Molecular Events Occurring During Softening of Strawberry Fruit

Abstract

Changes in fruit texture taking place during ripening, described as softening, are mainly due to alterations in structure and/or composition of the cell wall. Several non-covalent interactions between the three carbohydrate polymers of the cell wall, cellulose, pectins and hemicellulose, and many structural proteins and ions, enable a complex structure. During softening, the disassembly of the cell wall structure takes place, mediated by a complete set of cell wall degrading enzymes or proteins. Softening is a coordinated event that requires the orchestrated participation of a wide variety of proteins. Plant hormones and a set of transcription factors are the organizers of this multi-protein effort. Strawberry is a non climacteric fruit that softens intensively during the last stages of development. The Chilean strawberry fruit (Fragaria chiloensis), the maternal relative of the commercial strawberry (F. × ananassa), softens even faster than commercial strawberry. Softening of the Chilean strawberry fruit has been studied at different levels: changes in cell wall polymers, activity of cell wall degrading enzymes and transcriptional changes of their genes, providing a general view of the complex process. The search for the 'orchestra director' that could coordinate softening events in strawberry fruit has been focussed on hormones like ABA and auxins, and more precisely the relation ABA/AUX. These hormones regulate the expression of many cell wall degrading enzyme genes, and this massive transcriptional change that takes place involves the participation of key transcriptional factors (TF). This review provides an update of the present knowledge regarding the softening of strawberry fruit. Nevertheless, the entire softening process is still under active research especially for the great influence of texture on fruit quality and its high impact on fruit shelf life, and therefore it is expected that new and promising information will illuminate the field in the near future.

Keywords: cell wall; fruit ripening; softening; strawberry; transcription factor.

Figure 1

Diagram showing the temporal expression profile of cell wall degrading genes during ripening development of two strawberry fruit species: (A) F.× ananassa, and (B) F. chiloensis. Cell wall degrading enzymes correspond to Pectin methylesterase (PME), α-Arabinofuranosidase (AFase), Polygalacturonase (PG), Pectate lyase (PL), β-Galactosidase (βGal), Xyloglucan endotransglycosilase/ hydrolase (XTH), Endoglucanase (EGase), Expansins (EXP) and Rhamnogalacturonan I lyase (RGL). Fruit stages for F.×ananassa correspond to: LG, large green; SW, small white; LW, large white; P, pink/turning; R, red. Fruit stages for F. chiloensis correspond to: LG, large green (large size fruit with red achenes); T, turning (large size fruit with white receptacle and red achenes); and R, ripe fruit (full size fruit with pink receptacle and red/brown achenes). (C) Scheme showing changes in auxins and ABA levels in F.×ananassa fruit; in gray the ABA/AUX ratio (adapted from Symons et al., 2012).