Isolation and characterisation of two wheat beta-expansin genes expressed during male gametophyte development

Two novel beta-expansin genes, TaEXPB1 and TaEXPB2, were isolated from wheat microspores by suppression subtractive hybridisation. Northern blot and reverse transcription PCR analyses showed that the expression of both genes was restricted to early stages of male gametophyte development (from microspores to immature pollen). A homology search showed high similarity of the newly discovered genes to generative beta-expansins in grass pollen (group 1 pollen allergens). Southern hybridisation revealed that the isolated genes belong to a distinct group within the subfamily of beta-expansin genes in the wheat genome. A comparison of full-length cDNAs with the corresponding genomic sequences showed that there are two introns in the TaEXPB1 gene, whereas TaEXPB2 has three introns. Both genes were predicted to encode highly similar basic proteins (pI 9.0) with molecular masses of approximately 29 kDa consisting of a signal peptide, catalytic, and polysaccharide binding domains, which include conserved cysteines and tryptophans and motifs characteristic for beta-expansins.