A tomato gene expressed during fruit ripening encodes an enzyme of the carotenoid biosynthesis pathway.

In the initial stages of carotenoid biosynthesis in plants the enzyme phytoene synthase converts two molecules of geranylgeranyl diphosphate into phytoene, the first carotenoid of the pathway. We show here that a tomato (Lycopersicon esculentum) cDNA for a gene (Psy1) expressed during fruit ripening directs the in vitro synthesis of a 47-kDa protein which, upon import into isolated chloroplasts, is processed to a mature 42-kDa form. The imported protein is largely associated with membranes, but it can be easily solubilized by dilution or by treatment at high pH. A plasmid construct containing prokaryotic promoter and ribosome-binding sequences fused to the Psy1 cDNA complements the carotenoidless phenotype of a Rhodobacter capsulatus crtB mutant. We conclude that Psy1 encodes phytoene synthase and that this enzyme is a peripheral plastid membrane protein.     

Ribonucleic acid metabolism during the development and ripening of tomato fruits

The accumulation of RNA in the outer locule tissue of tomato fruits was measured during development and ripening. Labelling studies suggest two peaks of synthesis, the first during early development and the second just before the onset of ripening (colour change). During the second period of increased RNA labelling the amount of total RNA per fruit either remains constant or starts to decline. Synthesis of rRNA and soluble RNA occurred at all stages. Polydisperse RNA containing polyadenylic acid was isolated and shown to direct the synthesis of protein in vitro. No significant changes in the amount of polyadenylic acid, relative to total RNA were detectable during the ripening period.     

The cloning of two tomato lipoxygenase genes and their differential expression during fruit ripening.

A membrane-associated lipoxygenase from breaker-stage fruit of tomato (Lycopersicon esculentum Mill.) was purified and partially sequenced. Using degenerate oligonucleotides corresponding to portions of this sequence, a cDNA was amplified by PCR and used to screen a breaker fruit cDNA library. Two clones, tomloxA and tomloxB, were isolated and one of these (tomloxA) corresponded to the isolated protein. Genomic clones were isolated and sequence data from these were used to obtain the 5' ends of the cDNAs. The 2.8-kb cDNAs encode proteins that are similar in size and sequence to each other and to other plant lipoxygenases. DNA blot analysis indicated that tomato contains three or more genes that encode lipoxygenase. RNA blot analysis showed that tomloxA is expressed in germinating seeds as well as in ripening fruit, where it reached its peak during breaker stage. tomloxB appears to be fruit specific and is at its highest level in ripe fruit.     

Endo-beta-mannanase activity increases in the skin and outer pericarp of tomato fruits during ripening

Activity of endo-beta-mannanase increases during ripening of tomato (Lycopersicon esculentum Mill.) fruit of the cultivar Trust. beta-Mannoside mannohydrolase is also present during ripening, but its pattern of activity is different from that of endo-beta-mannanase. The increase in endo-beta-mannanase activity is greatest in the skin, and less in the outer and inner pericarp regions. This enzyme is probably bound to the walls of the outermost cell layers of the fruit during ripening, and it requires a high-salt buffer for effective extraction. The enzyme protein, as detected immunologically on Western blots, is present during the early stages of ripening, before any enzyme activity is detectable. The mRNA for the enzyme is also present at these stages; endo-beta-mannanase may be produced and sequestered in a mature-sized inactive form during early ripening. Most non-ripening mutants of tomato exhibit reduced softening and lower endo-beta-mannanase activity, but a cause-and-effect relationship between the enzyme and ripening is unlikely because some cultivars which ripen normally do not exhibit any endo-beta-mannanase activity in the fruit.     

Polygalacturonase-inhibiting proteins in plant fleshy fruits during their ripening and infections

During ripening of fleshy fruits, changes in tissue consistency are largely due to the functioning of the enzyme polygalacturonase (PG) digesting polygalacturonan in cell-wall pectin. Polygalacturonase-inhibiting proteins (PGIP) have been found in plants as proteins interacting with PG, which is secreted by pathogenic microorganisms. PGIP are glycoproteins comprising sequences enriched in leucine repeats. Since PG is one of the main factors of pathogenicity, it is supposed that PGIP are involved in processes hampering plant disease development. PGIP presence in the apoplast of essentially all plant tissues implies their involvement in biochemical processes occurring in the cell walls. This review considers PGIP role in plant fleshy fruits, where the cell-wall composition and structure are of importance for fruit ripening, storage, and resistance to diseases.     

