Molecular characterization and transcriptome profiling of expansin genes isolated from Calotropis procera fibers

The Calotropis procera seed fibers provide an excellent model system to study the genes involved in fiber elongation, fineness and strength. Expansins constitute one of the important gene families involved in plant cell expansion and other cell wall modification processes. Four homologs of Expansin A gene i.e. CpEXPA1, CpEXPA2, CpEXPA3 and CpEXPA4 were isolated from the cDNA library obtained from fast growing Calotropis procera fibers. These homologs represented typical Expansin A family. Each of them had two conserved domains including GH45 like domain and the putative polysaccharide binding domain. The deduced amino acid sequences of the homologs indicated three conserved motifs: i) eight cysteine residues at N-terminus, ii) four tryptophan residues at C-terminus and iii) a Histidine-Phenylalanine-Aspartate motif in the center of the sequence. The presence of N-terminal signal peptide consisting of hydrophobic amino acids and a transmembrane region in all these expansin isoforms suggests their cotranslational insertion into the endoplasmic reticulum and then transportation to the cell wall by secretory pathway. The relative quantification of the four expansins in root, stem, fiber and leave tissues indicated that the transcripts of CpEXPA1, CpEXPA2, CpEXPA3 and CpEXPA4 are variably transcribed in these tissues. The lowest transcription of all the four Expansin A isoforms was observed in elongating roots indicating that root tissue might be having specific expansins other than those confined to air grown organs.

Identification of differentially expressed genes in developing cotton fibers (Gossypium hirsutum L) through differential display

Cotton fibers are differentiated, non-dividing cells that originate from the epidermal layer of developing ovules. To identify genes involved in cotton fiber development, we performed non-radioactive differential display reverse transcriptase PCR (DDRT-PCR) on the purified mRNA. This technique was tested on mRNA isolated from five different developmental stages of cotton fiber including 0, 5, 10, 15 and 20 DPA (days after pollination). The mRNA purified from total RNA was reversibly transcribed using three anchored oligo-dT primers. Polymerase chain reaction (PCR) amplification of each cDNA preparation was carried out in combination with seven arbitrary primers. The amplified products were resolved on 1 percent agarose gel containing ethidium bromide. DNA was extracted from seventeen differentially expressed bands and cloned in pTZ57R/T vector. The sequencing and BLAST search analysis indicated that 12 of the differentially expressed genes matched the previously characterized genes, while 3 of them matched the uncharacterized sequences of cotton fiber expressed sequence tags (ESTs) reported previously to be associated with cotton fiber and 2 of the clones had homology with putative proteins. The technique can be used to efficiently identify differentially expressed genes and can be expanded to large scale studies by increasing the number of random decamers.