This work characterized the role of the R2R3-MYB10 transcription factor (TF) in strawberry fruit ripening. The expression of this TF takes place mainly in the fruit receptacle and is repressed by auxins and activated by abscisic acid (ABA), in parallel to the ripening process. Anthocyanin was not produced when FaMYB10 expression was transiently silenced in fruit receptacles. An increase in FaMYB10 expression was observed in water-stressed fruits, which was accompanied by an increase in both ABA and anthocyanin content. High-throughput transcriptomic analyses performed in fruits with downregulated FaMYB10 expression indicated that this TF regulates the expression of most of the Early-regulated Biosynthesis Genes (EBGs) and the Late-regulated Biosynthesis Genes (LBGs) genes involved in anthocyanin production in ripened fruit receptacles. Besides, the expression of FaMYB10 was not regulated by FaMYB1 and vice versa. Taken together, all these data clearly indicate that the Fragaria × ananassa MYB10 TF plays a general regulatory role in the flavonoid/phenylpropanoid pathway during the ripening of strawberry.

Using cDNA microarrays, a comprehensive investigation of gene expression was carried out in strawberry (Fragaria ϫ ananassa) fruit to understand the flow of events associated with its maturation and non-climacteric ripening. We detected key processes and novel genes not previously associated with fruit development and ripening, related to vascular development, oxidative stress, and auxin response. Microarray analysis during fruit development and in receptacle and seed (achene) tissues established an interesting parallelism in gene expression between the transdifferentiation of tracheary elements in Zinnia elegans and strawberry. One of the genes, CAD, common to both systems and encoding the lignin-related protein cinnamyl alcohol dehydrogenase, was immunolocalized to immature xylem cells of the vascular bundles in the strawberry receptacle. To examine the importance of oxidative stress in ripening, gene expression was compared between fruit treated on-vine with a free radical generator and non-treated fruit. Of 46 genes induced, 20 were also ripening regulated. This might suggest that active gene expression is induced to cope with oxidative stress conditions during ripening or that the strawberry ripening transcriptional program is an oxidative stress-induced process. To gain insight into the hormonal control of non-climacteric fruit ripening, an additional microarray experiment was conducted comparing gene expression in fruit treated exogenously with auxin and control fruit. Novel auxin-dependent genes and processes were identified in addition to transcriptional programs acting independent of auxin mainly related to cell wall metabolism and stress response.

The strawberry (Fragaria 3 ananassa 'Chandler') fruit undergoes a fast softening during ripening. Polygalacturonase (PG) activity is low during this process, but two ripening-related PG genes, FaPG1 and FaPG2, have been cloned. Both genes were up-regulated during fruit ripening and were also negatively regulated by auxin. To further assess the role of FaPG1 on strawberry softening, transgenic plants containing an antisense sequence of this gene under the control of the 35S promoter (APG lines) were obtained. Sixteen out of 30 independent transgenic lines showed fruit yields similar to those of the control. Several quality parameters were measured in ripe fruits from these 16 lines. Fruit weight was slightly reduced in four lines, and most of them showed an increase in soluble solid content. Half of these lines yielded fruits significantly firmer than did the control. Four APG lines were selected, their ripened fruits being on average 163% firmer than the control. The postharvest softening of APG fruits was also diminished. Ripened fruits from the four selected lines showed a 90% to 95% decrease in FaPG1 transcript abundance, whereas the level of FaPG2 was not significantly altered. Total PG activity was reduced in three of these lines when compared with control fruits. Cell wall extracts from APG fruits showed a reduction in pectin solubilization and an increase in pectins covalently bound to the cell wall. A comparative transcriptomic analysis of gene expression between the ripened receptacle of the control and those of the APG fruits (comprising 1,250 receptacle expressed sequence tags) did not show any statistically significant change. These results indicate that FaPG1 plays a central role in strawberry softening.

