In plants, organ shape is determined by organised and regulated control of cell expansion together with cell division. Emerging data support the view that the direction and magnitude of the enlargement of the primary cell wall largely determine the expansion pattern and thereby the final shape and size of the cells. Cell wall proteins are believed to play important roles in regulating cell wall extensibility which is a key parameter determining cell expansion. Among cell wall proteins studied to date, expansins are unique in the ability to induce immediate cell wall extension in vitro and cell expansion in vivo. Expansin genes have been identified in many plant species and make up a large superfamily that is divided into two major families α- and β-expansins, on the basis of the sequence divergence and biochemical activity. Studies of expansin gene expression indicate that the different expansin genes are expressed in different organs, tissues and cell type, and they respond distinctively to such treatments as hormones, light, pollination. The reverse genetics approach based on transposon mutagenesis was used to study the function of Petunia hybrida expansin genes. Four expansin genes (PhEXP2, PhEXP3, PhEXP4, PhEXP5) and the corresponding insertion mutants were isolated by the screening of large populations of W138 petunia plants, characterised by high copy number and high transposition activity of dTph1 element. Expression analysis of these genes in different plant organs and phenotypical analysis of the expansin insertion mutants were performed.
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