BMC Genomics 2009, 10:415 http://www.biomedcentral.com/1471-2164/10/415 ever, ESTs encoding Ras small GTPase and RAB1X were identified in both the genotypes. GTP binding proteins are reported to be associated with a wide range of growth and developmental processes [36] besides their role in plant defense [37]. Genes involved in metabolism Developmental changes and stress responses are often correlated with or result in adjustments in various meta- bolic pathways. The functional class of metabolism com- prised of about 14.56% of the unigenes and represented the largest class apart from NSH and hypothetical pro- teins. The genes present in this class represented several biochemical pathways such as carbohydrate, fatty acid, energy and protein metabolism besides nitrogen and nucleotide metabolism. The genes involved in carbohy- drate metabolism comprised alcohol dehydrogenase (ADH), glyceraldehyde3-phosphate dehydrogenase (GAPDH), enolase, fructose-bisphosphate aldolase, malate dehydrogenase, phosphoenol pyruvate (PEP) car- boxylase and triosephosphate isomerase and were present in both the genotypes. Most of these enzymes are known to show altered expression in response to stress [38,39]. Though very little is known about the direct role of these proteins in defense, many such enzymes are involved in maintaining metabolite pool that may drive the meta- bolic processes to overcome such stresses. Polygalactouro- nase inhibiting protein (PGIP) was present in both the susceptible and resistant genotypes. The PGIPs are cell wall proteins, which act against fungal polygalactouro- nases that are important pathogenecity factors [40]. Besides their classical function, they also form long chain oligogalactouronides and facilitate the activation of plant defense responses [41]. We observed the presence of 1,3- glucanase, a PR 2 family protein [42], which was earlier shown to be expressed in response to pathogen infection [43]. The genes involved in lipid biosynthesis, for example, acyl Co-A synthetase and acyl Co-A binding proteins were found to be present in both the genotypes. The enzyme acyl Co-A synthetase has been shown to be induced in response to compatible and incompatible interactions between Xanthomonas and Capsicum annum [44], while Acyl Co-A binding proteins play an important role in intracellular transport and formation of acyl Co-A pools [45]. ESTs encoding desaturases in the CaUnigene set showed varied substrate specificity between the two geno- types. For example, oleate and sterol desaturases were found in the susceptible genotype and putative desatu- rase-like protein was present in the resistant one. Desatu- rases catalyze the formation of double bonds in lipids thereby increasing the lipid fluidity, which might be required as an adjustment associated with membrane sta- bility under progressive wilting. ESTs corresponding to other lipid metabolism genes such as serine C-palmitoyl- transferase, was also found to be present in stressed tis- sues. Identification of sterol metabolism related enzymes such as sterol methyl transferases and oxysterol binding proteins suggest the occurrence of lipid modifications during pathostress. Presence of HMG Co-A reductase which catalyzes the synthesis of mevalonate, a specific precursor of plant defense metabolite sesquiterpene phy- toelexins, is indicative of its role in cell defense. Several amino acids act as precursors for the synthesis of specialized metabolites, during varied cellular adapta- tions. ESTs encoding enzymes involved in amino acid metabolism, for example, arginine decarboxylase, aspar- tate aminotransferase, and cysteine synthase were identi- fied in both the genotypes. Recent evidence suggests that arginine decarboxylase, involved in putrescine biosynthe- sis, is induced in response to various environmental stresses [46,47]. Proline dehydrogenase and many pro- line-rich proteins were identified in resistant genotype. Proline dehydrogenase is the rate-limiting enzyme in pro- line degradation and serves important functions in stress responses [48]. Nucleotide metabolism related genes such as nuleoside diphosphate kinase (NDK), adenine nucleotide transloca- tor and polynucleotide phosphorylase were also identi- fied in both the genotypes. Previously, it has been shown that the overexpressing NDK provides higher ability to eliminate H2O2, indicating its potential role in the man- agement of reactive oxygen species under stress [49]. Genes involved in cellular transport and homeostasis Stress-induced reorganization and spatial distribution of many key metabolites in plants require efficient transport machinery. It is well known that Arabidopsis has diverse array of genes for multi-efflux transport and response to stress signals, and rice has more secondary transporter genes for carbohydrate and nutrient transport [50]. Vari- ous transport associated genes were identified in this study. Of these, clathrin adaptor complexes and putative polyol transporter protein 4 were expressed in the suscep- tible genotype while multidrug resistance protein, Sec Y protein, ABC transporter family protein, Ctr copper trans- porter, SKS3 copper ion binding protein showed expres- sion only in the resistant genotype. Although few of these transport associated proteins showed genotype specificity, others like general substrate transporter, intracellular chloride channel, phosphate transporter 5, were repre- sentative of both the genotypes. ABC transporters and multidrug resistance genes are known to function in plant defense [51,52], while the role of other above mentioned genes in plant defense is yet to be discovered. The chloride channel was reported to be activated by CDPK in guard cell [53] whereas the chloride transporter has been found Page 6 of 21 (page number not for citation purposes)