Plant Molecular Biology 39: 161–169, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. 161 Differential expression of expansin gene family members during growth and ripening of tomato fruit David A. Brummell ∗ , Mark H. Harpster and Pamela Dunsmuir DNA Plant Technology, 6701 San Pablo Avenue, Oakland, CA 94608, USA ( ∗ author for correspondence) Received 22 April 1998; accepted in revised form 11 August 1998 Key words: expansin, fruit growth, fruit softening, gene expression, Lycopersicon esculentum Abstract cDNA clones encoding homologues of expansins, a class of cell wall proteins involved in cell wall modification, were isolated from various stages of growing and ripening fruit of tomato (Lycopersicon esculentum). cDNAs derived from five unique expansin genes were obtained, termed tomato Exp3 to Exp7, in addition to the previously described ripening-specific tomato Exp1 (Rose et al. (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 ripening. Exp4 mRNA was present only in green expanding fruit, whereas Exp5 mRNA was present in expanding fruit but had highest levels in full-size maturing 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 not in roots. Exp6 and Exp7 mRNAs were present at much lower levels than mRNAs of the other expansin genes, and were detected only in expanding or mature green fruit. The results indicate the presence of a large and complex expansin gene family in tomato, and suggest that while the expression of several expansin genes may contribute to green fruit development, only Exp1 mRNA is present at high levels during fruit ripening. Introduction The growth of expanding plant cells is controlled largely by the pliancy of the primary cell wall, which is composed of rigid cellulose microfibrils coated with, and tethered together by, a matrix of hemicellulosic glycan polysaccharides, all embedded in a coextensive matrix of pectins with smaller amounts of structural glycoproteins [3]. Since the extensibility of the cell wall constrains expansion, cell wall loosening is nec- essary in order to allow turgor-driven cell enlargement to occur [7, 8]. The walls of expanding vegetative tissues possess numerous enzyme activities that may contribute to a modification of cell wall mechanical The nucleotide sequence data reported will appear in the EMBL, GenBank and DDBJ Nucleotide Sequence Databases under the accession numbers AF059487 (tomato Exp3), AF059488 (tomato Exp4), AF059489 (tomato Exp5), AF059490 (tomato Exp6) and AF059491 (tomato Exp7). properties, including endo-1,4-β -glucanases [17], xy- loglucan endotransglycosylases [12], and expansins [29]. Of these, only expansins exhibit the ability to evoke growth when added to cell walls, as shown by increased cell wall loosening when purified expansin protein was added either to heat-inactivated hypocotyl cell walls in vitro or to apical meristems in vivo [11, 29]. In addition to vegetative tissues some expansins are also expressed in fruit, since in tomato the mRNA of a divergent expansin gene accumulated specifically during the ripening process [32]. While the activities of endo-1,4-β -glucanases [19, 26] and xyloglucan en- dotransglycosylases [1, 26] have been found both in growing green and in ripening red tomato fruit, the potential contribution of expansins to the growth of developing green fruit has not yet been examined. Expansins are a highly conserved group of cell wall-localized proteins that appear to mediate changes in cell wall plasticity during cell expansion or dif-