Rab28 [65] , carrot ECP31 [66] and Medicago trunculata MtPM25 [67] among others.

3.6. LEA Group 6

PvLEA18 protein from bean (Phaseolus vulgaris) was the first protein described from this group (Pfam PF03168) [68] . To date, 36 genes of this family have been described from vascular plants. The proteins in this group are characterized by their small size (approximately 7 - 14 kD) [19] . A distinctive characteristic of the group 6 LEA protein characterized is its accumulation in the elongation or growing regions of common bean roots and hypocotyls, which showed lower water potentials than the non-growing regions [68] .

3.7. LEA Group 7

The ASR proteins, considered to be members of the LEA family, are small, heat-stable, and intrinsically unstructured proteins [69] [70] . The group of this gene has been identified from various species of dicotyledonous and monocotyledonous plants [70] as well as from gymnosperm species. However, no ASR-like genes are found in Arabidopsis. They share physiochemical properties with other LEA proteins and they accumulate in seeds during late embryogenesis and in response to water-limiting conditions [70] .

4. Biochemical Properties and Structure of LEA Proteins

Most of the biochemical properties of LEA proteins arise from their hydrophilic nature and based amino acid composition. Although significant similarities have not been found between the members of the different groups, a unifying and outstanding feature of most of them is their high hydrophilicity and high content of Gly and a lack or low proportion of Cys and Trp residues, and a preponderance of certain amino acid residues such as Ala, Glu, Lys/Arg, and Thr [16] [32] [55] [71] . The high hydrophilicity is likely to be responsible for their lack of conventional secondary structure in the hydrated state. Most of them exist principally as randomly coiled proteins in solution. While structure modeling and structure prediction programs suggest that at least some LEA proteins from particular families contain defined conformations [51] [55] [57] all hydrophilic LEA proteins studied experimentally have revealed a high degree of unordered structure in solution. This has led them to be considered as intrinsically unstructured proteins [72] -[81] . Similarly, the ability of LEA proteins to remain soluble at elevated temperatures can be attributed to their hydrophilic, unstructured nature. Heat-induce aggregation of proteins results from partial denaturation and association through exposed hydrophobic regions, something that cannot occur in hydrophilic and natively unfolded protein [39] [82] .

LEA proteins are variable in size, ranging from 5 to 77 kD among most groups. They can be acid, neutral or basic. Group 1 proteins are acidic to neutral; group 2 comprises proteins with different pIs and groups 3 are neutral to basic [31] . The aberrant mobility of this type of proteins in SDS-PAGE is rather caused by the less interaction between SDS and charged amino acid residues [83] . Since the discovery than group 1 protein Em, from wheat, was flexible in conformation with little secondary structure (i.e. α-helice or β-sheet) and 70% of the protein behaving as random coil [84] most LEA proteins have been found to have an unfolded structure in the hydrated state [39] . LEA proteins from groups 1, 2 and 3 are predicted to be at least 50% unfolded. Lack of conventional secondary structure means that members of the major LEA protein groups are included in the large class of protein variously called “natively unfolded”, “intrinsically disordered” or “intrinsically unstructured” [85] -[87] and it is thus not surprising that attempts to crystallize purified LEA proteins for X-ray crystallography have reportedly been unsuccessful [72] [84] . Disorder prediction programs (http://www.pondr.com) suggest that 27% - 41% of all eukaryotic proteins contain unstructured region ≥50 residues long and that 6% - 17% of polypeptides are wholly disordered [85] ; therefore, many proteins are natively unfolded or contain natively unfolded domains. The disordered proteome is likely to be a rich source of unforeseen activities, highlighting new functions of both biological and potentially pharmaceutical significance [88] . Nevertheless, some LEA proteins show some secondary structure and they have some structural elements in equilibrium with unstructured states. Thus, a polyproline-type II (PPII) extended, left-handed helices have been described in groups 1 and 2 from soybean [76] , group 2 from Arabidopsis [89] and in group 3 from an anydrobiotic nematode [72] . Many natively unfolded proteins are known to undergo increased folding under some conditions, usually when they bind a partner molecule or cation [87] . Environmental conditions can also affect folding and several LEA proteins become more structured when dried [39] .

LEA proteins exhibit the remarkable ability to become more ordered and to develop secondary structure as dehydration proceeds. For animal LEA proteins, Tunnacliffe’s group first demonstrated this phenomenon by using Fourier-transform infrared (FTIR) spectroscopy [72] . FTIR spectroscopy allows for the assessment of protein secondary structure in the dry state by using the profile of the amide-I band, which provides information on the relative contributions of α-helix, β-sheet, and turn structures. A group 3 LEA protein from Typha latifolia become largely α-helical when dried rapidly; slow drying resulted in intermolecular β-sheet formation, as well as α-helix [80] . Similarly, the group 3 LEA proteins AavLEA1 from nematode Aphelenchus avenae [72] and LEAM from pea mitochondria [90] also gain structure on drying. Boudet et al. (2006) [91] used FITR to study both group 1 (MtEm6) and group 5 (MtPm25) proteins from M. trunculata and found them both to have increased folding in the dried state. Koag et al. (2003) [92] have also reported gain of structure, α-helical, when cowpea DHN1 is incubated with small unilamellar vesicles (SUVs).