Strawberry, a small fruit crop of great importance throughout the world, has been considered a model plant system for Rosaceae, and is susceptible to a large variety of phytopathogenic organisms. Most components and mechanisms of the strawberry defense network remain poorly understood. However, from current knowledge, it seems clear that the ability of a strawberry plant to respond efficiently to pathogens relies first on the physiological status of injured tissue (pre-formed mechanisms of defense) and secondly on the general ability to recognize and identify the invaders by surface plant receptors, followed by a broad range of induced mechanisms, which include cell wall reinforcement, production of reactive oxygen species, phytoalexin generation and pathogenesis-related protein accumulation. Dissection of these physiological responses at a molecular level will provide valuable information to improve future breeding strategies for new strawberry varieties and to engineer strawberry plants for durable and broad-spectrum disease resistance. In turn, this will lead to a reduction in use of chemicals and in environmental risks. Advances in the understanding of the molecular interplay between plant (mainly those considered model systems) and various classes of microbial pathogens have been made in the last two decades. However, major progress in the genetics and molecular biology of strawberry is still needed to uncover fully the way in which this elaborate plant innate immune system works. These fundamental insights will provide a conceptual framework for rational human intervention through new strawberry research approaches. In this review, we will provide a comprehensive overview and discuss recent advances in molecular research on strawberry defense mechanisms against pathogens.

Plant phenolics have drawn increasing attention due to their potential nutritional benefits. Although the basic reactions of the phenolics biosynthetic pathways in plants have been intensively analyzed, the regulation of their accumulation and flux through the pathway is not that well established. The aim of this study was to use a strawberry (Fragaria × ananassa) microarray to investigate gene expression patterns associated with the accumulation of phenylpropanoids, flavonoids, and anthocyanins in strawberry fruit. An examination of the transcriptome, coupled with metabolite profiling data from different commercial varieties, was undertaken to identify genes whose expression correlated with altered phenolics composition. Seventeen comparative microarray analyses revealed 15 genes that were differentially (more than 200-fold) expressed in phenolics-rich versus phenolics-poor varieties. The results were validated by heterologous expression of the peroxidase FaPRX27 gene, which showed the highest altered expression level (more than 900-fold). The encoded protein was functionally characterized and is assumed to be involved in lignin formation during strawberry fruit ripening. Quantitative trait locus analysis indicated that the genomic region of FaPRX27 is associated with the fruit color trait. Down-regulation of the CHALCONE SYNTHASE gene and concomitant induction of FaPRX27 expression diverted the flux from anthocyanins to lignin. The results highlight the competition of the different phenolics pathways for their common precursors. The list of the 15 candidates provides new genes that are likely to impact polyphenol accumulation in strawberry fruit and could be used to develop molecular markers to select phenolics-rich germplasm.

Two genomic clones corresponding to putative pectate lyase genes (plA and plB) were isolated and characterized in strawberry (Fragaria x ananassa cv. Chandler). The corresponding ORFs for the plA and plB genes revealed deduced proteins of 451 and 439 amino acids, respectively, that differ from that of the previously isolated strawberry plC gene. Southern blot analysis has shown that while the plB gene is a single copy gene, the plA gene is probably encoded by a small multigene family. By using specific probes corresponding to the untranslated 3' terminal region of the pl genes, and QRT-PCR methodology, the spatio-temporal expression pattern of both strawberry pl genes have been compared with that of the plC gene. The three transcripts were specifically expressed only in fruit and mainly during the ripening stages. Moreover, the expression of the plA and plB genes was induced in green de-achened fruit, but this increase was reduced by the external application of auxins as was the expression of plC. The expression of both pl genes was also strongly reduced in harvested fruit kept in controlled atmosphere (CA) containing high CO(2) levels. Immunolocalization studies using antibodies raised against the strawberry PL proteins placed the proteins in the cell wall of parenchymatic cells of the fruit receptacle. The role of pl genes in cell-wall disassembly and fruit ripening softening is discussed.

