Fruit Ripening

Abscission has been characterised as showing similarities with pod dehiscence, but it was originally identified as having many features in common with fruit ripening. They all exhibit forms of cell separation due to the actions of cell wall hydrolases, regulated by phytochromes (eg. ethylene). Fruit ripening was a relatively easy system to study because all the cells within the fruit are regulated to co-ordinate structural and textural changes, mainly as a result of cell wall re-organisation (Huber, 1983). Therefore as all the fruit cell walls are affected the differentially expressed mRNAs and their protein products can be easily isolated and analysed. Subsequently cellulase, polygalacturonase (PG) and pectin methyl esterase (PME) were cloned and used in probe hybridisations to elucidate the process of gene expression during abscission and dehiscence, with respect to their confinement to a discrete layer of cells (abscission zone, AZ).

As for abscission and dehiscence, anti-sense down-regulation of ripening specific genes was used to study what happens by effectively removing their respective proteins from the fruit because anti-sense mRNA reduces the amount of translatable, sense mRNA to less than 1% of its original level [for both abscission and dehiscence, tissue specific expression of anti-sense mRNA is used to ensure only the areas of interest (ie. abscission and dehiscence zones respectively) are affected, thus limiting the influence of other factors on the test system]. The ripening/ softening of tomatoes is an important agronomic character as it causes significant commercial losses due to short shelf-lives from over ripening. Therefore such genes involved in ripening have been down-regulated (Gray et al, 1992), for example the PG gene has been down-regulated in the fruit of one line of tomato by anti-sense expression (Smith et al, 1988), resulting in delayed ripening. This technique has now been applied to form a commercial variety, known as 'FlavrSavr'. PME has similarly been down-regulated in tomato (Hall et al, 1993) to also only delay the process of ripening.

Ethylene is believed to play the major role out of all the plant hormones during ripening, abscission and dehiscence in oilseed rape (Sexton & Roberts, 1982; Meakin & Roberts, 1990b) and other crops (Meakin, 1990). Morgan et al (1992) revealed that the synthesis of ethylene increases prior to abscission, which supports the evidence collected by Reid (1985) who suggested many years ago that ethylene is involved in cell separation. The enzymes that catalyse ethylene biosynthesis have been identified and their anti-sense down-regulation resulted in the inhibition of fruit ripening, indicating the importance of ethylene induced regulation (Oeller et al, 1991; Hamilton et al, 1990). The exact mechanism by which ethylene acts may be by either an on/off or a more complicated multiple control process. More recent investigations have characterised the proteins involved in the ethylene response-transduction pathway in many different species, as well as for the different cell separation events (eg. dehiscence). For example, the genes ETR1 and ERS from Arabidopsis thaliana code for proteins that show similarities to a bacterial two-component-like transduction system and have distinct ethylene binding domains. Therefore the exact role of ethylene is being elucidated and requires further research to completely identify the pathways involved.