Association between Elemental Content and Fruit Ripening in rin and Normal Tomatoes

Analysis of Ca and other inorganic ions in the pericarp of rin, a nonripening mutant, and normal tomato (Lycopersicon esculentum Mill) fruits revealed significant differences in their accumulations at advanced stages of fruit development. During early stages of fruit development, soluble Ca was higher in Rutgers and there were no detectable changes in the accumulation patterns of the other inorganic ions. In the mutant rin, bound Ca continued to increase with age and it was twice as high as compared to earlier stages. In the normal tomato, bound Ca decreased about 3-fold at later stages of development. Mg and Mn also showed some changes similar to Ca. K continued to increase with age and the mutant rin had lower levels than Rutgers throughout development. Other ions such as P, Zn, Cu, and Co were similar in the mutant and normal fruits. These results are interpreted as indicating that high levels of bound divalent cations in the mutant rin may be associated with an altered membrane and cell wall and play a role in fruit ripening.     

Ethylene and fruit ripening

The latest advances in our understanding of the relationship between ethylene and fruit ripening are reviewed. Considerable progress has been made in the characterisation of genes encoding the key ethylene biosynthetic enzymes, ACC synthase (ACS) and ACC oxidase (ACO) and in the isolation of genes involved in the ethylene signal transduction pathway, particularly those encoding ethylene receptors ( ETR ). These have allowed the generation of transgenic fruit with reduced ethylene production and the identification of the Nr tomato ripening mutant as an ethylene receptor mutant. Through these tools, a clearer picture of the role of ethylene in fruit ripening is now emerging. In climacteric fruit, the transition to autocatalytic ethylene production appears to result from a series of events where developmentally regulated ACO and ACS gene expression initiates a rise in ethylene production, setting in motion the activation of autocatalytic ethylene production. Differential expression of ACS and ACO gene family members is probably involved in such a transition. Finally, we discuss evidence suggesting that the NR ethylene perception and transduction pathway is specific to a defined set of genes expressed in ripening climacteric fruit and that a distinct ETR pathway regulates other ethylene-regulated genes in both immature and ripening climacteric fruit as well as in non-climacteric fruit. The emerging picture is one where both ethylene-dependent and -independent pathways coexist in both climacteric and non-climacteric fruits. Further work is needed in order to dissect the molecular events involved in individual ripening processes and to understand the regulation of the expression of both ethylene-dependent and -independent genes.     

Comparison of Propylene-induced Responses of Immature Fruit of Normal and rin Mutant Tomatoes

Continuous application of propylene to 40 to 80% mature fruits of normal tomato strains (Lycopersicon esculentum Mill.) advanced ripening in fruits of all ages by at least 50%. Although preclimacteric respiration was stimulated by propylene treatment, there was no concomitant increase in ethylene production. Once ripening commenced, the rates of endogenous ethylene production were similar in both propylene-treated and untreated fruits. Continuous exposure to propylene also stimulated respiration in immature fruits of rin, a nonripening mutant. Although respiration reached rates similar to those during the climacteric of comparable normal fruits there was no change in endogenous ethylene production which remained at a low level. Internal ethylene concentrations in attached 45 to 75% mature fruits of rin and a normal strain were similar. It is suggested that the onset of ripening in normal tomato fruit is not controlled by endogenous ethylene, although increased ethylene production is probably an integral part of the ripening processes.     

Stock-Scion Interactions of Normal and Fruit Ripening Mutants rin and nor in Tomato

Scions of the non-ripening rin and nor tomato strains (Lycopersicum esculentum Mill.) were grafted on normal understock plants (cv. Rutgers) in an effort to study the influence of roots and vegetative tissue on the ripening behavior of the tomato fruit. Receiprocal grafts of ‘Rutgers’ scions on rin and nor understocks as well as grafted and ungrafted controls were also established. No alteration in the ethylene, and CO₂ evolution and color development of either mutant fruits on normal understock or of normal fruits on mutant understock occurred. We suggest that the inability of rin and nor mutant fruits to ripen normally stems either from the presence in mutant fruit of a non-translocatable ripening inhibitor, or from the absence of a non-translocatable ripening factor.     

