**7. Stress genes**

**6.4. Acyl-CoA binding protein**

110 Abiotic and Biotic Stress in Plants - Recent Advances and Future Perspectives

**6.5. Catechol-O-methyltransferase**

Acyl-CoA binding protein (ACBP) is a highly conserved protein. In yeast it is involved in transportation of acyl-CoA esters from the fatty acid synthetase to acyl-CoA-consuming process [70]. The protein, which was first identified in mammals, acts as a neuropeptide that prevents binding of diazepam/endozepine to GABA receptor system [71-72] and is also involved in regulation of several acyl-CoA-dependent processes [73]. In addition, ACBP is involved in many other functions, which include regulating biosynthesis of fatty acid, functional regulation of enzymes and genes, intracellular acyl-CoA pool regulation, acyl-CoA esters donation required for β-oxidation and vesicular trafficking [74], and in regulation of mcalpain [75]. In plants *Arabidopsis thaliana*, different types of ACBPs are encoded [76], such as the ACBP1 and ACBP2, which are membrane-associated proteins [77-80]; ACBP3, which is the extracellularly targeted protein [81]; and ACBP4, ACBP5, and ACBP6, which are the cytosolic proteins [82-83]. ACBPs are involved in abiotic stress tolerance in plants. The ACBP2 in *Arabidopsis* was shown to be involved in heavy metal (Cd) tolerance [84-85]. ACBP6 and ACBP1 are functionally involved in increased freezing tolerance [78],[86]. In shrimp, ACBP functions in antibacterial [87] and antiviral response [88]. The ACBP gene in *P. monodon* was identified to be differentially expressed in the SSH libraries constructed from the gut tissues of both low (3ppt) and high (55ppt) salinity stressed shrimps. The complete cDNA sequence of ACBP consisted of 273 bp ORF coding for 90 amino and showed ligand-binding conserved domains similar to the other members of ACBP family. At 2 weeks post 3 ppt salinity stress conditions, a significant increase in the ACBP transcripts expression was observed in gills and muscle tissues with highest levels in the gut tissues (28.08-fold). Similar increase in the gene expression levels was observed in shrimps exposed to high salinity stress conditions of 55 ppt in gills and muscle tissues with gut tissues revealing high (11.95-fold) levels of gene expression [89]. These

results suggest a functional role of ACBP gene during salinity stress in shrimps.

O-methyltransferase (OMT) is an enzyme found in a wide range of organisms such as microbes [90], where it is involved in antibiotic biosynthesis [91], and in fungi, where it is involved in biosynthesis of aflatoxins [92]. The OMT found in plants are well characterized for their functional role in O-methylation during biosynthesis of lignin, stress resistance, and disease tolerance [93]. In crustaceans, farnesoic acid O-methyltransferase (FAMeT) catalyzes farnesoic acid methylation resulting in production of isoprenoid methyl farnesoate, which is involved in metabolic and physiological regulation [94]. Catechol-O-methyltransferase (COMT), which is a type of O-methyltransferases, helps in catalyzing the transfer of methyl group to the hydroxyl group of catechol compounds from S-adenosyl-L-methionine. In higher animals, the COMT helps in catalysis of methylation of various macromolecules that are involved in different functional and regulatory purposes and is present in soluble and membrane-bound forms [95]. In shrimps, the construction of suppression subtractive hybridization (SSH) libraries from *P. monodon* gill tissues resulted in identification of COMT gene as one of the differentially expressed genes subjected to salinity stress. The COMT gene was differentially regulated in both the SSH libraries generated from low and high salinity conditions. The ORF of COMT gene of 666 bp size revealed the coding protein with 221 amino acids [96]. The *P. monodon* COMT showed the conserved domains present in superfamily of S-adenosylmethio‐

### **7.1. Heat shock proteins**

Heat shock proteins (HSPs) initially discovered in *Drosophila melanogaster* are a highly con‐ served set of polypeptides present in both prokaryotic and eukaryotic cells. They are generally involved in conferring thermotolerance as molecular chaperones by refolding the denatured proteins and also respond against various other stresses. They play a crucial role in organisms' response to heat shock and cellular stress. In addition, the HSPs are also important for cellular damage protection and in maintaining cellular homeostasis [98]. In aquatic animals, the HSPs respond to environmental pollutants, abiotic stress, and are involved in disease resistance against viral and bacterial pathogens. The functional role and significance of HSP in farmed aquatic animals is demonstrated in stimulating the immune response [99]. In shrimp (*P. monodon*), some of the HSPs such as HSP21 have been characterized for gene expression against WSSV infection [100]; HSP70 gene expression was found to increase in the shrimp hemocytes after heat shock treatment [101]; and HSP90 gene expression has been related with the ovarian maturation [102]. The transcriptional levels of HSP21, HSP70, and HSP90 were inducible under the heat shock and responded upon bacterial exposure in *P. monodon* [103]. Expression of HSP70, which is one of the widely studied HSP in aquatic organisms, was high during shortterm hyperthermic treatment when compared to hypoxic and osmotic stress in *P. monodon* [104]. The significant increase in gene expression level HSP21 in the gut and muscle tissues of *P. monodon* exposed to low salinity stress (3 ppt) conditions, indicates its functional role in osmotic stress in shrimps [20].

In conclusion, the construction of SSH cDNA library in response to low (3 ppt) and high (55 ppt) salinity stress in shrimp (*P. monodon*) led to identification of various differentially expressed genes. The significant up-regulation expression of several genes at transcription level in gills, gut, antennal gland, and muscle tissues of shrimp in response to two-week postsalinity stress condition indicates their functional role in gene pathways and regulatory mechanism of osmotic stress at the molecular level.