Eugenol is a volatile phenylpropanoid that contributes to flower and ripe fruit scent. In ripe strawberry (Fragaria 3 ananassa) fruit receptacles, eugenol is biosynthesized by eugenol synthase (FaEGS2). However, the transcriptional regulation of this process is still unknown. We have identified and functionally characterized an R2R3 MYB transcription factor (EMISSION OF BENZENOID II [FaEOBII]) that seems to be the orthologous gene of PhEOBII from Petunia hybrida, which contributes to the regulation of eugenol biosynthesis in petals. The expression of FaEOBII was ripening related and fruit receptacle specific, although high expression values were also found in petals. This expression pattern of FaEOBII correlated with eugenol content in both fruit receptacle and petals. The expression of FaEOBII was repressed by auxins and activated by abscisic acid, in parallel to the ripening process. In ripe strawberry receptacles, where the expression of FaEOBII was silenced, the expression of CINNAMYL ALCOHOL DEHYDROGENASE1 and FaEGS2, two structural genes involved in eugenol production, was down-regulated. A subsequent decrease in eugenol content in ripe receptacles was also observed, confirming the involvement of FaEOBII in eugenol metabolism. Additionally, the expression of FaEOBII was under the control of FaMYB10, another R2R3 MYB transcription factor that regulates the early and late biosynthetic genes from the flavonoid/phenylpropanoid pathway. In parallel, the amount of eugenol in FaMYB10-silenced receptacles was also diminished. Taken together, these data indicate that FaEOBII plays a regulating role in the volatile phenylpropanoid pathway gene expression that gives rise to eugenol production in ripe strawberry receptacles.The octoploid cultivated strawberry (Fragaria 3 ananassa) is one of the most economically important, nonclimacteric, soft fruits, in which volatile compounds influence fruit flavor and aroma. Both characteristics contribute to the fruit organoleptic traits and are crucial factors to determine fruit quality.At present, extensive surveys on the components that contribute to strawberry flavor have been performed. In these studies, more than 360 volatiles have been identified (Latrasse, 1991;Nijssen, 1996;Zabetakis and Holden, 1997), but only 15 to 20 of them in wild varieties of strawberry are believed to be essential for sensory quality, together with nonvolatile sugars and organic acids (Schieberle and Hofmann, 1997). In contrast, in cultivated varieties of strawberry, only about six odor-active compounds have been identified as contributors to fruit flavor (Raab et al., 2006;Ulrich et al., 2007). Strawberry aroma is the result of the combined perception of fruity (ethyl butanoate, ethyl hexanoate, and methyl 2-methylbutanoat), green (Z-3-hexenal), sweaty (butanoic acid and 2-methylbutanoic acid), peach-like (g-decalactone), and caramel-like [4-hydroxy-2,5-dimethyl-3(2H)-furanone and 2,5-dimethyl-4-methoxy-3(2H)-furanone] flavor notes (Pyysalo et al., 1979;Larsen and Poll, 1992). Several wild strawb...

Short-chain esters contribute to the blend of volatiles that define the strawberry aroma. The last step in their biosynthesis involves an alcohol acyltransferase that catalyses the esterification of an acyl moiety of acyl-CoA with an alcohol. This study identified a novel strawberry alcohol acyltransferase gene (FaAAT2) whose expression pattern during fruit receptacle growth and ripening is in accordance with the production of esters throughout strawberry fruit ripening. The full-length FaAAT2 cDNA was cloned and expressed in Escherichia coli and its activity was analysed with acyl-CoA and alcohol substrates. The semi-purified FaAAT2 enzyme had activity with C1-C8 straight-chain alcohols and aromatic alcohols in the presence of acetyl-CoA. Cinnamyl alcohol was the most efficient acyl acceptor. When FaAAT2 expression was transiently downregulated in the fruit receptacle by agroinfiltration, the volatile ester production was significantly reduced in strawberry fruit. The results suggest that FaAAT2 plays a significant role in the production of esters that contribute to the final strawberry fruit flavour.