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.

Rapid Alkalinization Factor (RALF) gene family genomic structure and transcriptional regulation during host-pathogen crosstalk in Fragaria vesca and Fragaria x ananassa strawberry

“…Furthermore, the N-terminus sequence alignment of FaRALF1-1, FaRALF3-1, FaRALF7-1, FaRALF8-1a and FaRALF13 peptides with AtRALF23 (Fig 4A) shows that residues directly involved in AtRALF23 binding with AtLLG2 are conserved, suggesting that FaRALF peptides in strawberry fruits may also interact with the Fxa LLG2 homolog and Fxa FER Malectin Like Receptor Kinase (MRLK) to a heterotypic complex. Consistently, the homology models of FaRALF3-1 peptide interaction with FaMRLK47 - the MRLK mostly expressed in Fxa fruits [50] and with the Fxa LLG2 homolog (maker-Fvb3-4-snap-gene-34.65), shows that the structural components necessary to bind MRLK47 and LLG2 are conserved in FaRALF3 (Fig 4B and C), suggesting a similar binding mechanism and complex formation. In Arabidopsis, At RALF23 binding to FERONIA and LLG proteins leads to negative immunity response regulation, however further analyses will be required to test FaRALF3-1 effectiveness binding to MRLK47 in fruits.…”

“…In a recent work, Jia et al [50] analyzed the expression of the woodland strawberry ( F. vesca ) Malectin Receptor Like Kinases (MRLK) also known as the Catharanthus roseus RLK-like proteins (CrRLK1Ls). F. vesca MRLKs are encoded by more than 60 genes, and more than 50% of these are expressed during fruit development.…”

“…The majority of fruit FvMRLK genes are expressed at high level only at the early stage of fruit ripening, and decrease at ripe stages. In particular, Jia et al [50], showed that transiently silencing and overexpression of MRLK47 in strawberry fruit, severely affected ripening regulation. Moreover, a recent report describes how MRKL47 changes the sensitivity of ripening-related genes to ABA, a key hormone for strawberry fruit ripening [51].…”

Continuous Exposure to Ethylene Differentially Affects Senescence in Receptacle and Achene Tissues in Strawberry Fruit

“…Ethylene plays a key role in the development of strawberry color, in the accumulation of taste-related compounds (flavonoids, phenolics, organic acids, and sugars), and in the softening process (Villarreal et al, 2010;Merchante et al, 2013;Li et al, 2016a;Villareal et al, 2016;Moya-León et al, 2019). Moreover, it has been reported that ethylene biosynthesis and signaling genes are differentially expressed during ripening of strawberry, and that some Ethylene Response Factors (ERFs) showed an expression pattern similar to those found in a climacteric fruit (Trainotti et al, 2005;Sun et al, 2013;Sánchez-Sevilla et al, 2017).…”

“…The high perishability of strawberry represents a bottleneck for expanding the fresh market supply chain (Terry et al, 2011). Strawberry is classified as a non-climacteric fruit and its ripening is mainly regulated by the abscisic acid (ABA)/auxins ratio (Moya-León et al, 2019). In particular, auxins have been associated with ripening inhibition, while ABA accumulation has been linked with promotion of strawberry fruit ripening (Jia et al, 2011;Symons et al, 2012;Leng et al, 2014;Chen et al, 2015;Chen et al, 2016;Giné-Bordonaba and Terry, 2016;Jia et al, 2016;Li et al, 2016b;Medina-Puche et al, 2016).…”