Abstract
The strawberry (Fragaria × ananassa) is one of the most preferred fresh fruit worldwide, accumulates numerous flavonoids but has limited shelf life due to excessive tissue softening caused by cell wall degradation. Since lignin is one of the polymers that strengthen plant cell walls and might contribute to some extent to fruit firmness monolignol biosynthesis was studied in strawberry fruit. Cinnamoyl-CoA reductase (CCR), cinnamyl alcohol dehydrogenase (CAD), and a peroxidase (POD27) gene were strongly expressed in red, ripe fruit whereas a second POD gene was primarily expressed in green, immature fruit. Moreover, FaPOD27 transcripts were strongly and constitutively induced in fruits exposed to Agrobacterium infection. Gene expression levels and enzymatic activities of FaCCR and FaCAD were efficiently suppressed through RNAi in FaCCR- and FaCAD-silenced strawberries. Besides, significantly elevated FaPOD transcript levels were detected after agroinfiltration of pBI-FaPOD constructs in fruits. At the same time, levels of G-monomers were considerably reduced in FaCCR-silenced fruits whereas the proportion of both G- and S-monomers decisively decreased in FaCAD-silenced and pBI-FaPOD fruits. Development, firmness, and lignin level of the treated fruits were similar to pBI-intron control fruits, presumably attributed to increased expression levels of FaPOD27 upon agroinfiltration. Additionally, enhanced firmness, accompanied with elevated lignin levels, was revealed in chalcone synthase-deficient fruits (CHS−), independent of down- or up-regulation of individual and combined FaCCR. FaCAD, and FaPOD genes by agroinfiltration, when compared to CHS−/pBI-intron control fruits. These approaches provide further insight into the genetic control of flavonoid and lignin synthesis in strawberries. The results suggest that FaPOD27 is a key gene for lignin biosynthesis in strawberry fruit and thus to improving the firmness of strawberries.
Highlights
The strawberry (Fragaria × ananassa) is a highly perishable fruit, with a short shelf life due to increased cell wall degradation during the late stages of ripening, which results in soft fruits (Lefever et al, 2004)
In addition to LC-UVESI-MSn analyses of individual enzymatic assays, a GST-FaCCR reaction containing a mixture of equal molar amounts of three substrates, yielded three major peaks that were identified as caffeic aldehyde (3,4dihydroxycinnamaldehyde), p-coumaraldehyde, and coniferaldehyde, respectively
The activity of GST-FaCCR toward the different CoA esters was calculated from the peak areas and indicated that feruloyl-CoA (100%) was the preferred substrate compared with both caffeoyl-CoA (3%) and p-coumaroyl-CoA (3%) (Fig. S5)
Summary
The strawberry (Fragaria × ananassa) is a highly perishable fruit, with a short shelf life due to increased cell wall degradation during the late stages of ripening, which results in soft fruits (Lefever et al, 2004). Fruit firmness is an important target for genetic engineering to improve the quality of strawberries and to prolong the shelf-life of fresh fruit in markets (Chapple and Carpita, 1998; Manning, 1998). As fruit softening is associated with cell wall disassembly (Seymour and Gross, 1996) several studies aimed at slowing down this degradation process to improve the texture of strawberries. The pectate lyase gene is highly expressed during strawberry fruit ripening and is considered to play an important role in pectin decomposition (Dominguez-Puigjaner et al, 1997; MedinaEscobar et al, 1997). Transgenic strawberry fruits harboring antisense pectate lyase genes resulted in reduced pectate lyase activity, as well as increased fruit firmness (Jiménez-Bermúdez et al, 2002; Quesada et al, 2009)
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