Li and He (2009) [93] utilized a 66-amino-acid fragment of AavLEA1 and documented, through molecular dynamics simulation, many of these properties. Water was removed from 83.5 wt% to 2.4 wt%. As water is removed, the protein assumes progressively a more folded conformation. At 83.5 wt% LEA protein is completely solvated. At 50% water between 83.5% and 50.4%, the protein is unstructured. In this range and below this point, water molecules no longer are sufficient to fully solvate the protein. At less than 20% water the protein becomes more dehydrated and begins to adopt a significant amount of secondary structure. α-helical structure is apparent, and hairpin-like structures are formed. At 2.4% water the structure is very similar to that in the complete absence of water. The propensity of some LEA proteins to gain structure under some conditions, may be a general property of these proteins, and may have important functional implications in their physiological roles [94] .

5. Sub Cellular Localization and Expression Profiles

In plants, LEA proteins have been found localized in cytoplasm, nucleus, mitochondrion, chloroplast, endoplasmic reticulum, vacuole, peroxisome and plasma membrane [39] . The different LEA groups show no preference for a specific subcellular localization. Most LEA proteins from the different groups are accumulated during embryo development in the dry seed.

In plants, group 1 LEA proteins are found mostly in seeds and they are not induced by stress conditions in vegetative tissues. They accumulate during seed development and they are considered as embryo-specific LEA proteins [95] [96] .

Group 2 LEA proteins accumulate during seed desiccation and in response to water deficit induced by drought, low temperature or salinity [73] [97] [98] . Most of them accumulate in all tissues upon water deficit although there are those that preferentially respond to particular stress conditions. Some dehydrins are strongly accumulated in response to low-temperature treatments but not to drought or salinity [99] ; other group 2 members are not induced in response to low temperatures, while a small number of dehydrins show an unusual constitutive expression [100] -[102] . However, it is not possible to assign a specific accumulation pattern to particular group or subgroup. In addition, not all are ABA-induced and in some cases their response to stress is mediated by more than one pathway, one of which may be ABA dependent [97] [103] [104] . Many group 2 LEA proteins accumulate in the cytoplasm, and some of them are also localized to the nucleus. For nucleus directed SK2 proteins, the phosphorylated S-segment and the RRKK sequence are relevant for their nuclear localization [105] [106] . Some dehydrins are also found in other cell compartments, including the vicinity of the plasma membrane, mitochondria, vacuole, and endoplasmic reticulum [35] [100] [101] [107] [108] .

Expression and transcriptomic analysis of plant group 3 LEA proteins show their accumulation in mature seeds and in response to dehydration, salinity, or low temperatures [109] -[112] . Some members also respond to hypoxia [113] or to high-excitation pressure imposed by high light [112] .

As for LEA proteins from other groups, the expression of group 3 LEA proteins appears to be regulated by ABA during specific developmental stages and/or upon stress conditions [114] [115] . Group 3 D-7 LEA protein from cotton accumulates to a concentration of about 200 mM in mature cotton embryos [116] . Studies of seeds have localized group 3 LEA proteins to the cytoplasm and protein storage vacuoles, as is the case for HVA1 from barley Hordeum vulgare [117] whereas PsLEAm is distributed within the mitochondrial matrix of pea seeds [118] . Group 3 proteins are also detected in vegetative tissues; WAP27A and WAP27B are abundantly accumulated in endoplasmic reticulum of cortical parenchyma cells of the mulberry tree (Morus bombycis) during winter [119] ; and WCS19 accumulates specifically in wheat leaves and rye (Secale cereale) during cold acclimation, where it was localized within the chloroplast stroma [112] .

LEA proteins from group 4 in some classifications are partially included in group 3. Cotton D-113 was found homogeneously distributed in all embryo tissues at a concentration of nearly 300 mM [116] . Later, similar proteins were found to accumulate in vegetative tissues in response to water deficit. In tomato (Solanum lycopersicum) plants, group 4 LEA transcripts (LE25) accumulated in leaves in response to water deficit and ABA [120] . In Arabidopsis vegetative tissues, the transcripts of the group 4 LEA proteins also accumulated in response to water-deficit treatments [64] [121] .

