ReviewCell wall modifications during fruit ripening: when a fruit is not the fruit
Section snippets
Introduction: the significance of fruit ripening and associated textural modifications
Once regarded as a senescent phenomenon, fruit ripening is now considered as a well coordinated and genetically determined process of tissue differentiation. Events like pigment accumulation and volatile production are included among universal ripening changes but do not occur usually in senescence (Brady, 1987). Fruit ripening is a crucial physiological process for plants, since it represents the terminal stage of development in which the matured seeds are released. Therefore, it is the
Botanical, compositional and developmental differences between fruits
Different fruits differ markedly in their botanical origin, polysaccharide and protein composition, cell wall structure, enzymatic metabolism, growing and ripening pattern, or softening behaviour. These differences that reflect pulp firmness, rate of softening and overall texture are now recognized not only between different species but also between different cultivars, varieties and selections from the same species.
Different fruits have distinct botanical origins. The botanical definition of
Cell wall modifications in structure and composition during fruit ripening
Changes in the structure of the cell wall are associated with dissolution of the middle lamella and disruption of the primary cell wall (Crookes & Grierson, 1983). Structural changes in pectin, hemicellulose and cellulose together are assumed to be responsible for the alteration of cell wall structure during ripening-related loss of firmness (Huber, 1983, Seymour et al., 1990). These changes include not only solubilisation and depolymerisation of the polysaccharides but also rearrangements of
Parallels and differences in the temporal pattern of ripening-related cell wall modifications in distinct fruits
Knowledge about the temporal pattern in which these modifications take place is important to the overall understanding of the whole process. Using “Charentais” melon (C. melo L.), a fruit which softens in a very short period of time, a sequential pattern of polysaccharide modifications was proposed for the first time (Rose et al., 1998). The results obtained suggest that early events in melon fruit softening are associated with the regulated disassembly of a tightly bound fraction of xyloglucan
The expression and activity of cell wall-modifying enzymes
The plant cell wall contains many enzymes able to modify matrix polysaccharides, including several types of endoglycanases ordered to cleave the backbone of matrix hemicelluloses or pectins; glycosidases that may remove side-chains, thus allowing greater interactions between polysaccharide backbones; transglycosylases that may cut hemicelluloses and ligate them together, or esterases and acetylases that can remove methyl or acetyl groups from pectins and cleave ester linkages between
Probing the function of ripening-related cell wall-modifying enzymes using altered genetic backgrounds
Results from genetically modified lines in which fruit ripening-associated cell wall genes have been suppressed or overexpressed and analysis of gene expression in known ripening-impaired mutants, have provided more direct information about the possible function of each gene family and isoform in ripening. This sub-section reviews and discusses the literature concerning the results of the manipulation of individual cell wall-modifying genes in fruits, and its outcome in fruit softening and
Methodological constrains to the study of cell wall-related activities
The role of each enzyme cannot be explained by studying a single specific isoform since the presence of several isoforms, with distinct patterns of expression, may mask the total activity in a given developmental stage. Hence, assays for monitoring the changes in the activity during the development of the fruit are still informative and needed to complement the studies of genetic expression. Examination of the gene expression and enzyme activity data in the literature often shows a poor
Non-enzymatic control of cell wall modifications during ripening
More recently, it was suggested that ripening could occur as the result of non-enzymatic modifications of the cell wall components, both pectins and xyloglucans. Membrane permeabilisation occurring early in ripening, leads to the release of ascorbate into the apoplast, where it may trigger apoplastic hydroxyl production via the Fenton reaction and this radical is potentially involved in non-enzymatic scission of plant cell wall polysaccharides.
During ripening of tomato, excised pieces of living
Taking advantage of natural variation for improving texture in fruits
This review points out important differences concerning the softening behaviour of individual fruits. This aspect means that each fruit cultivar from a given species should be regarded to have putatively some specificities in relation to softening-related cell wall metabolism. This implies that a substantial amount of work must be undertaken in order to clarify specific aspects of fruit softening, in order to fully understand the mechanism. However, these natural occurring differences can be
Conclusions and prospects
Dietary guidelines recommending the consumption of fresh fruits will not be succeeded if consumer dissatisfaction with the product quality limits fruit consumption. However, contemporary traditional postharvest approaches for fruits are not sufficient to meet the increasing consumer's demand on quality due to the development of undesirable characteristics. Increasing the storage life of fruits through the development of new techniques aimed to reduce the rate of deterioration while maintaining
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