Differential expression of expansin gene family members during growth and ripening of tomato fruit

cDNA clones encoding homologues of expansins, a class of cell wall proteins involved in cell wall modification, were isolated from various stages of g rowing and ripening fruit of tomato (Lycopersicon esculentum). cDNAs derive d from five unique expansin genes were obtained, termed tomato Exp3 to Exp7 , in addition to the previously described ripening-specific tomato Exp1 (Ro se et nl. (1997) Proc Natl Acad Sci USA 94: 5955-5960). Deduced amino acid sequences of tomato Exp1, Exp4 and Exp6 were highly related, whereas Exp3, Exp5 and Exp7 were more divergent. Each of the five expansin genes showed a different and characteristic pattern of mRNA expression. mRNA of Exp3 was present throughout fruit growth and ripening, with highest accumulation in green expanding and maturing fruit, and lower, declining levels during ripe ning. Exp4 mRNA was present only in green expanding fruit, whereas Exp5 mRN A was present in expanding fruit but had highest levels in full-size maturi ng green fruit and declined during the early stages of ripening. mRNAs from each of these genes were also detected in leaves, stems and flowers but no t in roots. Exp6 and Exp7 mRNAs were present at much lower levels than mRNA s of the other expansin genes, and were detected only in expanding or matur e green fruit. The results indicate the presence of a large and complex exp ansin gene family in tomato, and suggest that while the expression of sever al expansin genes may contribute to green fruit development, only Exp1 mRNA is present at high levels during fruit ripening.