Transcripts from group 5 LEA proteins accumulate during the late stage of seed development and in response to stress conditions, such as drought, UV light, salinity, cold, and wounding [33] [122] -[124] . In maize, Rab28 has been found accumulated in the nucleolus of scutellar cells of mature dry embryos [125] . In Medicago trunculata, MtPM25 is highly induced by drought [91] .

6. LEA Protein Functions

There is an extensive bibliography showing the correlation between the expression of LEA proteins or their genes with stress resistance [20] [31] [60] . Many studies show the protection conferred by LEA proteins during salt and osmotic stress. Introduction of heterologous LEA proteins into plants and microorganisms results in an enhanced stress tolerance. Transgenic approaches have shown that over-expression of LEA proteins from different species in Arabidopsis, tobacco, rice, wheat, maize, lettuce or cabbage produces improve abiotic stress resistant phenotypes [126] -[131] . However, the precise molecular function of LEA proteins is still unclear and so far LEA proteins have been suggested to act as stabilizers, hydration buffers, membrane protectants, antioxidants, organic glass formers and/or ion chelators [39] .

6.1. Protein Protection

LEA proteins have the capacity to protect target proteins from inactivation and aggregation during water stress. A role in protein stabilization is supported by the fact that some LEA proteins preserve enzyme activity in vitro after partial dehydration, desiccation or freezing [72] [132] [133] . One mechanism of protection is the prevention of water stress induced aggregation of proteins [74] [134] -[137] .

Many proteins, including the enzymes citrate synthase and lactate dehydrogenase, form insoluble aggregates when dried or frozen, but aggregation is reduced in the presence of LEA proteins from groups 1, 2 and 3. Group 2 proteins also prevent protein aggregation from heat stress [74] . This protein anti-aggregation activity extends to the protection of complex mixtures of proteins, such as the water-soluble proteomes of human and nematode cells [134] . Due to their hydrophilic, unstructured nature, LEA proteins themselves are not susceptible to aggregation on desiccation, freezing or boiling [60] . The antiaggregation properties of group 3 LEA proteins have been demonstrated also in living cells; mammalian cells overexpressing aggregation-prone proteins reduce the formation of aggregates in the presence of nematode A. avena LEA protein [134] . The group of Tunnacliffe has proposed that LEA proteins may exert a “molecular shield” activity [41] [135] . In the increasing crowded environment of the dehydrating cytoplasm LEA proteins could decrease the interaction between partially denatured polypeptides and avoid their aggregation. The shield proteins might also have a space-filling role and help to prevent collapse of the cell as its water is lost. Another functional hypothesis is the chaperone activity [74] . The anti-aggregation activity of LEA proteins resembles a “holding” molecular chaperone, which function in the cell would be to stabilize passively protein species in a partially unfolded state, preventing aggregation while the stress lasts. They resemble holding chaperones, in their functioning without ATP, in contrast to classical folding chaperones which require ATP. But they are distinct in that they lack structure and do not form transient complexes with their client proteins through hydrophobic surfaces, since they are hydrophilic [64] [133] . The two functional mechanisms may not be so distinct and in the context of desiccation tolerance both activities may contribute to damage avoidance [60] .

6.2. Membrane Protection

During desiccation membrane protection is essential to preserve the cellular and organellar integrity. Some LEA proteins could contribute with sugars to H-bonding networking and protect membranes in the dry state [38] . LEA proteins being highly hydrophilic are not expected to interact with cellular membranes in hydrated conditions, but interaction cannot be excluded through α-helices in a dehydrating cell. Among group 2 LEA proteins, wheat WCOR414 and Arabidopsis LTi29 acidic dehydrins were immunodetected in the vicinity of the plasma membrane during cold acclimation [100] [138] and maize dehydrins were observed in association to membranes of protein and lipid bodies [139] . Maize native and recombinant dehydrin DHN (Rab17) binds in vitro to anionic lipid vesicles and binding produces an increase in amphipathic α-helix (a structural element that interacts to membranes and proteins); both binding and gain in conformation were attributed to the K-segment [140] [141] . Other dehydrins, ERD10 and ERD14, bind in vitro to acidic phospholipid vesicles [74] . Recently, the contribution of dehydrin K-segments from group 2 Lti30 LEA protein has been analyzed in vitro in the binding to membranes and the relevance of the flanking His side chains as regulators of the interaction between the K-segments and membranes in a pH-dependent manner has been shown [142] .

Group 3 LEA protein, LEAM, is located in pea mitochondria and is able to interact with a membrane and afford protection in the dry state [90] . When drying, this unstructured polypeptide is able to fold into amphipathic α-helices. The interaction between LEAM and phospholipids and the protective effect of LEAM was demonstrated by differential scanning colorimetry using a liposome desiccation assay [90] . To summarize, although their hydrophylicity and lack of structure suggest that LEA proteins should act in soluble compartments of cells, specific LEA proteins upon folding during desiccation or freezing may contribute to membrane protection [60] .

