The role of the ripening-specific expansin Exp1 protein in fruit softening and cell wall metabolism was investigated by suppression and overexpression of Exp1 in transgenic tomato plants. Fruit in which Exp1 protein accumulation was suppressed to 3% that of wild-type levels were firmer than controls throughout ripening. Suppression of Exp1 protein also substantially inhibited polyuronide depolymerization late in ripening but did not prevent the breakdown of structurally important hemicelluloses, a major contributor to softening. In contrast, fruit overexpressing high levels of recombinant Exp1 protein were much softer than controls, even in mature green fruit before ripening commenced. This softening was correlated with the precocious and extensive depolymerization of structural hemicelluloses, whereas polyuronide depolymerization was not altered. These data are consistent with there being at least three components to fruit softening and textural changes. One component is a relaxation of the wall directly mediated by Exp1, which indirectly limits part of a second component due to polyuronide depolymerization late in ripening, perhaps by controlling access of a pectinase to its substrate. The third component is caused by depolymerization of hemicelluloses, which occurs independently of or requires only very small amounts of Exp1 protein. INTRODUCTIONPlant cell growth is constrained by the cell wall, whose rigid cellulose microfibrils are held together by three structurally independent but interacting matrices (Carpita and Gibeaut, 1993). The matrix glycans, or hemicelluloses, are polysaccharides composed of neutral sugars, a major constituent of which in dicotyledonous species is xyloglucan. Xyloglucan molecules coat and cross-link cellulose microfibrils and, by extensive noncovalent bonding, anchor the microfibrils relative to one another. The xyloglucan-cellulose framework is itself embedded in a pectin matrix composed of polyuronide molecules, together with a domain of structural glycoproteins. Precisely how cell wall components and their intermolecular interactions are changed to allow wall loosening during growth or remodeling during cell development is not known, but a group of highly conserved cell wall proteins called expansins have been implicated in many aspects (Cosgrove, 1998).Expansins are encoded by large multigene families (Shcherban et al., 1995; Cho and Kende, 1997; Cosgrove et al., 1997;Brummell et al., 1999a), and the expression of expansin mRNA and protein is correlated with growth in many tissues of the plant, including hypocotyls (McQueen-Mason et al., 1992), coleoptiles (Li et al., 1993), internodes (Cho and Kende, 1997), leaves (Keller and Cosgrove, 1995), roots (Wu et al., 1996), and green fruit (Brummell et al., 1999a). However, expansins also may play a role in the cell wall modification involved in other aspects of plant development. In tomato, expansin Exp1 mRNA is expressed at high levels specifically during fruit ripening (Rose et al., 1997) and is the major expansin gene family ...

Promoter DNA sequences from a petunia chlorophyll a/b binding protein gene were fused to octopine synthase DNA sequences and the resulting chimaeric genes were introduced into petunia and tobacco cells. Populations of transformed regenerated petunia plants containing the chimaeric genes were examined so that the expression of any particular construction could be compared between independent transformants. Substantial variation was observed between transformants in the level of chimaeric gene expression. In general, transcriptional fusions in which a linker sequence interrupted the 5′‐untranslated region gave rise to less chimaeric mRNA accumulation than a translational fusion. In the most actively expressing transformants the amount of mRNA from the introduced chimaeric genes was half that of the endogenous wild‐type gene. Transcription initiated at the same place in the chimaeric and endogenous genes. Construction of the translational cab/ocs fusion caused three amino acid changes in the octopine synthase protein and functional octopine synthase enzyme was absent from plants in which mRNA for the chimaeric gene was abundantly expressed.

Of the eight nuclear genes in the plant multi‐gene family which encodes the small subunit (rbcS) of Petunia (Mitchell) ribulose bisphosphate carboxylase, one rbcS gene accounts for 47% of the total rbcS gene expression in petunia leaf tissue. Expression of each of five other rbcS genes is detected at levels between 2 and 23% of the total rbcS expression in leaf tissue, while expression of the remaining two rbcS genes is not detected. There is considerable variation (500‐fold) in the levels of total rbcS mRNA in six organs of petunia (leaves, sepals, petals, stems, roots and stigmas/anthers). One gene, SSU301, showed the highest levels of steady‐state mRNA in each of the organs examined. We discuss the differences in the steady‐state mRNA levels of the individual rbcS genes in relation to their gene structure, nucleotide sequence and genomic linkage.

We have analyzed the polyadenylation sites for the small subunit of ribulose bisphosphate carboxylase and chlorophyll a/b binding protein genes of Petunia (Mitchell) and the bronze gene of Zea mays. Sequence analysis of multiple cDNA clones revealed that polyadenylation of the transcripts occurred at either 2 or 3 sites for all three groups of genes. In the examples where 3 polyadenylation sites were detected, the middle site was the one predominantly used. Putative polyadenylation signals preceding the poly A tails diverged significantly from the animal consensus sequence AATAAA. In all the genes examined the first A residue in the poly A tail of the cDNA clones corresponded to an A residue in the homologous genomic sequence.

The enzyme acetolactate synthase (ALS) is the target enzyme for the sulfonylurea and imidazolinone herbicides. We describe the isolation and characterization of the ALS genes from two herbicide‐resistant mutants, C3 and S4‐Hra, of Nicotiana tabacum. There are two distinct ALS genes in tobacco which are 0.7% divergent at the amino acid sequence level. The C3 mutant has a single Pro–Gln replacement at amino acid 196 in one ALS gene. This gene is termed the class I gene and is equivalent to the SuRA locus. The S4‐Hra mutant has two amino acid changes in the other ALS gene. This gene is termed the class II gene or the SuRB locus. The S4‐Hra mutant includes a Pro–Ala substitution at amino acid 196 and a Trp–Leu substitution at amino acid 573. Gene reintroduction experiments have confirmed that these amino acid substitutions are responsible for the herbicide resistance phenotypes. Transgenic plants carrying these genes are highly resistant to sulfonylurea herbicide applications.