LEA proteins are the Late Embryogenesis Abundant proteins. As the name suggests, LEA proteins were discovered in the later stages of the development of the embryo in cotton seeds. These proteins are found in animals and plants protecting other proteins from forming aggregation due to desiccation and cold caused by osmotic stresses and low-temperature stresses respectively. During dehydration, they protect mitochondrial membranes. In response to low-temperature stress, there hasn’t any solid mechanism established yet, but conformational changes in transcription factors or proteins of the integral membrane have been suggested instead. Though LEA proteins are abundant in seeds and pollens, these proteins were initially discovered being accumulating in cotton seeds during late embryogenesis. In plants, most LEA proteins and their mRNAs accumulate to high concentrations in embryo tissues during the last stages of seed development at the time of desiccation.

Characteristics of LEA proteins

LEA proteins are generally characterized as;

1. Highly hydrophilic, highly soluble, globular in nature
2. Having a biased amino acid composition i.e. rich in Ala, Gly and lacks Cyt and His
3. Having a high number of polar residues
4. LEA proteins are mainly low molecular weight (10-30 kDa) proteins
5. Suppress the proteins to form aggregation during osmotic and moisture stress, in which LEA proteins form helix structures
6. Prevents aggregation of proteins due to water stress

Classification of LEA proteins

LEA proteins are generally classified into seven groups (families) according to their amino acid sequence and corresponding mRNA homology, which are basically localized in the cytoplasm and nuclear region.

Group-1: Rich in glycine, accumulation in embryo

Group-2: Dehydrins, rich in polar amino acids, counter salt, cold and drought stress

Group-3: Rich in 11 mer-amino acid sequence, regulation by ABA

Group-4: Conserved N terminus, 70-80 residue long, found in dry embryos

Group-5: Rich in hydrophobic residues, counter wounding & UV stress, found in late stages of seed

Group-6: Highly conserved, hydrophilic with 4 motifs

Group-7: Hydrophilins, intrinsically unstructured, salt stress

Gene regulation and expressions

Most of the genes encoding the LEA proteins are having the abscisic acid response or low-temperature response factors in their promotor and are induced by moisture stress and low-temperature stress.

No tissue-specific LEA gene expression has been considered as one main regulatory mechanism on the basis of extensive studies with the model plant, Arabidopsis thaliana. The study of the regulatory mechanism of LEA gene expression is an important feature of modern plant molecular biology. LEA genes have been found in several plant spp. There are 51 genes encoding LEA proteins in arabidopsis. LEA protein synthesis, expression, and biological activities are regulated by many factors i.e. developmental stages, hormones, ion exchange, dehydration, signal transduction pathways, etc.

Functions of LEA proteins

There are extensive research outcomes found on the expression of LEA proteins or their genes protecting other proteins from aggregation during stress. It has also been observed that the introduction of heterogenous LEA proteins into plants and micro-organisms results in enhanced stress tolerance. Transgenic approaches have also shown over-expression of these proteins in different species of rice, wheat, maize, tobacco, cabbage, etc. improves the ability of their phenotypes to resist plant stress. LEA proteins act as a buffer for hydration, membrane protectant, anti-oxidants, chelator, stabilizer, etc. The general importance of LEA proteins for anhydrobiosis led us to develop different strategies to explore the role of these enigmatic proteins.

Protein protection

The main function of LEA protein is its capacity to protect target proteins from aggregation and deactivation during moisture stress. This protective mechanism is supported by in-vitro preservation of enzymes after partial dehydration and desiccation or freezing.

Membrane protection function

Membrane protection is vital to prevent cell damage from the desiccation to preserve organellar or cellular integrity. This may be supported by the sugar to H-bonding networking of LEA protein in the dry state.

Ion binding and anti-oxidant function

LEA proteins sequester ions because they contain many charged amino acid residues.

Other functions

LEA protein is believed to act as a buffer in slowing down the rate of water loss during dehydration. In a desiccating cell, the cytoplasm enters into the glassy state, which is an indispensable survival mechanism in seeds and pollens of plants when the water content declines to below 10%. LEA proteins accumulate in higher levels in seeds i.e. 2-4% of the water-soluble proteome, which increases the density of sugar glasses by strengthening the H-bonding of the LEA-sucrose mixture.

Conclusion

LEA proteins or Late Embryogenesis Abundant proteins are found to prevent other proteins from aggregation during desiccation due to different abiotic stresses i.e. osmotic stress, water stress, and low-temperature stress. They play certain roles like buffering of water or water status stabilization, cytosolic structure production, transporting nuclear-targeted proteins, prevention of membrane leakage, etc. LEA type genes are accumulated in plant seeds during the late stage of embryogenesis, e.g. In Barley, group 3 LEA gene, HVA1 was overexpressed to increase drought tolerance. This gene conferred tolerance to soil water deficit and salt stress in transgenic rice in which the gene has transferred from barley to the rice plant.