Translocation and the alternative D-galacturonate pathway contribute to increasing the ascorbate level in ripening tomato fruits together with the D-mannose/L-galactose pathway

The D-mannose/L-galactose pathway for the biosynthesis of vitamin C (L-ascorbic acid; AsA) has greatly improved the understanding of this indispensable compound in plants, where it plays multifunctional roles. However, it is yet to be proven whether the same pathway holds for all the different organs of plants, especially the fruit-bearing plants, at different stages of development. Micro-Tom was used here to elucidate the mechanisms of AsA accumulation and regulation in tomato fruits. The mRNA expression of the genes in the D-mannose/L-galactose pathway were inversely correlated with increasing AsA content of Micro-Tom fruits during ripening. Feeding L-[6-(14)C]AsA to Micro-Tom plants revealed that the bulk of the label from AsA accumulated in the source leaf was transported to the immature green fruits, and the rate of translocation decreased as ripening progressed. L-Galactose feeding, but neither D-galacturonate nor L-gulono-1,4-lactone, enhanced the content of AsA in immature green fruit. On the other hand, L-galactose and D-galacturonate, but not L-gulono-1,4-lactone, resulted in an increase in the AsA content of red ripened fruits. Crude extract prepared from insoluble fractions of green and red fruits showed D-galacturonate reductase- and aldonolactonase-specific activities, the antepenultimate and penultimate enzymes, respectively, in the D-galacturonate pathway, in both fruits. Taken together, the present findings demonstrated that tomato fruits could switch between different sources for AsA supply depending on their ripening stages. The translocation from source leaves and biosynthesis via the D-mannose/L-galactose pathway are dominant sources in immature fruits, while the alternative D-galacturonate pathway contributes to AsA accumulation in ripened Micro-Tom fruits.     

类胡萝卜素的生物合成途径及生物学功能研究进展

类胡萝卜素具有重要的生物学功能,一直是研究的热点。综述近十年来对植物类胡萝卜素的生物合成途径,并对其生物学功能参与光系统的组装、非光化学猝灭、基因表达、抗氧化性和维持膜的稳定性等五个方面的研究进展作一介绍。     

Evidence for the involvement of sucrose phosphate synthase in the pathway of sugar accumulation in sucrose-accumulating tomato fruits

To better understand the mechanism of sugar unloading and sugar concentration in hexose- and sucrose-accumulating tomato fruits (Lycopersicon chmielewskii and L. esculentum, respectively) and to determine the causes of the late accumulation of sucrose present in sucrose-accumulating tomato fruits, the assimilation of [³H](fructosyl)-sucrose was studied. Key enzymes involved in carbohydrate metabolism were also assayed. The results demonstrated that the low level of sucrose present in young fruits accumulates directly without undergoing hydrolysis, suggesting a symplastic pathway for sucrose unloading. By contrast, the large quantity of the sucrose present in ripe sucrose-accumulating fruits originates from hydrolysis and resynthesis, suggesting an apoplastic pathway for sucrose unloading. The increase in sucrose level observed in sucrose-accumulating fruits is associated with a gradual decline in invertase activity and an increase in sucrose phosphate synthase activity. This latter enzyme seems to play a key biochemical role in the accumulation of sucrose and the establishment of a high sugar content in tomato fruits.     

Light signaling genes and their manipulation towards modulation of phytonutrient content in tomato fruits

Due to its economic importance, ease of genetic manipulation, cultivation and processing, the tomato plant has been a target for increasing and diversifying content of fruit phytonutrients by transgenic and non-transgenic approaches. The tomato high pigment (hp) mutations exemplify the latter alternative and due to their positive effect on fruit lycopene content, they were introgressed into elite tomato germplasm for cost effective extraction of this important carotenoid. Interestingly, hp mutant fruits are also characterized by higher fruit levels of other functional metabolites, phenotypes caused by mutations in central genes regulating light signal-transduction. This gene identification suggests that modulation of light signaling machinery in plants may be highly effective towards manipulation of fruit phytonutrients but has never been thoroughly reviewed. This review therefore summarizes the progress which has been made on this valuable approach, emphasizing the consequences of transgenic modulation of light signaling components on the functional properties of the tomato fruit.     

Quantitative changes in tRNA during ethylene induced ripening (ageing) of tomato fruits

Iso-accepting forms of tRNA^met, tRNA^leu, tRNA^lys, and tRNA^tyr were isolated from combined walls and septa of tomato fruits at 5 consecutive stages of ethylene induced ripening. Changes in the relative amount of some tRNA^leu and tRNA^lys were discerned 10hr after exposure to ethylene. Individual patterns of change for each of several iso-acceptor tRNAs were evident throughout the ripening sequence. Maximal changes were: tRNA^lys, ?66.3%; tRNA^leu, ?24.8%; and tRNA^met, +26.7%.