6.3. Ion Binding and Antioxidant Function

One consequence of dehydration is the increase in concentration of intracellular components, including ions. Increased ionic concentration can affect macromolecular structure and function. It has been proposed that LEA proteins, because of their many charged amino acid residues might act to sequester ions [55] [100] . A dehydrin- like protein from celery is located in the vacuole and binds Ca⁺⁺ when phosphorylated [108] . Acidic group 2 proteins, ERD10, COR47 and ERD14 also exhibit phosphorylation-dependent Ca⁺⁺ binding. Phosphorylation sites are located in the serine motif [143] . Group 2 LEA proteins also bind a number of other metal ions; the interaction was proposed to occur through His residues which are over-represented in most group 2 LEA proteins [144] . Binding of metals ions by group 2 LEA proteins may be linked to the antioxidant properties reported for citrus CuCOR19 protein and in vitro scavenging activity for hydroxyl radicals [145] . LEA proteins might reduce oxidative stress in dehydrating cells by scavenging ROS and/or by sequestering metal ions that generate ROS [39] .

6.4. Other Functions

LEA proteins might act as hydration buffers, slowing down the rate of water loss during dehydration; during partial drought, osmotic or freezing stress, hydration buffers allow sufficient water activity for proteins to retain function [23] [32] . Using a knockout mutant of Arabidopsis, whose seed exhibited premature dehydration, a role for group1 LEA protein Atm6 as hydration buffer was proposed [95] . In a desiccating cell, when the water content falls below 10% on a dry weight basis, the cytoplasm vitrifies and enters in the “glassy state” [146] . In plants the formation of intracellular glasses is indispensable for survival in the dry state (seeds and pollens). LEA proteins accumulate to high levels in seeds (2% - 4% of the water soluble proteome) [116] and they increase the density of the sugar glasses by strengthening the hydrogen-bonding of the sucrose/LEA mixture [146] . Thus a potential role of LEA proteins is their contribution to the formation of biological glasses.

While among LEA proteins, some proteins which display similar sequences show different structure and in vitro properties, a prevalent issue is that a single LEA protein might have more than one function. The chloroplast LEA-like protein COR15am protects both membranes and proteins [137] [147] ; the mitochondrial group 3 protein LEAM, also exhibits both functions [90] ; group 2 from Citrus shows ionic binding, antioxidant and nucleic acid binding properties [145] [148] . This versatility may be a general feature of LEA proteins. Performance of more than one function, “moonlightening”, is not uncommon among proteins and it is more likely to evolve in unfolded, rather than in folded proteins [78] .

7. Biotechnological Applications of LEA Genes

Numerous transgenic studies revealed a positive effect of LEA gene expression on plant stress tolerance. In general, the phenotypes of the transgenic plants show enhanced stress tolerance, often related to drought or salt stress. Most studies report enhanced growth rates and reduced wilting of the aerial parts under stress under laboratory conditions and in some field trials, demonstrating a real potential of LEA proteins in engineering crops more tolerant to stress [126] . Apart from agronomical purposes, LEA proteins could be useful for other biotechnological applications in relation to their capacity to prevent aggregation of proteins. The use of a group 3 LEA protein as a fusion partner facilitates recombinant expression of recalcitrant proteins in E. coli in a soluble form [149] . Moreover, the anti-aggregation properties of another group 3 LEA protein have been applied to reduce the formation of in vivo aggregates; thus, coexpression of aggregation-prone proteins containing long polyglutamine (polyQ) or polyalamine (polyA) sequences, with group 3 AavLEA1 LEA protein in mammalian cells reduces substantially the expansion of protein aggregates associated to neurodegenerative diseases [150] .

8. Conclusion and Future Prospects

With increasing data from diverse research fields, LEA appears to be an amazingly versatile family of proteins presumably due to their intrinsically unstructured character. They exhibit myriads of functions (e.g., chaperone, cryoprotective, antifreeze, radical-scavenging, ion-binding functions) when exposed to various stress factors, including drought, high-salinity stress, low-temperature stress, heavy-metal stress, and perhaps also to biotic stresses. Despite the relevant progress made toward understanding the role of LEA, the molecular mechanisms through which they can enhance stress tolerance remain unknown. Future work should broadly examine other LEA to learn whether LEA-dependent regulatory mechanisms modulate stress responses.

Acknowledgements

This work was supported jointly by grants from the Ministry of Higher Education and Scientific Research, Tunisia and the Agence Espagnole de cooperation Internationale (AECI) Officina Técnica de Cooperacion, Spain.

Conflict of Interest

The authors declare that they have no conflict of interest.

References

NOTES

^*Corresponding author.