Effect of Sodium Chloride on Fruit Ripening of the Nonripening Tomato Mutants nor and rin

Tomato (Lycopersicon esculentum Mill) plants of the nonripening mutant nor, the ripening-inhibited mutant rin, and the normal cultivar `Rutgers' were grown in nutrient solution supplemented with 3 grams per liter NaCl from the time of anthesis. In plants treated with NaCl, all the ripening parameters of the fruits of the nor mutant increased, but those of the rin mutant did not. The ripening of the fruits of the NaCl-treated nor plants was characterized by the development of a red color and taste, increased pectolytic activity, and increased evolution of CO₂ and ethylene. These changes do not normally take place in nor under control conditions. The values of these ripening parameters in nor were lower than those of the normal Rutgers fruits. In addition, both in nor and rin and in the normal variety, exposure of the plants to NaCl shortened the developmental period of the fruit, decreased the fruit size, and increased the concentrations of total soluble solids, Na⁺, Cl⁻, reducing sugars, and titratable acids in the fruit. The role of NaCl in overcoming the inability of nor to ripen is discussed.     

Upregulation of two ripening‐related genes from a non‐climacteric plant (pepper) in a transgenic climacteric plant (tomato)

Activities of promoters from the capsanthin/capsorubin synthase and fibrillin genes, which are molecular markers for ripening in the non-climacteric pepper fruits, have been studied in transgenic tomato plants that produce fruits of the climacteric type (characterized by an increase in respiration and ethylene production). The promoters of both genes were strongly upregulated during tomato fruit ripening in a manner similar to the induction of these genes in pepper fruits. Induction occurred at the mature green stage preceding ripening (a stage when ethylene production and respiration are known to rise in tomato fruits). Ethylene positively influenced the expression of both genes in tomato. Other plant growth regulators, namely abscisic acid, auxin and polyamines, did not alter gene expression. In contrast, water loss strongly induced both promoters. This dehydration-mediated gene induction was inhibited by mitochondrial respiration inhibitors (mainly of the alternative oxidase). A slight positive effect with light, apparently not linked to normal photosynthesis but rather to photooxidative stress, was also observed. Taken together, the data indicate that activation of oxidase systems, leading to changes in the cellular redox balance, mediates the induction of both genes in tomato. Various cellular compartments are likely to be contributors to this process, which leads to the developmental regulation of nuclear genes encoding plastid-located proteins.     

Polygalacturonase Isozymes and Pectin Depolymerization in Transgenic rin Tomato Fruit

We have previously described the construction and expression of a chimeric gene that allows developmentally regulated expression of tomato (Lycopersicon esculentum) polygalacturonase in ripening-impaired, mutant (rin) tomato fruit (JJ Giovannoni, D DellaPenna, AB Bennett, RL Fischer [1989] The Plant Cell 1: 53-63). We now show that expression of the chimeric polygalacturonase gene in rin tomato fruit resulted in the accumulation of all three polygalacturonase isozymes (PG1, PG2A, and PG2B). Polyuronide solubilization and polyuronide depolymerization both reached their maximal levels in transgenic rin fruit prior to the appearance of PG2 isozymes. These results demonstrate that PG1, PG2A, and PG2B all arise by differential processing of a single gene product and further suggest that the PG1 isozyme is sufficient to carry out both polyuronide solubilization and depolymerization in vivo.     

Transplantation Studies with Immature Fruit of Normal, and rin and nor Mutant Tomatoes

The aim of the work reported herein was to determine whether the lack of normal ripening in fruits of rin and nor tomato mutants is due to the presence of ripening inhibitors or to the lack of ripening factors in the fruit. A fruit tissue transplantation technique was developed for this purpose.     

Fruit cuticle lipid composition during development in tomato ripening mutants

Recent studies suggest that fruit cuticle is an important contributing factor to tomato (Solanum lycopersicum) fruit shelf life and storability. Moreover, it has been hypothesized that variation in fruit cuticle composition may underlie differences in traits such as fruit resistance to desiccation and microbial infection. To gain a better understanding of cuticle lipid composition diversity during fruit ontogeny and to assess if there are common features that correlate with ripening, we examined developmental changes in fruit cuticle wax and cutin monomer composition of delayed‐ripening tomato fruit mutants, ripening inhibitor (rin) and non‐ripening (nor) and delayed‐ripening landrace Alcobaça. Previous reports show that fruit ripening processes such as climacteric ethylene production, cell wall degradation and color change are significantly delayed, or do not occur, in these lines. In the study presented here, however, we show that fruits from rin, nor and Alcobaça have cuticle lipid compositions that differ significantly from normal fruits of Ailsa Craig (AC) even at very early stages in fruit development, with continuing impacts throughout ripening. Moreover, rin, nor and the Alcobaça lines show quite different wax profiles from AC and each other throughout fruit development. Although cutin monomer composition differed much less than wax composition among the genotypes, all delayed‐ripening lines possessed higher relative amounts of C₁₈ monomers than AC. Together, these results reveal new genetic associations between cuticle and fruit development processes and define valuable genetic resources to further explore the importance of cuticle in fruit shelf life.