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